ASNT NDT Handbook Volume 9 visual Testing

weld concavity is acceptable (Fig. 21). If fillet weld) plus 0.75 or 1.5 mm (0.03 or
the center point does not make contact 0.06 in.), depending on the standard
with the weld metal, the concavity is being used. The actual weld throat minus
excessive and the weld is unacceptable. the allowable convexity must be less than
or equal to the theoretical throat (Fig. 22).
A convex weld surface bulges or curves
outward (Fig. 10b). Excessive convexity in In axially aligned members of different
a butt joint is the same as excessive materials, thicknesses are required so that
reinforcement. A few codes have accepted the slope in the transition zone does not
convexity not greater than 0.1 times the exceed some specified amount. The
measured fillet weld leg length (or the transition is accomplished by chamfering
longer leg in the case of an unequal leg the thicker part, tapering the wider part,
sloping the weld metal or a combination
FIGURE 21. Desirable fillet weld profiles: (a) diagram of weld of these. Figures 23 to 25 show examples
components; (b) weld cross section. of transition weld requirements adapted
from the American Welding Society’s
(a) Actual throat Convexity Structural Welding Code,6 which requires
that the transition zone not exceed
25 mm (1 in.) in 64 mm (2.5 in.).

Theoretical throat FIGURE 23. Transition formed by sloping
weld surface: (a) center line alignment;
Legs (b) offset alignment.

(b) 45 degrees Size (a)
Size
Size 25 mm (1 in.)
Size C 64 mm (2.5 in.)

64 mm (2.5 in.)
25 mm (1 in.)

(b)

25 mm (1 in.)
64 mm (2.5 in.)

FIGURE 22. Acceptable fillet weld profiles: (a) diagram of weld
components; (b) weld cross section.

(a) Actual throat Convexity FIGURE 24. Transition formed by sloping weld surface and
chamfering: (a) center line alignment; (b) offset alignment.

(a) Remove after welding

Theoretical throat 25 mm (1 in.)

64 mm (2.5 in.)

(b) Legs 64 mm (2.5 in.) Remove after welding
25 mm (1 in.) Remove after welding
Size Size
Size Size (b)

C 25 mm (1 in.)
64 mm (2.5 in.)

C

Electric Power Applications of Visual Testing 245

A simple gage (Fig. 26) may be used for has tails some standards, such as
determining acceptable transitions. The Section VIII of the ASME Boiler and
gage is positioned with the flat edge on Pressure Vessel Code, base the size of the
the thicker base metal and the sloped bore to include the tail.
edge over the transition zone. To be
acceptable, the slope of the transition Surface porosity is usually visible with
zone must follow the slope of the gage. the naked eye if the test area is properly
illuminated. Surface porosity is generally
Weld Joint Discontinuities undesirable, but the fabrication document
should be consulted for acceptance
Porosity standards. In cases where standards are
given for porosity (Sections III and VIII of
Porosity is a group of gas pockets or voids the ASME Boiler and Pressure Vessel Code,3
(Fig. 27), inside or on the surface of weld ANSI/ASME B31.1,7 AWS D1.1M6),
metal. Porosity is commonly observed as acceptance limits are based on either
spherically shaped voids that are maximum single pore diameter, number
uniformly scattered, clustered or linear. of pores below a given size per unit area,
Uniformly scattered porosity consists of or aggregate pore diameter per unit area.
individual pores which can vary in A rule with increments of 0.8 mm (0.03
diameter from microscopic to easily in.) or less is needed to make this
visible. Cluster porosity is a grouping of determination. A low power magnifying
small pores. Linear porosity typically lens may also be useful for measuring
occurs in the root pass. When porosity pores.

FIGURE 25. Transition by chamfering thicker part: (a) center Overlap
line alignment (particularly applicable to web plates);
(b) offset alignment (particularly applicable to flange plates). Overlap or cold lap is the protrusion of
weld metal beyond the toe or root of the
(a) Chamfer before welding weld (Figs. 28 and 29). Overlap is
generally not allowed in a completed weld
25 mm (1 in.) because the notch formed between the
crown and base metal concentrates
63 mm (2.5 in.) stresses.

63 mm (2.5 in.) Chamfer before welding The presence of overlap is determined
25 mm (1 in.) Chamfer before welding by visually examining the weld
metal-to-base metal transition at the toe
(b) of the weld. The transition should be
smooth, without rollover of weld metal.
25 mm (1 in.)
63 mm (2.5 in.) Consider a tangent to the weld metal
at the intersection between the weld
metal and base metal. If no overlap is
present, the angle between the tangent to
the weld metal and the base metal is
greater than or equal to 90 degrees. If
overlap is present, the angle between the
tangent to the weld metal and the base
metal is less than 90 degrees.

FIGURE 27. Porosity beneath weld surface: (a) longitudinal
section view; (b) cross section view.

(a)

FIGURE 26. Example of acceptable slope of taper where
4:1 slope is specified.

4:1 template

(b)

Template shows Cut out for
more than weld joint
4:1 taper

246 Visual Testing

Undercut acceptance criteria for undercut. For
example, Section III of the ASME Boiler
Undercut is a groove formed at the toe or and Pressure Vessel Code3 has required that
root of a weld when base metal is melted undercut be less than 0.8 mm (0.03 in.)
away and left unfilled by weld metal deep; welders and inspectors must consult
(Fig. 30). Undercut can be difficult to the edition specified by contract. AWS D1
measure accurately. When a gage is used gives acceptable limits for undercut.6
to measure the depth of undercut in butt
joints, the body of the gage is held to Inadequate Joint Penetration
straddle the weld crown and a pointed tip
on a semicircular dial is placed in the Inadequate penetration is a failure of base
undercut. material to fuse with filler metal in the
root of the weld joint (Fig. 31). Where full
Undercut results in a reduction of base penetration is specified by the
metal thickness and may materially construction drawing, the inspector must
reduce the strength of a joint, particularly visually determine that the root has been
the fatigue strength. For this reason, completely filled with weld metal.
excessive undercut is undesirable in a
completed weld and most codes specify Cracks

FIGURE 28. If overlap is present, the angle between the Surface cracks detectable by visual testing
tangent to the weld metal and the base metal should be less may occur longitudinally or transversely
than 90 degrees. Acceptable profiles without overlapping: along the weld root, face, toe and heat
(a) in V joint; (b) in butt joint. affected zone (Fig. 32) or in arc strikes
outside the weld. A low power (5×)
(a) magnifying lens is the best tool for
visually detecting surface cracks. Care
90 degrees must be exercised when using a magnifier
— minor surface irregularities can be
accentuated, affecting the accuracy of
interpretation.

With few exceptions, no visible surface
cracks are allowed in completed welds.

(b) FIGURE 30. Presence of undercut: (a) in fillet weld joint; (b) in
butt weld joint.

90 degrees (a)

FIGURE 29. Presence of unacceptable overlap: (a) in V joint; (b)
(b) in fillet joint.

(a)

<90 degrees

(b)
FIGURE 31. Inadequate joint penetration.

<90 degrees

Electric Power Applications of Visual Testing 247

Arc Strikes continuous. If the outside fillet is
continuous, there is reasonable assurance
Arc strikes are caused by unintentional that a sound joint has been made.
rapid heating of the base metal or weld
metal and subsequent rapid cooling of the Soldered Joints
molten material (Fig. 33). Melting of the
base metal and deposition of filler metal Inadequate wetting is the most common
are often associated with strikes caused by problem encountered with soldering,
a welding arc. resulting in incomplete coverage of the
surface with solder. Nonwetting is
Arc strikes may also be caused by a apparent during visual testing by the
poorly connected welding ground clamp appearance of the original surface finish.
or by instruments used for magnetic Dewetting is the flow and retraction of
particle testing. The extremely high heat solder, caused by contaminated surfaces,
input that occurs during an arc strike can dissolved surface coatings or overheating
cause localized hardness and cracking. before soldering.

Visual Testing of Brazed Dewetting may look like nonwetting
and Soldered Joints but it is identified by a colored residual
film with beads of solder where the solder
Brazed Joints receded. Excessive movement of the
soldered joint during solidification may
In power plants, brazed joints are materially weaken the joint and is visually
commonly used on instrumentation lines determined by a frosty appearance.
in low criticality systems, called Class III
systems in the ASME Boiler and Pressure Visual testing is the most widely used
Vessel Code.3 Lap joints are the most method for nondestructive inspection of
common configurations for these soldered joints. Reference standards are
applications. Before brazing, the used to aid the inspector in checking for
soundness of these joints is ensured by general design conformance, wetting,
control of cleanliness, of joint overlap and finished cleanliness of the product and
of the joint gap. the quality and quantity of solder.

The objective in visual testing of Acceptance Standards
brazed lap joints is to determine that a
continuous flow of braze filler metal has Every feature of the joint configurations
been achieved. If filler metal is applied and dimensions subjected to visual testing
from the same side of the joint as is has an associated quantitative tolerance
visually examined, there is no assurance limit or qualitative criteria of
of penetration within the joint, even if acceptability. These are established with
the appearance of the fillet is good. the drawing or incorporated into the
Ultrasonic testing may be used to requirements of the governing document.
determine filler metal penetration in such Discontinuities also have quantitative and
cases. qualitative criteria.

To simplify verification of a sound The inspector should record all test
braze, good joint design requires that filler results. Acceptability may be determined
metal is placed inside the joint before at the site or later, depending on the
brazing. Visual testing is then conducted specified requirements for the application.
on the outside fillet to determine if it is
FIGURE 33. Arc strikes on work pieces and
FIGURE 32. Typical weld cracks. portions of multipass weld.

Transverse crack in weld and heat Arc strikes
affected zone

Crater crack

Arc strikes

Throat crack

Toe crack

Root crack

248 Visual Testing

Recording and Reporting In the record of results, identify all
Visual Test Results measurements to be recorded, provide
space for all recorded measurements with
Visual test results are recorded by using identification, and provide space for
forms developed for specific applications. recording indications.
If no form is provided by the customer or
regulation, the inspector should develop Through standard recording formats
one that provides the following data: and prepared forms, the inspector and
(1) joint identification and description, supervisor can determine if all required
(2) joint inspector’s identification, observations have been made properly.
(3) testing dates, (4) instrument and
equipment identification (may refer to
procedure), (5) procedure identification
(and checklist) and (6) record of the test
results.

Electric Power Applications of Visual Testing 249

PART 2. Visual Testing of Various Components8,9

Visual Testing of Reactor The purpose of the test is to determine
Pressure Vessels how the pressure vessel system has been
operating. Many small component failures
Visual testing of a reactor vessel and its have been detected by such tests,
internal components is one of the most preventing serious failures. It is almost
critical operations in any inspection impossible to make a complete visual test
program. Not only is the test closely of anything as complex as pressure vessel
audited by the Nuclear Regulatory internals. The visual inspector has limited
Commission and the American Nuclear time available because time in the vessel
Institute (ANI), it is usually done on a may be expensive during a refueling
critical outage schedule, so there is no outage. Some limitations are set by
opportunity to substitute operations if equipment, and each test technique has
there are problems with the inspection. drawbacks.

A Level III inspector is typically Boiler Code Visual Test Categories
involved in all phases of the test,
beginning with the selection of Nuclear plants are examined on a routine
equipment, the training of test personnel, basis according to the requirements of
supervision of the test itself and the final their license, the laws of the state in
interpretation and review of the results. which they are operated and special
The text below presents situations typical requirements of the Nuclear Regulatory
of such a visual inspection and suggests Commission. Most of these tests must
techniques that might help resolve meet ASME Boiler and Pressure Vessel Code
anticipated problems. requirements in the inservice test
program. In certain instances, visual tests
Applicable Code are requested by the Nuclear Regulatory
Commission when component failures are
Visual tests of reactor vessel internals are suspected.
performed according to the rules of the
plant’s inservice test program, which is in Before the 1977 edition of the ASME
turn based on visual tests required in Boiler and Pressure Vessel Code, visual tests
appropriate standards. It is the were conducted to determine the general
responsibility of the inspector to perform condition of a part, component or surface.
the tests so that the requirements of the These tests were made according to
standard and Nuclear Regulatory Section V, Article 9, with an additional
Commission bulletins are satisfied. requirement for lighting sufficient to
resolve a 0.75 mm (0.03 in.) wide black
The primary requirement is that the line on an 18 percent neutral gray card.
visual test be done according to a written
procedure and that a checklist be used to After the 1980s, Section XI of the ASME
plan and perform the test. Possible Boiler and Pressure Vessel Code divided
checklist items are in Table 1. visual tests into three categories: VT-1, to
determine the condition of a part,
Visual tests of the vessel are performed component or surface; VT-2, to detect
to determine the condition of critical leakage from pressure retaining
inner surfaces. Such critical surfaces components or abnormal leakage during
include high stress points at the junctions system pressure or functional inspection;
of nozzles and the vessel or nozzles and VT-3, to determine the general
the cladding. Cladding is critical because mechanical and structural condition of
it protects the vessel from corrosion. components and supporting structures
and to determine conditions relating to
The visual inspection of a reactor vessel component or device operability.
may involve many techniques. Vessel tests
may be made with the vessel empty — or VT-1. The VT-1 test (for condition of part,
filled with water with the internal component or surface) is required on the
components in place — by remote following components, according to
viewing or direct viewing (in any such Section XI of the ASME Boiler and Pressure
test, it is impossible for the inspector to Vessel Code.
place his unprotected eye close to the test
site).

250 Visual Testing

1. The reactor vessel bushings and are uncovered. The inspector should form
closure washers are available for close, the habit of making a very close
direct visual observation when the inspection for leaks during other visual
head is removed from the reactor. The tests.
bushings, part of the head, may
require that the inspector wear There is a tendency for some inspectors
protective clothing to avoid inhaling to limit themselves to the specialized test
loose contaminant. The washers underway, neglecting other visual
should present no contamination evidence. Inspectors should remember
problem because they are usually kept that the reactor is in theory a watertight
separate from the head in special bins. and gastight system. Any water where
Volumetric examination of the vessel none is expected, no matter how small
bushing area and ultrasonic testing of the amount, is cause for concern.
vessel bushing areas may be required.
VT-3. The VT-3 test (for mechanical and
2. All bolts, studs and nuts of the reactor structural conditions) is used to determine
vessel are kept clear of the head and the general condition of the vessel outside
are straightforward to inspect. The of the beltline region and the core
root of the threads in the studs may be supports. This test can be video recorded.
fouled with a threading compound The object of the test is to disclose
nearly impossible to remove, making a corrosion or broken, worn or missing
close visual test impractical. The eddy components and may require physical
current technique for surface testing of measurements. Test personnel are
these components may be used. qualified by the same program as VT-1
Volumetric and surface examinations and VT-2.
of the studs and surface examination
of the nuts are needed, in addition to TABLE 1. Typical visual inspection checklist for pressurized
ultrasonic testing of reactor vessel water nuclear reactor's vessel internals. Each component
head bolts. listed gets complete visual inspection.

