ASNT NDT Handbook Volume 2_ Liquid Penetrant

NONDESTRUCTIVE TESTING Third Edition

HANDBOOK

Volume 2

Liquid Penetrant
Testing

Technical Editor
Noel A. Tracy

Editor
Patrick O. Moore

American Society for Nondestructive Testing

NONDESTRUCTIVE TESTING Third Edition

HANDBOOK

Volume 2

Liquid Penetrant
Testing

Technical Editor
Noel A. Tracy
Editor
Patrick O. Moore

® American Society for Nondestructive Testing

FOUNDED 1941

Copyright © 1999
AMERICAN SOCIETY FOR NONDESTRUCTIVE TESTING, INC.
All rights reserved.

No part of this book may be reproduced, stored in a retrieval system or transmitted, in any form or by any means —
electronic, mechanical, photocopying, recording or otherwise — without the prior written permission of the publisher.
Nothing contained in this book is to be construed as a grant of any right of manufacture, sale or use in connection with
any method, process, apparatus, product or composition, whether or not covered by letters patent or registered
trademark, nor as a defense against liability for the infringement of letters patent or registered trademark.
The American Society for Nondestructive Testing, its employees and the contributors to this volume are not responsible
for the authenticity or accuracy of information herein, and opinions and statements published herein do not necessarily
reflect the opinion of the American Society for Nondestructive Testing or carry its endorsement or recommendation.
The American Society for Nondestructive Testing, its employees, and the contributors to this volume assume no
responsibility for the safety of persons using the information in this book.

American Society for Nondestructive Testing, Incorporated
1711 Arlingate Lane
PO Box 28518
Columbus, OH 43228-0518
(614) 274-6003; fax (614) 274-6899
www.asnt.org

Library of Congress Cataloging-in-Publication Data

Liquid penetrant testing / technical editor, Noel A. Tracy ; editor, Patrick O.

Moore

p. cm. -- (Nondestructive testing handbook. Third edition ; v. 2)

Includes bibliographical references and index.

ISBN-13 978-1-57117-028-6

ISBN-10 1-57117-028-6

1. Penetrant inspection. I. Tracy, Noel A. II. Moore, Patrick O.

III. Series: Nondestructive testing handbook (3rd ed.) ; v. 2.

TA417.55.L57 1999

620.1’127--dc21 99-34056

CIP

First printing 09/99.
Second printing 11/03.
Third printing 07/06, with new impositions for pages ii, 84.
Fourth printing 11/08.

Published by the American Society for Nondestructive Testing
PRINTED IN THE UNITED STATES OF AMERICA

President’s Foreword

Liquid Penetrant Testing is the second The existence of Liquid Penetrant Testing
volume of the third edition of the is testimony to the commitment of the
Nondestructive Testing Handbook. This American Society for Nondestructive
volume continues to advance the series’ Testing (ASNT) to its missions of
mission of disseminating information providing technical information and
about the technology. instructional materials and of promoting
nondestructive testing technology as a
Nondestructive testing contributes to profession.
public safety and to our quality of life in
countless ways. The technology has made Robert E. Green, Jr.
possible those advances in technology ASNT National President (1998-99)
that are the hallmark of this turn of the
century.

Technology typically relies on things
that can be counted, on numbers — on
measurements and data that can be
quantified, processed and stored by
computer. In such an age, liquid
penetrant testing occupies a special place
because it is a qualitative method that has
defied quantification. At the same time,
the method remains extremely sensitive,
reliable, cost effective and useful to
industry. Because liquid penetrant testing
relies so much on the training and
experience of the human inspector, an
authoritative handbook is especially
important.

ASNT has been fortunate that the
Technical Council’s Penetrant Committee
is superbly qualified to provide the
expertise needed to rewrite and review a
book of such importance and scope. The
collaboration between the volunteers and
staff in the writing and review of this
volume has made productive use of
ASNT’s volunteer resources. Scores of
authors and reviewers have donated
thousands of hours to this volume.

Liquid Penetrant Testing was produced
under the guidance of ASNT’s Handbook
Development Committee. A special note
of thanks is extended to Handbook
Development Director Gary Workman; to
recent Penetrant Committee Chairs
William E. Mooz, Vilma G. Holmgren,
Brian MacCracken and Michael L. White;
to Technical Editor Noel A. Tracy; and to
Handbook Editor Patrick Moore for their
dedicated efforts.

Liquid Penetrant Testing iii

Foreword

The Aims of a Handbook for instance, may have little bearing on a
practical examination. Other parts of a
The volume you are holding in your hand handbook are specific to a certain
is the second in the third edition of the industry. Although a handbook does not
Nondestructive Testing Handbook. Now is a pretend to offer a complete treatment of
good time to reflect on the purposes and its subject, its value and convenience are
nature of a handbook. not to be denied.

Handbooks exist in many disciplines of The present volume is a worthy
science and technology, and certain addition to the third edition. The editors,
features set them apart from other technical editors and many contributors
reference works. A handbook should and reviewers worked together to bring
ideally give the basic knowledge necessary the project to completion. For their
for an understanding of the technology, scholarship and dedication I thank
including both scientific principles and them all.
means of application.
Gary L. Workman
The typical reader may be assumed to Handbook Development Director
have completed three years of college
toward a degree in mechanical
engineering or materials science and
hence has the background of an
elementary physics or mechanics course.
Occasionally an engineer may be
frustrated by the difficulty of the
discussion in a handbook. That happens
because the assumptions about the reader
vary according to the subject in any given
chapter. Computer science requires a sort
of background different from nuclear
physics, for example, and it is not possible
for the handbook to give all the
background knowledge ancillary to
nondestructive testing.

A handbook offers a view of its subject
at a certain period in time. Even before it
is published, it starts to get obsolete. The
authors and editors do their best to be
current but the technology will continue
to change even as the book goes to press.

Standards, specifications,
recommended practices and inspection
procedures may be discussed in a
handbook for instructional purposes, but
at a level of generalization that is
illustrative rather than comprehensive.
Standards writing bodies take great pains
to ensure that their documents are
definitive in wording and technical
accuracy. People writing contracts or
procedures should consult real standards
when appropriate.

Those who design qualifying
examinations or study for them draw on
handbooks as a quick and convenient way
of approximating the body of knowledge.
Committees and individuals who write or
anticipate questions are selective in what
they draw from any source. The parts of a
handbook that give scientific background,

iv Liquid Penetrant Testing

Preface

What could be simpler than directly in place of other, more expensive point
viewing a part with a suitable light to see sensitive techniques such as eddy current
an indication of a discontinuity produced testing. However, in some applications,
by dipping the part in a colored liquid, especially where residual compressive
washing excess liquid off with a water stresses exist, the surface opening of a
hose and drying the part? As one gains discontinuity may be too small for
experience with liquid penetrant testing, reliable liquid penetrant testing. A good
a more appropriate question may come to example is a small fatigue crack.
mind: how can a simple technique be so
complex? Liquid penetrant materials are
constantly being improved to meet
Because the liquid penetrant test is general or specific application
fundamentally simple and the equipment requirements, to make the test process
(if any) is easy to operate, untrained more forgiving or to satisfy new
observers usually think they can save time environmental concerns. In some
and money by borrowing some liquid applications water washable liquid
penetrant materials and performing the penetrants are as sensitive as
test themselves. However, as training and postemulsifiable types. Equipment is also
experience show, the best materials are improving. Properly designed and
worthless without strict adherence to monitored automated processing systems
processing guidelines that direct and have the potential to more carefully
control the test from the preparation of control the liquid penetrant testing
the inspection surface (including the process while alleviating the monotony
crack surface), to the visual examination experienced by an inspector who applies
of the part to locate an indication. the test steps repetitively.
Furthermore, the materials themselves
require some basic care so that they don’t The technology of liquid penetrant
spoil. Even experienced inspectors must testing lacks a reliable and objective
avoid the trap of apparent simplicity, scientific test for evaluating the sensitivity
which breeds complacency. Inattention to of a liquid penetrant test. Photometers
processing details and materials have been used in the laboratory to assign
maintenance will result in a test that will arbitrary sensitivity levels to liquid
fail because it is out of control. The penetrant systems by measuring the
editing of this volume has attempted to luminance of fluorescent indications.
emphasize these issues. However, even with precisely controlled
processing parameters, the luminance
Perhaps complex is the wrong word to measurements on a set of low cycle
describe the test; methodical is probably a fatigue cracks have been reproducible
better descriptor. Apparent disadvantages only within 20 percent.
of the test can usually be overcome by
modifying a step or applying a different Part of the problem has been the
set of steps. For example, inspectors could difficulty of correlating the physical and
work in tandem when testing large areas, chemical phenomena and properties of
for which process control is more liquid penetrants to practical liquid
difficult, or use liquid penetrant that penetrant test sensitivity. This difficulty
requires more than just water to remove it influenced the decision to limit the
if removal of the liquid penetrant from theoretical discussions in this volume to
shallow cracks is a concern. In another the practical characteristics of liquid
situation the requirement for strict penetrant materials. Because an inspector
process control may be turned into an is ultimately concerned about the
advantage in that methodical adjustments presence or absence of a relevant
in the process can adjust the sensitivity of indication, the more that is understood
the test so that only the relevant about how the test process affects those
discontinuities are detected. characteristics, the more likely a visible
indication will be produced.
Despite its subtlety liquid penetrant
testing does work. Large areas, small areas, The future of liquid penetrant testing is
plane surfaces, multifaceted surfaces, all sure to include continued efforts to bring
can be inspected quickly and machine vision and decision making up
economically. Because of this advantage it to the level of competency achieved with
is tempting to use liquid penetrant testing the human eye and brain. Initially,
questions of economic feasibility will

Liquid Penetrant Testing v

have to be answered in light of the
current economic advantages of liquid
penetrant testing, but cheaper technology
that works will be used.

The Technical Editor is indebted to the
committee members, contributors,
reviewers who volunteered to help
assemble this book. The aim was to build
on the work of those who contributed to
previous editions, updating the technical
content while preserving the
technological story line of lessons learned.
The guidance and assistance of the ASNT
staff is also gratefully acknowledged.
Noel A. Tracy
Technical Editor

vi Liquid Penetrant Testing

Editor’s Preface

The third edition of the Nondestructive Acknowledgments
Testing Handbook continues as the second
edition did, with a volume on liquid Handbook Development
penetrant testing. This third edition Committee
volume is indebted to the preceding
edition’s volume in many ways. Much of Gary L. Workman, University of Alabama,
the text is the same, despite significant Huntsville
additions and alterations.
Michael W. Allgaier, GPU Nuclear
The technical content of this third Albert S. Birks, AKZO Nobel Chemicals
edition volume differs in several ways Richard H. Bossi, Boeing Company, Seattle
from that of the second. (1) Pages have Lisa Brasche, Iowa State University
been added to cover new applications, Lawrence E. Bryant, Jr., Los Alamos
such as filtered particle testing of
aerospace composites and quality control National Laboratory
of down hole oil field tubular assemblies. William C. Chedister, Circle Chemical Co.
(2) A new section on probability of James L. Doyle, Northwest Research
detection may help some facilities to
evaluate their inspection procedures. Associates, Inc.
(3) The introduction includes new Allen T. Green, Acoustic Technology
information on method history and
measurement units. (4) The text reflects Group
the fact that materials degrading to the Robert E. Green, Jr., The Johns Hopkins
environment have been regulated.
(5) A comprehensive glossary is provided. University
(6) An extensive bibliography lists liquid Matthew J. Golis, Advanced Quality
penetrant testing publications, more than
some practitioners of the method might Concepts
have expected. (7) This third edition Frank A. Iddings
volume pays more attention to standards Charles N. Jackson, Jr.
documents than did the second edition; John K. Keve, DynCorp Tri-Cities Services
references to current standards have been Lloyd P. Lemle, Jr., BP Oil Company
added throughout. Xavier P.V. Maldague, University Laval
Michael L. Mester, The Timken Company
The contributors and reviewers all Paul McIntire, American Society for
brought their gifts individually to this
project — collectively they made it better Nondestructive Testing
than a product of one person could be. Ronnie K. Miller, Physical Acoustics
Among these volunteers, the editors
would like to thank William E. Mooz for Corporation
the time he invested in careful reading of Scott D. Miller, Saudi Aramco
the entire book. Patrick O. Moore, American Society for

ASNT is indebted to Technical Editor Nondestructive Testing
Noel A. Tracy and to all the technical Stanley Ness
experts listed at the end of this foreword. Ronald T. Nisbet, IESCO
(Please note that people listed as Louis G. Pagliaro, Technical Associates of
contributors were also reviewers but are
listed only once, as contributors.) Charlotte
Emmanuel P. Papadakis, Quality Systems
It is difficult to overstate the
contributions of staff members Hollis Concepts
Humphries and Joy Grimm to the art, J. Thomas Schmidt, J.T. Schmidt
layout and text of the book. I would also
like to thank Publications Manager Paul Associates
McIntire for his support throughout Fred Seppi, Williams International
production. Amos G. Sherwin, Sherwin Incorporated
Kermit S. Skeie, Kermit Skeie Associates
Patrick O. Moore Roderic K. Stanley, Quality Tubing
Editor Holger H. Streckert , General Atomics
Stuart A. Tison, Millipore Corporation
Noel A. Tracy, Universal Technology

Corporation
Mark F.A. Warchol, Aluminum Company

of America
Glenn A. Washer, Turner-Fairbank

Highway Research Center
George C. Wheeler, Materials & Processes

Consultants

Liquid Penetrant Testing vii

Contributors Ronald T. Nisbet, IESCO
Clifford D. Smith, Smith’s NDT Services
James S. Borucki, Gould Bass NDT Clint E. Surber, Boeing Company, Seattle
Art Cedillos, Palomar Plating Company Michael L. Turnbow, Tennessee Valley
Jeffrey F. Cook, JFC NDE Engineering
Robert L. Crane, Air Force Research Authority
Alexander Waluszko, UVP, Incorporated
Laboratory Mark F.A. Warchol, Alcoa Technical
Charles W. Eick, Dassault Falcon Jet
John J. Flaherty, Flare Technology Center
Matthew J. Golis, Advanced Quality
Multimedia Contributors
Concepts
Bruce C. Graham Thomas H. Bennett, Howmet Corporation
Frank V. Gricus, Reynolds Metals Charles J. Hellier, III, Hellier Associates

Company
Donald J. Hagemaier, Boeing Company,

Long Beach
Norman J. Hendle
Vilma G. Holmgren, Magnaflux Division

of Illinois Tool Works
Dennis G. Hunley, Quality Assurance

Corporation
Robert J. Lord, Jr., Boeing Company,

St. Louis
Brian A. MacCracken, Pratt & Whitney
Joseph L. Mackin, International Pipeline

Inspectors Association
William E. Mooz, Met-L-Chek Company
Stanley Ness
Samuel J. Robinson, Sherwin Incorporated

— East
David J. Ross, Spectronics Corporation
Ward D. Rummel, D&W Enterprises
J. Thomas Schmidt, J.T. Schmidt

Associates
Amos G. Sherwin, Sherwin Incorporated
Kermit A. Skeie, Kermit Skeie Associates
Dennis S. Smith, Boeing Company, Long

Beach
Jack C. Spanner, Sr., Spanner Engineering
Holger H. Streckert, General Atomics
Richard Z. Struk, Shellcast Foundries
Noel A. Tracy, Universal Technology

Corporation
Roger D. Wallace, Newport News

Shipbuilding and Dry Dock Company
Michael L. White, Met-L-Chek Company

Reviewers

Robert A. Baker
Thomas H. Bennett, Howmet Corporation
Lisa Brasche, Iowa State University
Patrick Dubosc, Babb Company SA
Rob Hagen, NDT Europa BV
William O. Holden, William Holden

Company
Stephen C. Hoyt, American Society for

Nondestructive Testing
James F. Jackson
Brian F. Larson, Iowa State University
Richard D. McGuire, National Board of

Boiler and Pressure Vessel Inspectors
Gregory F. Monks, QC Technologies,

Incorporated

viii Liquid Penetrant Testing

Contents

Chapter 1. Introduction to Liquid Chapter 4. Care and Maintenance of
Penetrant Testing . . . . . . . . . . . . 1 Liquid Penetrant Test Materials . 99

Part 1. Nondestructive Testing . . . . 2 Part 1. Importance of Maintenance
Part 2. Management of Liquid of Liquid Penetrant
Materials . . . . . . . . . . . . 100
Penetrant Testing . . . . . . . . 7
Part 3. Personnel Selection and Part 2. Care and Maintenance of
Liquid Penetrant Testing
Qualification for Liquid Materials in Storage . . . . 101
Liquid Penetrant
Testing . . . . . . . . . . . . . . . 12 Part 3. Care and Maintenance of
Part 4. History of Liquid Penetrant Liquid Penetrants
Testing . . . . . . . . . . . . . . . 19 in Use . . . . . . . . . . . . . . 103
Part 5. Units of Measure for
Nondestructive Testing . . 27 Part 4. Care and Maintenance of
Liquid Penetrant
Chapter 2. Principles of Liquid Emulsifiers and Removers
Penetrant Testing . . . . . . . . . . . 33 in Use . . . . . . . . . . . . . . 106

Part 1. Elements of Liquid Part 5. Care and Maintenance of
Penetrant Testing . . . . . . . 34 Developers in Use . . . . . 107