3. The reactor vessel internal Core barrel assembly
attachments within the beltline region 1. Core barrel alignment key
can be visually tested by using a video 2. Core barrel thermal pad mounting
monitor. 3. Core barrel irradiation specimen basket
4. Core barrel lower internals head alignment pin
The visual inspector is expected to be 5. Core barrel outlet nozzle/vessel interface surface
(1) trained and qualified and (2) able to 6. Core barrel midplane girth weld
demonstrate vision acuity and color 7. Instrumentation mounted in core barrel
discrimination. Details of qualification 8. Upper internals alignment key in core barrel
and certification should be specified in 9. Baffles in core barrel
the contract, written practice and
regulations. 10. Guide tube attachment to upper core plate
11. Upper core plate
VT-2. The VT-2 test (for evidence of leaks) 12. Upper core plate alignment keyway
can be performed every time an inspector 13. Alignment of keyway upper support plate
works around the reactor vessel, 14. Core barrel
particularly in the area of penetrations or Supporting components
underneath the vessel (in boiling water 15. Upper support plate instrument conduits and supports
reactors). The VT-2 test is recorded during 16. Lower core plate showing fuel assembly alignment pins, flow
a pressure test of the system.
holes, in core instrument guides, supports and attachments
The VT-2 inspector looks for stains, 17. Lower core support columns and instrument guides
dislodged insulation and water on the 18. Support column instrument guide tube attachments to lower
floor or dripping from overhead
insulation. A leak in a vessel nozzle or safe core support plate
end can result in water traveling along the 19. Upper internal keyway insert attachments
insulation until it has a path for escape, 20. Fuel pin and support column attachment upper internals
so the vessel inspector should be alert to 21. Upper internals guide types and support columns
conditions in piping around the vessel. 22. Guide tube note: split pin attachments
23. Baffle bolting
Vertical insulated components may be 24. Upper tube assemblies on upper support plate with
examined at the lowest point where
leakage may be detected. Horizontal instrument port guide post
insulated components are examined at 25. Secondary core support assembly and instrument guide tubes
each insulation joint. If accessible, the 26. Reactor vessel internal surfaces
VT-2 test is performed directly on the 27. Interior attachments and core support structures
vessel. If this is not possible, the test may 28. Core support structures
be made on the surrounding areas and
floor, looking for evidence of leakage.

Much of the VT-2 leak test is performed
by the visual inspectors when the plant is
cold. These individuals have close access
to the nozzles and safe ends when they

Electric Power Applications of Visual Testing 251

Proper performance of a remote visual recommended for this type of reactor calls
test depends on appropriate fixturing and for visual testing of internals at ten year
lighting. A video camera on a long pole is intervals, as defined in the ASME Boiler
like a pendulum — unless care is taken to and Pressure Vessel Code.
stabilize it, the inspector can spend as
much time waiting for the camera to stop During a normal refueling operation
swinging as for completing the test itself. on a pressurized water reactor, the head
and control rod drive assembly are
Test Procedures
FIGURE 35. Pressurized water reactor vessel.
A direct VT-1 test should be performed
with the eyes no more than 600 mm 1 13
(24 in.) away from the test object, at an
angle no greater than 30 degrees to the 2 14
surface. The illumination on the object is 15
such that a 0.75 mm (0.03 in.) wide line 3 16
inscribed on an 18 percent neutral gray
card can be seen by the inspector. Remote 4 17
viewing aids such as video cameras work 18
if conditions equivalent to direct 5 19
inspection can be met. 20
6 21
It is very unlikely that a gray card test
could be used with an underwater closed 7 22
circuit television system. In some cases, 23
the closed circuit television must resolve a 8
stretched wire of a known diameter placed 24
against the test material (Fig. 34). 9

Pressurized Water Reactor 10
Internals
11
All of the internals of the pressurized
water reactor (Fig. 35) may be removed 12
from the vessel for testing. To do this, the
fuel must also be removed. Consequently, Legend
the inservice inspection program 1. Control rod drive shaft.
2. Lifting lug.
FIGURE 34. Calibration standard for remote 3. Upper support plate.
visual test. 4. Internals support ledge.
5. Core barrel.
Pole connection to handling tool 6. Outlet nozzle.
7. Upper core plate.
Wire 8. Reactor vessel.
9. Lower instrumentation guide tube.
Aluminum frame
10. Bottom support forging.
11. Radial support.
12. Tie plates.
13. Control rod drive mechanism.
14. Thermal sleeve.
15. Closure head assembly.
16. Hold-down sharing.
17. Inlet nozzle.
18. Fuel assemblies.
19. Baffle.
20. Former.
21. Lower core plate.
22. Irradiation specimen guide.
23. Neutron shield pad.
24. Core support columns.

252 Visual Testing

removed and stored. Access to the lower With the components removed from
portions of the vessel must, in many the vessel, a television camera can view
cases, be through openings left by fuel the upper nozzles and interior piping.
that has been removed. In some cases, Visual inspection of these items is very
access openings for a television camera important and must be carefully
may be provided so that a small video performed. A positioning fixture is
unit may be inserted between the thermal required for close examination of the
shield wall and the vessel. piping.

Before beginning the test, the inspector The remaining fuel in the vessel can be
should have a written procedure and a a cable trap. The tops of the fuel bundles
checklist. The checklist helps to ensure have many closely grouped springs, nuts,
that nothing is overlooked (Table 2). antirotation devices and other hardware.
It is very easy to snag a small cable if it is
At the end of the ten year interval, the allowed to loop into the fuel. This can be
pressurized water reactor internals are prevented by attaching floats to the cable.
completely removed from the vessel and
stored separately. At this time, they are The inspector working over the open
examined in detail. Because this test is reactor vessel must take extreme care that
repeated over such long time intervals, it equipment does not become dislodged
is important that complete records be and fall into the vessel. Operating
maintained. Because an index and personnel make sure that the inspector
inspection logs may become separated has all pockets taped, eyeglasses taped on
from a video tape, it is a good idea to and all small tools moved away from the
make the tape self identifying. If the vessel. It may be worthwhile to situate the
recorder has a voice channel, it can be inspector and the data recorder away from
used. A title block, made by writing the the vessel and to have an operator
essential data on a card before the taping, position the camera on the bridge, using a
should be required. small monitor and intercom.

Boiling Water Reactor Internals Two types of remote camera
positioning equipment are a camera
When the boiling water reactor is opened positioner mounted to the trolley of the
for refueling, the head, the steam refueling bridge and a submersible
separator and the steam dryer are remotely operated vehicle.
removed and placed on stands. The head
is stored out of the water on the refueling 1. A video camera positioner (Fig. 37)
building floor and the other components supports and positions an underwater
are on stands in a holding pool, which camera and lighting system in X, Y
may be gated off from the reactor vessel. and Z coordinates. The unit’s design
Fuel may or may not be completely allows stable positioning of the
removed. A cross section of a typical camera. The horizontal inspection
boiling water reactor is shown in Fig. 36. boom (Fig. 38) attaches to the
It is over 24 m (80 ft) from the refueling bottom-most extension tube and is
bridge to the bottom of the vessel and it is designed for scanning the reactor
6 m (20 ft) from the vessel flange to the pressure vessel circumference from the
refueling bridge. center of the vessel. This position
obviates moving the refueling bridge
or trolley for circumferential
examinations.

Table 2. Reactor vessel and internals visual testing.

Test Object Test Category Extent Every 10 Years

Partial penetration weld VT-2 25 percent of nozzles
Pressurizer heater penetration VT-2 all nozzles
Vessel nozzles VT-2 25 percent of nozzles
Control rod nozzles VT-2 25 percent of nozzles
Instrumentation nozzles VT-2 25 percent of nozzles
Flange surfaces VT-1 all when disassembled
Nuts, bushings and washers VT-1 all
Bolts, studs, nuts VT-1 all
Vessel interior VT-3 accessible areas
Interior attachment within beltline VT-1 accessible welds
Interior attachment beyond beltline VT-3 accessible welds
Core support structure VT-3 accessible surfaces
Pressure retaining boundary VT-2 coolant system leak test
Pressure retaining boundary VT-2 coolant system hydrotest

Electric Power Applications of Visual Testing 253

FIGURE 36. Boiling water reactor vessel. 2. The remotely operated vehicle is
controlled manually, with the
1 12 controller connected to the vehicle by
a coaxial cable. A power supply is
2 13 connected to the vehicle by an
umbilical cable. The vehicle has a pan
3 14 and tilt color camera, twin lights, two
4 15 horizontal thrusters and one vertical
5 16 thruster.
6
7 17 Inservice inspection components
8 18 include core spray sparger and core spray
19 piping, feedwater sparger, top guide, core
9 20 plate, steam dryer, moisture separator,
shroud head bolts, fuel support pieces,
21
22 FIGURE 37. Video camera positioner.

23 Light control console Motion control
console

Quick detachable Quick clamp
support clamps turning handle

Vertical drive winch Refueling bridge
trolley

Support frame

Swivel pin connection

10 Rotator/light cable
Camera cable
11
24 360 degree pan rotation
Legend
1. Vent and head spray. Winch cable
2. Steam outlet.
3. Core spray inlet. Quick connect extension
4. Low pressure coolant injection inlet. tube
5. Core spray sparger.
6. Jet pump assembly.
7. Fuel assemblies.
8. Jet pump/recirculation water inlet.
9. Vessel support skirt.

10. Control rod drives.
11. In-core flux monitor.
12. Steam dryer lifting lug.
13. Steam dryer assembly.
14. Steam separator assembly.
15. Feedwater inlet.
16. Feedwater sparger.
17. Core spray line.
18. Top guide.
19. Core shroud.
20. Control blade.
21. Core plate.
22. Recirculation water outlet.
23. Shield wall.
24. Control rod drive hydraulic lines.

Camera support yoke

150 degree tilt

254 Visual Testing

stub tubes and clad patches. Components in remotely operated vehicles. For high
requiring visual testing because of license radiation fields, however, metal oxide
agreements or regulatory requirements semiconductor chip cameras can be
vary from plant to plant. Typical replaced with tube cameras.
augmented inspection components are
the intermediate range monitors, source Handheld cameras will probably always
range monitors, local power range be needed. The technicians performing
monitors, control rod drive return nozzle the visual testing of reactor vessel
shroud and shroud separator mating internals should get hands-on training
surface. with the camera equipment as well as
training on what to look for during a
The expense of remote camera visual test. Skill is needed to position a
handling equipment is justified when camera and record a steady picture in a
inexperienced personnel are visually timely manner.
testing the internals of a reactor vessel.
Often the remote handling equipment Familiarization with Visual
can provide steadier pictures and can Recording Means
speed up the visual tests. Some remotely
operated vehicles are designed specifically Visual test data are recorded as video. The
for the nuclear industry. These vehicles inspector should remember that light is
are small enough to get to areas difficult slower in water than in air, so the focal
to access with handheld cameras and settings on the camera must be modified.
produce pictures superior in steadiness It also means that a test cannot be
and clarity. qualified on the bench: it must be
qualified underwater.
The environment of the visual test
mandates camera selection. Charge Where possible, a zoom lens on the
coupled device cameras start to lose the camera simplifies the test because it is
image as the radiation field increases and very difficult to increment a camera on a
would not be a good choice for visual long pole. A camera fixture lets the
testing of reactor vessel internals. Metal camera be tilted manually.
oxide semiconductor chip cameras
provide color images and have been used A video test should be planned and
performed systematically. If the test is
FIGURE 38. Horizontal inspection boom for video camera. recorded, the counter readings can be
marked on an appropriate drawing to
3 index each view.
9
The video record has the advantage of
4 7 recording motion. This is important when
1 8 11 attempting to analyze certain visual
9 discontinuity indications. For example, a
2 56 crack may cause a surface disruption that
12 produces shadows whereas a stain or
10 small scratch does not.

13 15 It must be remembered, however, that
the detail in video images may be set to a
14 low quality. Older analog video screens
had fewer than 300 horizontal lines,
Legend much less than digital images. Even with
digital images, an image which is a single
1. Boom control console. frame captured from a video file may have
2. Boom drive motor. a lower resolution than expected in a still
3. Reactor pressure vessel and vertical extension tubes. digital photograph.
4. Bottom extension tube.
5. Boom adapter. For some records, it is possible to make
6. Horizontal boom. usable photographs from a video monitor
7. Top view of feedwater sparger or core spray header. if the camera exposure suits the monitor
8. Buoyancy chamber. screen and the speed of the camera is kept
9. Camera support yoke. slower than half the frame frequency.
10. Core spray spargers. Monitor brightness usually does not
11. Camera. match the surroundings, however, and a
12. Travel. proper exposure of the entire scene is
13. Top guide. unfavorable for the monitor. Even if the
14. Core shroud. picture is not as detailed as desired, it may
15. Pressure vessel wall. still be used as a guide when the video is
replayed later.

Visual Test Reporting

The test report should be a record of all
pertinent data resulting from the test, as
well as the procedures, personnel and

Electric Power Applications of Visual Testing 255

equipment used. The report may be a Diffuser elements for pumps are
specific form for each test object or a visually inspected for erosion and cracks
general form made specific by the entries in the same way as the impeller.
of the inspector. The test report and Discontinuity indications are referred to
photographic or video files may be filed as the pump manufacturer for disposition if
hard copy and archived electronically. If the component is to be reused.
possible, the filed copy of the report
should indicate the file names and The dimensions of sleeves and rings
locations of the remaining records of the should be verified against the
test for future reference. manufacturer’s tolerances and replaced if
worn beyond tolerance. Excessive
Drawings included as part of the test clearances reduce the pump’s efficiency
report should indicate the file names and and can cause hydraulic imbalance.
location of photos taken or recorded scans
of the video camera (Fig. 39). Video A total indicator reading should be
counter readings can be entered on the taken on the pump shaft to ensure its
drawing to facilitate review of the test. straightness. Even slightly warped or bent
shafting may precipitate excessive
Visual Testing of Pumps vibration during use. Shaft bearing
journals should be checked for proper
Condensate and Boiler Feed finishes. Total indicator reading or runout
Pumps is the deviation from a perfect form and is
normally detected by full rotation. A
Always refer to the dismantling procedure runout tolerance applied to a surface
outlined in the manufacturer’s means that the considered surface must
instructions. During disassembly, check lie within a tolerance zone normal to the
the impellers for signs of erosion and true profile of the component. The zone
cavitation damage. Typically, the areas limits are separated by a distance equal to
most affected are near the inlet of the the specified total tolerance.
impeller vane. The vane inlet should be
smooth and radiused. Vanes exhibiting Because runout is applied as a
abnormal wear may be blunt edged, composite form of related features having
jagged and, if cavitation was present, a common axis, measurements should be
porous. Each impeller is also inspected taken under a single setup. Runout is a
using liquid penetrant techniques over geometric form control that can include
100 percent of its surface. All cracks are such qualities as roundness, straightness,
repaired or the impeller is replaced. flatness and parallelism of surfaces. When
reviewing the condition of a shaft or
FIGURE 39. Drawing with video and photographic record element, never try to mount the shaft in
locations in horizontal cross section of reactor vessel. the original machining centers. It is likely
that the centers could be damaged
Nozzle through erosion or corrosion, making true
readings difficult.
Jet pump and riser
brace pair Always try to set journal surfaces of the
shaft on a set of V blocks. The readings
Jet Pump 1 Video counter 000–035 Riser (typical) should be taken on all bearing and ring
2 035–150 Jet pump (typical) surfaces with a dial indicator. In any case,
3 150–200 the pump manufacturer should be
4 200–315 consulted for acceptable runout
5 315–850 tolerances.