Part 2. Liquid Penetrant Testing Part 6. Quality Control Tests for
Processes . . . . . . . . . . . . . 42 Liquid Penetrant
Materials . . . . . . . . . . . . 109
Part 3. Principles of Emulsification
and Removal of Excess Part 7. Quality Control Tests for
Surface Liquid Test Systems and
Penetrant . . . . . . . . . . . . . 48 Procedures . . . . . . . . . . . 118

Part 4. Principles of Application Chapter 5. Interpretation of Liquid
and Operation of Penetrant Indications . . . . . . . 125
Developers . . . . . . . . . . . . 55
Part 1. Inspector Functions and
Part 5. Inspection and Terminology for
Interpretation of Liquid Interpretation and
Penetrant Indications . . . 59 Evaluation of Liquid
Penetrant Indications . . 126
Part 6. Field Techniques for Liquid
Penetrant Testing . . . . . . . 65 Part 2. General Interpretation of
Liquid Penetrant
Part 7. Maintenance of Liquid Indications . . . . . . . . . . 133
Penetrant Test Systems . . 69
Part 3. Processing Effects
Part 8. Health and Safety Influencing Liquid
Penetrant Indications . . 136
Precautions . . . . . . . . 71
Part 4. Establishing Acceptance
Chapter 3. Characteristics of Liquid Standards for Liquid
Penetrant and Processing Penetrant Indications . . 140
Materials . . . . . . . . . . . . . . . . . . 83
Part 5. Interpretation of Liquid
Part 1. Liquid Properties of Liquid Penetrant Indications of
Penetrant . . . . . . . . . . . . . 84 Cracks . . . . . . . . . . . . . . 142

Part 2. Liquid Penetrant Part 6. Interpretation of Liquid
Removal . . . . . . . . . . . . . 87 Penetrant Indications of
Laminar
Part 3. Color of Liquid Discontinuities . . . . . . . 147
Penetrants . . . . . . . . . . . . 90
Part 7. Interpretation of Liquid
Part 4. Action of Developers . . . . 92 Penetrant Indications of
Part 5. Viewing Indications . . . . . 95 Porosity and Leaks . . . . . 149

Part 8. Nonrelevant and False
Liquid Penetrant
Indications . . . . . . . . . . 151

Liquid Penetrant Testing ix

Part 9. Recognition Training of Chapter 10. Liquid Penetrant System
Liquid Penetrant Chemistry and Effluent Waste . 287
Inspectors . . . . . . . . . . . 153
Part 1. Effects of Sulfur and
Part 10. Specifications and Guides Chlorine Impurities in
for Evaluation of Liquid Liquid Penetrant
Penetrant Indications . . 154 Materials . . . . . . . . . . . . 288

Chapter 6. Surface Preparation and Part 2. Mechanisms of Stress
Cleaning . . . . . . . . . . . . . . . . . 161 Corrosion Cracking of
Austenitic Stainless
Part 1. Effects of Test Object Steels . . . . . . . . . . . . . . . 295
Surface Contamination
or Irregularities . . . . . . . 162 Part 3. Methods for Sulfur and
Halogen Analysis of
Part 2. Procedures for Cleaning Liquid Penetrant
Surfaces before Liquid Materials . . . . . . . . . . . . 300
Penetrant Testing . . . . . . 167
Part 4. Techniques for Control of
Part 3. Procedures for Postcleaning Pollution from Liquid
Test Objects after Liquid Penetrant Waste . . . . . . . 306
Penetrant Testing . . . . . . 178
Part 5. Recycling of Water Effluent
Part 4. Cleaning Requirements for and Postemulsifiable
Fluorescent Liquid Liquid Penetrant . . . . . . 314
Penetrant Testing in
Aircraft Overhaul . . . . . . 181 Part 6. Clarification and
Distillation Recovery of
Part 5. Influence of Mechanical Waste Water . . . . . . . . . . 318
Processing on Effectiveness
of Liquid Penetrant Chapter 11. Filtered Particle Testing . 325
Testing . . . . . . . . . . . . . . 184
Part 1. Principles of Filtered
Chapter 7. Liquid Penetrant Testing Particle Testing . . . . . . . 326
Equipment . . . . . . . . . . . . . . . . 201
Part 2. Mechanisms of
Part 1. Processing Equipment for Operation of Filtered
Liquid Penetrant Particle Tests . . . . . . . . . 328
Testing . . . . . . . . . . . . . . 202
Part 3. Design and Selection of
Part 2. Large Scale Automated Filtered Particle Test
Aerospace Liquid Media . . . . . . . . . . . . . . 331
Penetrant Test
Equipment . . . . . . . . . . . 217 Part 4. Filtered Particle Test
Equipment . . . . . . . . . . . 333
Part 3. Lighting for Liquid
Penetrant Testing . . . . . . 226 Part 5. Prewetting and Associated
Phenomena . . . . . . . . . . 335
Part 4. Mechanized Scanning of
Fluorescent Liquid Part 6. Interpretation of Filtered
Penetrant Indications . . 239 Particle Test
Indications . . . . . . . . . . 337
Chapter 8. Comparators and Reference
Panels . . . . . . . . . . . . . . . . . . . 245 Part 7. Filtered Particle Testing
of Carbon Matrix
Part 1. Cracked Metal Components . . . . . . . . . 339
Comparator Blocks . . . . 246
Chapter 12. Liquid Penetrant Testing
Part 2. Surface Cracked in Primary Metals Production . 343
Nickel-Chrome Liquid
Penetrant Test Panels . . . 254 Part 1. Liquid Penetrant Testing
in Foundries . . . . . . . . . 344
Part 3. Liquid Penetrant
Comparators with Surface Part 2. Liquid Penetrant Testing
Indentations Simulating of Ferrous Metals . . . . . . 350
Discontinuities . . . . . . . 259
Part 3. Liquid Penetrant Testing
Part 4. Test Panels for in Light Alloy
Measurement of Foundries . . . . . . . . . . . . 352
Background Levels . . . . . 264
Chapter 13. Electric Power Applications
Chapter 9. Liquid Penetrant Testing of Liquid Penetrant Testing . . . 361
Crack Detection Capabilities
and Reliability . . . . . . . . . . . . . 275 Part 1. Applications of Liquid
Penetrant Testing in
Electric and Power
Plants . . . . . . . . . . . . . . 362

x Liquid Penetrant Testing

Part 2. Liquid Penetrant Testing of Chapter 16. Liquid Penetrant Testing
Pressure Components of Glossary . . . . . . . . . . . . . . . . . . 433
Nuclear Power Plants . . . 364
Chapter 17. Liquid Penetrant Testing
Part 3. Examples of Liquid Bibliography . . . . . . . . . . . . . . 453
Penetrant Indications of
Discontinuities in Reactor Index . . . . . . . . . . . . . . . . . . . . . . . . 483
Piping . . . . . . . . . . . . . . 369
Figure Sources . . . . . . . . . . . . . . . . . . 493
Part 4. Personnel Performing
Liquid Penetrant Testing Movie Sources . . . . . . . . . . . . . . . . . . 494
in Power Plants . . . . . . . 374

Chapter 14. Aerospace Applications of
Liquid Penetrant Testing . . . . . 379

Part 1. Liquid Penetrant Testing
Process Specifications
in Aerospace
Manufacturing . . . . . . . . 380

Part 2. Liquid Penetrant
Performance without
Developer . . . . . . . . . . . 386

Part 3. Applications of Liquid
Penetrant Testing to
Liquid Oxygen Systems . 392

Part 4. Fluorescent Liquid
Penetrants with Depth
Sensing Capabilities . . . . 396

Part 5. Low Viscosity Liquid
Penetrant Testing of
Braze Bond Open Face
Honeycomb Seals . . . . . . 398

Part 6. Ultrasonic Enhancement
of Liquid Penetrant
Indications of Cracks in
Aerospace Structural
Weldments . . . . . . . . . . 402

Part 7. Applications of Liquid
Penetrant Testing in
Aircraft Fleet
Maintenance . . . . . . . . . 404

Part 8. Liquid Penetrant Testing
of Radioisotope Heat
Source Capsules for Deep
Space Missions . . . . . . . . 407

Chapter 15. Various Applications of
Liquid Penetrant Testing . . . . . 415

Part 1. Liquid Penetrant Testing
of Metal Cutting Tools . . 416

Part 2. Liquid Penetrant Testing
of Oil Field Down Hole
Tubular Parts . . . . . . . . . 419

Part 3. Marine Applications of
Liquid Penetrant
Testing . . . . . . . . . . . . . . 421

Part 4. Liquid Penetrant Testing
of Automotive Parts . . . . 425

Part 5. Liquid Penetrant Testing
of Plastic Materials . . . . . 429

Liquid Penetrant Testing xi

Multimedia Contents

Chapter 2. Principles of Liquid Movie. False indications. . . . . . . . 151
Penetrant Testing . . . . . . . . . . . 33 Movie. Nonrelevant indications

Movie. Bleeding suggests can mask relevant
discontinuity severity. . . . 35 ones. . . . . . . . . . . . . . . . 151
Movie. Nonrelevant indication
Movie. Fluorescent liquid from part geometry. . . . .151
penetrant. . . . . . . . . . . . . 36
Chapter 6. Surface Preparation and
Movie. Liquid penetrant seeps Cleaning . . . . . . . . . . . . . . . . . 161
into discontinuity. . . . . . . 36
Movie. Postcleaning. . . . . . . . . . .178
Movie. Solvent removes excess
liquid penetrant from Chapter 12. Liquid Penetrant
part surface. . . . . . . . . . . . 37 Testing in Primary Metals
Production . . . . . . . . . . . . . . . . 343
Movie. Nonaqueous wet
developer enhances Movie. Rejectable
visible dye contrast. . . . . . 38 discontinuity. . . . . . . . . .347

Movie. Hydrophilic prerinse. . . . . 43 Movie. Porosity in casting. . . . . . 353
Movie. Fluorescent bleedout
Movie. Dip in hydrophilic
emulsifier; dwell. . . . . . . . 43 reveals shrinkage. . . . . . .353

Movie. Water wash. . . . . . . . . . . . 43

Movie. Developer application. . . . 43

Movie. Viewing of developed
indications. . . . . . . . . . . . 43

Movie. Developer is applied. . . . . . 44

Movie. Wipe part. . . . . . . . . . . . . . 45

Movie. Visible red dye liquid
penetrant bleeds out. . . . . 45

Movie. Indication in root pass
of weld. . . . . . . . . . . . . . . 46

Movie. Water wash. . . . . . . . . . . . 50

Movie. Developer application. . . . 56

Movie. Nonaqueous wet
developer enhances
visible dye contrast. . . . . . 58

Movie. Shake the spray can. . . . . . 58

Chapter 3. Characteristics of Liquid
Penetrant and Processing
Materials . . . . . . . . . . . . . . . . . . 83

Movie. Visible red dye liquid
penetrant bleeds out. . . . . 90

Chapter 5. Interpretation of Liquid
Penetrant Indications . . . . . . . 125

Movie. Fluorescent bleedout
reveals shrinkage. . . . . . 128

Movie. Quenching cracks. . . . . . . 130

Movie. Linear discontinuity. . . . . 133

Movie. Open and partially
open cracks. . . . . . . . . . . 133

Movie. Pitting and porosity. . . . . 133

Movie. Porosity in casting. . . . . . 149

Movie. Process control can
mask discontinuities. . . . 151

Liquid Penetrant Testing xii

Figure Sources

The following list indicates owners of figures at time of submittal. Chapter 12

Chapter 1 Figure 1 — Reynolds Metals Company, Richmond, VA.
Figures 1-2 — D&W Enterprises, Limited, Littleton, CO. Figures 2-3 — Magnaflux Division of Illinois Tool Works, Glenview, IL.
Figures 3-5, 7-11, 14 — Magnaflux Division of Illinois Tool Works,
Chapter 13
Glenview, IL.
Figure 6 — Alys Alburger Braun, Somis, CA. Figures 1-4 — Battelle Memorial Institute, Columbus, OH.
Figure 5 — Southwest Research Institute, San Antonio, TX.
Chapter 2 Figures 6-7 — Stone and Webster, Boston, MA.
Figures 1-11 — Magnaflux Division of Illinois Tool Works, Glenview, IL.
Chapter 14
Chapter 3
Figures 1-11 — Magnaflux Division of Illinois Tool Works, Glenview, IL. Figure 1-6 — Boeing Company, Long Beach, CA.
Figure 8 — Air Force Research Laboratory, Wright-Patterson Air Force
Chapter 4
Figures 1-2, 5, 7, 9 — Magnaflux Division of Illinois Toolworks, Glenview, IL. Base, OH.
Figures 3-4, 6 — Sherwin Incorporated, South Gate, CA. Figures 9-14 — Rockwell International, Canoga Park, CA.
Figure 8 — Met-L-Chek, Santa Monica, CA.
Chapter 15
Chapter 5
Figures 1-6, 8, 12-15, 18-25 — Magnaflux Division of Illinois Toolworks, Figure 1 — Magnaflux Division of Illinois Tool Works, Glenview, IL.
Figure 2-3 — International Pipe Inspectors Association, Houston, TX.
Glenview, IL. Figure 4 — Chrysler Corporation, Detroit, MI.
Figures 7, 26 — Allied Signal Aerospace Company [formerly AiResearch Figure 5 — Gregory F. Monks, QC Technologies, Incorporated, Noblesville, IN.
Figure 6 — Dennis G. Hunley, Quality Assurance Corporation, Indianapolis, IN.
Manufacturing Division, Garrett Corporation], Los Angeles, CA.
Figure 16-17 — Turbodyne Technologies, Incorporated, Woodland Hills, CA.

Chapter 6
Figures 2-11 — Boeing Company, Long Beach, CA.
Figures 12-25 — Boeing Company, St. Louis, MO.

Chapter 7
Figures 1-4f, 4h, 6-8a, 25-26, 31, 34, 36-43 — Magnaflux Division of Illinois

Tool Works, Glenview, IL.
Figures 8b, 8c — Sherwin Incorporated, South Gate, CA.
Figures 9-15 — Boeing Company, St. Louis, MO.
Figure 17-24 — Northrop Grumman Corporation, Los Angeles, CA.
Figures 33, 35a-35b — Spectronics Corporation, Westbury, NY.
Figures 35c — Ely Chemical Company, Aurora, IL.

Chapter 8
Figures 2-4 — Sherwin Incorporated, South Gate, CA.
Figures 5-7, 9-12 — Boeing Company, St. Louis, MO.
Figure 13 — Turco Products, Incorporated, Long Beach, CA.

Chapter 9
Figures 1-7 — D&W Enterprises, Limited, Littleton, CO.

Chapter 10
Figures 1-6 — Westinghouse Hanford, Hanford, WA.
Figure 7 — Magnaflux Division of Illinois Tool Works, Glenview, IL.
Figures 8-10 — Sherwin Incorporated, South Gate, CA.

Chapter 11
Figures 1-13 — Magnaflux Division of Illinois Tool Works, Glenview, IL.
Figures 14-15 — Robert L. Crane, Air Force Research Laboratory,

Wright-Patterson Air Force Base, Ohio.

493

Movie Sources

All video is copyrighted by ASNT or reproduced by permission of the Chapter 6
copyright holders. The following list indicates copyright ownership at time of
submittal. The participation of ASNT Past President Charles N. Hellier, III as Movie. Postcleaning — American Society for Nondestructive Testing,
narrator and instructor in many of these movies is gratefully acknowledged. Columbus, OH.

Chapter 2 Chapter 12

Movie. Bleeding suggests discontinuity severity — Hellier Associates, Movie. Rejectable discontinuity — Hellier Associates, Incorporated, Niantic, CT.
Incorporated, Niantic, CT. Movie. Porosity in casting — American Society for Nondestructive Testing,

Movie. Fluorescent liquid penetrant — Hellier Associates, Incorporated, Columbus, OH.
Niantic, CT. Movie. Fluorescent bleedout reveals shrinkage — ASM International, Materials

Movie. Liquid penetrant seeps into discontinuity — ASM International, Park, OH.
Materials Park, OH.

Movie. Solvent removes excess liquid penetrant from part surface — ASM
International, Materials Park, OH.

Movie. Nonaqueous wet developer enhances visible dye contrast — ASM
International, Materials Park, OH.

Movie. Hydrophilic prerinse — Howmet Castings, Whitehall, MI.
Movie. Dip in hydrophilic emulsifier; dwell — Howmet Castings,

Whitehall, MI.
Movie. Water wash — Howmet Castings, Whitehall, MI.
Movie. Developer application — Howmet Castings, Whitehall, MI.
Movie. Viewing of developed indications — Howmet Castings, Whitehall, MI.
Movie. Developer is applied — American Society for Nondestructive Testing,

Columbus, OH.
Movie. Wipe part — American Society for Nondestructive Testing,

Columbus, OH.
Movie. Visible red dye liquid penetrant bleeds out — American Society for

Nondestructive Testing, Columbus, OH.
Movie. Indication in root pass of weld —Hellier Associates, Incorporated,

Niantic, CT.
Movie. Water wash — Howmet Castings, Whitehall, MI.
Movie. Developer application — Howmet Castings, Whitehall, MI.
Movie. Nonaqueous wet developer enhances visible dye contrast — ASM

International, Materials Park, OH.
Movie. Shake the spray can — Hellier Associated, Incorporated, Niantic, CT.
Movie. Nonaqueous wet developer enhances visible dye contrast — ASM

International, Materials Park, OH.

Chapter 3

Movie. Visible red dye liquid penetrant bleeds out — American Society for
Nondestructive Testing, Columbus, OH.

Chapter 5

Movie. Fluorescent bleedout reveals shrinkage — ASM International,
Incorporated, Materials Park, OH.

Movie. Quenching cracks — Hellier Associate, Incorporated, Niantic, CT.
Movie. Linear discontinuity — American Society for Nondestructive Testing,

Columbus, OH.
Movie. Open and partially open cracks — American Society for Nondestructive

Testing, Columbus, OH.
Movie. Pitting and porosity — American Society for Nondestructive Testing,

Columbus, OH.
Movie. Porosity in casting — American Society for Nondestructive Testing,

Columbus, OH.
Movie. Process control can mask discontinuities — American Society for

Nondestructive Testing, Columbus, OH.
Movie. False indications — Hellier Associates, Incorporated, Niantic, CT.
Movie. Nonrelevant indications can mask relevant ones — American Society

for Nondestructive Testing, Columbus, OH.
Movie. Nonrelevant indication from part geometry — American Society for

Nondestructive Testing, Columbus, OH.

494

1

CHAPTER

Introduction to Liquid
Penetrant Testing

John J. Flaherty, Flare Technology, Incorporated, Elk
Grove Village, Illinois
Ward D. Rummel, D&W Enterprises, Limited, Littleton,
Colorado
Amos G. Sherwin, Sherwin Incorporated, South Gate,
California
Holger H. Streckert, General Atomics, San Diego,
California
Noel A. Tracy, Universal Technology Corporation,
Dayton, Ohio

PART 1. Nondestructive Testing1

Nondestructive testing (NDT) has been sampling. Sampling (that is, less than 100
defined as comprising those test methods percent testing to draw inferences about
used to examine or inspect a part or the unsampled lots) is nondestructive
material or system without impairing its testing if the tested sample is returned to
future usefulness.1 The term is generally service. If the steel is tested to verify the
applied to nonmedical investigations of alloy in some bolts that can then be
material integrity. returned to service, then the test is
nondestructive. In contrast, even if
Strictly speaking, this definition of spectroscopy used in the chemical testing
nondestructive testing includes of many fluids is inherently
noninvasive medical diagnostics. X-rays, nondestructive, the testing is destructive if
ultrasound and endoscopes are used by the samples are poured down the drain
both medical and industrial after testing.
nondestructive testing. Medical
nondestructive testing, however, has come Nondestructive testing is not confined
to be treated by a body of learning so to crack detection. Other discontinuities
separate from industrial nondestructive include porosity, wall thinning from
testing that today most physicians never corrosion and many sorts of disbonds.
use the word nondestructive. Nondestructive material characterization
is a growing field concerned with material
Nondestructive testing is used to properties including material
investigate specifically the material identification and microstructural
integrity of the test object. A number of characteristics — such as resin curing, case
other technologies — for instance, radio hardening and stress — that have a direct
astronomy, voltage and amperage influence on the service life of the test
measurement and rheometry (flow object.
measurement) — are nondestructive but
are not used specifically to evaluate Nondestructive testing has also been
material properties. Radar and sonar are defined by listing or classifying the
classified as nondestructive testing when various techniques.1-3 This approach
used to inspect dams, for instance, but conveys a sense of nondestructive testing
not when they are used to chart a river that is a practical sense in that it typically
bottom. highlights methods in use by industry.

Nondestructive testing asks “Is there Purposes of
something wrong with this material?” Nondestructive Testing
Various performance and proof tests, in
contrast, ask “Does this component Since the 1920s, the art of testing without
work?” This is the reason that it is not destroying the test object has developed
considered nondestructive testing when from a laboratory curiosity to an
an inspector checks a circuit by running indispensable tool of production. No
electric current through it. Hydrostatic longer is visual testing of materials, parts
pressure testing is another form of proof and complete products the principal
testing, one that may destroy the test means of determining adequate quality.
object. Nondestructive tests in great variety are in
worldwide use to detect variations in
Another gray area that invites various structure, minute changes in surface
interpretations in defining nondestructive finish, the presence of cracks or other
testing is future usefulness. Some material physical discontinuities, to measure the
investigations involve taking a sample of thickness of materials and coatings and to
the inspected part for testing that is determine other characteristics of
inherently destructive. A noncritical part industrial products. Scientists and
of a pressure vessel may be scraped or engineers of many countries have
shaved to get a sample for electron contributed greatly to nondestructive test
microscopy, for example. Although future development and applications.
usefulness of the vessel is not impaired by
the loss of material, the procedure is The various nondestructive testing
inherently destructive and the shaving methods are covered in detail in the
itself — in one sense the true test object — literature, but it is always wise to consider
has been removed from service objectives before details. How is
permanently.

The idea of future usefulness is relevant
to the quality control practice of

2 Liquid Penetrant Testing

nondestructive testing useful? Why do life were not well known. After relatively
thousands of industrial concerns buy the short periods of service some of these
testing equipment, pay the subsequent aircraft suffered disastrous failures.
operating costs of the testing and even Sufficient and proper nondestructive tests
reshape manufacturing processes to fit the could have saved many lives.
needs and findings of nondestructive
testing? As technology improves and as service
requirements increase, machines are
Modern nondestructive tests are used subjected to greater variations and to
by manufacturers (1) to ensure product wider extremes of all kinds of stress,
integrity and, in turn, reliability; (2) to creating an increasing demand for
avoid failures, prevent accidents and save stronger or more damage tolerant
human life; (3) to make a profit for the materials.
user; (4) to ensure customer satisfaction
and maintain the manufacturer’s Engineering Demands for Sounder
reputation; (5) to aid in better product Materials
design; (6) to control manufacturing
processes; (7) to lower manufacturing Another justification for nondestructive
costs; (8) to maintain uniform quality tests is the designer’s demand for sounder
level; and (9) to ensure operational materials. As size and weight decrease and
readiness. the factor of safety is lowered, more and
more emphasis is placed on better raw
These reasons for widespread profitable material control and higher quality of
use of nondestructive testing are sufficient materials, manufacturing processes and
in themselves, but parallel developments workmanship.
have contributed to its growth and
acceptance. An interesting fact is that a producer of
raw material or of a finished product
Increased Demand on Machines sometimes does not improve quality or
performance until that improvement is
In the interest of greater speed and rising demanded by the customer. The pressure
costs of materials, the design engineer is of the customer is transferred to
often under pressure to reduce weight. implementation of improved design or
This can sometimes be done by manufacturing. Nondestructive testing is
substituting aluminum alloys, magnesium frequently called on to deliver this new
alloys or composite materials for steel or quality level.
iron, but such light parts may not be the
same size or design as those they replace. Public Demands for Greater Safety
The tendency is also to reduce the size.
These pressures on the designer have The demands and expectations of the
subjected parts of all sorts to increased public for greater safety are apparent
stress levels. Even such commonplace everywhere. Review the record of the
objects as sewing machines, sauce pans courts in granting higher and higher
and luggage are also lighter and more awards to injured persons. Consider the
heavily loaded than ever before. The stress outcry for greater automobile safety, as
to be supported is seldom static. It often evidenced by the required auto safety
fluctuates and reverses at low or high belts and the demand for air bags,
frequencies. Frequency of stress reversals blowout-proof tires and antilock braking
increases with the speeds of modern systems. The publicly supported activities
machines and thus parts tend to fatigue of the National Safety Council,
and fail more rapidly. Underwriters Laboratories, the
Occupational Safety and Health
Another cause of increased stress on Administration and the Federal Aviation
modern products is a reduction in the Administration in the United States, as
safety factor. An engineer designs with well as the work of similar agencies
certain known loads in mind. On the abroad, are only a few of the ways in
supposition that materials and which this demand for safety is expressed.
workmanship are never perfect, a safety It has been expressed directly by
factor of 2, 3, 5 or 10 is applied. However, passengers who cancel reservations
because of other considerations, a lower following a serious aircraft accident. This
factor is often used that depends on the demand for personal safety has been
importance of lighter weight or reduced another strong force in the development
cost or risk to consumer. of nondestructive tests.

New demands on machinery have also Rising Costs of Failure
stimulated the development and use of
new materials whose operating Aside from awards to the injured or to
characteristics and performance are not estates of the deceased and aside from
completely known. These new materials costs to the public (e.g. evacuation due to
create greater and potentially dangerous chemical leaks), consider briefly other
problems. As an example, an aircraft part factors in the rising costs of mechanical
was built from an alloy whose work
hardening, notch resistance and fatigue

Introduction to Liquid Penetrant Testing 3

failure. These costs are increasing for materials and structures without
many reasons. Some important ones are disruption or impairment of serviceability.
(1) greater costs of materials and labor; Nondestructive testing makes it possible
(2) greater costs of complex parts; for internal properties or hidden
(3) greater costs due to the complexity of discontinuities to be revealed or inferred
assemblies; (4) greater probability that by appropriate methods.
failure of one part will cause failure of
others because of overloads; (5) trend to Nondestructive testing is becoming
lower factors of safety; (6) probability that increasingly vital in the effective conduct
the failure of one part will damage other of research, development, design and
parts of high value; and (7) part failure in manufacturing programs. Only with
an automatic production machine, appropriate nondestructive testing
shutting down an entire high speed, methods can the benefits of advanced
integrated, production line. When materials science be fully realized. The
production was carried out on many information required for appreciating the
separate machines, the broken one could broad scope of nondestructive testing is
be bypassed until repaired. Today, one available in many publications and
machine is tied into the production of reports.
several others. Loss of such production is
one of the greatest losses resulting from Classification of Methods
part failure.
In a report, the National Materials
Applications of Nondestructive Advisory Board (NMAB) Ad Hoc
Testing Committee on Nondestructive Evaluation
adopted a system that classified
Nondestructive testing is a branch of the techniques into six major method
materials sciences that is concerned with categories: visual, penetrating radiation,
all aspects of the uniformity, quality and magnetic-electrical, mechanical vibration,
serviceability of materials and structures. thermal and chemical-electrochemical.3 A
The science of nondestructive testing modified version is presented in Table 1.1
incorporates all the technology for
detection and measurement of significant Each method can be completely
properties, including discontinuities, in characterized in terms of five principal
items ranging from research specimens to factors: (1) energy source or medium used
finished hardware and products in service. to probe object (such as X-rays, ultrasonic
By definition, nondestructive testing waves or thermal radiation); (2) nature of
methods are means for inspecting the signals, image and/or signature
resulting from interaction with the object

TABLE 1. Nondestructive testing method categories. Objectives
Categories

Basic Categories

Mechanical-optical color; cracks; dimensions; film thickness; gaging; reflectivity; strain distribution and magnitude; surface
finish; surface flaws; through-cracks

Penetrating radiation cracks; density and chemistry variations; elemental distribution; foreign objects; inclusions; microporosity;
misalignment; missing parts; segregation; service degradation; shrinkage; thickness; voids

Electromagnetic-electronic alloy content; anisotropy; cavities; cold work; local strain, hardness; composition; contamination;
corrosion; cracks; crack depth; crystal structure; electrical and thermal conductivities; flakes; heat
treatment; hot tears; inclusions; ion concentrations; laps; lattice strain; layer thickness; moisture content;
polarization; seams; segregation; shrinkage; state of cure; tensile strength; thickness; disbonds

Sonic-ultrasonic crack initiaion and propagation; cracks, voids; damping factor; degree of cure; degree of impregnation; degree of
sintering; delaminations; density; dimensions; elastic moduli; grain size; inclusions;
mechanical degradation; misalignment; porosity; radiation degradation; structure of composites; surface stress;
tensile, shear and compressive strength; disbonds; wear

Thermal and infrared bonding; composition; emissivity; heat contours; plating thickness; porosity; reflectivity; stress; thermal
conductivity; thickness; voids

Chemical-analytical alloy identification; composition; cracks; elemental analysis and distribution; grain size; inclusions; macrostructure;
porosity; segregation; surface anomalies

Image generation Auxiliary Categories
Signal image analysis
dimensional variations; dynamic performance; anomaly characterization and definition; anomaly
distribution; anomaly propagation; magnetic field configurations

data selection, processing and display; anomaly mapping, correlation and identification; image enhancement;
separation of multiple variables; signature analysis

4 Liquid Penetrant Testing

(attenuation of X-rays or reflection of discontinuities. Volumetric methods
ultrasound, for example); (3) means of include radiography, ultrasonic testing,
detecting or sensing resultant signals acoustic emission testing, certain infrared
(photoemulsion, piezoelectric crystal or thermographic techniques and less
inductance coil); (4) method of indicating familiar methods such as
and/or recording signals (meter deflection, acoustoultrasonic testing and magnetic
oscilloscope trace or radiograph); and resonance imaging. Through-boundary
(5) basis for interpreting the results (direct methods described include leak testing,
or indirect indication, qualitative or some infrared thermographic techniques,
quantitative and pertinent dependencies). airborne ultrasonic testing and certain
techniques of acoustic emission testing.
The objective of each method is to Other less easily classified methods are
provide information about the following material identification, vibration analysis
material parameters: and strain gaging.

1. discontinuities and separations (cracks, No one nondestructive testing method
voids, inclusions, delaminations etc.); is all-revealing. That is not to say that one
method or technique of a method is
2. structure or malstructure (crystalline rarely adequate for a specific object or
structure, grain size, segregation, component. However, in most cases it
misalignment etc.); takes a series of test methods to do a
complete nondestructive test of an object
3. dimensions and metrology (thickness, or component. For example, if surface
diameter, gap size, discontinuity size cracks must be detected and eliminated
etc.); and the object or component is made of
ferromagnetic material, then magnetic
4. physical and mechanical properties particle testing would be the obvious
(reflectivity, conductivity, elastic choice. If that same material is aluminum
modulus, sonic velocity etc.); or titanium, then the choice would be
liquid penetrant or electromagnetic
5. composition and chemical analysis testing. However, for either of these
(alloy identification, impurities, situations, if internal discontinuities were
elemental distributions etc.); to be detected, then ultrasonics or
radiography would be the selection. The
6. stress and dynamic response (residual exact technique in either case would
stress, crack growth, wear, vibration depend on the thickness and nature of
etc.); and the material and the type or types of
discontinuities that must be detected.
7. signature analysis (image content,
frequency spectrum, field Value of Nondestructive Testing
configuration etc.).
The contribution of nondestructive
Terms used in this block are further testing to profits has been acknowledged
defined in Table 2 with respect to specific in the medical field and computer and
objectives and specific attributes to be aerospace industries. However, in
measured, detected and defined. industries such as heavy metals, though
nondestructive testing may be grudgingly
The limitations of a method include promoted, its contribution to profits may
conditions required by that method: not be obvious to management.
conditions to be met for method Nondestructive testing is sometimes
application (access, physical contact, thought of only as a cost item. One
preparation etc.) and requirements to possible reason is industry downsizing.
adapt the probe or probe medium to the When a company cuts costs, two
object examined. Other factors limit the vulnerable areas are quality and safety.
detection and/or characterization of When bidding contract work, companies
discontinuities, properties and other add profit margin to all cost items,
attributes and limit interpretation of including nondestructive testing, so a
signals and/or images generated. profit should be made on the
nondestructive testing. However, when
Classification Relative to Test production is going poorly and it is
Object anticipated that a job might lose money,
it seems like the first corner that
Nondestructive testing methods may be production personnel will try to cut is
classified according to how they detect nondestructive testing. This is
indications relative to the surface of a test accomplished by subtle pressure on
object. Surface methods include liquid nondestructive testing technicians to
penetrant testing, visual testing, grid and accept a product that does not quite meet
moiré testing, holography and a code or standard requirement. The
shearography. Surface/near-surface attitude toward nondestructive testing is
methods include tap, potential drop,
magnetic particle and electromagnetic
testing. When surface or
surface/near-surface methods are applied
during intermediate manufacturing
processes, they provide preliminary
assurance that volumetric methods
performed on the completed object or
component will reveal few rejectable

Introduction to Liquid Penetrant Testing 5

gradually improving as management nondestructive testing at the end of a
comes to appreciate its value. manufacturing process. This approach will
ultimately increase production costs.
Nondestructive testing should be used When used properly, nondestructive
as a control mechanism to ensure that testing saves money for the manufacturer.
manufacturing processes are within design Rather than costing the manufacturer
performance requirements. It should money, nondestructive testing should add
never be used in an attempt to obtain profits to the manufacturing process.
quality in a product by using

TABLE 2. Objectives of nondestructive testing methods.