Details 700–850 Babbitted surfaces must be inspected
14 photos for smoothness and wear. Babbitt, also
called white metal, is a soft alloy of lead or
Nozzle tin and can be scored by tiny particles.
Liquid penetrant tests are also used to
180 degrees check babbitt for lamination or separation
from its backing.

The pump casing should be visually
inspected for erosion or washout. Casing
contours should be smooth and
continuous. Pits or ridges can reduce
efficiency and accelerate casing wear.
Casing joints should be checked for
erosion and effective flange sealing.

All sleeve bearings should be visually
inspected for pitting, finish, scoring and
size. The design clearance should be
checked with feeler gages. Frictionless ball
or roller bearings must be inspected for
surface finishes on the rotating
assemblies.

256 Visual Testing

Circulating Water Pumps Because a valve rarely approaches
perfect performance, and because valve
Always refer to the dismantling procedure failure can be costly from an operational
outlined in the manufacturer’s instruction and safety standpoint, periodic tests are
manual. If the pump has rubber bearings, performed during the life of the valve to
use only fresh water for cleaning; solvents minimize failures.
may damage the rubber. After all
components have been cleaned, check Before visual testing of any valve, the
them and the pump casing for corrosion. inspector must first know the type of
Deposits or scaling must be removed valve, its function, temperatures and
before reassembly. pressures, how long it has been in service
and its maintenance history. With this
The components are checked for cracks information, the inspector can more
and signs of erosion or cavitation damage. accurately evaluate questionable
Particular attention is given to the inlet conditions.
and discharge areas of each impeller vane.
The vane tips and edges are inspected on Valves are dismantled always according
vertical column pumps with an impeller- to a detailed procedure or in accordance
to-impeller cone design. with the manufacturer’s technical
instruction manual. Documentation of
The diffuser vanes are inspected in the the as-found condition is very important
same manner as the impeller vanes. Areas step for determining continuance of
of wear or erosion should be repaired and service. The as-found condition is also
ground smooth. compared to the valve’s maintenance
record.
Check the shaft sleeves for scoring. A
scored sleeve accelerates bearing wear and Valves in Service
should be replaced. Polishing is acceptable
if the scoring is minor. Inservice testing allows the performance
of a complete visual test of the valve, plus
Sleeve, rubber or babbitted bearings any other nondestructive test that may be
should be checked for wear or damage. needed. It is best to remove the valve
Worn or damaged bearings are replaced. from the line. This permits a more
Visual inspection is also used to check detailed inspection by exposing more of
rubber and babbitt for adherence to the the internal surfaces. More reliable seat or
shell. The preferred method of checking shell leak tests are also possible with the
babbitt adherence is liquid penetrant valve offline.
testing.
The first step in performing an
Remove the packing and check the inservice test is to review the preservice,
shaft sleeve for scoring. Make sure that maintenance and past inspection records.
the stuffing box and the seal water piping
lines are flushed. Gate Valves

Check all running clearances with a Sealing surfaces of the wedge and body
feeler gage. Consult manufacturer’s are inspected for evidence of physical
instructions for allowable wear limits. damage (cracks, scratches, galling, wire
drawing, pits, indentations). If facilities
Inspect the water pump for erosion or are available, liquid penetrant testing is
loss of protective coatings (if used). Pits also performed. Guide surfaces (stuffing
and ridges can reduce efficiency and can box and yoke area) are inspected for
accelerate wear. Check flanges and welded evidence of wear or galling. The wear
joints for signs of erosion. Shafts should pattern on guide surfaces often reveals
be checked for runout. Visually inspect misalignment of working components.
shafts for corrosion or signs of washout. Excessive clearance in guide surfaces can
lead to excessive rubbing between seat
Visual Testing of Valves faces when the valve is closing, causing
premature leakage failure (Fig. 40).
A valve is a mechanical device that
controls flow into, inside of or out of In gate valves, the upstream seat of the
enclosed conduits such as piping and wedge and downstream body seat are the
tubing. When fully open, the perfect most likely places for erosion and wear.
valve offers no more flow resistance than The sealing surface behind removable seat
an equal length of pipe. Closed, the rings are good candidates for leakage from
perfect valve permits no fluid to pass. In erosion through wire drawing or steam
addition, it completely resists distortion cutting. The stuffing box is the second
from internal fluid gas pressure. Also most vulnerable spot in a valve. Remove
necessary is resistance to fluid dynamic the packing and check for corrosion in
effects, temperature, pressure drop, the walls of the box. Check the stem in
vibration, corrosion, wear, erosion by the area passing through the bonnet guide
small particles and damage from large surface and the packing gland for
objects in the fluid stream. This evidence of rubbing or corrosion. The
performance has to be constant over the
life of the valve.

Electric Power Applications of Visual Testing 257

stem in this area should have a surface the guide surfaces of the disk and seat
finish no greater than 0.8 µm ring or body for evidence of galling and
(3.2 × 10–5 in.) to ensure reasonable wear. In a globe valve, excessive clearance
packing life. Another area of concern is in the guides increases the possibility of
the stem-to-wedge connection. the bottom edge of the disk catching on
the top edge of the seat in the body and
In an outside stem and yoke gate valve, damaging one or both.
the stem-to-wedge connection must, in
addition to opening the valve, prevent the This is especially significant if the stem
stem’s turning for the stem nut to operate is not vertical during service. Check the
the valve. Check this connection for wear body seat for leakage behind the ring
or erosion. Excessive wear, allowing the when performing an offline pressure test.
stem to turn or to become disengaged, If it is necessary to resurface seat faces,
renders the valve inoperative. Check the verify that the clearance between the
stem-to-stem nut threads for excessive hand wheel and yoke permits full closure
wear. Excessive wear can be corrected only of the reconditioned valve. Remove the
by replacing the worn components. packing and check the walls of the
stuffing box for corrosion. Check the stem
Because stem threads can be observed in the stuffing box for evidence of
during an inservice test, a judgment can rubbing or corrosion. A 0.8 µm
be made regarding the rate of wear and (3.2 × 10–5 in.) finish is the maximum
the need for replacement. Stem nuts can permitted in the area through the
usually be replaced while the valve packing. In a globe valve, the stem-to-disk
remains in service but stem replacement connection is especially important for
requires removal of the valve from service. proper operation. The fit should be tight
Wear and refitting the wedge of a gate but not rigid. Excessive clearance at this
valve make the wedge fit lower in the point leads to erratic operation, excessive
body. Thus, when inspecting a valve, it is noise and accelerated wear and this
important to be sure that the connection should be carefully inspected
reconditioned wedge fits properly. There for excessive clearance. The stem-to-nut
must be adequate contact between the threads are an important test site.
wedge seats and body seats to prevent Excessive wear is corrected by replacement
leakage. of the component (Fig. 41).

Globe Valves and Stop Check FIGURE 41. Bolted bonnet globe nonreturn valve.
Valves

Sealing surfaces of the disk and body are
inspected for evidence of physical
damage: cracks, scratches, galling, wire
drawing, pits and indentations. Liquid
penetrants are used, if possible. Inspect

FIGURE 40. Bolted bonnet gate valve. Stem nut

Stem nut Stuffing box Stem Bonnet
Stem Bonnet Yoke sleeve Body

Yoke sleeve Body Disk or plug

Fixed seat
Seat face

Wedge

258 Visual Testing

Lift Check Valves and Swing Check Plug Valves and Butterfly Valves
Valves
Plug valves are tested the same way as ball
Lift check valves are inspected the same valves. In a tapered plug valve, wear and
way as globe valves, without the stem or refitting of the plug in the body causes
stuffing box (Fig. 42). the plug to fit lower. When inspecting a
valve of this type, it is important to be
In a swing check valve, inspect sealing sure that the reconditioned component
surfaces of the disk and body for evidence fits properly. Proper alignment between
of physical damage and perform a liquid the components in the plug and body
penetrant test if possible. Check for prevent excessive turbulence and pressure
leakage behind the seat ring and verify drop that may cause accelerated erosion.
that the disk is not installed backward.
Inspect the hinge pin, hinges and disk for In butterfly valves, inspect the seating
evidence of wear. Excessive clearance at surface of the disk for evidence of physical
these points can lead to misalignment of damage, wear or corrosion. Again, the
the seat surfaces and leakage (Fig. 43). horizontal plane at the centerline is the
most vulnerable point. Inspect the plastic
Ball Valves seal or body liner for evidence of damage
or cold flow. Alignment of the seal and
Sealing surfaces are visually inspected for disk when closed is vital to the
evidence of physical damage, wear or satisfactory operation of a butterfly valve.
corrosion that causes leakage. The Inspect the bearing surfaces and position
horizontal plane at the flow centerline is stops for excessive wear (Fig. 45).
the first location to show wear because it
always seals against the full differential Diaphragm Valves
pressure. Check the stuffing box and stem
thrust bearing surfaces for evidence of Inspect the sealing surface of the body
corrosion or wear (Fig. 44). partition for evidence of corrosion,
erosion or damage. Most importantly,
FIGURE 42. Bolted bonnet lift check valve. visually inspect the diaphragm for
evidence of aging and cracking, especially
Plug where it is retained by the body and
bonnet or on the surface in contact with
Cap the body partition. The stuffing box is
inspected the same way as it is in a globe
valve.

FIGURE 43. Bolted bonnet swing check valve. Visual Testing of Bolting

Hinge Bolts (Fig. 46) are visually tested to detect
conditions such as cracks, wear, corrosion,
Lap erosion or physical damage on the
surfaces of the components.
Body The test setup and environment are
specified in detail. For example, tests of
pressure vessel bolts may be done with
direct techniques when access is sufficient
to place the eye within 600 mm (24 in.)
of the test surface at an angle not less
than 30 degrees to the surface. Mirrors
may be used to improve the viewing
angle. Lighting, natural or artificial, must

FIGURE 44. Floating ball valve.

Ring Hinge pin
Disk
Closure device
Body

Electric Power Applications of Visual Testing 259

be sufficient to resolve a 0.75 mm 3. Secondary processing discontinuities
(0.03 in.) black line on an 18 percent are produced during manufacture of
neutral gray card. studs, washers, bolts and nuts.
Secondary processing includes
Remote visual tests may be substituted machining, grinding, heat treating,
for direct visual testing, using devices plating and other finishing operations.
such as telescopes, borescopes, cameras or
other suitable instruments. Such devices 4. Service induced discontinuities may be
must have resolution at least equivalent caused by vibration, tensioning and
to that of direct visual testing. corrosion.

Equipment and Preparation The presence of inherent, primary
processing and secondary processing
Steel rules, micrometers, vernier calipers, discontinuities is sometimes revealed in
depth micrometers, thread gages and service.
magnifying glasses are necessary for direct
visual testing of bolts. The most common locations for
fastener failures are in the head-to-shank
Visual tests often require clean surfaces fillet, through the first thread inside the
or decontamination for valid nut on threaded fasteners or at the
interpretation of results. Precautions transition from the thread to the shank.
should be taken before any cleaning Sources of failure are discontinuities in
process. the metal caused either by segregation in
the form of inclusions in the ingot or by
Discontinuity Classification folds, laps or seams that have formed
because of faulty working in the
Discontinuities in bolts, studs, washers semifinishing or finishing mills (Fig. 47).
and nuts fall into four classes.
FIGURE 46. Diagram of nondestructive test for bolting.
1. Inherent discontinuities originate
from solidification of metal in the D DS
ingot. Pipe and nonmetallic inclusions
are common and can lead to other B
types of discontinuities in service.

2. Primary processing discontinuities are
produced from the hot or cold
working of the ingot into rod and bar.

FIGURE 45. Butterfly valve. Edge of nut in Center drill hole
bolted position (where used)

J K

25 mm (1 in.) In place ultrasonic
testing (JKLM)
Threaded bushing
(where used) Face of
flange of
HG component
25 mm
(1 in.)
CB

DB DS

Inspect volume EF D Inspect volume
threads M A threads in
flange (ABCD)
in flange L
(EFGH)

Disk Stud

260 Visual Testing Legend

ABCD, EFGH = threaded area requiring visual testing

DB = diameter including threads
DS = diameter not including threads
LM = unthreaded distal end

Discontinuities Visible in Bolting Folds may occur during forging, at or
near the intersection of diameter changes.
Bursts in bolting materials may be
internal or external. Found in bars and Tool marks are longitudinal or
forgings, internal bursts are caused by circumferential shallow grooves produced
rupturing of metal extruded or forged at by the movement of manufacturing tools
temperatures too low or too high. over the fastener surface. A nick or gouge
External bursts often occur where forming is an indentation on the surface of a
is severe or where sections are thin fastener produced by forceful abrasion or
(Fig. 48). by impact of the fastener against other
components.
Seams are generally inherent in the raw
material from which a fastener is made. Shear breaks or shear cracks are open
Seams are usually straight or smoothly breaks in the metal at the periphery of a
curved discontinuities running parallel to bolt or nut, at about a 45 degree angle to
the longitudinal axis. the long axis. Shear breaks occur most
often with flanged products. They can be
Laps are surface discontinuities caused caused by overstressing the metal during
by folding of the metal. If laps occur in forging, insufficient ductility and high
threads, they generally show a pattern of strain rates.
consistency — they are located in the
same place on all threads of the nut or Necking down is a localized reduction
bolt. in area of a component in overload
conditions.
Surface tears occur along the length of
a bar or threaded fastener and are caused Erosion is destruction of metals or
by faulty extrusion dies or inadequate other materials by the abrasive action of
lubrication during extrusion. Surface tears moving fluids, usually accelerated by the
can resemble seams. presence of solid particles in suspension.

FIGURE 47. Common failures in threaded fasteners: Crevice corrosion cracks are a type of
(a) tension failures; (b) shear failures; (c) cracked nut. concentration cell corrosion — corrosion
of a metal caused by the concentration of
(a) dissolved salts, metal ions, oxygen or
other gases. It occurs in crevices or
Threads Head-to-shank Midgrip Dished head pockets remote from the principal fluid
fillet stream, with a resulting build up of
differential cells that ultimately cause
(b) deep pitting.

Recording

Any area where a visual test reveals
surface discontinuities (physical damage,
wear, cracks, gouges, corrosion, erosion,
misalignment, nicks, oxidation, scratches)
on studs, washers, nuts or bolts is
recorded on a data sheet regardless of
discontinuity size. Unspecified movement
and the looseness of bolts is also recorded.

If there are areas or indications that
cannot be easily recorded on a data form,
a sketch or photograph is included with
the report, to clarify the results.

FIGURE 48. Visible bolting discontinuities.