Objectives Attributes Measured or Detected

Discontinuites and separations

Surface anomalies roughness; scratches; gouges; crazing; pitting; inclusions and imbedded foreign material
Surface connected anomalies
Internal anomalies cracks; porosity; pinholes; laps; seams; folds; inclusions

cracks; separations; hot tears; cold shuts; shrinkage; voids; lack of fusion; pores; cavities;
delaminations; disbonds; poor bonds; inclusions; segregations

Structure molecular structure; crystalline structure and/or strain; lattice structure; strain; dislocation; vacancy;
Microstructure deformation

Matrix structure grain structure, size, orientation and phase; sinter and porosity; impregnation; filler and/or
reinforcement distribution; anisotropy; heterogeneity; segregation
Small structural anomalies
Gross structural anomalies leaks (lack of seal or through-holes); poor fit; poor contact; loose parts; loose particles; foreign objects

assembly errors; misalignment; poor spacing or ordering; deformation; malformation; missing parts

Dimensions and metrology linear measurement; separation; gap size; discontinuity size, depth, location and orientation
unevenness; nonuniformity; eccentricity; shape and contour; size and mass variations
Displacement; position film, coating, layer, plating, wall and sheet thickness; density or thickness variations
Dimensional variations
Thickness; density

Physical and mechanical properties

Electrical properties resistivity; conductivity; dielectric constant and dissipation factor
Magnetic properties polarization; permeability; ferromagnetism; cohesive force
Thermal properties conductivity; thermal time constant and thermoelectric potential
Mechanical properties compressive, shear and tensile strength (and moduli); Poisson’s ratio; sonic velocity; hardness; temper and

Surface properties embrittlement
color; reflectivity; refraction index; emissivity

Chemical composition and analysis

Elemental analysis detection; identification, distribution and/or profile
Impurity concentrations contamination; depletion; doping and diffusants
Metallurgical content variation; alloy identification, verification and sorting
Physiochemical state moisture content; degree of cure; ion concentrations and corrosion; reaction products

Stress and dynamic response heat treatment, annealing and cold work effects; residual stress and strain; fatigue damage and life (residual)
wear; spalling; erosion; friction effects
Stress; strain; fatigue corrosion; stress corrosion; phase transformation
Mechanical damage radiation damage and high frequency voltage breakdown
Chemical damage crack initiation and propagation; plastic deformation; creep; excessive motion; vibration; damping; timing of
Other damage
Dynamic performance events; any anomalous behavior

Signature analysis potential; strength; field distribution and pattern
Electromagnetic field isotherms; heat contours; temperatures; heat flow; temperature distribution; heat leaks; hot spots
Thermal field noise; vibration characteristics; frequency amplitude; harmonic spectrum and/or analysis; sonic and/or
Acoustic signature
ultrasonic emissions
Radioactive signature distribution and diffusion of isotopes and tracers
Signal or image analysis image enhancement and quantization; pattern recognition; densitometry; signal classification, separation

and correlation; discontinuity identification, definition (size and shape) and distribution analysis;
discontinuity mapping and display

6 Liquid Penetrant Testing

PART 2. Management of Liquid Penetrant
Testing4

Synopsis of Liquid indications and provides a contrasting
Penetrant Testing white background for visible (red) dye
liquid penetrant indications.
Liquid penetrant testing can be defined as 5. Visually examine surfaces for liquid
a physical and chemical nondestructive penetrant indications; interpret and
testing procedure designed to detect and evaluate the indications.
expose surface connected discontinuities 6. Postclean the part to remove process
in nonporous engineering materials. The residues if they will be detrimental to
method relies on the physical interaction subsequent operations or the part’s
between an appropriately formulated intended function. (In some cases, a
chemical liquid and the surface of a part. treatment to prevent corrosion may be
This interaction causes the liquid to enter required.)
surface cavities and later to emerge,
visually indicating the location and Liquid penetrant testing is popular
approximate size and shape of the surface because it is relatively easy to use and has
opening. The objective of liquid penetrant a wide range of applications. Because it
testing is to provide visual evidence of uses physical and chemical properties
cracks, porosity, laps, seams and other rather than electrical or thermal
surface discontinuities rapidly and phenomena, it can be used in the field,
economically with a high degree of far from power sources. Test equipment
reliability. This objective is facilitated by can be as simple as a small, inexpensive
formulating the liquid to include dyes kit of aerosol cans or as extensive as a
that are highly visible with either white or large mechanized and automated
near ultraviolet radiation. With proper installation. However, in all cases, the
technique, liquid penetrant testing will success of liquid penetrant testing
detect a wide variety of discontinuities depends on cleanliness of test surfaces, on
ranging in size from readily visible to absence of contamination or surface
microscopic. conditions that can interfere with liquid
penetrant entry into and subsequent
The liquid penetrant process consists of emergence from surface openings of
six basic steps. discontinuities, and on care by
inspectors/operators to ensure proper
1. Preclean and dry the test surfaces of processing techniques and observation of
the object to be tested. Cleaning is a test indications. Previous manufacturing
critical part of the liquid penetrant processes also may inhibit detection of
process and is emphasized because of some types of discontinuities by liquid
its effect on the test results. penetrants. For example, many seams and
Contaminants, soils or moisture, laps are forged shut by the hot rolling or
either inside a discontinuity or on the piercing processes that create these
part surface at the discontinuity elongated discontinuities; in addition,
opening, can reduce or completely local welding of the metal or trapping of
destroy the effectiveness of the test. heat treat byproducts within the opening
Etching and pickling are used at this can inhibit or prevent entry of the liquid
stage to remove smeared surface metal. penetrant, making testing ineffective.

2. Apply liquid penetrant to the test Liquid penetrant testing is also used for
surfaces and permit it to dwell on the leak testing. The same basic fundamentals
part surface for a period of time to apply but the liquid penetrant removal
allow it to enter and fill any step may be omitted. The container is
discontinuities open to the surface. either filled with liquid penetrant or the
liquid penetrant is applied to one side of
3. Remove excess liquid penetrant from the container wall. The developer is
the test surfaces. Care must be applied to the opposite side, which is
exercised to prevent removal of liquid visually examined after allowing time for
penetrant contained in discontinuities. the liquid penetrant to seep through any
leak points. This method may be used on
4. Apply a developer, which aids in thin parts where there is access to both
drawing any trapped liquid penetrant surfaces and where the discontinuity is
from discontinuities and spreading it expected to extend through the material.
on the test surface for improved
discontinuity detection. Developer
enhances the brightness of fluorescent

Introduction to Liquid Penetrant Testing 7

It may be further enhanced by the the inspector can visually see the
application of a pressure differential. discontinuity. Other methods such as
ultrasonic testing and eddy current
Reasons for Selecting testing have a signal output that has
Liquid Penetrant Testing to be compared to that of a reference
standard.
Some of the reasons for choosing liquid
penetrant testing are as follows. Disadvantages and
Limitations of Liquid
1. Liquid penetrant testing can quickly Penetrant Testing
examine all the accessible surfaces of
objects. Complex shapes can be 1. Liquid penetrant testing depends on
immersed or sprayed with liquid the ability of liquid penetrant to enter
penetrant to provide complete surface and fill discontinuities. Liquid
coverage. penetrant testing will only reveal
discontinuities open to the surface.
2. Liquid penetrant testing can detect
very small surface discontinuities. It is 2. Surfaces of objects to be tested must
one of the most sensitive be clean and free of organic or
nondestructive testing methods for inorganic contaminants that will
detecting surface discontinuities. prevent interaction of the penetrating
media with a surface. Organic surface
3. Liquid penetrant testing can be used coatings, such as paint, oil, grease or
on a wide variety of materials: ferrous resin, are in this category. Any coating
and nonferrous metals and alloys; that covers or blocks the discontinuity
fired ceramics and cermets; powdered opening will prevent liquid penetrant
metal products; glass; and some types entry. Even when the coating does not
of organic materials. Restrictions on cover the opening, material at the
materials imposed by the nature of the edge of the opening may affect entry
liquid penetrant process are covered in or exit of liquid penetrant and greatly
the discussion of limitations, below. reduce reliability of the test. Coatings
in the vicinity of a discontinuity will
4. Liquid penetrant testing can be also retain liquid penetrant, causing
accomplished with relatively background indications. Cleaning test
inexpensive, nonsophisticated surfaces is discussed in more detail
equipment. If the area to be tested is below.
small, the test can be accomplished
with portable equipment. 3. It is also essential that the inside
surface of discontinuities be free of
5. Liquid penetrant testing magnifies the materials such as corrosion,
apparent size of discontinuities, combustion products or other
making the indications more visible. contaminants that would restrict entry
In addition, the discontinuity of liquid penetrant. Because it is
location, orientation and approximate impossible to check inside
size and shape are indicated on the discontinuities, one must trust that
part, making interpretation and processes selected to clean test surfaces
evaluation possible. will clean inside surfaces of
discontinuities also.
6. Liquid penetrant testing is readily
adapted to volume processing, 4. Mechanical operations, such as shot
permitting 100 percent surface testing. peening, some types of plastic media
Small parts may be placed in baskets blasting (PMB), machining, honing,
for batch processing. Specialized abrasive blasting, buffing, brushing,
systems may be partially or fully grinding or sanding will smear or peen
automated to process many parts per the surface of metals. This mechanical
hour. working closes or reduces the surface
opening of existing discontinuities.
7. The sensitivity of a liquid penetrant Mechanical working (smearing or
testing process may be adjusted peening) also occurs during service
through appropriate selection of liquid when parts are in contact or rub
penetrant, removal technique and together. Liquid penetrant testing will
type of developer. This allows the not reliably detect discontinuities
liquid penetrant process to be adapted when it is performed after a
to characteristics (e.g., composition, mechanical operation or service use
surface condition) of the part that smears or peens the surface.
requiring testing and to be tailored to Depending on the severity of the
detect specified minimum allowable mechanical operation, chemical
discontinuities. Thus, inconsequential removal (etching) of smeared metal
discontinuities can be suppressed may restore test reliability.
whereas larger discontinuities of more
concern are indicated.

8. Liquid penetrant testing is a direct
nondestructive testing method in that

8 Liquid Penetrant Testing

5. Unless special procedures are used, discontinuities) produced by other
liquid penetrant testing is impractical nondestructive test techniques. For
on certain materials, such as some instance, a forged steel bracket might
types of anodized aluminum surfaces, have such a sharp change of section
other protective coatings and certain that test processes such as magnetic
nonmetallic parts. Liquid penetrant particle tests would show nonrelevant
rapidly enters pores of the material indications in that area. Or a weld
and becomes trapped. This can result might be so located that a change in
in an overall background fluorescence thickness could mask the X-ray
or color that could mask any potential indication of a crack.
discontinuity indications. In addition, 3. When the size or shape of the surface
removal of the liquid penetrant may discontinuity is such that it can escape
not be possible after the test. detection by other techniques. An
aluminum forging may have a forging
6. Penetrants, emulsifiers and some types lap, almost sealed shut, which is so
of developers have very good wetting small that only liquid penetrant
and detergent properties. They can act testing can show it. A tiny crack in the
as solvents for fats and oils. They also radius of a turbine bucket is very
can clean ferrous materials so difficult to find by any means other
thoroughly that rust will begin almost than liquid penetrant testing.
immediately if corrosion inhibitor is 4. When parts are to be tested in
not applied. If allowed to remain in locations where electric power is not
contact with human skin for extended available or is too expensive or too
periods, they may cause irritation. inconvenient to use or where
electricity creates a safety hazard.
Reasons for Using Liquid When the surface of parts must be
Penetrant Testing checked in the field, perhaps in
remote areas, liquid penetrant systems
Liquid penetrant testing is often the first are especially advantageous.
nondestructive testing method
management considers for testing of However, if the test material is
industrial products (1) because it often ferromagnetic, magnetic tests may offer
requires minimal capital expenditure for advantages over liquid penetrant tests in
implementation and (2) because it can detection of near-surface discontinuities
accommodate a variety of test object and surface discontinuities that do not
materials, shapes and sizes, test locations permit entry of a liquid penetrant. If tests
and environmental conditions. However, for internal and surface soundness are
the outcome of a liquid penetrant test is required, then liquid penetrant testing
largely dependent on the human operator. can be used in addition to other
Liquid penetrant testing is a labor nondestructive test methods such as
intensive method. Therefore, labor penetrating radiation, ultrasonic or eddy
constitutes a high recurring test cost. In current tests.
short, the quality of the test and the cost
of testing warrant considerable Choosing Which Liquid
management attention to the selection, Penetrant Process
training, management and audit of liquid
penetrant testing personnel. When the decision to use liquid penetrant
testing has been made, which system
Liquid penetrant testing detects only should be selected? There are varieties of
those discontinuities that are present on penetrants, liquid penetrant removal
or are open to the surface of the part. systems and developers; there are also
Therefore, if only surface discontinuities many ways to combine these different
are of interest, liquid penetrant processes liquid penetrant system elements. The
may be appropriate in the following manager must consider the specific
situations. application. For the great majority of
parts, the conventional water washable or
1. When the test material is a postemulsifiable penetrants are suitable
nonmagnetic metal (such as annealed and adequate. However, many factors
austenitic stainless steel, titanium, must be considered when selecting the
aluminum, magnesium and copper optimum liquid penetrant system. For
alloys) or a nonmetallic material (such example, the preferred system should
as plastics or ceramics). Although (1) indicate significant discontinuities
magnetic particle testing is usually open to part surfaces; (2) not adversely
thought of for magnetic materials, affect the material or the part in later
liquid penetrant testing could also be service, (3) be affordable and (4) not take
used. so much time that it unreasonably
disrupts production schedules.
2. When the geometry of the part is such
that the shape itself may hide or
obscure indications (of surface

Introduction to Liquid Penetrant Testing 9

What Material Is in Test Parts? test sensitivity. Depending on the
quantity involved, it may be more
Most materials, both metallic and economical to install a conventional
nonmetallic, can be satisfactorily tested by liquid penetrant system for the majority
conventional fluorescent or visible dye of parts and send the one item with
penetrants. But austenitic stainless steels, stricter requirements to a commercial
nickel base alloys and titanium (at nondestructive testing laboratory for
elevated temperatures) are subject to stress processing. (In like manner, many
corrosion; for these metals, the liquid companies have their own X-ray machine
penetrant materials should be low in for normal applications but have a
sulfur and in halogens, especially commercial nondestructive test laboratory
chlorine. For materials harmed by water, a handle parts requiring very large X-ray
nonaqueous system is indicated. Some equipment.)
nonmetallic materials are affected by oil
base liquid penetrants or emulsifiers; for How Many Parts Are to Be Tested
these test objects, a water base liquid and How Often?
penetrant system should be selected.
After having established the size and type
What Type of Service Is Expected of discontinuity that must be found,
for Test Parts? managers may find that there are several
liquid penetrant systems that would
Ordinary liquid penetrant systems are probably be equally suitable. Therefore,
suitable for nearly all parts. But if the the choice in this case, will usually be
service involves exposure to liquid oxygen based on cost factors such as the capital
or gaseous oxygen or in nuclear investment required, the materials used,
applications, special penetrants should be the volume of parts to be processed, the
selected. necessity for treating waste water or the
plant space needed for test operations.
What Size of Discontinuity Is
Significant in Test Parts? Where Is Test to Be Performed?

The optimum liquid penetrant process Management must ask, especially if there
indicates surface discontinuities that are only a small number of parts to be
could adversely affect the serviceability of tested, whether it would be cheaper to use
test parts while minimizing nonrelevant a commercial nondestructive testing
indications from such causes as surface laboratory. Are commercial facilities easily
roughness. To select this optimum system, accessible or would it cause an
engineering must decide which unacceptable delay to ship parts into and
discontinuities are important: what type, out of the plant? How much space is
size and location will really affect service. available in the plant for liquid penetrant
Prior history from similar parts is very processing and where is it located? When
helpful in making such decisions. If the is the part to be checked: in receiving
operation is new or lacking good service inspection, in process, just before final
records, management can draw on the assembly, as the finished product or at
experience of liquid penetrant system several of these areas? Are there some
suppliers or reported information on the items that must be checked on test stands
capabilities of liquid penetrant testing. or high in the air, or in the field away
from other installations? For these,
For instance, if small shallow portable liquid penetrant kits may be the
discontinuities on the surface do not best selection.
affect the part, one might select a water
washable fluorescent liquid penetrant What Handling of Parts Is
process. But if shallow discontinuities are Involved?
of major concern, this is not the best
choice, because trapped liquid penetrant What is the range of size, shape and
can be washed out of shallow weight of part to be checked by liquid
discontinuities so the discontinuity penetrant? Are the parts large, heavy and
indications cannot be formed. Sometimes, unwieldy or are they small lightweight
one part will present a special problem items or a mixture of both? Are additional
because both large and small handling facilities required?
discontinuities are of consequence; a
special liquid penetrant may be the What Is Surface Condition of Test
solution in this case. Parts?

What Facility Should Test If most parts are checked in the as-cast
Occasional Critical Test Parts? condition, this will be a big factor in
selecting the best liquid penetrant system.
In typical cases, the great majority of parts If some are rough as-cast or as-forged parts
to be checked may be much alike, with and others are highly machined, the
only one critical item demanding utmost

10 Liquid Penetrant Testing

liquid penetrant test procedure may have
to be modified to get equally good results
on both types of surfaces.

Checklist of Factors for
Choosing Liquid Penetrant
Testing Technique

Managers are urged to consider the
following important factors and to discuss
them with suppliers of liquid penetrant
test equipment and processing materials
before reaching a decision on the type of
liquid penetrant testing system to install
or use:

1. test object material (aluminum,
magnesium, copper alloys, austenitic
stainless steel, nickel alloys, titanium,
iron, steel or others);

2. number of test parts or test areas to be
tested (per hour, per shift, per day or
per week);

3. size of test object to be handled
(largest dimension under 20 mm,
100 mm, 500 mm, 1 m, 2 m, 5 m,
10 m etc.);

4. weight of test objects to be handled
(1 to 300 g, 300 g to 1 kg, 1 to 3 kg, 3
to 10 kg, 10 to 30 kg, 30 to 100 kg
etc.).