Head Head Midgrip Threads

(c)

Shear break External burst

Electric Power Applications of Visual Testing 261

Visual Tests for Casting testing, a discontinuity must appear on an
Discontinuities accessible surface to be detectable. It is
important to plan and perform the visual
Because casting is a primary process, the test during the manufacturing cycle to
discontinuities associated with casting are provide opportunities for viewing all
considered to be inherent. These finished surfaces.
discontinuities include (1) hot tears,
(2) inclusions, (3) porosity, (4) unfused Bursts are internal forging
chills and unfused chaplets and (5) cold discontinuities. They appear as scaly,
shuts. ragged cavities inside the forging. Large
forgings generally receive secondary
1. Hot tears appear as ragged cracks or, in processing during the manufacturing
severe cases, as a group of cracks. Tears cycle. Trimming, descaling and machining
are always open to the surface and can are all processes that could expose a
be detected visually. hidden discontinuity such as a burst. A
trim cut on the end of a forged shaft is a
2. Inclusions in a casting may be good example. By scheduling visual
detectable during a visual test — a testing during the manufacturing cycle
visual inspector must draw after each individual operation,
conclusions based on a small portion discontinuities may be detected that
of the discontinuity being visible at would otherwise remain hidden.
the surface. Inclusions are usually sand
or refractory material and appear as Laps are folds of metal forced into the
irregularly shaped cavities containing surface of the component during forging.
a nonmetallic material. A lap can be shallow or very deep and its
appearance is that of an oddly shaped
3. Porosity appears as a series of crack on the surface. A lap indication can
hemispherical depressions in the vary from a tight, straight, linear
surface of a casting. This pockmarked discontinuity to a wide U shaped
appearance is easily recognized during indication. When a lap is viewed through
visual examination. a low magnification lens, the inside
surfaces often contain oxidized scale — a
4. Unfused chills and chaplets appear as gray, porous material.
irregularly shaped cavities on the
surface of the casting. The cavity Cracks are different from laps in that
varies from the entire shape of the cracks follow the stress distribution within
chill or chaplet, such as a square, to a the forging whereas laps do not. Both laps
portion of the shape, depending on and cracks can appear on the surface of a
the amount of fusion. Chills and forging as thin, jagged, linear indications.
chaplets differ in shape, so their
cavities also differ. Each foundry It is often helpful for the inspector to
selects the chills and chaplets it use a 5× to 10× magnifier. Lenses higher
prefers, so the shapes of these than 10× are typically large and difficult
discontinuities cannot be defined to keep steady.
here.
Rolling
5. Cold shuts appear as folds or smooth
cracklike discontinuities, depending Rolled products are probably the most
on the location and the severity. They common visual test objects. A visual
are usually visible on the surface of a inspector should become familiar with the
casting. rolling processes to identify
discontinuities by their location on the
Visual Testing of Forgings component. Visual tests of rolled products
are complicated by the fact that many
Forgings are often large but simple in complex structures are fabricated from
shape. They can usually be visually plate and rolled shapes.
inspected without complex viewing
equipment. The forging process usually Inherent discontinuities such as pipe,
occurs at an elevated temperature, so inclusions and gas holes are affected by
scaling or oxidation can sometimes be rolling, forming laminar discontinuities
found inside the discontinuity. parallel to the rolling direction. When any
of these discontinuities are moved to the
The processing discontinuities that a surface of a rolled product, a seam
visual inspector can detect on a forging (Fig. 49), stringer (Fig. 50) or crack can
are bursts, laps and cracks. form.

These primary processing The location of the discontinuity on
discontinuities are a result of the forging the component helps to classify it. Seams,
process. Inherent discontinuities caused cracks and stringers can appear anywhere
by pipe, porosity or inclusions could also on a rolled product. Seams and stringers
be detected if the forging process moves follow the direction of rolling.
them to an exposed surface. In visual Laminations detected by visual inspection
appear on the edges of plate or the ends
of pipe (Fig. 51). They are linear and

262 Visual Testing

parallel with the top and bottom surfaces Drawing, Extruding and Piercing
of the plate. On rolled shapes,
laminations are parallel to the rolling The discontinuities associated with
direction and appear on the edges of the drawing, extruding and piercing are all on
shape. Pipe causes laminations oxidized the surfaces of the component and
inside. Gas holes also cause laminations therefore detectable during a visual test.
that may be oxidized if oxygen was in the
hole. Inclusions cause laminations that Drawn products usually exhibit gross
contain a layer of the included material. failure if any discontinuity is present.
Because most drawn products have thin
Processing discontinuities encountered walls, failure usually appears as a
in rolled product include tears and cracks. through-wall break.
Such discontinuities exhibit characteristics
similar to some forging discontinuities, Extrusions can have surface
including an oxidized, scaly interior. discontinuities that appear as scrapes and
tears.
Visual testing must be performed
before fabrication hides portions of the Pierced pipe can contain slugs of metal
material. In addition to magnifiers (5× to that are easily identified. Severe score
10×), a mirror is useful for visual testing of marks usually lead to the slug.
rolled shapes with obstructed areas.
FIGURE 50. Rolled bar containing stringers or
FIGURE 49. Seam in billet: (a) typical location; inclusions elongated during the rolling
(b) magnified view of rough, oxidized process.
surface seam.

(a)

(b) FIGURE 51. Typical I-beam lamination and
seam locations.

Laminations

Seam

Electric Power Applications of Visual Testing 263

References

1. Section 9, “Applications of Visual and Anderson, M.T., S.E. Cumblidge and
Optical Testing in the Electric Power S.R. Doctor. “An Assessment of Remote
Industries”: Part 1, “Joining Processes.” Visual Testing System Capabilities for
Nondestructive Testing Handbook, the Detection of Service Induced
second edition: Vol. 8, Visual and Cracking.” Materials Evaluation.
Optical Testing. Columbus, OH: Vol. 63, No. 9. Columbus, OH:
American Society for Nondestructive American Society for Nondestructive
Testing (1993): p 247-262. Testing (September 2005): p 883-891.

2. EPRI TM101PS, Visual Examination ASME Boiler and Pressure Vessel Code:
Technologies: Level 1, Practical/Specific. Section V, Nondestructive Examination.
Charlotte, NC: Electric Power Research Article 9, Visual Examination. New
Institute (1996). York, NY: ASME International (2007).

3. ASME Boiler and Pressure Vessel Code. ASME NQA 1, Quality Assurance
New York, NY: ASME International Requirements for Nuclear Facilities
(2008). Applications. New York, NY: ASME
International (2008).
4. ANSI/ASNT CP-189, Standard for
Qualification and Certification of AWS B1.11, Guide for the Visual
Nondestructive Testing Personnel. Examination of Welds. Miami, FL:
Columbus, OH: American Society for American Welding Society (2000).
Nondestructive Testing (2006).
Chang, Pan-Tze, I. Kaufman and
5. ASNT Central Certification Program D.-Y. Shyong. “Detection and Imaging
(ACCP), Revision 4 (March 2005). of Surface Cracks by Optical
Columbus, OH: American Society for Scanning.” Materials Evaluation.
Nondestructive Testing (2005). Vol. 45, No. 8. Columbus, OH:
American Society for Nondestructive
6. AWS D1.1M, Structural Welding Code — Testing (August 1987): p 943-950.
Steel. Miami, FL: American Welding
Society (2008). Chuang, K.C. “Application of the Optical
Correlation Measurement to Detection
7. ANSI/ASME B31.1, Power Piping of Fatigue Damage.” Materials
Systems. New York, NY: ASME Evaluation. Vol. 26, No. 6. Columbus,
International (2007). OH: American Society for
Nondestructive Testing (June 1968):
8. Hedtke, J. and R.[T.] Nademus. p 116-119.
Section 9, “Applications of Visual and
Optical Testing in the Electric Power Davis, Renée S. “Remote Visual Inspection
Industries”: Part 2, “Specific Visual in the Nuclear, Pipeline and
Inspection Applications.” Underwater Industries.” Materials
Nondestructive Testing Handbook, Evaluation. Vol. 48, No. 6. Columbus,
second edition: Vol. 8, Visual and OH: American Society for
Optical Testing. Columbus, OH: Nondestructive Testing (June 1990):
American Society for Nondestructive p 797-798, 800-803.
Testing (1993): p 263-276.
Pellegrino, Bruce A. “Remote Visual
9. EPRI TM102CG, Visual Examination Testing for Internal Pressure Vessel
Technologies: Level 2, General. Inspection.” Materials Evaluation.
Charlotte, NC: Electric Power Research Vol. 56, No. 5. Columbus, OH:
Institute (1996). American Society for Nondestructive
Testing (May 1998): p 606-609.
Bibliography
Samsonov, Peter. “Remote Visual
Abbott, P.J., K. von Felten and Inspection for NDE in Power Plants.”
B.A. Pellegrino. EPRI 2603, Materials Evaluation. Vol. 51, No. 6.
“Development of Next-Generation Columbus, OH: American Society for
Inner-Bundle Imaging Device for Nondestructive Testing (June 1993):
Triangular-Pitch, Secondary-Side Steam p 662-663.
Generator Visual Inspection.” 21st
Annual Steam Generator NDE Workshop
[Berkeley, CA, July 2002]. Charlotte,
NC: Electric Power Research Institute.

264 Visual Testing

11

CHAPTER

Aerospace Applications of
Visual Testing

Donald R. Christina, Boeing Company, Charleston Air
Force Base, South Carolina
Bruce L. Bates, Yucaipa, California

Part 2 is a work of the United States government and not subject to copyright.

PART 1. Visual Testing of Aircraft Structure1,2

Visual testing is the oldest and most to obtain as great a magnification as
economical form of nondestructive desired. The distance from lens to object
testing. Visual testing is the primary is adjusted until the object is in the lens’s
method used in aircraft maintenance, and depth of field and is in focus. The
such tests can reveal a variety of simplest form of a microscope is a single
discontinuities. Generally, these tests converging lens, often referred to as a
cover broad surfaces of the aircraft. simple magnifier.

Detailed tests of small areas are Magnifying lenses are discussed in this
conducted with optical instruments. volume’s chapter on direct visual testing.
These instruments have two functions:
(1) to magnify discontinuities that cannot Borescopes
be detected by the unaided eye and (2) to
permit visual checks of areas not A borescope is a precise optical
accessible to the unaided eye. Such tests instrument with built-in illumination. It
include the use of magnifiers and can be used to visually check internal
borescopes. The text below details areas and deep holes and bores.
optically aided tests of structures where Borescopes are available in rigid and
access is poor for other nondestructive flexible models from 2.5 mm (0.1 in.) in
tests. A short description of optical diameter to 19 mm (0.75 in.) in diameter
instruments is given below, with examples and meters (several feet) in length. These
where such devices are used to inspect devices are generally provided with fixed
aircraft structures and operating diameters and fixed working lengths, with
mechanisms. optical systems designed to provide direct,
right angle, retrospective and oblique
It is important to know the type of vision. These orientations are described in
discontinuities that may develop and to this volume’s chapter on indirect
recognize the areas where such problems techniques of visual testing. Generally, the
may occur. Many discontinuities are diameter of the borescope is contingent
revealed during visual tests of aircraft. on the diameter of the hole or bore being
Rust stains reveal corroded steel. Paint inspected. The borescope length is
blisters around fasteners in wing skins governed by the distance between the
reveal underlying intergranular corrosion available access and the distance to the
of the skin. And fuel leaks in the lower test area. The choice of viewing angle is
wing skin can indicate a cracked spar cap determined by discontinuity type and
or skin. Even the pilot’s walk-around location.
visual inspection can reveal serious faults.
For example, an improper attitude of the Microborescopes
aircraft can indicate a failed main landing
gear attachment fitting, and a floating A microborescope is an optical device, of a
spoiler can reveal a failed torsion bar. The kind resembling a medical endoscope. It is
following text discusses and illustrates much like a borescope but with superior
corrosion pitting, stress corrosion cracks optical systems and high intensity cold
and fatigue cracks. light piped to the working tip through
fiber optic bundles. Other useful features
Optical Aids include constant focus from about 4 mm
(0.15 in.) to infinity. Actually, when the
Magnifying devices and lighting aids are tip is about 4 mm (0.15 in.) from the test
used and the general area is checked for surface, a magnification factor of about
cleanliness, presence of foreign objects, 10× is achieved. Microborescopes are
security of the component, corrosion and available in diameters down to 1.7 mm
cracks or other damage. In many cases, (0.07 in.) and in lengths from 100 to
the area to be inspected is cleaned before 150 mm (4 to 6 in.).
visual inspection.
Flexible Fiber Optic Borescopes
Magnifying Lenses
Flexible fiber optic borescopes permit
An optical microscope is a combination of manipulation of the test instrument
lenses used to magnify an image. The around corners and through passages with
object is placed close to the lens in order several directional changes. Woven

266 Visual Testing

stainless steel sheathing protects the inspection can be directly compared to
image relay bundle during repeated current results for quick identification of
flexing and maneuvering. These devices areas where surface features have
are designed to provide sharp, clear changed. The optical setup for the
images of components and interior diffracted light technique consists of a
surfaces that are normally impossible to light source, a retroreflective screen and
inspect. Remote end tip deflection allows the object being inspected (Fig. 1). The
the viewer to thread the fiber optic surface being inspected must be reflective.
borescope through a complex series of Both flat and moderately curved surfaces
bends. The end tip is deflected with a can be inspected using this technique.
rotating control mounted on the handle. The diffracted light effect can be
Most of these devices have a wide angle explained with geometric optics. If a flat
objective lens that provides a 100 degree surface with an indentation is inspected,
field of view and tip deflection of the light striking the indentation is
±90 degrees. They all have a fiber optic deflected. It then strikes the retroreflective
image bundle and are equipped with a screen at a point removed from the light
focus control to bring the subject into rays reflected from the area surrounding
sharp focus over a wide range of viewing the indentation. The retroreflective screen
distances. The working lengths are attempts to return all these rays to the
normally from 0.6 to 3.7 m (2 to 12 ft) points on the inspected surface from
with diameters from 3 to 13 mm (0.12 to which they were first reflected. However,
0.5 in.). the screen, consisting of numerous glass
beads, returns a cone of light to the
Microelectronic Video Borescopes surface. This imperfection of the
retroreflective screen creates the diffracted
An electronic sensor embedded in the light effect. By backlighting the
movable tip of the probe transmits signals discontinuity, the technique increases the
to a video processor, where the image is light intensity on one side of the
sent to a monitor. The video borescope indentation and reduces it on the
has a bright, high resolution color image opposite side.
with no distortions or spots. The device
does not have an eyepiece like other The edge of light technique is a related
borescopes. It has a freeze frame feature test. Raking light from an oblique angle
that allows closer viewing of the image. creates shadows from irregularities on the
The image can be electronically test surface. Image processing helps
transferred for permanent documentation. identify surface anomalies such as pitting,
Grids or measurement references may be impact damage, fatigue deformation and
entered into the margin of the image and pillowing of laminates.4,5
can become part of the permanent record.
The image may be magnified for precise FIGURE 1. Optical setup for diffracted light technique.
viewing. The field of view is up to
90 degrees and the probe tip has four-way Screen
articulation. Presently, the smallest probe
diameter is 10 mm (0.37 in.) with 1.2 m (4 ft) 90 degrees to screen
working lengths up to 30 m (100 ft). Surface

Diffracted Light Lens
Lamp
A technique using diffracted light has
been developed for visualizing surface 30 degrees 1.2 m (4 ft)
distortions, depressions or protrusions as 1.5 m (5 ft)
small as 10 µm (0.0004 in.). A real time
technique particularly applicable to rapid
inspection of large surfaces, the diffracted
light technique has been used to inspect
automobile body panels and metal
working dies. Commercial versions of
diffracted light equipment have ranged
from a manual handheld system, in
which the operator directly views the
inspected part, to systems with video
cameras and computer based image
processors. This technique has been used
to inspect composite structures for barely
visible impact damage.3,4
Computer based image processing has
been applied to diffracted light technique
images. An image from a previous

Aerospace Applications of Visual Testing 267

Typical Applications revealed that failures were caused by
fatigue cracks that initiated at the bell
Following are examples of visual tests crank-to-collar attachment holes (Fig. 3).
used by airline maintenance personnel to With the bell crank installed on the
ensure the structural integrity of transport aircraft, access to the crack location is
aircraft. extremely poor, preventing the use of
ultrasonic, eddy current or radiographic
Torsion Bar Core Corrosion Pitting nondestructive testing. This leaves the
fiber optic borescope as the only option
Stress corrosion cracks can cause failure in for inspecting this component. A right
high strength steel spoiler torsion bars. In angle flexible borescope with deflecting
one investigation, corrosion pitting on tip, 5 mm (0.2 in.) in diameter and about
the inside surface of the torsion bar cavity 1 m (40 in.) in length, is recommended
(bore) was found to have led to stress for this test. Before the inspection, the
corrosion cracks and subsequent failure bore of the bell crank is cleaned with
(Fig. 2). It was determined that torsion bar solvent to remove grease. The tip of the
failure can be greatly minimized by flexible borescope is placed in the forward
removing corrosion from the bore within end of the bell crank and fed aft until it is
repairable limits and resealing the cavity aligned with the two attachment holes. To
with a polysulfide sealant (to prevent control the position of the flexible
moisture ingress). For this particular case, borescope, it can be placed within a stiff
a service bulletin was issued requesting but flexible plastic or thin metal tube.
operators to remove the torsion bars, When the operator has the tip aligned
clean the exterior surface and perform a with the attachment bolts, it can be
magnetic particle test for cracks. If the test rotated to inspect for cracks originating at
shows the bar is cracked, it is scrapped. If the attachment holes. If cracks are
no cracks are detected, sealant (if present) detected, the bell crank is removed from
and primer are removed from the bore the aircraft. If no cracks are detected,
and it is visually inspected for corrosion repetitive tests can be conducted until the
pits using a 70 or 90 degree borescope. If bell crank is replaced with a better
corrosion pits are detected, the bore is component.
reamed to a maximum oversize and
reinspected. If the second borescope test Spoiler Lubrication Hole Cracks
reveals pits, the component is scrapped. If
no pits are detected, the bore is cleaned Failures of the link or fitting assemblies of
with solvent, given two coats of corrosion the slat drive mechanism may be reported
inhibiting primer and then filled with by operators. In one investigation, the
polysulfide sealant. The torsion bar is failures were found to be caused by
then radiographed to verify sealant fatigue cracks generated at the inner
integrity. surface of the lubrication holes in the link

Slat Drive Crank FIGURE 3. Flexible borescopic visual test of slat drive bell
crank.
Instances of slat bell crank failures were
reported by operators. Investigation Slat drive mechanism
assembly
FIGURE 2. Borescopic visual test of spoiler torsion bars.
A

Bell crank A
Forward
Bore inspection
for pits Typical crack
Collar
Borescope Test site

Borescope Eyepiece Typical
Eyepiece crack

Cable drum

268 Visual Testing

and fitting assemblies (Fig. 4). The initial This test required removal of the
service bulletin called for an ultrasonic lubrication fittings and grease before
transverse wave test to detect subsurface inserting the 3 mm (0.125 in.) eddy
cracks adjacent to the lubrication holes. current probe into the holes. Borescope
This test proved to be unreliable (cracks tests were considered when the
were missed) and was replaced by an eddy microborescopes discussed above became
current test of the bores of three holes. available commercially. If the visual test is
performed, the lubrication fittings and
FIGURE 4. Borescopic visual test of spoiler grease are removed and the bores are
actuating mechanism: (a) link assembly; cleaned with solvent. The visual test is
(b) fitting asembly. performed using a 70 degree forward
oblique borescope, 2.7 mm (0.11 in.) in
(a) Typical cracks diameter by 180 mm (7 in.) long. Crack
indications may appear at the inboard
Link and outboard sides of the lubrication
assembly bores. Cracks, if detected, are not
acceptable and the component must be
Typical crack replaced.

Wing Cracks under Panel

Fatigue cracks may occur in the wing rear
spar cap web and doubler under the
trapezoidal panel attachment fitting.
These fatigue cracks may occur in both
the web and doubler or in each member
separately. The cracks originate at the
lower edge of both members (Fig. 5).
Direct access to the cracked areas of the
web and doubler require removal of the
trapezoidal fitting. If both members are

70 degree forward FIGURE 5. Wing rear spar doubler and web crack location:
oblique borescope (a) from above; (b) from side.

Light source (a)

(b) Typical crack Forward
Tee cap

Inboard

Rear spar web

Fitting assembly Doubler or Fitting
web crack
Typical crack Up
Borescope Doubler Inboard

(b)

Light source Crack position 1 Edge of doubler and web

Aerospace Applications of Visual Testing 269

cracked, fuel may leak from under the their lengths must be determined by use
fitting, indicating the existence of of a 3.2 mm (0.125 in.) maximum
through-thickness cracks. Other methods diameter rigid borescope or flexible
of nondestructive testing cannot be used borescope (Fig. 7).
because of the poor access caused by the
fitting. The doubler and web are made To perform this test, the fastener
from clad, wrought aluminum sheet about common to the rudder skin and the rib
4 mm (0.16 in.) thick. Access to the area is flange (opposite the cracked flange) must
aft of the rear spar plane, inboard and be removed. A 0 degree borescope is
outboard of the trapezoidal panel and inserted through the open hole and the
fitting. The visual test can be performed opposite flange radius and web are
with a 0 degree borescope along with a 70 inspected to determine crack position and
to 90 degree borescope, 300 to 480 mm length. An alternative approach requires
(12 to 19 in.) in length and 4 to 5 mm removal of a fastener common to the
(0.16 to 0.20 in.) in diameter. The test rudder skin and the cracked flange that is
area is cleaned using a cotton swab wetted judged to be about 50 mm (2 in.) beyond
with solvent. The area is first viewed using the crack image on the radiograph. A
the 0 degree borescope to check for cracks flexible borescope or rigid retrograde
in the radius areas of the doubler and web borescope is inserted through the open
(position 1, Fig. 5b). If a crack exists and it
does not run under the fitting, its length FIGURE 6. Borescopic visual test of wing rear spar doubler
in the doubler may be determined by a and web for cracks under fitting: (a) view looking inboard;
liquid penetrant or high frequency eddy (b) view looking forward at lower portion of rear spar left
current test using a shielded surface side.
probe. To measure the length of a crack in
the web (foremost member) under an (a) Trapezoidal panel
uncracked doubler requires removal of the
fitting and application of a low frequency Up Rear spar
eddy current test. Detecting a crack in the
doubler or web hidden by the fitting Forward
(Fig. 6b, positions 2 and 3) requires the
use of the 70 or 90 degree borescope. The Cracks
area is cleaned with a cotton swab wetted
with solvent and the borescope is inserted Wing skin
through the small opening between the
forward side of the fitting and the aft side Borescope
of the doubler (Fig. 6). To measure the
length of doubler cracks under the fitting Trace
at position 2 or 3 requires ultrasonic
transverse wave (angle beam) techniques. (b) Fitting

Rudder Flange Cracks Crack position 2 Crack position 3
Borescope Up
Radiographic tests can reveal cracks
developing in the rib flanges of the Inboard
rudder. In one case, analysis determined
that cracking of the rib flanges resulted
from acoustically induced vibration. It
was also determined that installation of
stiffeners on the rudder ribs strengthens
the rudder and minimizes the possibility
of further crack development. A service
bulletin was issued giving criteria for
flyable crack lengths based on the number
of cracked ribs and the length of the
cracks. Unrepaired rib flange cracks may
cause cracks to occur in the rudder skins,
thereby requiring more extensive repairs.
A radiographic test is first conducted and
if cracks are detected in the rib flanges at
or adjacent to the skin attachment
fastener holes, their lengths must be
determined. The cracks may run upward
into the flange radius and progress into
the rib area, where their lengths may be
difficult to determine from radiographs
alone (Fig. 7). If cracks occur within or
progress into the flange upper radius,

270 Visual Testing

hole and articulated to allow viewing of detected, the beam is removed from the
the rib flange, radius and web to aircraft and the pits are removed by
determine crack position and length. If reaming the affected holes to a maximum
cracks exceed flyable length, the ribs are oversize. Beams showing slight corrosion
repaired. If cracks are of tolerable length, may continue in service for a limited time
an easily replaced fastener is installed in provided that periodic borescope and
the open holes to allow for repetitive ultrasonic transverse wave tests are made
evaluation. to detect possible stress corrosion cracks
that may originate at a pit. Figure 9a
Landing Gear Pitting Corrosion shows an acceptable condition at the
inner bore chamfer. Figure 9b shows a
Three instances of main landing gear typical heat treat pit — an acceptable
truck beam assembly failures were condition away from the inner bore
reported as resulting in major secondary chamfer. Figure 9c shows unacceptable
damage to the aircraft. Investigation corrosion pitting at the inner pivot bore
revealed that failure was a result of stress intersection. Figure 9d shows acceptable
corrosion fracture that initiated at or machining marks (rifling) in the bore
immediately adjacent to the intersection outer edge, close to the lubrication fitting
of the lubrication hole and the pivot bore threaded area.
(Fig. 8). The stress corrosion fracture in
the lubrication hole results from severe Wing Spar Cap Cracks
pitting caused by inadequate lubrication.
If this condition is not corrected, the The purpose of this test is to check the
truck beam assemblies are vulnerable to lower spar cap forward tang for fatigue
failure. Removing corrosion or pitting cracks at the fastener locations. The area
from the surface of the four pivot bore of interest is located at the four wing
lubrication holes and increasing the pylons that support the jet engines. The
frequency of lubrication minimizes the total inspection for fatigue cracks at the
possibility of failure and extends the pylons includes X-ray, ultrasonic
service life of the beam assemblies. transverse wave and borescope tests. In
Inservice testing for corrosion pitting order to accomplish the ultrasonic and
requires removal of the lubrication fitting borescope tests inboard of the pylons, an
and grease from each of four holes in the access hole is made in the wing leading
subject beam. The internal surface of each edge, as shown in Fig. 10. In addition, a
bore is checked using a 0 degree (forward smaller access hole is made in the
looking) 2.8 mm (0.11 in.) diameter footstool fitting, so that the fasteners
borescope. If corrosion or pitting is located under the fitting can be visually
revealed, the hole is checked a second inspected. Inspection of the six fasteners
time with a 70 or 90 degree (lateral) at the inboard side of the fitting is
borescope. When heavy pitting is

FIGURE 7. Determining crack length in rudder ribs by visual test.

Flange upper Rigid
radius borescope

D Typical rib web

Fiber optic Fiber optic
light source light source

Rudder skin

Flange upper D
radius Typical
cracks
Flexible
Rigid borescope Flexible
borescope borescope

Cracked
Up flange

Aerospace Applications of Visual Testing 271

accomplished using a flexible borescope (Fig. 10). These tests have been successful
with a minimum length of 0.6 m (2 ft). in detecting small cracks at the forward
and aft side of the fasteners and larger
The inspector places his hand through cracks that propagate to the leading edge
the access hole and positions the tip of (forward) or vertical leg (aft) of the cap.
the 90 degree flexible borescope forward
and aft of each fastener to detect fatigue FIGURE 9. Surface conditions possible in
cracks in the spar cap forward tang. The typical main landing gear truck beam:
area under the footstool fitting is (a) acceptable bore chamfer; (b) typical heat
inspected using two techniques: (1) the treat pit; (c) severe bore chamfer corrosion;
seven fastener locations are inspected (d) machining marks (rifling).
using a 0.6 m (2 ft) long, 90 degree, rigid
borescope or a flexible borescope (a)
supported by semirigid plastic tubing and
(2) the forward edge of the spar cap is
inspected with a flexible borescope in a
semirigid tube bent into a J shape

FIGURE 8. Case history of corrosion leading to component
failure: (a) main landing gear truck beam; (b) fractured
surface of failed component caused by failure to apply
lubricant. The lubrication hole is visible opening on the
inside surface.

(a) Lube holes

(b)

(c)
(b)

(d)

272 Visual Testing

Closing limited. The examples given are typical of
other such applications for maintenance
As indicated above, visual tests use testing of aircraft structures and operating
magnifiers and borescopes. These devices mechanisms. Visual testing is a viable and
are typically used on aircraft to detect economical method that helps monitor
corrosion and cracks where access is the structural integrity of inservice
aircraft.

FIGURE 10. Borescopic visual test of wing lower forward spar cap tang for fatigue cracks.

Footstool fitting

Forward Footstool fitting

Pylon fitting

Inboard Fastener
Typical crack
Spar cap Aft
forward edge Inboard Flexible borescope

Access hole J tube guide
Light source
Spar forward tang

Access hole
Inboard side of footstool fitting

Pylon fitting and footstool fitting left side

Aerospace Applications of Visual Testing 273

PART 2. Visual Testing of Jet Engines6

Distinctive Features of trailing edge repairs should be smoothed,
Engine Inspection or blended, with a minimum edge
specified for that component.
Details observed in engine inspection
differ from airframe inspection and Blades may be damaged by impact with
generally relate to surface finish and the foreign objects. The use of a substantial
means for inferring underlying number of blades repaired to or near
conditions. Inspections within protective maximum limits, or use of blades having
hoods and housings often require many repaired areas, may adversely affect
complex visual aids such as light guides, compressor efficiency and therefore
borescopes and special inspection engine performance. If two or more
fixturing. blended blades are found grouped, an
attempt should be made to redistribute
The engine components have surface these blades evenly throughout the
textures different from that of the rolled engine.
and formed surfaces in airframes and so
are inspected with different criteria. Many When checking blades for foreign
engine anomalies are related to surface object damage, they should be visually
features of the material, and engine tested for arc burn. Arc burn is evidenced
manuals describe visible surface by a small circular or semicircular heat
characteristics in terms of the process that affected area on the blade surface that
caused them and its effect on satisfactory may contain shallow pitting, remelting or
performance of the part. As a result, much cracking. Blades with arc burn are not
guidance relating to engine visual acceptable for further service.
inspection is related to manufacturing
rather than operational and maintenance Blending is intended to ensure that all
inspection. Some anomalies, such as subsurface as well as surface damage is
forging cracks or rolling marks, are found removed. It is critically important that the
first in the manufacturing process; other blending procedures be followed carefully
anomalies, such as corrosion and fatigue whether in the shop or on the wing.
damage, may be found by maintenance Proper blending must be performed in
inspectors after some flights. successive stages: removal of the visible
damage, confirmation by eddy current or
Another difference related to engine fluorescent penetrant testing that all
inspection is associated with the surface damage has been removed and,
environment in which engines are finally, removal of additional material to
expected to operate. Many engine parts ensure the removal of subsurface
are expected to survive intense heat. Some deformed material.
superficial clues to engine condition are
leaks and evidence of smoke. Peening After refurbishment or repair, the blade
tends to remove evidence of heat should be visually tested with bright light,
discoloration and to smooth damage, mirrors, a 3× magnifier, fingertips and
making it less apparent. fingernails, looking specifically for small
depressions in surface contour. Observed
Turbine Blades conditions may be compared with
standard documentation and reference
Blades are evaluated from a standpoint of photographs.
structural integrity. The blade is
engineered to transfer combustion energy Electrical Discharge Damage
to the engine compressor (providing
mostly air as driving force) and, in the Electrical discharge damage occurs in two
process, experiences severe thermal and forms.
mechanical loading. In an aerospace
engine, the material integrity of turbine 1. Strong shapes such as ovals, crescents
blades must be high to sustain the service or rectangles depressed into the blade
life. All blade surfaces must be smooth, surface might be caused by contact
and all evidence of previous leading and with a rod or wire carrying an
electrical charge.