5. location at which testing is required
(receiving department, in process
along production line, during
assembly, as final inspection, in field
during erection, in service or at
maintenance bases);

6. types of discontinuities to be detected:
small, deep, large, shallow, clustered or
scattered; porosity, cracks, seams, laps,
roughness;

7. surface condition of parts to be tested
(as-cast, as-forged, machined, ground,
lapped, polished, plated, painted,
corroded, oily, engine varnish, eroded,
scratched, scaled);

8. conditions to which parts will be
subjected after testing (medical,
nuclear, liquid or gaseous oxygen
systems; welding, plating or finishing
operation; high temperature,
aerospace, industrial or transportation
uses; inaccessible locations; consumer
products; etc.).

Introduction to Liquid Penetrant Testing 11

PART 3. Personnel Selection and Qualification
for Liquid Penetrant Testing4

Qualification of Liquid Liquid Penetrant Testing
Penetrant Testing Dependence on Management
Capabilities Interest

Reliable liquid penetrant testing in Inspectors with the proper temperament,
industry requires (1) qualification of the discipline, vigilance and integrity must be
test method for revealing critical selected and sustained by management to
conditions in test parts to be evaluated; ensure consistent and reliable
(2) qualification of the test personnel for performance in the testing environment.
the method and performance level The effectiveness of liquid penetrant
required; and (3) management of test testing depends on management policy,
operations to ensure consistent personnel selection and motivation and
performance. direct interest and involvement.

Productivity in liquid penetrant testing However, liquid penetrant testing is
requires consistent and reliable test also process dependent. Proper selection
performance at minimum cost. and control of liquid penetrant process
variables are mandatory to qualify a liquid
Skill and Integrity of Liquid penetrant testing process to reveal critical
Penetrant Testing Personnel conditions in the part to be tested and to
establish inspector confidence that the
The success of a liquid penetrant test task objectives of detection of critical
depends primarily on the personnel discontinuities are attainable. (Variables in
performing the varying processing steps the liquid penetrant process are described
and on their interpretation of the cause in detail elsewhere in this volume.)
and criticality of indications produced by
the test process. Qualification, skill and Requirements for Liquid
excellence in performance are important Penetrant Testing
to reliable liquid penetrant testing. Personnel

Liquid penetrant testing involves Liquid penetrant testing requires that test
skillful and judicious processing to personnel be (1) physically and mentally
produce visible evidence of the soundness qualified to perform the required
of a part. Vigilant observation and processing steps; (2) skilled and
decision making based on this visual experienced in performing the actual
evidence is used to ascertain acceptance of processing steps; (3) alert to consistent
test parts for service. Thus, liquid processing and resultant indications;
penetrant testing is people dependent. In (4) motivated to perform with vigilance at
most cases, the skill and integrity of the the required detection and reliability
liquid penetrant inspector must be equal level; and (5) capable of consistent,
to or greater than that of the operator independent and unbiased decisions
who produced the part. Unlike the based on interpretation of resultant liquid
production of the part, there is usually no penetrant indications.
tangible evidence (no permanent record)
of the quality of testing completed. The Qualification of Personnel for
reliability of and confidence in the test Liquid Penetrant Testing
(and ultimately in the part) depend on
the test operator. The test operator is a Personnel qualification depends on
professional and must be selected and requirements of the test to be performed,
treated as one. Management inattention the individual capabilities of the inspector
to human factors in liquid penetrant and the management philosophy and skill
testing will waste production resources applied. Qualification for one liquid
and may compromise the reliability of the penetrant testing operation may not be
hardware submitted for testing. reasonable or valid for a second operation.
Personnel qualification cannot be
separated from the operating facility.
Thus, the nondestructive test method and

12 Liquid Penetrant Testing

management factors must be considered operation. In general, the inspector
for each application. should able to perform the test with little
physical discomfort and minimal fatigue
The apparent simplicity of liquid during the test.
penetrant testing is deceptive. Very slight
process variations during the performance Ensuring Good Vision of
of a test can invalidate the test by Liquid Penetrant
counteracting the formation of Inspectors
indications. It is essential that personnel
performing liquid penetrant testing be Liquid penetrant testing requires visual
trained and experienced in the liquid examination of part surfaces under
penetrant process. All individuals who conditions unique to the test process. The
apply liquid penetrant materials or conditions under which the human eye is
examine components for penetrant used play a large role in determining the
indications should be qualified. behavior of the eye and must always be
taken into account. No single test or set of
Qualification requires classroom and tests has been devised to screen operators
practical training, passing marks on for all conditions and characteristics
examinations and experience. Typical important to liquid penetrant testing.
qualification requirements are contained Factors that have been identified as
in ANSI/ASNT CP-189, Standard for influencing the visual capabilities of
Qualification and Certification of liquid penetrant inspectors include vision
Nondestructive Testing Personnel;5 in EN acuity, vernier acuity, color vision, motion
473, Qualification and Certification of NDT detection, distance perception and dark
Personnel — General Principles;6 in ISO adaptation. The characteristic of the eye
9712, Nondestructive Testing — that is probably of greatest interest to the
Qualification and Certification of Personnel;7 optical engineer is its ability to recognize
and in AIA/NAS 410, NAS Certification & small, fine details. Vision acuity is defined
Qualification of Nondestructive Test and measured in terms of the angular size
Personnel.8 Another widely used document of the smallest character that can be
intended as a guideline for employers to recognized.
establish their own written practice for
the qualification and certification of their Filtered mercury arc ultraviolet sources
nondestructive testing personnel is ASNT emit a substantial amount of 405 nm
Recommended Practice No. SNT-TC-1A.9 In (violet) radiation only dimly seen by the
the 1990s, the American Society for human eye. The plastic lens implants that
Nondestructive Testing introduced the result from cataract surgery are
ASNT Central Certification Program.10 transparent at 405 nm. An inspector with
such plastic lens implants is thus
Physical Qualifications of inspecting against a bright violet light
the Liquid Penetrant background and can easily overlook tiny
Inspector fluorescent indications. Goggles that filter
ultraviolet radiation would be appropriate.
A liquid penetrant inspector must be
physically qualified to perform liquid Vision Acuity Examinations
penetrant testing reliably. The inspector’s
capability depends on the inspector’s Various eye test methods have been
health, physical strength and dexterity, devised and are used routinely in industry
good vision and consistency in to measure vision acuity. The vision
performance. acuity of the liquid penetrant operator
must be sufficient to detect and evaluate
Motor Ability and Dexterity of the the indications produced by the test
Liquid Penetrant Inspector process. To confirm natural or corrected
vision, eyes should be examined initially
The liquid penetrant inspector’s and periodically to ensure continuing
coordination must be sufficient to enable performance. Specifications and industrial
performance of precise, time dependent standards set requirements for vision
execution of all processing steps. Liquid acuity and the frequency of its
penetrant inspectors must have the reverification for liquid penetrant testing
physical capabilities necessary for timely personnel.
application of fluids to test objects or for
timely movement of test parts through Ensuring Adequate Color Vision
various processing fluid exposures.
Inspectors must also be able to visually The inspector must perceive color
test all critical areas of the test objects for brightness and contrast to perform liquid
evidence of anomalies. Physical strength, penetrant testing with visible dye (color
motor ability and dexterity requirements contrast) penetrants. One of the
vary with the hardware to be tested and challenging unsolved problems of vision
with the nature of the production

Introduction to Liquid Penetrant Testing 13

is determination of the response of the After a short time, the eyes adjust or adapt
human eye to colors that differ in hue to the low light level and objects in the
and saturation as well as brightness. dark room become visible. The time
required for dark adaptation before visual
Tests of inspectors for color blindness examination for indications can be
are required by some liquid penetrant performed varies with the individual and
specifications, with varying criteria for depends on the overall health and age of
acceptance. Although such tests provide the individual operator. A dark adaptation
an indication of operator capability for time of 1 min is usually sufficient for
performing liquid penetrant testing, they fluorescent liquid penetrant testing.
do not definitively measure operator (Complete dark adaptation may take as
capability. If color vision tests indicate a long as 20 min.)
color deficiency, the operator should be
tested on his ability to distinguish actual Suitable red lens eyeglasses may reduce
liquid penetrant indications. Color the required time for dark adaption but
blindness is usually inherited. Most forms should not be worn during inspection.
of inherited color blindness can occur
without being associated with other kinds Disuse of Photosensitive Eye
of visual anomalies. Color blindness is Glasses
usually found in both eyes but is
occasionally confined to one eye. There Photosensitive eyeglasses darken in the
are also acquired types of color blindness presence of ultraviolet radiation. Lens
that may affect performance. The type darkening is proportional to the amount
and severity of color blindness will of incident radiation. Although this type
determine the capability of an inspector of lens has advantages in sunlight
for liquid penetrant testing. In some cases, conditions, such glasses can decrease the
the type and severity of color blindness ability of an operator to perform
will permit the inspector to perform fluorescent liquid penetrant testing. Use
fluorescent liquid penetrant testing (color of photosensitive eyeglass lenses by liquid
contrast discrimination not required) even penetrant operators is not permitted. On
though the inspector is incapable of the other hand, suitable red eyeglasses
performing testing with color contrast may aid dark adaptation. Eyeglass frames
visible dye penetrants. that fluoresce and cause an unnecessary
background during fluorescent liquid
Ensuring Adequate Brightness penetrant testing should not be used.
Discrimination
Magnification in Evaluation of
Brightness discrimination is necessary for Small Liquid Penetrant Indications
fluorescent liquid penetrant inspectors.
The sensitivity of fluorescent liquid Testing using optical magnification can
penetrant is usually measured in terms of assist vision acuity of liquid penetrant
the brightness of the indications it inspectors, particularly when small
produces. Although color blindness is the indications are involved. A 10×
usual cause for variation in discrimination magnifying glass or 10× magnifier on a
of brightness or contrast in white light, supporting stand should be included in
other factors may contribute. Inherited the standard equipment available to
anomalies, eye disease and external liquid penetrant inspectors. In this
factors such as photosensitive eyeglasses connection, the value of a broad field
may affect brightness discrimination. This microscope for critical viewing cannot be
factor is evaluated by observance of actual overemphasized. The microscope is an
liquid penetrant indications. indispensable tool for the analysis of
microindications (with 20× to 100×
Ensuring Adequate Dark enlargements) as well as for determining
Adaptation the extent and nature of macroindications
(with 5× to 20× enlargements).
The lowest level of brightness that can be
seen or detected is determined by the Mental and Skill
light level to which the eye has become Qualifications Required of
accustomed. When the illumination level the Liquid Penetrant
is reduced, the pupil of the eye expands, Inspector
admitting more light. The retina of the
eye becomes more sensitive by switching Liquid penetrant testing requires skillful,
from cone vision to rod vision and also by repetitive performance of multiple
an electrochemical mechanism involving processing steps and the interpretation of
rhodopsin, the visual purple pigment. the resultant visual indications. To qualify
This process is called dark adaptation. as a liquid penetrant inspector, the
operator must not only understand and
Everyone has had the experience of not
being able to see a thing on passing from
a brightly lighted room into a dark one.

14 Liquid Penetrant Testing

be familiar with test principles and production techniques, indications of a
execution steps but also with factors that relevant nature and specification
may influence the success and/or results interpretation comprise the elements of
of the liquid penetrant testing process. the training process for liquid penetrant
Liquid penetrant testing principles and inspectors. Skill in execution of liquid
practices are discussed in detail penetrant testing must then be developed
throughout this volume. The inspector by practice. Ideally, practice would
should be able to answer written and include experimental cross sectioning of
verbal questions concerning processing tested parts to determine the nature of
principles and to demonstrate a thorough discontinuities whose indications were
knowledge of the subject. Demonstrated observed and whose indications were
knowledge should include interpretation missed. This process of metallurgical
of causes for failure of the liquid evaluation is too costly in most industrial
penetrant process to reveal material operations, and practical skill must be
anomalies. This knowledge should cover gained by working with an experienced,
test procedures used and the production knowledgeable and skilled inspector.
history of the articles to be tested. Practice should develop skill in executing
the liquid penetrant testing process, skill
Ensuring Inspector in performing the process and confidence
Knowledge of Types and in the outcome of the process.
Locations of Indications
Sought After knowledge of the liquid penetrant
process has been demonstrated by verbal
Liquid penetrant testing reveals a variety and written examination and after skill in
of material surface conditions. Knowledge the methods has been developed by
of the source and kinds of material practice, the inspector should be able to
anomalies to be revealed by the liquid demonstrate ability in liquid penetrant
penetrant process is required to use liquid tests of typical production hardware
penetrant process principles effectively. In containing known and documented
short, the inspector must know where to discontinuities. Demonstrations should be
look for indications, how to interpret the repeated periodically to ascertain a
indications and how to judge measure of performance and confidence
acceptability of test objects for use in in the test. Skill and technique must be
service. The liquid penetrant inspector developed for each liquid penetrant
must be able to recognize relevant system for each part type. Skill in testing
indications and to distinguish relevant of one part type, e.g., forgings, cannot be
indications from nonrelevant (noise) assumed in testing of a second part type,
indications. The inspector must then e.g., weldments. Skill and technique for
evaluate relevant indications with respect varying situations are developed by
to the requirements of the product being experience and no substitute for
tested. This skill is gained by observation experience is known.
and experience in liquid penetrant
testing. Attaining Productivity by Judicious
Application of Tools Available
The liquid penetrant inspector must be
able not only to perform the required Liquid penetrant testing must detect
liquid penetrant process and locate and discontinuities reliably and economically
evaluate liquid penetrant indications but to be of value in the production process.
the inspector must also determine the The liquid penetrant inspector must in
frequency and severity of discontinuities turn perform the test reliably in the
and analyze these with respect to shortest practical time to be productive.
specification requirements. At first glance Productivity is measured both by the
this task appears to be straightforward. For number of tests completed and by the
inspectors who work for a producer who reliability of detection of rejectable
checks similar parts each day to company discontinuities, without rejection of
specifications, this may be true. However, acceptable test objects.
the situation is quite different for
inspectors working for commercial Test process output may be increased
laboratories. Interpretation and grading of by the addition of tools, jigs, materials
liquid penetrant test indications must be and parts sequencing in the same manner
done in accordance with requirements of as any other production process step.
the particular part involved and may Learning curves and shop efficiency
involve understanding and interpretation factors familiar to those dealing with
of both written and verbal instructions. other shop operations are applicable to
liquid penetrant testing. However, check
Acquiring written and verbally and balance measurements with respect to
demonstrated knowledge of liquid operator performance are not as simple
penetrant testing principles, part and available as are those for other
processes. No increase in productivity is
gained unless the reliability and

Introduction to Liquid Penetrant Testing 15

FIGURE 1. Aluminum alloy flat plate by crack length: (a) as machined; (b) after etch; (c) after
proof.

(a)

100

90

Probability of Detection (percent) 80

70

60

50

40

30

20

10

0

0 1.3 2.5 3.8 5.1 6.4 7.6 8.9 10.2 11.4 12.7 14.0 15.2 16.5 17.8 19.1
0 (0.05) (0.10) (0.15) (0.20) (0.25) (0.30) (0.35) (0.40) (0.45) (0.50) (0.55) (0.60) (0.65) (0.70) (0.75)

Actual Crack Length, mm (in.)
(b)

100

Probability of Detection (percent) 90

80

70

60

50

40

30

20

10

0
0 1.3 2.5 3.8 5.1 6.4 7.6 8.9 10.2 11.4 12.7 14.0 15.2 16.5 17.8 19.1
0 (0.05) (0.10) (0.15) (0.20) (0.25) (0.30) (0.35) (0.40) (0.45) (0.50) (0.55) (0.60) (0.65) (0.70) (0.75)

Actual Crack Length, mm (in.)
(c)

100

Probability of Detection (percent) 90

80

70

60

50

40

30

20

10

0

0 1.3 2.5 3.8 5.1 6.4 7.6 8.9 10.2 11.4 12.7 14.0 15.2 16.5 17.8 19.1
0 (0.05) (0.10) (0.15) (0.20) (0.25) (0.30) (0.35) (0.40) (0.45) (0.50) (0.55) (0.60) (0.65) (0.70) (0.75)

Actual Crack Length, mm (in.)

Legend

= predicted probability of detection
X = hit data

© Copyright 1999, D&W Enterprises, Littleton, CO.

16 Liquid Penetrant Testing

FIGURE 2. Aluminum alloy flat plate by crack depth: (a) as machined; (b) after etch; (c) after
proof.

(a)

100

90

Probability of Detection (percent) 80

70

60

50

40

30

20

10

0 1.3 2.5 3.8 5.1
0 (0.05) (0.10) (0.15) (0.20)
0
Actual Crack Depth, mm (in.)
(b)

100

90

Probability of Detection (percent) 80

70

60

50

40

30

20

10

0 1.3 2.5 3.8 5.1
0 (0.05) (0.10) (0.15) (0.20)
0
Actual Crack Depth, mm (in.)
(c)

100

90

Probability of Detection (percent) 80

70

60

50

40

30

20

10

0

0 1.3 2.5 3.8 5.1
(0.20)
0 (0.05) (0.10) (0.15)

Actual Crack Depth, mm (in.)

Legend

= predicted probability of detection
X = hit data

© Copyright 1999, D&W Enterprises, Littleton, CO.