2. Small overlapping craters or pits are
coarse linear indications.

Regardless of form, electrical discharge
damage is local, unlike erosion pitting.

274 Visual Testing

Damage boundaries do not have raised FIGURE 11. Airfoil inspection limits.
material as does damage from foreign
objects. 22
1
Blades suspected of electrical discharge
damage must be removed from service 6
immediately and blue etch anodize 4
inspected to confirm damage. Electrical
discharge damage is not repairable by 12 6
blending. Electrical discharge damage
produces a heat affected area that may A A
extend 1.5 mm (0.06) in.) deeper than 3
visible damage. Blades exhibiting 7
electrical discharge damage should be 8
removed from service.
11 10
Low Pressure Blade Assembly
94 5
Consult applicable procedures whenever 6
foreign object damage is suspected.
Consult applicable specifications for Plane Z
functional arrangement and part number
applicability, and see general inspection Tangent Tangent
data.
90 degrees 13 90 degrees
Inspect low pressure compressor blade
assembly for damage. Section A-A
chord length measurement
Inspect airfoil for damage: (1) sustained
indentations with compressed material Legend
and raised edges, (2) deformation with
small radii or ragged edges and (3) tip curl 1. Blade tip blend radius — 38.100 mm (1.500 in.) maximum.
damage. 2. Leading or trailing edge and tip blend area — blend to be 19.050 mm

Damage should be blended within (0.750 in.) deep maximum.
limits specified for the part. Limits shown 3. 495.681 mm (19.515 in.).
in Fig. 11 refer to maximum material 4. Critical area — depth of blend to be 6.350 mm (0.250 in.) maximum
removal allowed during blending and not
to actual depth of damage before except for airfoil root section. See index 5 dimension.
blending. Damage at or near maximum 5. 25.400 mm (1.000 in.) — damage in airfoil-to-platform radius area
limits shown are not repairable if
blending will exceed these limits. cannot be blended except per airfoil root section blend limits.
6. No damage or blending permitted in shroud-to-airfoil radius or
Inspect leading and trailing edges for
foreign object damage such as nicks and platform-to-airfoil radius.
dents. These discontinuities must be 7. Concave and convex blade surface blend area — depth of blend in this
blended.
area to be 0.762 mm (0.030 in.) maximum (round bottom).
1. Damage outside critical area should be 8. Leading edge blend area — depth of blend to be 12.77 mm (0.500 in.)
blended if it exceeds 0.51 mm
(0.020 in.) in depth. maximum.
9. 160.020 mm (6.300 in.).
2. Damage within critical area should be 10. 469.900 mm (18.500 in.).
blended if it exceeds 0.127 mm 11. 571.800 mm (22.500 in.).
(0.005 in.) in depth. 12. 12.700 mm (0.500 in.), pour places.
13. Minimum chord length to be 176.530 mm (6.950 in.) within
3. Damage not exceeding 0.127 mm
(0.005 in.) in depth need not be 160.020 mm (6.390 in.) of leading edge platform. Chord length must be
blended if material is not torn. measured tangent to blade leading and trailing edges and parallel to
phase Z.
No visible cracks or tears are permitted. Note: Damage which affects both index 1 and index 2 must be blended by
combining both limits to produce a smooth blend.
Injection Molded Blades7

Ceramic materials have been developed
for many uses. Visual techniques are used
for ceramic blades, along with liquid
penetrant, radiographic and ultrasonic
techniques. For a ceramic turbine blade of
silicon nitride and 6 percent glass, liquid
penetrant tests are used to detect and
locate cracking and porosity.8
Radiography is used to detect several types
of inclusions as well as subsurface
cracking. Visual techniques are also
commonly used for inspection of these
injection molded blades.

Aerospace Applications of Visual Testing 275

There are several ways visual and adjusted on the basis of operator
optical tests may be used with advanced experience and consultation with the
ceramic components. In one application, local regulatory agency.
optical magnifications from 5× to 40×
have been used for crack detection.8 Engines get very hot. Temperatures of
Cracking previously located with parts to be borescopically tested should be
penetrants is typically not detected permitted to cool after engine operation.
visually but open surface cracks along a Damage may result to borescopic
turbine blade’s split lines are visible equipment if equipment is exposed to gas
(Fig. 12). It is possible that such cracks path or metal temperatures above 66 °C
occur during removal from the mold and (150 °F). A rule of thumb is that, if
are subsequently sintered smooth so that adjacent cases are too hot to touch, then
they do not hold liquid penetrant. visual testing should be delayed.

Visual techniques may also be used in Borescopic equipment for visual testing
this application to locate pores in the should satisfy the standards, procedures
turbine blade’s thick sections. and specifications in force for the engine
and workplace. Equipment and associated
Borescopy hardware includes power source, light
cables and adapter required during
Borescopic testing of the engine borescopy. The specification sets the
represents a significant aid to effective quality and functional standards for this
maintenance. Borescope access ports are equipment. For example, the following
incorporated at many locations along the borescopic equipment has been
engine gas path, allowing detailed recommended: (1) low magnification rigid
examination of critical internal engine borescope with 6.8 mm (0.270 in.) barrel
areas (Fig. 13). diameter, (2) high magnification rigid
borescope with 11.3 mm (0.444 in.) barrel
Indirect visual testing with video maximum diameter and (3) flexible
borescopes or other electric viewing borescope 6.8 mm (0.270 in.) cable
devices must be performed in a protected maximum diameter. Optional borescope
area. In wet weather, precautions must be equipment enhances the test capability
used to prevent damage to equipment or and eases the task for the inspector. These
electric shock to the operator. Also, items range from clamping equipment to
operation of visual test equipment beyond hold the borescope in position to optical
its recommended limits may not be video equipment to record the visual test
economically justifiable because of for later review.
impaired performance or reduced service
life. For some engines, full 360 degree
borescopic testing of the high pressure
The regular inspection interval is part rotor can be accomplished by rotor
of the written maintenance procedure. A
reduced inspection interval may be half FIGURE 13. Borescope access ports (right side).
that value. Visual test intervals may be
233 degrees 215 degrees
FIGURE 12. Visually detected open crack on
aerospace engine ceramic turbine blade. Fifth stage stator Second stage vane

Fourth stage stator Diffuser case
(bleed valve opening) Access ports

245 degrees Sixth stage stator 338 degrees
277 degrees
231 degrees
225 degrees

Tenth stage stator

214 degrees

276 Visual Testing

cranking of the gearbox. A pneumatic
motor actuated by foot or hand will free
the operator to use both hands to position
and adjust the borescope. It is
recommended that rotation be stopped as
each rotor blade is positioned properly for
inspection. Tilting will allow thorough
appraisal of each blade’s condition.

Aerospace Applications of Visual Testing 277

PART 3. Visual Testing of Composite Materials9

Visual and optical testing are important Because of inhomogeneity, it is
methods to the composite materials important to choose an appropriate scale
industry as primary inspection methods, factor when viewing composite materials
as quick checks and as backups to other with magnification. Many visual tests are
forms of nondestructive testing. The need performed with 5× to 10× power. This
for visual testing often results from several relatively low power evaluation is a
associated needs, such as the desire to balance of close viewing of the small
visualize an anomalous situation, the surface imperfections and retention of the
desire to perform a low cost evaluation ability to view a significant surrounding
and the desire to visualize or document area for perspective and comparison.
the findings of another nondestructive Figure 14 shows a questionable area on a
evaluation method. cross section of an aramid polyester
composite. Without proper lighting and
Visual testing is often used with other scale factor, it appears that the area in the
forms of nondestructive evaluation — center of the figure has two matrix cracks.
looking at a composite structure’s surface Figure 15 shows the same area, with
for pits or voids, for example, before higher magnification and with better
performing radiography. Despite its low lighting. The two lines are not matrix
technology and low cost, a visual cracks. Figure 16 shows actual matrix
evaluation may be all that is required by cracks (notice the prominent crevice).
engineering personnel in assessing quality
or service life conditions. Several topics Because a visible light image can be
will be covered in the following text: readily viewed, documentation of an
problems in the applications of visual evaluation is important. Often the visual
testing, what visual testing can detect in test will be requested and documentation
composite materials, anomalous situations required because of an anomaly detected
calling for visual testing, specific with another form of nondestructive
applications and recent image processing evaluation. Both film based and video
techniques for visual testing. photography are used extensively. Low
power magnification film photography is
Problem Areas useful to document surface discontinuities
(Fig. 17); microscope photography is
Visual testing of composite materials is useful in imaging discontinuities that
more difficult than that of most other have been cross sectioned and polished
engineering materials, due to the (Fig. 18). Video photography is useful in
heterogeneous nature of composites. A documenting inaccessible areas,
perfectly good structure may have particularly when moving the imaging
variations in resin, causing color device from the external structure into the
variations on the exterior surface, surface inaccessible area while the video recorder
roughness or waviness due to woven is turned on.
materials used to apply pressure during
the layup, or benign surface features such FIGURE 14. Possible matrix cracks.
as small voids or wrinkles.
1 mm (0.04 in.)
Preparation of the part to be viewed is
important in visual testing of composites.
Often, structures cannot be viewed
immediately after manufacture and edges
and bag side surfaces must be trimmed
extensively to remove excess resin. After
these trimming operations there may be
dust and residue on the surfaces. If the
edge of the structure must be viewed,
sanding or polishing of the edge may be
necessary. After the cleaning, good
lighting is essential. Straight-on lighting
may be needed to see the features at the
bottom of a void; or oblique lighting, to
distinguish between a protruding or
concave surface feature.

278 Visual Testing

Conditions of Interest in A more thorough evaluation may be
Composite Materials required by engineering personnel. This
may entail looking for resin rich or resin
Visual testing is the first evaluation a starved areas, blisters or delaminations in
composite structure receives after the exterior plies, surface voids and edge
manufacture. This may be simply a quick separations. If the matrix material is
evaluation of the condition of the semitransparent, this evaluation may also
structure after removal from the include looking for internal voids by
autoclave. After removal of the tooling shining a high intensity light through the
and bagging materials, the surfaces of the structure and viewing in a darkened area.
component may be viewed for excessive
wrinkles, holes, gaps between adjacent Visual evaluations often are required at
plies, surface voids, cracks, buckling and fixed inservice intervals. Structures that
other conditions. This visual check is experience stresses from weather,
normally done without magnification. temperature fluctuations, handling or
Just as the actual part itself is viewed, the impact damage are routinely checked for
tooling and bond forms may be viewed damage initiation or growth.
for discontinuities and alignment Delaminations, cracks and buckling can
problems. These evaluations are normally occur. This visual evaluation may reveal
performed by manufacturing personnel. suspicions about an area and may
necessitate followup nondestructive
testing. Often the visual indication is the

FIGURE 15. Coloration change only, no cracks. FIGURE 17. Voids exposed on composite surface.

25 mm (1 in.)

FIGURE 18. Voids in cross section of composite.

FIGURE 16. Actual matrix cracks.

Aerospace Applications of Visual Testing 279

first sign of something wrong, and then common light source is a small, bright
the followup nondestructive evaluation flashlight. It is portable and its beam is
reveals the details. easily directed to the viewing area. The
alcohol is used as a visible penetrant to
Applications enhance the contrast on edge
discontinuities and voids. After wiping
Visual testing may be all that is required the alcohol onto the composite surface,
to evaluate a structure in low stress the surface will dry very quickly, whereas
applications. For example, thin sheets of the alcohol trapped in cracks or voids will
composite material bonded to a take longer to dry. The dark marks go
honeycomb core may show visibly the away as the alcohol evaporates. The
large voids and delaminations that need inking dyes are useful in imaging small
to be detected. Even though the structure surface breaking cracks or voids,
may be used in an aerospace application, particularly when trying to document the
it may not be necessary to spend large cracks with photography. A small amount
dollar amounts performing sophisticated of dye is poured onto a cloth and wiped
nondestructive evaluation procedures. An over the area. After the residual surface
example of this type of structure may be dye is wiped away, some dye stays trapped
the fold down door on the overhead in the cracks, contrasting them against
baggage compartment in an airliner. As an the background color. Figure 19 shows
added test, a sonic tap test may be surface cracks in polyester resin after an
performed (also a low cost evaluation). impact damage event.

Visual testing may be done on large, Of course, contacting materials must be
costly aerospace structures as a first look compatible, not triggering corrosion for
to confirm that extensive discontinuities example.
do exist, making it uneconomical to
continue with more nondestructive Impact Damage and Exfoliation in
testing. If no surface voids, excessive Composite Panels
wrinkles or buckles are noted, ultrasonic
testing may be used to look for Impact damage and exfoliation are
delaminations and radiographic conditions that cause anomalies in the
techniques may be used to look for surface contours of composite panels like
voids/porosity. The cost of the ultrasound those used in aviation. The diffracted
and radiography may be avoided if the light technique described above has been
first visual evaluation discovers rejectable investigated for possible use in evaluating
discontinuities. the condition of such panels.

Borescope assisted visual testing has FIGURE 19. Dye enhancement of surface cracks.
been used to augment other methods of
nondestructive evaluation, where probes 10 mm (0.4 in.)
or film placements are in areas difficult to
access. Some ultrasound composite testing
applications have mounted probes on
borescope devices that are inserted into
accessible apertures of closed structures.
Viewing the borescope image allows
placement of the probe in an otherwise
inaccessible area.

Visual testing is often the first
opportunity to detect damage to aircraft
from hail impact, runway debris or
lightning strikes. Looking at the structure
for dents, buckled areas or discoloration is
part of preflight inspection. Some aircraft
radomes (antenna housings) have had
excessive static discharges that have
resulted in burns to the composite. These
burns are visually detected; then the
composite structure is measured by
ultrasound and radiography, and the
results are provided to engineering
personnel.

Magnifiers, light sources, inking dyes
and wetting agents such as alcohol are all
necessary tools for performing visual
testing on composite materials. As noted
above, 5× to 10× seems to be an
appropriate viewing magnification. A

280 Visual Testing

Graphite epoxy panels of three definitely shows corrosion in four of the
thicknesses (8, 24 and 48 plies) using a six. The remaining two fastener holes
[0/45/90/–45]s layup were numbered and probably would have indicated corrosion
each was cut into three specimens (A, B if a more sensitive ultrasonic technique
and C). The specimens were then painted had been used.
and ultrasonically C-scanned. No
discontinuities were found. The specimens FIGURE 21. Ultrasonic and visual test results
were inspected with the diffracted light for exfoliation corrosion around steel
technique before being subjected to fasteners in wrought aluminum plate:
impact energy ranging from 0.7 to 21.7 J. (a) ultrasonic C-scan; (b) ambient light;
The resulting indentations ranged from (c) diffracted light.
nonvisible to barely visible. The depths of
the indentations were measured and the (a)
specimens were ultrasonically C-scanned
to detect and measure delaminations. The
diffracted light images of the specimens
are shown in Fig. 20 along with the
C-scans.10 The test results clearly show
that the diffracted light technique can
detect barely visible impact damage in
composite laminates and can also detect
cold worked holes in wrought aluminum
panels and fatigue cracks extending from
holes in wrought aluminum panels.3,4,10

Figure 21 shows the result of
exfoliation corrosion detection under
ambient lighting, ultrasonic C-scan and
diffracted light technique imaging. The
test object is not a composite but
illustrates the techniques. The diffracted
light technique shows corrosion around
six of the twelve fastener holes. C-scan

FIGURE 20. Damage 6.1 µm deep and
18.6 mm wide caused by 4.1 J impact on
carbon epoxy panel: (a) ultrasonic C-scan;
(b) diffracted light image.

(a)

(b)

(b) (c)

Aerospace Applications of Visual Testing 281

Image Processing Summary

Recent advances and lowering costs of Visual testing as applied to composite
image processing tools have made their structures has been discussed, showing
use in visual testing more popular. Some the importance of the method as a
commercially available video borescopes primary nondestructive evaluation
now can be attached to computer systems method and as a supplement to one or
with the ability to capture, store and more other nondestructive evaluation
manipulate the images. This ability to methods. Composite structures have some
manipulate the images makes visual unique problem areas that require
testing very powerful in visualizing faint attention during visual testing. Several
images, in tracking features over time by applications were briefly presented, as was
storing and comparing images and by the importance of visual testing to the
using measurement capabilities. verification of discontinuities noted by
other methods. Looking to the future, it
Captured images of a composite can be said that image processing
structure can be filtered, denoised and techniques will play an increasing role in
enhanced. Faint matrix cracks can be the choice of visual testing as applied to
scaled up, contrast enhanced, composite materials.
pseudocolored, have an edge detecting
filter applied to them—all to make them
more visible. The captured image can be
measured to determine the length of a
void or crack.

282 Visual Testing

References

1. Bates, B.[L.], D.[R.] Christina and D.[J.] 8. Dunhill, T. “Some Experiences of
Hagemaier. “Optically Aided Visual Applying NDE Techniques to Ceramic
Testing of Aircraft Structure.” Materials and Components.”
Nondestructive Testing Handbook, Nondestructive Testing of
second edition: Vol. 8, Visual and High-Performance Ceramics. Westerville,
Optical Testing. Columbus, OH: OH: American Ceramic Society (1987):
American Society for Nondestructive p 268.
Testing (1993): p 292-301.
9. Bailey, D. “Visual Testing of Composite
2. Hagemaier, D.J., B.[L.] Bates and Materials.” Nondestructive Testing
D.R. Christina. “Optically Aided Visual Handbook, second edition: Vol. 8,
Inspection of Aircraft Structure.” Visual and Optical Testing. Columbus,
Materials Evaluation. Vol. 46, No. 13. OH: American Society for
Columbus, OH: American Society for Nondestructive Testing (1993):
Nondestructive Testing p 328-330.
(December 1988): p 1696-1701, 1707.
10. Komorowski, J. and R. Gould. “A
3. Komorowski, J.P., D.L. Simpson and Technique for Rapid Inspection of
R.W. Gould. “Enhanced Visual Composite Aircraft Structures for
Technique for Rapid Inspection of Impact Damage.” AGARD Conference
Aircraft Structures.” Materials Proceedings. No. 462. Neuilly sur Seine,
Evaluation. Vol. 49, No. 12. Columbus, France: North Atlantic Treaty
OH: American Society for Organization, Advisory Group for
Nondestructive Testing (1991): Aerospace Research and Development
p 1486-1490. (1990).

4. Komorowski, J.P. and D.S. Forsyth. Bibliography
“The Role of Enhanced Visual
Inspections in the New Strategy for Bailey, W.H. “Requirements for Visual
Corrosion Management.” Aircraft Acuity in the Aerospace Industry.”
Engineering and Aerospace Technology. Materials Evaluation. Vol. 46, No. 13.
Vol. 72, No. 1. Bradford, West Columbus, OH: American Society for
Yorkshire, United Kingdom: Emerald Nondestructive Testing (December
(2000): p 5-13. 1988): p 1625-1630.

5. Liu, Z., D.S. Forsyth and T. Marincak. Boyd, Cliff and M. Lyman. “Video
“Preprocessing of Edge of Light Borescope Technology Helps Aircraft
Images: Towards a Quantitative Engine Maintenance.” Materials
Evaluation.” Nondestructive Evaluation Evaluation. Vol. 51, No. 5. Columbus,
and Health Monitoring of Aerospace OH: American Society for
Materials and Composites II (San Diego, Nondestructive Testing (May 1993):
CA, March 2003). SPIE Proceedings p 543.
Vol. 5046. Bellingham, WA:
International Society for Optical Drury, C.G. “Human Reliability in Civil
Engineering (Society of Photo-Optical Aircraft Inspection.” Research and
Instrumentation Engineers) (2003): Technology Organization Meeting
p 30-38. Proceedings 32, The Human Factor in
System Reliability — Is Human
6. FAA 43-204, Visual Inspection for Performance Predictable? [Human
Aircraft. Advisory circular. Washington, Factors and Medicine Panel Workshop,
DC: United States Government Siena, Italy, December 1999]. Hanover,
Printing Office, for Federal Aviation MD: National Aeronautics and Space
Administration (1997). Administration, Center for AeroSpace
Information, for North Atlantic Treaty
7. “Visual Tests of Ceramics.” Organization (2000).
Nondestructive Testing Handbook,
second edition: Vol. 8, Visual and
Optical Testing. Columbus, OH:
American Society for Nondestructive
Testing (1993): p 328-330.

Aerospace Applications of Visual Testing 283

Good, G.W. and V.B. Nakagawara. Final McGarry, G. “Penetrants and Fiber Optic
Report [FAA Grant 02-G-031], Vision Borescope Help Ensure Safety of
Standards and Testing Requirements for Shuttle Training Aircraft.” Materials
Nondestructive Inspection (NDI) and Evaluation. Vol. 52, No. 10. Columbus,
Testing (NDT) Personnel and Visual OH: American Society for
Inspectors. Washington, DC: Federal Nondestructive Testing
Aviation Administration, Aviation (October 1994): p 1180.
Maintenance and Human Factors
(2005).

Katzoff, S. “The Surface-Tension Method
of Visually Inspecting
Honeycomb-Core Sandwich Plates.”
Nondestructive Testing [Materials
Evaluation]. Vol. 18, No. 2. Columbus,
OH: American Society for
Nondestructive Testing (March-April
1960): p 114-118.

284 Visual Testing

12

CHAPTER

Techniques Allied to
Visual Testing

Stephen L. Meiley, Champion International, West
Nyack, New York (Part 2)
Donald Parrish, Southern Company Services,
Birmingham, Alabama (Part 3)
Donald J. Roth, National Aeronautics and Space
Administration, Glenn Research Center, Cleveland,
Ohio (Part 1)
Michael A. Urzendowski, Valero Energy, San Antonio,
Texas (Part 2)

PART 1. Indications Not from Visual Testing

As a nondestructive test method, visual 2. Visual tests are often used to inspect
testing is defined by its interrogating and or verify the data of the other tests. It
indicating energy: light, in the visible part might be said, for example, that
of the electromagnetic spectrum. Light radiography is ultimately a visual test
and images are used to display, observe, of the radiograph — to determine that
analyze, communicate and record the the radiographic images are properly
results of all nondestructive tests. exposed, that the areas of interest are
free of artifacts and that the images
Visibility criteria are specified for can show the characteristics of
magnetic particle tests, liquid penetrant interest. As another example, wet
tests and some leak tests. Vision acuity is magnetic particle tests are not reliable
verified for some inspectors who use unless the bath has passed a
magnetic particle and ultrasonic testing. preliminary visual test so that the
Light levels, indication sizes, viewing inspector can see discontinuity
angles, color sensitivity and many other indications.
phenomena pertaining to human vision
are strictly controlled to achieve reliable Visual Aspects of Leak
accuracy in visual tests as well as other Testing
nondestructive testing techniques.
Leak testing is done by detecting a tracer
Visual testing is linked to these other medium, a gas or liquid, that has escaped
methods by shared hardware. from confinement. The tracer can be an
added fluid or in some cases the fluid that
1. Borescopes, the basic tools of visual the vessel is designed to hold. Testing is
testing, are often used to view done visually, aurally or electronically.
obstructed magnetic particle or liquid Occasionally, tracers are designed to
penetrant test indications. interact with materials applied or
naturally present outside the vessel, to
2. Magnifiers are used to visually test produce highly visible evidence of
material surfaces, as well as to study leakage. The visual portion of a typical
the details in radiographs and to leak test is that which determines the
measure indications in magnetic presence and location of leakage. The rate
particle and liquid penetrant testing. of leakage and its effect on fluid flow may
be determined by visual observation of
3. Still and video cameras are used in meters and gages.
cineradiography, in the photographic
recording of various test results, in Visual tests are conducted to locate
machine vision, in remote television leakage from pressure retaining
pickups and in virtually all the components. In nuclear power plants,
automated nondestructive test visual tests are required for locating
methods. abnormal leakage from components with
or without leakage collection systems. For
4. Image processing software enhances certain nuclear power components, visual
the display and interpretation of, for testing is performed using the reactor
example, radiographic, ultrasonic or coolant (water) as the tracer medium. The
microwave test results. visual testing of noninsulated pressure
retaining components is performed by
Visual testing may be used with inspecting external, exposed surfaces for
another method, either before or after it. visible evidence of leakage.

1. Visual testing may be used Components whose external surfaces
qualitatively to identify problem areas are inaccessible for direct viewing are
that need follow-up with a examined by visually checking the
quantitative method. A corroded spot surrounding area, including drip pans or
identified visually may be flagged for surfaces beneath the components of
ultrasonic thickness testing, for interest. Color detection and color
example. differentiation are essential in certain leak
testing procedures. For example,

286 Visual Testing

bromocresol purple is a dye used in Penetrant developers may also suffer
chemical reaction leak testing with degradation that is visually detectable.
ammonia gas tracers. The dye is sprayed The best test of a developer is a visual
or brushed onto the outside surfaces of comparison with new material under
pressure vessel welds and allowed to dry. visible and ultraviolet light. Penetrant
After drying, the dye turns into a yellow contamination in dry developer causes
powder. The vessel is then pressurized bright color spots or bright fluorescent
with ammonia gas. If a leak exists, it is spots.
indicated by change in the color of the
powdered dye — from a light yellow to a During liquid penetrant tests, color
vivid purple. detection and color differentiation are
often critical to completion of the test.
Various visible color and fluorescent Certain visual data produced by penetrant
dyes are also used in leak testing. These testing procedures may correlate with
materials are sensitive to the specific aspects of material discontinuities.
concentration of active ions, either acid or In many codes, it is specified that
alkaline. Such ions determine the penetrant indications may be visually
hydrogen potential (pH) of a solution, detected in natural or artificial light.
and the pH value can be shifted by
addition of an acid or an alkali. A number Because penetrant tests normally rely
of useful indicator dyes are sensitive to on an inspector’s visual detection, the
small changes in pH and the most direct lighting for this procedure is very
way to use these effects is to use the dyes important. It not only affects the
with a tracer gas that produces a change sensitivity of the test method but it is also
in pH. This type of gas-phase leak an important factor in inspector fatigue.
indicator typically employs a liquid Visible light sources for penetrant tests are
applied onto areas suspected of leakage. identical to those specified for other
One dye suitable for producing visible visual test applications. Spectral
color leak indications is phenolphthalein. characteristics are usually not critical for
visible light sources but it may be better
Color differentiation is also needed in to use a light source deficient in the light
high voltage discharge leak testing. In this reflected by the penetrant and rich in the
technique, a spark coil is used to excite a other components of the visible spectrum.
visible glow discharge in systems where When such a light is used on a test object
the pressure is between 1 and 1000 Pa having good white developer background,
(10–2 to 10 torr). The tracer can be a gas the penetrant indication appears darker
such as carbon dioxide or a volatile liquid and maximum contrast is obtained.
such as benzene, acetone or methyl Floodlights are advantageous for large and
alcohol. When the tracer gas enters the relatively flat test surfaces. On intricate or
system through a leak, the color of the small test objects, manually directed
discharge changes from blue or purple spotlights may be the most effective
(the color of air) to the color characteristic visible light source.
of the tracer (Table 1). Visual detection of
this color change is required to interpret TABLE 1. Discharge colors in gases and vapors at low
the leak test. pressures.

Visual Aspects of Liquid Gas Negative Glow Positive Indication
Penetrant Testing
Air blue red
In liquid penetrant tests, visual Argon
techniques are used to compare Bromine blue deep red (violet)
sensitivities of penetrant systems, to Carbon dioxide
detect discontinuity indications and to Carbon monoxide yellow/green red
verify the cleanliness of the testing Chlorine
materials. When a hydrophilic emulsifier Helium blue white
bath is used, visual monitoring of the Hydrogen
bath can provide clues to its condition. A Iodine green/white white
fresh solution of emulsifier in water Krypton
exhibits a typical color in visible light and Lithium green light green
in ultraviolet light. Traces of fluorescent Mercury
penetrant contamination in the bath will Methane pale green violet/red
darken its color and cause the Neon
fluorescence to shift. High levels of Nitrogen pink/bright blue pink (rose)
penetrant contamination cause a Oxygen
hydrophilic emulsifier solution to become Potassium orange/yellow peach
cloudy and, at higher levels, free Sodium
penetrant can be seen to float on the bath Xenon colorless green
surface.
white bright red

green (gold/white) green/blue (green)

colorless red/violet

red/orange red/orange (blood red)

blue yellow (red/gold)

yellow/white lemon

green green

yellow/green (white) yellow

colorless blue/white

Techniques Allied to Visual Testing 287

The proper intensity of visible light is Subdued lighting in the viewing area is
determined by the requirements of the preferable to total darkness. The room
penetrant test. For gross discontinuities, a lighting must be arranged so that there
brightness level of 300 to 550 lx at the are no reflections from the surface of the
surface is typically sufficient. Intensity film.
levels of 1000 lx are necessary for small
but critical discontinuities. Magnifiers and densitometers may help
to view industrial radiographs. Scanning
Because liquid penetrant tests use the microdensitometry equipment can be
human eye as a detection device, the useful for certain industrial applications,
condition of the inspector’s eyes is an including focal spot measurements and
important test specification. For determination of total radiographic
fluorescent penetrant testing, the unsharpness. The relationship of density
inspector must be dark adapted and the differences to material thickness
intensity of the ultraviolet source must differences can be graphed and studied
reach specified minima. Photosensitive with an item of known thickness (a shim,
eye glasses must not be worn. The eye for example) on the radiograph.
itself will fluoresce under certain
conditions, causing temporarily clouded For an image to be digitally enhanced,
vision. it must be in digital form. Digital
radioscopy and computed tomography are
Optical technologies are used in inherently digital. Radioscopic images
penetrant tests in a variety of ways. For from luminescent screens (fluoroscopy) or
example, a flying spot laser can be used cathode ray tubes may be scanned and
for detection of penetrant test indications. presented in a two-dimensional digital
When the laser beam strikes fluorescent array of picture elements (pixels).
penetrant materials, a pulse of different Conventional film radiographs also may
wavelength light is generated. A be digitized. Once the radiographic image
photodetector converts this pulse into an is digitized, a variety of enhancement
electrical signal that is analyzed, using methods can be used, including
pattern recognition techniques, to brightness transfer functions, gradient
determine the discontinuity’s shape and removal, digital filtering, field flattening,
size. smoothing and a number of other
transforms.
Visual Aspects of
Radiography The coding of several scale intervals
with a different color results in an
Vision acuity is vital to the radiographic enhancement technique known as
interpretation process. Individual vision pseudocolor. Though these color images are
can and does vary from test to test striking to the viewer, the technique has
depending on physiological and the disadvantage of presenting color
psychological factors. Annual vision changes that do not necessarily
acuity examinations cannot detect daily correspond to abrupt changes in optical
fluctuation or its influence on density on the image.
interpretation and the frequency of a
vision acuity examination may be Visual Aspects of Magnetic
specified. Particle Testing