Introduction to Liquid Penetrant Testing 17

consistency of the testing process are Measuring Inspector Performance
maintained. in Terms of Throughput and
Reliability
Ensuring Reliability of Test
Results Before measuring an inspector’s
performance, it is necessary to ensure that
When a test is performed, there are four rejectable discontinuities can be found by
possible outcomes: (1) a discontinuity can the testing process. This task is a part of
be found when a discontinuity is present; the design qualification for a new
(2) a discontinuity can be missed even production part. Reliable detection of
when a discontinuity is present; (3) a rejectable discontinuities should have
discontinuity can be found when none is been demonstrated by actual test or by
present; and (4) no discontinuity is found similarity to parts for which testing has
when none is present. A reliable testing been demonstrated.
process and a reliable inspector should
find all discontinuities of concern with no Because all discontinuities are not
discontinuities missed (no errors as in equally detectable, the type of
case 2, above) and no false callouts discontinuity to be detected must be the
(case 3, above). basis for testing qualification. For
example, machining tears may be easily
To achieve this goal, the probability of detected by a given liquid penetrant
finding a discontinuity must be high and process whereas grinding cracks may be
the inspector must be both proficient in completely missed by that specific
the testing process and motivated to process. The liquid penetrant testing
perform a maximum efficiency. A reckless process must be qualified to the part
inspector may accept parts that contain produced.
discontinuities, with the resultant
consequences of possible inservice part Because discontinuity size affects
failure. A conservative inspector may detection, the size of discontinuity to be
reject parts that contain actual detected must be the basis for test
discontinuities but the inspector also may qualification. Figures 1 and 2 and Table 3
reject parts that do not contain show actual demonstrated liquid
discontinuities, with the resultant penetrant detection reliability as a
consequences of unnecessary scrap and function of fatigue crack size in
repair. Neither inspector is doing a good aluminum alloy flat plate. A discontinuity
job. size is shown at the inflection point of the
curve. This process would be expected to
reliably detect discontinuities greater than
the threshold size. Acceptance criteria
should be based on discontinuities of this
size or larger.

TABLE 3. Probability of detection for fluorescent liquid penetrant testing of aluminum flat
plates (see Figs. 1 and 2). False calls were not documented.

Data Set Condition Quantity Threshold for 90 Percent
of Cracks Probability of Detection
Detected/Present
mm (in.)

Crack Length as machined 233/311 7.95 mm (0.313 in.)
C1001C (Fig. 1a) after etch 260/311 2.69 mm (0.106 in.)
C1002C (Fig. 1b) after proof testing 280/306 1.50 mm (0.059 in.)
C1003C (Fig. 1c)
as machined 233/311 3.56 mm (0.140 in.)
Crack Depth after etch 260/311 0.79 mm (0.031 in.)
C1001C (Fig. 2a) after proof testing 280/306 0.38 mm (0.015 in.)
C1002C (Fig. 2b)
C1003C (Fig. 2c)

18 Liquid Penetrant Testing

PART 4. History of Liquid Penetrant Testing11

Through their efforts, a number of found. There were no standards, no
creative people made significant penetration time, nothing about the
contributions to the technology of liquid quality of the materials or even what
penetrant testing, a major method of kinds of materials to use. People who used
nondestructive testing. The story should it could find gross discontinuities, but the
be especially interesting to readers results were not consistent. The
familiar with the green stuff, with its interesting thing was that, although the
features and problems, with its approach found and located
tremendous capabilities and its frustrating discontinuities, it had little to do with the
limitations. It is the story of problems and innovations that led to the useful and
how ingenious people solved them. practical techniques routinely used in the
fluorescent technique of liquid penetrant
Oil and Whiting testing. With the introduction of the
magnetic particle method in the 1930s,
It is generally agreed that the origins of the oil-and-whiting technique became
the liquid penetrant testing method lie in obsolete.4
the railroad industry in a technique
known as the oil-and-whiting method.12 Need for Nonferrous
This approach was in use early in this Surface Testing
century and possibly even before. It was a
craft that was used to test critical steel In the mid-1930s, the use of aluminum
components for discontinuities. and other nonmagnetic metals was
increasing significantly. The need for a
Basically, the oil-and-whiting simple and effective nondestructive
technique worked as follows. The part to testing method for locating surface
be tested was cleaned and immersed in discontinuities in these materials was
dirty crankcase oil diluted in kerosene.12 obvious. Carl E. Betz, F.B. Doane and
(The oil used in the large railroad engines Taber DeForest were a few of the people
was very heavy, on the order of 600 who were experimenting with everything
weight, and experience showed that dirty and anything that might solve this
oil worked best.) The part was then industrial need. Techniques using brittle
allowed to drain and was cleaned with a lacquer, electrolysis, anodizing, etching
solvent. After this, it was covered with and color contrast penetrants were all
whiting (powdered chalk).13 The evaluated and generally discarded with
entrapped oil would then bleed into the the exception of the anodizing process.14
whiting (Fig. 3). It also has been reported This was used by the military for detailing
that alcohol based or water based chalk cracks in critical aluminum airframe
suspension was used as the developer.4 members. A specification was generated to
define and control this process.15
The technique was widely used but
never refined. Results were not consistent, In the fall of 1941, Carl E. Betz (Fig. 4)
and only gross discontinuities could be received a telephone call from a person
who identified himself as Robert C.
FIGURE 3. In the early 1930s, the oil-and-whiting method is Switzer (Fig. 5). Switzer said that he had
used in a railroad shop on a locomotive coupler. Bleeding of developed a new method for the detection
oil indicates crack. of surface cracks, and the claims had just
been allowed on the patent application.
He knew of Magnaflux and thought it
might be interested in some sort of
license.

At that time, Betz was in charge of
Magnaflux’s sales office in New York, NY.
Switzer had assumed that Magnaflux was
centered out of New York whereas it was
actually located in Chicago. This turned
out to be a fortunate mistake. Because of
Betz’s previous work on the development
of nondestructive testing methods for

Introduction to Liquid Penetrant Testing 19

FIGURE 4. Carl E. Betz. FIGURE 6. James R. Alburger.

FIGURE 5. Robert C. Switzer. Joseph Switzer had a magic act that
consisted of an invisible presentation in
nonmagnetic materials, he was quick to which the performers were all dressed in
realize the potential value of Switzer’s black. When a particular event was to be
claims. The phone call lasted several shown, it was painted white. The stage
hours and was soon followed by a visit to was darkened, and white lights were used
Switzer by A.V. DeForest and his son so that only the objects that were painted
Taber.10 white were visible. Switzer had the idea of
using ultraviolet radiation and fluorescent
Early Applications of Fluorescence paint instead of white paint and white
lights.17 Ultimately Joseph Switzer and his
Fluorescent liquid penetrant is partially brother Robert set up a business called
rooted in the theater. John “Pop” Switzer Brothers Ultra-Violet Laboratories
Shannon supplied liquid penetrant and started to manufacture and sell
material to Flo Ziegfield who staged shows fluorescent equipment and chemicals in
using fluorescent costumes and lighting. August 1934.
James Alburger (Fig. 6), Shannon’s son-in-
law, eventually branched out from The Switzer brothers abilities were
theatrical and esoteric fluorescent brought to the attention of Continental
applications into liquid penetrant.16 Lithograph Corporation, Cleveland, Ohio.
Continental was a subsidiary of Warner
In another part of the country teenager Brothers motion pictures and did all of
the printing for Warner’s advertising. In
1936 the brothers moved to Cleveland,
licensed Continental in all fields of
fluorescence and continued their
development work.

A very interesting use for fluorescence
that had significant later implications for
nondestructive testing was in law
enforcement. The Federal Bureau of
Investigation and the postal authorities
used some of Switzer’s materials and
equipment to track suspects. A superfine
pigment that had brilliant fluorescent
properties was often dusted on evidence
such as currency and was used as an
invisible branding mark. If the suspect
stole the evidence, the fine invisible
pigment was usually smeared all over the
suspect, the suspect’s household and in
many cases members of the suspect’s
family. Because the pigment was invisible
under normal lighting, the suspect was
not aware that he was being tracked.

20 Liquid Penetrant Testing

When the law enforcement people closed patents on discontinuity detection. He
in and used ultraviolet radiation in a was impressed with the scope of coverage
darkened area, the results were and decided to investigate the company
devastating and confessions usually further. He read everything he could
occurred on the spot. about Magnaflux while prosecuting his
patent application.
Invention of Fluorescent
Liquid Penetrant The patent office finally notified
Switzer in the summer of 1941 that his
One of the important observations made claims were allowed and that the patent
was that it was difficult to remove the would be issued. Switzer immediately
fluorescent material. It was difficult called Carl Betz at Magnaflux’s New York
enough to remove the material even office.
when you were aware of it, so the chances
of accidental removal were practically Successful Demonstration of
zero. It was noted that you could scrub Liquid Penetrant Testing
and scrub, and the stuff would stay in the
cracks and pores of your hands. This The two people that came to Switzer’s
observation was the key to the later use of development were also very remarkable
fluorescent materials for nondestructive individuals (Fig. 7). A.V. DeForest had
testing. already made his name as the chief
inventor of the magnetic particle
It turned out that Robert Switzer, process.19 Taber DeForest’s many
Joseph’s brother, had more interest
finding uses for fluorescent materials. In FIGURE 7. The DeForest brothers: (a) A.V.
late 1937, Switzer became aware of a DeForest; (b) Taber DeForest.
serious problem that one of the local (a)
casting companies was having with Ford
Motor Company. It seemed that a very (b)
large batch of aluminum castings was
found to contain a significant number of
discontinuities, specifically surface cracks.
The cracks did not become visible until
the parts were machined.

Ford Motor was in the process of
attempting to collect damages for all
machining expenses as well as the cost of
the parts. Switzer remembered the
experience with the superfine fluorescent
pigment remaining in the cracks and
pores of his hands. He thought he might
be able to use that property of the fine
material to detail the surface cracks. He
had a friend who worked with the
company and was able to obtain samples
of the castings for his experiments.

Like most good ideas, it did not work at
first. His initial approach was to use the
pigment that was used in the crime
detection effort. But surface cracks in
castings were not the same as the cracks in
one’s hands. Switzer persisted and
developed the idea of using liquids to carry
the fluorescent material into the cracks.

He was not satisfied with his results
and continued his developmental efforts.
The work was done mostly in his home
because both Continental and his brother
Joseph Switzer were not interested in this
activity. On August 17, 1938, Robert
Switzer applied for a patent on the
process.18 Robert asked Joseph for his
share of the patent filing fee, which
amounted to fifteen dollars. Joseph
refused to spend the money.

Switzer did his own patent work and,
while performing the search, ran across a
number of Magnaflux Corporation

Introduction to Liquid Penetrant Testing 21

significant contributions to the Liquid Penetrant Testing Process
nondestructive testing field were yet to
come. However, he had been working on The process used that day was very similar
a visible dye liquid penetrant for several in many ways to the process used today.
years with no measurable success.12 The part was cleaned in a strong
chlorinated solvent, probably carbon
Switzer met with his guests at the tetrachloride. The part was then immersed
Continental Lithograph laboratories and in the liquid penetrant solution for more
spent most of the day trying to than 10 min. Sometimes the part was
demonstrate his process. Things did not heated. After penetration, it was rinsed
go well. His samples were castings that he with a strong solvent and was wiped dry
hoped contained surface discontinuities. until clean while being observed under
In his earlier work, after liquid penetrant ultraviolet radiation. It was then subjected
testing revealed a discontinuity, Switzer to repeated hammer blows. The purpose
had to section and thereby destroy the of the last operation was to drive the
test sample. So he would start the test entrapped material out of the surface
without knowing whether the sample cracks into the openings on the surface.
contained discontinuities. As mentioned Then the part was examined for
above, he was never completely satisfied discontinuities under ultraviolet radiation.
with his results.
The number of variables was significant
At the end of many frustrating hours, and results were not very repeatable.
A.V. DeForest quietly suggested that Magnaflux’s experience with magnetic
Robert Switzer try a sample that DeForest particle testing had shown that
pulled from an old leather satchel. This repeatability was a key to acceptance. It
was a sample that DeForest relied on for was decided to continue the development
comparison between various experimental effort to make the processing techniques
magnetic particle materials. He knew more suitable to the production
where each discontinuity was located. environment.
Switzer processed the part. As the
indications became visible, all three were Magnaflux Corporation and the Switzer
astounded by the results. Discontinuities Brothers then entered into a license
were displayed that DeForest had not seen agreement through Continental
before (Fig. 8). They looked back at the Lithograph.12 A joint development effort
castings and quickly noted that the was initiated. Many formulations were
surfaces of these parts had been peened, tried and many different techniques were
so any discontinuities present were evaluated. The astounding results formed
probably closed, making the samples very the basis for developments years later. The
unsuitable for liquid penetrant testing. Magnaflux effort was led by Greer Ellis
The effort that Switzer had spent (Fig. 9) and Taber DeForest. The
improving the process from 1937 to 1941 development work at Switzer Brothers was
had paid off: the method was very done by Joseph Switzer (Fig. 10), who was
sensitive to fine discontinuities. now very interested, and by R.A. Ward.12

FIGURE 8. Photograph of first penetrant test FIGURE 9. Greer Ellis.
sample.

22 Liquid Penetrant Testing

FIGURE 10. Joseph Switzer (left) and Robert Switzer. FIGURE 11. The first liquid penetrant testing unit, model ZA1,
delivered on October 23, 1942.

Water Washable Liquid essentially described the hydrophilic
Penetrant remover process. This was not introduced
commercially until 1963.
The most significant development to
come from this joint effort was the water Developers
washable liquid penetrant. A line of
equipment for handling and processing The introduction of developers shortly
parts was also developed. In June 1942, followed.21 This was the result of efforts of
water washable materials and equipment Harry Kuhrt and Taber DeForest and was
were offered for sale (Fig. 11). A flood of heavily influenced by whiting in the
inquiries resulted and a number of oil-and-whiting process. The first
companies started using the process. The commercial products were dry, yellowish
first equipment sale occurred in October materials consisting mainly of talc. The
1942 to Aluminum Industries, Cincinnati, wet developer process did not appear until
Ohio, for the testing of aluminum late in 1944.11
castings.12
By 1942, a fluorescent water washable
Some of the early applications were for liquid penetrant and a dry developer were
the testing of aluminum propeller blades commercially available. Processing
and engine oil cases. Pistons, valves, spark equipment was available and many trials
plug housings and critical airframe in industrial plants had been completed.
structures also were tested. The most The first sale had taken place.
significant early use of the process was in
the maintenance and overhaul of aircraft Development of Dye
engines. The method worked very well Liquid Penetrant
and in a short time gained wide
acceptance.11,12 During this period, a need arose for a less
complex technique of surface crack
A patent on the water washable process testing for nonferrous components. The
was applied for in June 1942 and was fluorescent liquid penetrant testing
issued in July 1945 to R.A. Ward.20 It is process as it existed required holding
interesting to note that in column five of tanks, a water supply, electricity and a
the patent’s text, Ward states that the darkened area for viewing the indications.
emulsifier agent could be used in the
wash water and need not be included in
the testing agent. Thus postemulsification
was mentioned long before it became a
commercial reality. Also, in describing the
inclusion in the wash water, Ward had

Introduction to Liquid Penetrant Testing 23

A great deal of development work was located. The area was very bright in the
going on in fluorescent liquid penetrant California daylight. Smith Sparling tried
testing technology: sensitivity and various ways to use the fluorescent
reliability were significantly improved, but techniques, but to no avail. She
the techniques were becoming more remembered her experience with the
complex in the process. The water oil-and-whiting technique in her early
washable technique was relatively simple railroad days. She decided to try to
and reasonably fast, but something even improve the process.
simpler was needed.
She stated in one conversation, “All
A Northrop metallurgist named housewives know this about cleaning
Rebecca Smith (later Rebecca Smith porcelain sinks. Dirt will remain in the
Sparling) in the mid-1940s had developed cracks in the sink and will be highlighted
a process that satisfied these needs.4,12 against the white background of the
This was the dye liquid penetrant testing porcelain.” Smith Sparling enlisted the
technique, which has become one of the help of a Northrop chemist by the name
most commonly used techniques for of Loy Sockman (Fig. 13),22 as well as a
surface discontinuity detection. young man by the name of Elliot Brady,
and pursued the development of this idea.
Smith Sparling (Fig. 12) graduated from
Vanderbilt University Nashville, FIGURE 13. Rebecca Smith Sparling and Loy
Tennessee, in 1932 with a master’s degree Sockman (1950).
in physical chemistry. During the next ten
years, she worked at a number of forging
companies and in 1941 was in southern
Wisconsin, working for Lakeside Malleable
Casting. At this location, she had her first
detailed introduction to the
oil-and-whiting technique. Lakeside was
involved in the manufacture of castings
for both the railroad and the automotive
industries. She became familiar with the
process.11

FIGURE 12. Rebecca Smith Sparling (1949).

In the mid-1940s, Smith Sparling was Smith Sparling would suggest an
working in the Turbodyne Engine approach and the chemists would work
Division of Northrop Corporation. She on it. She would then evaluate and
was involved in the final testing of jet comment on their progress and they
engines and was concerned with certain would modify and innovate. This process
crack formation occurring on turbine continued for a number of years and
blades during the test cycle. The engines resulted in the first practical dye liquid
were located at the top of a gantry some penetrant.19 A patent was applied for by
3 m (10 ft) off the floor, close to the Sockman in March 1949.23 Smith Sparling
ceiling where overhead windows were was also involved in setting up a number
of standards that were eventually to
become industry references for the dye
liquid penetrant testing technique.

This new technique filled an important
market need that Northrop recognized.
The technology and patents were sold to
Turco Products. Sockman left Northrop
and founded the company Met-L-Chek in
the early 1950s; there he developed and
sold a dye liquid penetrant. Magnaflux
offered their version of dye liquid
penetrant in 1952.19

24 Liquid Penetrant Testing

A few years later, an interesting point overwash problem of the water washable
developed with respect to the Sockman process. A great deal of effort had been
patent: a patent interference occurred expended in an attempt to solve this
between it and claims filed by Robert problem, but with little success.
Switzer. To understand this fully, the
original claims filed by Switzer in August Parker realized that the overwash was
1938 must be remembered. In this patent caused by the emulsifier’s being mixed in
application, Switzer included claims for the liquid penetrant. For certain types of
visible as well as fluorescent penetrants. gross discontinuities, the wash water
The patent office required division of the could penetrate the discontinuity and
fluorescent from the visible claims as one emulsify the retained liquid penetrant,
of the early office actions. The fluorescent and the wash cycle would remove the
patent was issued in 1941 to Switzer, as fluorescence. Parker removed the
stated earlier. emulsifier from the liquid penetrant and
applied it later as a separate, independent
Robert Switzer filed the visible claims operating step, so shallow, open cracks
in 1945 as a continuation of the earlier could be detected easily with no overwash
filing. The interference occurred after problem.
Sockman’s patent was issued in 1954. To
complicate the issue further, a number of In addition to this, a significant new
other similar patents for dye penetrants capability for increased sensitivity was
were issued in this period.24,25 After a added to liquid penetrant testing: the
number of years, the patent office ruled in emulsification time and wash time were
favor of the Switzer claims, allowing the now mutually exclusive variables. Each
early filing date and assigning a number variable could now be adjusted for the
of the issued visible dye liquid penetrant optimum time independently of the
patents to Switzer.11,18,26 other.

Postemulsification in Fluorescent The first postemulsification products
Liquid Penetrant Testing were introduced in 1953. A patent was
applied for by Taber DeForest and Parker
The next significant improvement in July 1954 on the process.28 In 1955,
occurred in 1952 with the development of Parker developed two new emulsifiers and
the postemulsification process.27 This applied for a patent in 1956.29
development was precipitated by the fact
that Rebecca Smith Sparling used visible Parker and DeForest also experimented
dye liquid penetrant to find cracks in with other ideas concerning the removal
turbine buckets that fluorescent liquid of the fluorescent background. Instead of
penetrant had missed.16 Tabor DeForest washing it off, they thought of quenching
and Donald Parker (Fig. 14), both of its fluorescence.30 This was eventually
Magnaflux, set out to improve the done by large quantities of ultraviolet
performance of fluorescent liquid radiation acting on the dye’s instability to
penetrant. Parker was concerned with the high doses of energy. This approach,
called the fugitive dye technique, was
FIGURE 14. Donald Parker (1955). patented by DeForest and Parker.
Although much time was spent trying to
make it practical, it was never used
commercially.31

Cascading Dyes

The next significant development was of
cascading dyes, first accomplished by
James Alburger. This became another
Parker effort.27 This approach permitted
another quantum leap in sensitivity.
Parker discovered that sensitivity could be
improved dramatically by combining
several different dyes in the liquid
penetrant. What occurred in this mixture
was a cascading effect or a linking between
the dyes. One dye had its absorption peak
in the ultraviolet range whereas its main
emission peak was in the blue range. The
other dye had its main absorption in the
blue and its main emission in the yellow
or yellow-green range. The cascading effect
had a net efficiency greater than that of a
single dye. A patent was applied for by
Parker and Joseph Switzer, and the process
was incorporated into commercial
products in 1954.32

Introduction to Liquid Penetrant Testing 25

Value and Price So, in 1961, twenty years after Switzer
had the first successful demonstration of
In the mid-1950s, an interesting event fluorescent penetrants, both fluorescent
pointed out the relationship and and visible dye penetrants were being
communication between users and used by a number of industries. Materials
product developers. After the of great sensitivity were available.
breakthrough in cascading dyes, Parker Industry and government standards were
decided to develop the ultimate liquid being established. Water washable and
penetrant. This would not be a commercial postemulsification processes were in use,
product; rather, it would represent what a and developers were in common use.
liquid penetrant could do. It would be a
laboratory material — a standard that all Other Contributors
new products would be referenced
against. It contained such a high level of A number of other people have
dye that the selling price would be contributed to the development of liquid
prohibitive.20 Parker developed the liquid penetrant testing technology during this
penetrant, and it was good. One day, he period. Frank Catlin of Magnaflux worked
demonstrated it to personnel of a large on both fluorescent and dye penetrants in
aircraft manufacturer and showed them the early years.13 Ferdi Stern of Magnaflux
how it could find very minute cracks on worked with Greer Ellis on the field trials
their turbine blades. The user’s comment of the early penetrants. Orlando Molino
on Parker’s material was that they of Rockwell developed and patented the
couldn’t afford not to use it. So it became high resolution liquid penetrant testing
a product. In fact, with some small process in the late 1950s. About the same
refinements, it still is in use today. time major improvements in the
sensitivity of water washable liquid
Military Standards penetrants were introduced by James
Alburger, who was also the first to
The first military specification for liquid commercially introduce hydrophilic
penetrant testing was AN-1-30a, issued on emulsifier a little later. J. Thomas Schmidt
September 18, 1946.33 It covered both the of Magnaflux developed and refined the
process and the material. It was a very quality control and test procedures for
general specification and allowed a great product uniformity in the 1960s. People
deal of latitude. A detailed process like Al Robinson at General Electric,
specification, MIL-I-6866, was issued in Victor McBride at Pratt & Whitney, O.E.
1950.34 Stutsman of Wright Field and William
Hitt of Douglas critiqued materials and
A key milestone occurred when the refined the processes. Other people,
military material specification unnamed here, made contributions to
MIL-I-25135 was issued on August 6, liquid penetrant testing in this period.
1956.35 This last specification resulted
from a meeting attended by industry and As a result of the efforts of all of these
military experts in 1955.9 As a result of pioneering people, there has been a
this meeting, the first specific military dramatic increase in the reliability of our
standards for liquid penetrant testing aircraft engines and airframes. The
materials were established. Some contributions of these people have made
important long term policies were possible also some of the very significant
specified, such as the concept of families advances in aircraft performance that
of penetrants. This standard lasted for have occurred in this period. Application
thirty years with only minor changes. of liquid penetrant testing has followed
the increased use of a variety of
Quantitative Approaches nonferrous metals in critical applications
Used in the nuclear, automotive, space,
ordnance and electronic fields.
In the late 1950s and early 1960s, a great
deal of work was done quantitatively to The next twenty-five years were to see
analyze the performance of penetrants. the following developments:
Many data were generated. Details such as (1) development of hydrophilic remover,
effect of liquid penetrant film thickness, (2) development of the high sensitivity
ultraviolet radiation absorption and water washable products, (3) refinements
theoretical effects of developers were of military standards, (4) introduction of
studied and applied to new liquid automatic test techniques and
penetrant development. The industry (5) investigations of heat fade and
moved toward a more scientific phase of statistical techniques of measurement.
development.36 Because of its sensitivity and economy,
liquid penetrant testing will remain one
of the most important nondestructive
testing methods in the twenty-first
century.

26 Liquid Penetrant Testing

PART 5. Units of Measure for Nondestructive
Testing

Origin and Use of the SI through national standards organizations
System and specialized information compiled by
technical organizations.36,37
In 1960 the General Conference on
Weights and Measures devised the Multipliers
International System of Units. Le Systeme
Internationale d’Unites (SI) was designed so Very large or very small numbers with
that a single set of interrelated units are expressed by using the SI
measurement units could be used by all multipliers, prefixes of 103 intervals
branches of science, engineering and the (Table 7) in science and engineering. The
general public. Without SI, this multiplier becomes a property of the SI
Nondestructive Testing Handbook volume unit. For example, a millimeter (mm) is
could have contained a confusing mix of 0.001 meter (m). The volume unit cubic
Imperial units, obsolete centimeter (cm3) is (0.01)3 or 10–6 m3.
centimeter-gram-second (cgs) metric Unit submultiples such as the centimeter,
system version units and the units decimeter, dekameter (or decameter) and
preferred by certain localities or scientific hectometer are avoided in scientific and
specialties. technical uses of SI because of their
variance from the 103 interval. However,
SI is the modern version of the metric dm3 and cm3 are in use specifically
system and ends the division between because they represent a 103 variance.
metric units used by scientists and metric
units used by engineers and the public. TABLE 5. Derived SI units with special names.
Scientists have given up their units based
on centimeter and gram and engineers Quantity Units Symbol Relation
made a fundamental change in
abandoning the kilogram-force in favor of to Other
the newton. Electrical engineers have SI Unitsa
retained their ampere, volt and ohm but
changed all units related to magnetism. Frequency (periodic) hertz Hz 1·s–1
The main effect of SI has been the
reduction of conversion factors between Force newton N kg·m·s–2
units to one (1) — in other words, to
eliminate them entirely. Pressure (stress) pascal Pa N·m–2

Table 4 lists seven base units. Table 5 Energy joule J N·m
lists derived units with special names.
Table 6 gives examples of conversions to Power watt W J·s–1
SI units. In SI, the unit of time is the
second (s) but hour (h) is recognized for Electric charge coulomb C A·s
use with SI.
Electric potentialb volt V W·A–1
For more information, the reader is
referred to the information available Capacitance farad F C·V–1

Electric resistance ohm Ω V·A–1

Conductance siemens S A·V–1

Magnetic flux weber Wb V·s

Magnetic flux density tesla T Wb·m–2

Inductance henry H Wb·A–1

TABLE 4. Base SI units. Luminous flux lumen lm cd·sr

Illuminance lux lx lm·m–2

Quantity Unit Symbol Plane angle radian rad 1

Length meter m Radioactivity becquerel Bq 1·s–1
Mass kilogram kg
Time second s Radiation absorbed dose gray Gy J·kg–1
Electric current ampere A
Temperaturea kelvin K Radiation dose equivalent sievert Sv J·kg–1
Amount of substance mole mol
Luminous intensity candela cd Solid angle steradian sr 1

Time hour h 3600 s

Volumec liter L dm3

a. Kelvin can be expressed in degrees celsius a. Number one expresses dimensionless relationship.
(°C = K – 273.15). b. Electromotive force.
c. The only prefixes that may be used with liter are milli (m) and micro (µ).

Introduction to Liquid Penetrant Testing 27

TABLE 6. Examples of conversions to SI units.

Quantity Measurement in Non-SI Unit Multiply by To Get Measurement in SI Unit

Area square inch (in.2) 645 square millimeter (mm2)
Distance nanometer (nm)
angstrom (Å) 0.1 millimeter (mm)
Energy kilojoule (kJ)
inch (in.) 25.4 joule (J)
Specific heat watt (W)
British thermal unit (BTU) 1.055 kilojoule per kilogram per kelvin (kJ·kg–1·K–1)
Force (torque, couple)
Force or pressure calorie (cal) 4.19 joule (J)
Frequency (cycle) kilopascal (kPa)
Illuminance British thermal unit per hour (BTU·h–1) 0.293 hertz (Hz)
lux (lx)
Luminance British thermal unit per pound 4.19 lux (lx)
candela per square meter (cd·m–2)
Radioactivity per degree Fahrenheit (BTU·lbm–1·°F–1) 1.36 candela per square meter (cd·m–2)
Ionizing radiation exposure foot-pound (ft-lbf) 6.89 candela per square meter (cd·m–2)
Mass pound force per square inch (lbf·in.–2) 1/60 candela per square meter (cd·m–2)
Temperature (difference) cycle per minute candela per square meter (cd·m–2)
Temperature (scale) candela per square meter (cd·m–2)
footcandle (ftc) 10.76 gigabecquerel (GBq)
millicoulomb per kilogram (mC·kg–1)
phot (ph) 10 000 kilogram (kg)
degree celsius (°C)
candela per square foot (cd·ft–2) 10.76 degree celsius (°C)
(°F – 32)/1.8) + 273.15 kelvin (K)
candela per square inch (cd·in.–2) 1 550

footlambert 3.426

lambert 3 183 (= 10 000/π)

nit (nt) 1

stilb (sb) 10 000

curie (Ci) 37

roentgen (R) 0.258

pound (lbm) 0.454
degree fahrenheit (°F) 0.556

degree fahrenheit (°F) (°F – 32)/1.8

TABLE 7. SI multipliers. should be observed. For example, the
meanings of the prefix m (milli) and the
Prefix Symbol Multiplier prefix M (mega) differ by nine orders of
magnitude.
yotta Y 1024
zetta Z 1021 SI Units to Express
exa E 1018 Particular Quantities in
peta P 1015 Nondestructive Testing
tera T 1012
giga G 109 Pressure
mega M 106
kilo k 103 The pascal (Pa), equal to one newton per
hectoa h 102 square meter (1 N·m–2), is used to express
deka (or deca)a da pressure, stress etc. It is used in place of
decia d 10 units of pound force per square inch
centia c 10–1 (lbf·in.–2), atmosphere, millimeter of
milli m 10–2 mercury (mm Hg), torr, bar, inch of
micro µ 10–3 mercury (in. Hg), inch of water (H2O) and
nano n 10–6 other units. The text must indicate
pico p 10–9 whether gage, absolute or differential
femto f 10–12 pressure is meant.
atto a 10–15
zepto z 10–18 Volume
yocto y 10–21
10–24 The cubic meter (m3) is the only volume
measurement unit in SI. It takes the place
a. Avoid these prefixes (except in dm3 and cm3) for of cubic foot, cubic inch, gallon, pint,
science and engineering. barrel and more. In SI, the liter (L) is also
approved for use. The liter is a special
Note that 1 cm3 is not equal to 0.01 m3. name for cubic decimeter (1 L = 1 dm3 =
Also, in equations, submultiples such as 10–3 m3). Only the milli (m) and micro (µ)
centimeter (cm) or decimeter (dm) should prefixes may be used with liter.
be avoided because they disturb the
convenient 103 or 10–3 intervals that The fundamental units of time,
make equations easy to manipulate. temperature, pressure and volume are
expressed every time movement of a fluid
In SI, the distinction between upper (liquid or gas) is measured.
and lower case letters is meaningful and

28 Liquid Penetrant Testing

Light TABLE 8. Abrasive particle size and sieve apertures.

Light is electromagnetic radiation in the _M__e_s_h__D__e_s_ig_n__a_t_io_n__ Nominal ISO _____S_ie_v_e__L_i_n_e_s____
visible range of wavelength or frequency. USA FEPA Sieve Aperture per (per

Vision requires a source of Grit Grit (µm) 10 mm 1.0 in.)
illumination. The light source is measured
in candela (cd), defined as the luminous 70 to 80 213 212 to 180 30.3 (76.9)
intensity in a given direction of a source 80 to 100 181 180 to 150 39.4 (100.0)
that emits monochromatic radiation of 100 to 120 151 150 to 125 43.7 (111.1)
540 × 1012 hertz (540 THz) at a radiant 120 to 150 126 125 to 106 49.2 (125.1)
intensity of 1/683 watt per steradian 140 to 170 107 106 to 90 59.1 (150.1)
(about 0.0015 W·sr–1).
from being indicated in tests. Finish also
The luminous flux in a steradian (sr) is affects adherence of materials to a surface.
measured in lumen (lm). The In preparation of test panels for
measurement in lumen is the product of evaluation, the type and size of abrasive
candela and steradian (1 lm = 1 cd·sr). blasting is often specified.

A light flux of one lumen (1 lm) Abrasive particle size (coarse versus
striking one square meter (1 m2) on the fine) in the United States has traditionally
surface of the sphere around the source been specified using industry accepted
illuminates it with one lux (1 lx), the gage numbers that correspond to the
unit of luminance, or brightness. If the number of lines (or wires) per inch in
source itself is scaled to one square meter sieves used to sift the abrasive grit. A
and emits one candela (1 cd), the number of standards govern grit
luminance of the source is 1 cd·m–2. specifications in the abrasive industries —
for example, ANSI B 74.16 for industrial
Some terms have been replaced in SI. diamonds. Table 8 shows several of the
Illumination is now illuminance. many levels of grit designations based on
Transmission factor is now transmittance. particle size.39
Meter-candle is now lux. Nit is candela per
square meter (cd·m–2). Organizations that issue standards in
this area include the American National
Old units are to be converted (Table 6). Standards Institute (ANSI), the American
Footcandle (ftc) and phot (ph) now Society for Testing and Materials (ASTM),
convert to lux (lx). Stilb (sb), footlambert the Federation of the European Producers
and lambert convert to candela per square of Abrasives (FEPA) and the International
meter (cd·m–2). For measurement of Standardization Organization (ISO).
wavelength, nanometer (nm) obviates
angstrom (Å): 10 Å = 1 nm.

Viscosity

Dynamic viscosity is expressed in SI by the
pascal second. An older unit is the
poise (P), or centipoise (cP): 100 cP = 1 P =
0.1 Pa·s.

Kinematic viscosity is expressed in SI as
square meter per second, equivalent to
the dynamic viscosity divided by mass
density. An older unit is the stokes (St):
1 cSt = 0.01 St = 1 mm2·s–1; 1 St =
0.0001 m2·s–1.

Porosity

Porosity is reported as a ratio of volume to
volume and can be expressed as a
percentage. For example, if hydrogen
content in aluminum is measured as
2.5 mm3·g–1, this value reduces to
2.50 mm3·(0.37 cm3)–1 × 1000 mm3·cm–3 =
0.675 or about 0.7 percent. Therefore the
hydrogen content should be reported as
6.75 mm3·cm–3 in volume for a porosity
of 0.7 percent.

Abrasives and Finish Treatment

Surface abrading is a matter of concern in
liquid penetrant testing because grinding,
shot peening and sand blasting can close
surface breaking cracks and so keep them

Introduction to Liquid Penetrant Testing 29

References

1. Nondestructive Testing Handbook, 13. Swan, L.K. “NDT Afloat: The
second edition: Vol. 10, Nondestructive Development of Nondestructive
Testing Overview. Columbus, OH: Testing at Newport News
American Society for Nondestructive Shipbuilding.” Materials Evaluation.
Testing (1996). Vol. 44, No. 8. Columbus, OH:
American Society for Nondestructive
2. Wenk, S.A. and R.C. McMaster. Testing (July 1986): p 908-911.
Choosing NDT: Applications, Costs and
Benefits of Nondestructive Testing in Your 14. Boisvert, B.W. “The Fluorescent
Quality Assurance Program. Columbus, Penetrant Hydrophilic Remover
OH: American Society for Process.” Report 81-463-3. Dayton,
Nondestructive Testing (1987). OH: Universal Technology
Corporation (1981).
3. Nondestructive Testing Methods.
TO33B-1-1 (NAVAIR 01-1A-16) 15. MIL-I-8474, Anodizing Process for
TM43-0103. Washington, DC: Inspection of Aluminum Alloys and Parts
Department of Defense (June 1984). (18 September 1946).

4. Nondestructive Testing Handbook, 16. Skeie, K.S. Forty Years with Green
second edition: Vol. 2, Liquid Penetrant Fingernails. Manuscript (1999).
Tests. Columbus, OH: American
Society for Nondestructive Testing 17. Taylor, F.J. “The Coldfire Boys.”
(1982). Saturday Evening Post (1 January 1949):
p 29, 44.
5. ANSI/ASNT CP-189, Standard for
Qualification and Certification of 18. Switzer, R.C. Flaw Detector. United
Nondestructive Testing Personnel. States Patent 2 259 400 (applied for
Columbus, OH: American Society for August 1938; issued October 1941).
Nondestructive Testing.
19. Thomas, W.E. Magnaflux Corporation:
6. EN 473, Qualification and Certification A History. Chicago, IL: Magnaflux
of NDT Personnel — General Principles. Corporation (1979).
Brussels, Belgium: European
Committee for Standardization. 20. Ward, R.A. Method and Compositions for
Locating Surface Discontinuities. United
7. ISO 9712, Nondestructive Testing — States Patent 2 405 078 (filed June
Qualification and Certification of 1942: issued July 1946).
Personnel. Geneva, Switzerland:
International Organization for 21. Hill, C. Letter to John Clarke (25 July
Standardization. 1942).

8. AIA NAS 410, Certification and 22. Mooz, W.E. “Retrospective of Loy W.
Qualification of Nondestructive Test Sockman.” Materials Evaluation.
Personnel. Washington, DC: Aerospace Vol. 50, No. 8. Columbus, OH:
Industries Association of America American Society for Nondestructive
(May 1996). Testing (August 1992): p 961-964.

9. ASNT Recommended Practice No. 23. Sockman, L.[W.] et al. Dye Solution
SNT-TC-1A. Columbus, OH: American Flaw Inspection Method. United States
Society for Nondestructive Testing. Patent 2 667 070 (filed March 1949;
issued January 1954).
10. ASNT Central Certification Program
(ACCP), Revision 3 (November 1997). 24. Bloom, R. et al. Method of Detecting
Columbus, OH: American Society for Flaws in Metal Articles. United States
Nondestructive Testing (1998). Patent 2 420 646 (filed January 1945;
issued May 1947).
11. Flaherty, J.J. “History of Penetrants:
The First 20 Years, 1941-1961.” 25. Stokely, J.M. Flaw Detection Fluid.
Materials Evaluation. Vol. 44, No. 12. United States Patent 2 478 951 (filed
Columbus, OH: American Society for May 1944; issued August 1949).
Nondestructive Testing (November
1986): p 1371-1374, 1376, 1378, 1380, 26. Interoffice memo to Robert Switzer.
1382. Cleveland, OH: Dayglo Color
Corporation (April 1965).
12. Betz, C.E. Principles of Penetrants.
Chicago, IL: Magnaflux Corporation 27. Parker, D. Laboratory notebook no. 1.
(1963): p 5-13. Chicago, IL: Magnaflux (unpublished).

28. DeForest, T. et al. Method of Detecting
Surface Discontinuities. United States
Patent 2 806 959 (filed February 1956;
issued September 1957).

30 Liquid Penetrant Testing

29. Parker, D. et al. Water Emulsifiable
Composition. United States Patent
2 978 418 (filed February 1956; issued
April 1961).

30. Parker, D. Chicago unpublished
laboratory report C-28. Chicago, IL:
Magnaflux (April 1952 and October
1952).

31. DeForest, T. et al. United States Patent
2 774 886, Method of Detecting Surface
Discontinuities (filed November 1952;
issued December 1956).

32. Switzer, J. et al. Fluorescent Penetrant
Inspection Materials and Methods.
United States Patent 2 920 203 (filed
September 1955; issued January 1960).

33. Military specification AN-I-30a,
Fluorescent Method of Inspection
(September 18, 1946).

34. MIL-I-6866, Penetrant Method of
Inspection (August 7, 1950).

35. MIL-I-25135, Inspection Materials:
Penetrants (August 6, 1956).

36. Thomas, W.E., “An Analytic Approach
to Penetrant Performance” (1963
Lester Honor Lecture). Nondestructive
Testing. Vol. 21, No. 6. Columbus, OH:
American Society for Nondestructive
Testing (November-December 1963):
p 354-368.

37. IEEE/ASTM SI 10-1997, Standard for Use
of the International System of Units (SI):
The Modernized Metric System. West
Conshohocken, PA: American Society
for Testing and Materials (1996).

38. Taylor, B.N. Guide for the Use of the
International System of Units (SI). NIST
Special Publication 811, 1995 edition.
Washington, DC: United States
Government Printing Office (1995).

39. B 74.16, Checking the Size of Diamond
and Cubic Boron Nitride Abrasive Grain.
New York, NY: American National
Standards Institute (1995).

Introduction to Liquid Penetrant Testing 31

2

CHAPTER

Principles of Liquid
Penetrant Testing

Noel A. Tracy, Universal Technology Corporation,
Dayton, Ohio
James S. Borucki, Gould Bass NDT, Pomona, California
Art Cedillos, Palomar Plating Company, Escondido,
California
John J. Flaherty, Flare Technology, Elk Grove Village,
Illinois
Bruce C. Graham, Arlington Heights, Illinois
Donald J. Hagemaier, Boeing, Long Beach, California
Robert J. Lord, Boeing, St. Louis, Missouri
Stanley Ness, Mission Viejo, California
J.T. Schmidt, J.T. Schmidt Associates, Incorporated,
Crystal Lake, Illinois
Kermit S. Skeie, Kermit Skeie Associates, San Dimas,
California
Amos G. Sherwin, Sherwin Incorporated, South Gate,
California

PART 1. Elements of Liquid Penetrant Testing

Basic Liquid Penetrant the part surface at the discontinuity
Testing Process opening, can reduce or completely
destroy the effectiveness of the test.
The basic principles of the liquid 2. Apply liquid penetrant to the test surfaces
penetrant testing process have not and permit it to dwell on the part
changed from the oil-and-whiting days. surface for a period of time to allow it
These principles are shown in Fig. 1. As to enter and fill any discontinuities
the liquid penetrant testing process open to the surface.
evolved, additional steps were specified. 3. Remove the excess liquid penetrant from
Presently, the process consists of six basic the test surfaces. Care must be exercised
steps. to prevent removal of liquid penetrant
contained in discontinuities.
1. Preclean and dry the test surfaces of the 4. Apply a developer, which aids in
object to be inspected. Cleaning is a drawing any trapped liquid penetrant
critical part of the liquid penetrant from discontinuities and spreading
process and is emphasized because of that liquid penetrant on the test
its effect on the test results. surface to improve the visibility of
Contaminants, soils or moisture, indications. The developer also
either inside the discontinuity or on provides a contrasting background on
a part surface, especially for
FIGURE 1. Basic liquid penetrant process: (a) nonfluorescent indications.
apply liquid penetrant; (b) remove excess; 5. Visually examine surfaces for liquid
(c) apply developer. penetrant indications; interpret and
evaluate the indications.
(a) 6. Postclean the part to remove process
residues if they will be detrimental to
Liquid subsequent operations or the part’s
penetrant intended function. (In some cases, a
treatment to prevent corrosion may be
(b) required.)

These six basic steps are followed
regardless of the type of fluorescent or
visible color dye used to form the liquid
penetrant indications. The fourth step,
application of a developer, is sometimes
omitted when testing newly
manufactured parts with fluorescent
liquid penetrants but at the cost of lower
test sensitivity due to reduced visibility of
indications.

(c) Indication Materials That Can Be
Inspected by Liquid
Developer Penetrant Tests

Liquid penetrant Liquid penetrant testing is one of the best
test methods for all types of
discontinuities open to accessible surfaces
in solid nonpermeable materials.
Laminations and lack of bond between
layers of laminated materials can be
detected with liquid penetrants only if
these laminar discontinuities extend to
exposed edges or holes where the liquid
penetrant can enter and indications can
be observed. Liquid penetrant testing has

34 Liquid Penetrant Testing

MOVIE. been used with excellent success on discontinuities. Thus, detection of
Bleeding ferrous and nonferrous metals and alloys, inconsequential discontinuities can be
suggests fired ceramics and cermets, powdered minimized whereas larger
discontinuity metal products, glass and some types of discontinuities of more concern are
severity. plastics and synthetic organic materials. indicated.
Liquid penetrants are also used for
detection of leaks in tubing, tanks, welds Disadvantages and
and components. Limitations of Liquid
Penetrant Testing
Reasons for Selecting
Liquid Penetrant Testing 1. Liquid penetrant testing depends on
the ability of liquid penetrant to enter
Some of the reasons for choosing liquid and fill discontinuities. Penetrant
penetrant testing are as follows. testing will only reveal discontinuities
open to the surface.
1. Liquid penetrant testing can quickly
examine all the accessible surfaces of 2. Surfaces of objects to be inspected
objects. Complex shapes can be must be clean and free of organic or
immersed or sprayed with liquid inorganic contaminants that will
penetrant to provide complete surface prevent interaction of the penetrating
coverage. media with a surface. Organic surface
coatings, such as paint, oil, grease or
2. Liquid penetrant testing can detect resin, are in this category. Any coating
very small surface discontinuities. It is that covers or blocks the discontinuity
one of the most sensitive opening will prevent liquid penetrant
nondestructive test methods for entry. Even when the coating does not
detecting surface discontinuities. cover the opening, material at the
edge of the opening may affect entry
3. Liquid penetrant testing can be used or exit of liquid penetrant and greatly
on a wide variety of materials: ferrous reduce reliability of the test. Coatings
and nonferrous metals and alloys; in the vicinity of a discontinuity will
fired ceramics and cermets; powdered also retain liquid penetrant, causing
metal products; glass; and some types background fluorescence. Cleaning
of organic materials. Restrictions on test surfaces is discussed in more detail
materials imposed by the nature of the below.
liquid penetrant process are covered in
the discussion of limitations, below. 3. It is also essential that the inside
surface of discontinuities be free of
4. Liquid penetrant testing can be materials such as corrosion, fluids,
accomplished with relatively combustion products or other
inexpensive, nonsophisticated contaminants that would restrict entry
equipment. If the area to be inspected of liquid penetrant. Because it is
is small, the test can be accomplished impossible to check inside
with portable equipment. discontinuities, one must trust that
processes selected to clean test surfaces
5. Liquid penetrant testing magnifies the will clean inside surfaces of
apparent size of discontinuities discontinuities also.
making the indications more visible.
In addition, the discontinuity 4. Mechanical operations, such as shot
location, orientation and approximate peening, plastic media blasting (PMB),
size and shape are indicated on the machining, honing, abrasive blasting,
part, making interpretation and buffing, brushing, grinding or sanding
evaluation possible. will smear or peen the surface of
metals. This mechanical working
6. Liquid penetrant testing is readily closes or reduces the surface opening
adapted to volume processing, of existing discontinuities. Mechanical
permitting 100 percent surface working (smearing or peening) also
inspection. Small parts may be placed occurs during service when parts are
in baskets for batch processing. in contact or rub together. Penetrant
Specialized systems may be partially or testing will not reliably detect
fully automated to process many parts discontinuities when it is performed
per hour. after a mechanical operation or service
use that smears or peens the surface.
7. The sensitivity of a liquid penetrant In some cases chemical removal
testing process may be adjusted (etching) of smeared metal may
through appropriate selection of liquid restore test reliability.
penetrant, removal technique and
type of developer. This allows the 5. Unless special procedures are used,
liquid penetrant process to be adapted liquid penetrant testing is impractical
to characteristics (e.g., composition,
surface condition) of the part
requiring testing and to be tailored to
detect specified minimum rejectable

Principles of Liquid Penetrant Testing 35

MOVIE. on porous materials, such as some provides a high contrast background for
Fluorescent types of anodized aluminum surfaces, the colored liquid penetrant when viewed
liquid other protective coatings and porous under the appropriate light. Red dye is
penetrant. nonmetallic parts. Penetrant rapidly most common, although some blue dye is
enters pores of the material and also used.
MOVIE. becomes trapped. This can result in an
Liquid overall background fluorescence or Dual Mode (Both Visible and
penetrant color that could mask any potential Fluorescent) Liquid Penetrant
seeps into discontinuity indications. In addition,
discontinuity. removal of the liquid penetrant may Dual mode liquid penetrants contain dyes
not be possible after the test. that are both colored under white light
6. Penetrants, emulsifiers and some types and fluorescent under ultraviolet
of developers have very good wetting radiation. However, the intensities of the
and detergent properties. They can act visible color (usually red) and the
as solvents for fats and oils. They also fluorescent color (usually orange) are less
can clean ferrous materials so than the colors produced by the single
thoroughly that rust will begin almost mode visible and fluorescent liquid
immediately if corrosion inhibitor is penetrants respectively.
not applied. If allowed to remain in
contact with human skin for extended Classification of Liquid
periods, they may cause irritation. Penetrants by Removal
Method
Classification of Liquid
Penetrants by Dye Type A liquid penetrant is further classified by
the technique used to remove it from the
Liquid penetrants are generally classified surface of a part after it has been on the
by type according to the dye contained in part a specified amount of dwell time
the liquid penetrant. The liquid penetrant during the test process. The liquid
testing process relies on liquid penetrant penetrants are formulated and
entering a discontinuity and subsequently manufactured for specific removal
being drawn back out and made easily techniques designed to minimize removal
visible on the surface of a part. The of the liquid penetrant that has seeped
amount of liquid penetrant material into a discontinuity. Each removal
entrapped in discontinuities is usually technique has advantages and
very small. If the discontinuity is to be disadvantages, discussed below.
detected, the very small amount of liquid
penetrant must be highly visible. In the Water Washable Liquid Penetrant
oil-and-whiting days, it was found that
used or dirty oil was much more visible Most liquid penetrants contain an oil base
than clean machine oil. Today chemists insoluble in and immiscible with water.
make liquid penetrants visible by This means that the excess liquid
dissolving highly colored dyes in a penetrant on a part cannot be removed
penetrating oil or other vehicle. Based on with water. However, some liquid
the dye, liquid penetrants are classified as penetrants are carefully compounded
one of the three types described below. mixtures of an oil base and an emulsifier,
and others have water or a surfactant as a
Fluorescent Liquid Penetrant base rather than oil. Manufacturers
provide these alternative formulations in
Fluorescent liquid penetrants contain ready-to-use liquid penetrants, which may
fluorescent dye that emits yellowish green be removed with water immediately after
light when exposed to near ultraviolet the liquid penetrant dwell. Depending on
radiation (with a wavelength of 320 to requirements imposed on the liquid
400 nm). This property is termed penetrant testing procedure by applicable
fluorescence. Very small quantities of process specifications, the removal may be
fluorescent liquid penetrant will emit accomplished by wiping the part surface
highly visible indications when exposed with a wet lintfree cloth (after wiping
to ultraviolet radiation. with a dry lintfree cloth first), by directing
a controlled spray onto the part or by
Visible Liquid Penetrant dipping and agitating the part in water.

Visible dye or color contrast liquid Postemulsifiable Liquid Penetrant
penetrants contain a dye that is visible
under natural or white light. The visibility When used in the postemulsification
is further enhanced during the liquid process, liquid penetrants can be
penetrant process by the application of a formulated to optimize their penetrating
white developer. The white developer and visibility characteristics for higher

36 Liquid Penetrant Testing