In radiographic testing, the physical In magnetic particle tests, the vision
measure of interest for vision acuity is the acuity of the inspector and the visibility
discontinuity as displayed on the film, of the test results are as important as they
regardless of how much it may differ from are for liquid penetrant tests.
the actual discontinuity in the test object.
Vision acuity tests can be based on Dry magnetic particles are
microdensitometric scans of commercially available in visible colors,
discontinuities taken directly from actual fluorescent colors and daylight fluorescent
radiographs. Vision acuity exams for colors. Visible particles are typically
radiographers may include factors such as available in gray, red, black, yellow, blue
the figure-to-ground relationship, and metallic pigments. Colors of visible
background luminance, contrast, line particles are chosen for individual testing
width, line length, viewing distance, blur, applications based on the highest contrast
line orientation and characteristics of the with the test object surface.
light source.
Daylight fluorescent colors were
During radiographic tests, viewing developed to increase visibility in those
conditions are very important. A finished applications where somewhat lessened
radiograph should be inspected under sensitivity is acceptable. Daylight
conditions that afford maximum visibility fluorescent colors have enhanced visibility
of detail together with maximum comfort in light from the sun, blue mercury vapor
and minimum fatigue for the interpreter. lamps or white fluorescent tubes. The
yellow light from sodium vapor sources

288 Visual Testing

does not excite fluorescence in these penetrant tests: illuminated rigid
magnetic particles. The dyes absorb borescopes, fiber optic borescopes and
photons of one energy and emit photons video borescopes. These devices are used
at a lower energy. in the same way, with the same
advantages and precautions, as in a purely
Much of the specified technique for visual test. The difference is that the
magnetic particle and other tests is object of interest is a magnetic particle
established to enhance visibility of the indication of a material discontinuity.
discontinuity indications. Choice of color, Light intensities for borescopes and
magnetization levels and application of remote viewing instruments must be set
the particles themselves are all done in to include losses between the eyepiece
ways that maximize visibility. In certain and the distal tip of the device. Quartz
magnetic particle applications, a thin fiber borescopes typically need less light
white lacquer is applied to the test object intensity than conventional borescopes
surface to make dark colored particles used for viewing visible particle
more visible (smaller particles are more indications. In a rigid borescope, light
responsive to magnetism and the addition transmission is governed by an optical
of a layer of pigment to the particles system. For video borescopes, the
decreases their sensitivity). sensitivity of the camera chip is the
critical factor. Reference standards and
Wet method particles are available in comparative tests with known
fluorescent and nonfluorescent forms. discontinuities are used to verify remote
Particle color is again chosen to increase viewing of magnetic particle indications.
contrast with the test object surface.
Visual Aspects of
Not only are particle characteristics Ultrasonic Testing
chosen to maximize visibility but light
levels are also carefully monitored. A light Ultrasonic imaging can be performed by
intensity of 1000 lx (100 ftc) can be used several techniques. The most common is
for viewing nonfluorescent magnetic that used by commercially available
particle indications. The optical light ultrasonic immersion C-scanning systems.
level, however, is sometimes a An ultrasonic wave of known amplitude is
compromise between operator fatigue and transmitted through a test material. The
visibility. On bright or reflective surfaces, final C-scan image is a representation of
high light intensities can cause glare that amplitude attenuation or energy lost by
interferes with vision and subsequent the ultrasonic wave during its trip from
interpretation. On darker surfaces or those the transmitter to the receiver.
covered with thin scale, the 1000 lx
(100 ftc) level may be barely adequate for C-scan images represent the energy lost
visibility. by the ultrasonic wave but, unfortunately,
not solely because of the internal
In wet method fluorescent tests, structure of the test object. The ultrasonic
standards require a minimum ultraviolet reflection coefficients at the water-to-
light intensity of 1000 mW·mm–2, with a object interfaces are generally assumed to
maximum allowable visible light intensity be constant but actually can vary widely.
at the surface of 20 lx (2 ftc). Even small These reflection coefficients must be
amounts of visible light can lower the included in the analysis to determine the
contrast of a fluorescent indication. actual energy loss or attenuation caused
by the inner structure of the test object.
Appropriate light levels are so critical They must also be used in determining
to the completion of magnetic particle the accuracy of the acoustic image as well
tests that visible light intensity as the accuracy of the imaging technique.
measurements are routinely performed
and logged. Typically, such entries are An ultrasonic transducer is scanned
made from a light meter that is itself over the test object surface while a load
calibrated every six months or at intervals cell gages coupling pressure. After
prescribed by the relevant specification. collection and subsequent fourier analysis
of the appropriate waveforms, velocity
Visual capabilities are critical to and attenuation can be determined
magnetic particle indication detection but accurately at different positions on the
they are also used to verify the quality of test object, in an organized array. The
the testing materials. In an agitation resulting data yield velocity and
system, where the particle bath constantly attenuation maps that are displayed on a
passes through a centrifugal pump, video system.
particles are subject to constant high
speed impact and shearing from the A typical data set, from which about 25
pump’s impeller. As breakdown of the images can be obtained, contains about
particles increases, test indications 2000 waveforms, 2000 fourier spectra and
become dimmer and background 40 000 attenuation and velocity values.
fluorescence is seen to increase
(indication-to-background contrast
diminishes).

Optical equipment used for magnetic
particle tests is the same as for liquid

Techniques Allied to Visual Testing 289

The original acquired data use about ultrasonic data. In the past, only a few
20 megabytes of computer memory. The measurements were taken at a small
reduced data set containing the number of positions to evaluate a material
attenuation, velocity and reflection ultrasonically. The data from these
coefficient2 images occupies about measurements were averaged and the
5 megabytes. standard deviation determined. These two
values (the average and the standard
Evaluating an image often requires deviation) were used to classify the test
tests of both the raw and reduced data for object but the two quantities may not
a particular point in the image. A data contain sufficient information for
retrieval program provides the necessary providing a full understanding of the
interface to allow immediate retrieval of internal structure of the test material.
data displayed in a video image overlay.
For example, the data for one point For example, from nine measurements,
(determined by the cursor location) in an the average ultrasonic attenuation and its
ultrasonic image of velocity are displayed standard deviation at 100 MHz in a
in an overlay. This display contains ceramic test object can be calculated as
waveforms and their fourier spectra, 0.11 and 0.03 Np·mm–1. But this
reflection and attenuation coefficients information does not describe all
(may be used to determine accuracy), cloudlike attenuation structures.
phase velocity as a function of frequency Ultrasonic attenuation in ceramics can be
and group velocity (determined by cross caused by diffractive scattering from
correlation). individual grain and pore boundaries. An
increase in the number or size of pores
All these data are inspected for produces an increase in attenuation. High
irregularities when evaluating a specific ultrasonic attenuation may also be caused
point on the ultrasonic image. For by diffractive scattering at regions
example, if a quantitative measure of the exhibiting large velocity (density)
density difference between two points is gradients, not increased porosity.
needed, it can be determined from the
group velocity at these points on the Closing
appropriate overlays. Also, the frequency
dependence of attenuation has been The importance of optical technologies to
related to the subsurface pore or the broad field of nondestructive testing is
discontinuity sizes,3 the state of the typified in the interplay of visual and
recrystallization in metals4 and the mean optical techniques with ultrasonic
grain size.5 Easy access to this type of technology, as detailed above. Visual
information is made possible by the use of testing has a long and vital history.
a video system. Despite their origins and their value in
the past, the visual and optical techniques
Role in Data Interpretation are also directly linked to the most
complex nondestructive testing
Video and computer systems also play a technologies of the present.
crucial role in the interpretation of

290 Visual Testing

PART 2. Replication6

Replication is a valuable tool for the Removal of loose surface particles is
analysis of fracture surfaces and usually done by wetting a piece of acetate
microstructures and for documentation of tape on one side with acetone, allowing a
corrosion damage and wear. There is also short period for softening and applying
potential for uses of replication in other the wet side of the tape to the area of
forms of surface testing. interest. Thicker tapes of 0.013 mm
(0.005 in.) work best for such cleaning
Replication is a method used for applications (thin tapes tend to tear).
copying the topography of a surface that Following a short period, the tape hardens
cannot be moved or one that would be and is removed. This procedure is
damaged if transferred. A police officer normally repeated several times until a
making a plaster cast of a tire print at an final tape removes no debris from the
accident scene or a scientist making a cast surface.
of a fossilized footprint are common
examples of replication. These replicas Fracture Surface Analysis
produce a negative topographic image of
the subject known as a single stage replica. The topography of fracture surfaces can be
A positive replica made from the first cast replicated and analyzed using an optical
to produce a duplicate of the original microscope, scanning electron microscope
surface is called a second stage replica. or transmission electron microscope. The
maximum useful magnification obtained
The technology of replication is using optical microscopes depends on the
described in the literature.7-9
FIGURE 1. Acetate tape replication
Many replicating mediums are producing negative image of surface:
commercially available. Most replica types (a) microstructure cross section; (b) softened
in nondestructive testing are cellulose acetate tape applied; (c) replica curing;
acetate or silicone rubber. Both have (d) replica removal.
advantages and limitations, and either
can provide valuable information without (a)
altering the test object.
(b)
Cellulose Acetate
Replication (c)

Acetate replicating material is used for (d)
surface cleaning, removal and evaluation
of surface debris, for fracture surface
microanalysis and for microstructural
evaluation. Single-stage replicas are
typically made, creating a negative image
of the test surface. A schematic diagram of
microstructural replication is shown in
Fig. 1.

Cleaning and Debris Analysis

Fracture surfaces should be cleaned only
when necessary. Cleaning is required
when the test surface holds loose debris
that could hinder analysis and that
cannot be removed with a dry air blast.

Cleaning debris from fracture surfaces
is useful when the test object is the debris
itself or the fracture surface. Debris
removed from a fracture can be coated
with carbon and analyzed using energy
dispersive spectroscopy. This provides a
semiquantitative analysis when a
particular element is suspected of
contributing to the fracture.

Techniques Allied to Visual Testing 291

roughness of the fracture but seldom As with the removal of surface debris,
exceeds 100×. The scanning electron it has been found that the thicker replicas
microscope has good depth of field at provide better results, for the same
high magnifications and is typically used reasons. The procedure for replication of
for magnification of 10 000× or less. The fracture surfaces is identical to that for
transmission electron microscope has debris removal. On rough surfaces,
been used to document microstructural however, difficulty may be encountered
details up to 50 000×. when trying to remove the replica. This
can cause replication material to remain
In general, scanning electron on the fracture surface but this can easily
microscope analysis of a replica provides be removed with acetone.
information regarding mode of failure
and, in most instances, is sufficient for Replicas, in the as-stripped condition,
completion of this kind of analysis. An typically do not exhibit the contrast
example of a replicated fracture surface is needed for resolution of fine microscopic
shown in Fig. 2. The transmission electron features such as fatigue striations. To
microscope is used in instances where improve contrast, shadowing or vapor
information regarding dislocations and deposition of a metal is performed. The
crystallographic planes is needed. Both metal is deposited at an acute angle to the
single stage (negative) and second stage replica surface and collects at different
(positive) replicas can be used for failure thicknesses at different areas depending
analysis. Some scanning electron on the surface topography. This produces
microscope manufacturers offer a reverse a shadowing that allows greater resolution
imaging module that provides positive at higher magnifications.
images from a negative replica. This
eliminates the need to think and interpret Shadowing with gold or other high
in reverse. This feature has also proven atomic number metals enhances the
valuable for evaluating microstructures electron beam interaction with the sample
through replication. and greatly improves the image in the
scanning electron microscope by reducing
FIGURE 2. Fracture surface replication shows the signal-to-noise ratio.
fatigue striations on surface: (a) 2000×;
(b) 10 000×. Scale has been modified during Microstructural Interpretation
publication.
To date, the greatest advances in the use
(a) of acetate replicas for nondestructive
testing have come from their use in
(b) microstructural testing and interpretation.
Replication is an integral part of visual
tests in the power generation industries as
well as in refining, chemical processing
and pulp and paper plants. Replication, in
conjunction with microstructural analysis,
is used to quantify microstrain over time
and to predict the remaining useful life of
a component. Future applications are not
limited by material type.

In industry, tests are carried out at
preselected intervals to assess the
structural integrity of components in their
systems. These components can be
pressure vessels, piping systems or
rotating equipment. Typically these
components are exposed to stresses or an
environment that limits their service life.
Replication is used to evaluate such
systems and to provide data regarding
their metallurgical condition.

Microstructural replication is done in
two steps: surface preparation followed by
the replication procedure. Surface
preparation involves progressive grinding
and polishing until the test surface is
relatively free of scratches (metallurgical
quality). Depending on the material type
and hardness, this can be obtained by
using a 1 to 0.05 mm (4 × 10–5 to
2 × 10–6 in.) polishing compound as the
final step. Electrolytic polishing can
increase efficiency if many areas are being

292 Visual Testing

tested. Surfaces can be electropolished shown in Figs. 3 to 6. Replication is used
with a 320 to 400 grit finish. for detection of high temperature creep
damage, stress corrosion cracking,
The disadvantages of electropolishing hydrogen cracking mechanisms, as well as
are that (1) the equipment is costly, the precipitation of carbides, nitrides and
(2) with most systems only a small area second phase precipitates such as sigma or
can be polished at one time and gamma prime. Replication is also used for
(3) pitting has been known to occur with distinguishing fabrication discontinuities
some alloy systems containing large from operational discontinuities.
amounts of carbides.
Strain Replication
Next, the polished surface is etched to
provide microstructural topographic Attempts have been made to replicate
contrast which may be necessary for strain to evaluate localized strain in
evaluation. Etchants vary with material materials exposed to elevated
type and can be applied electrolytically, temperatures and stresses over time
by swabbing or spraying the etchant onto (materials susceptible to high temperature
the surface. With some materials, a creep). The purpose is to monitor
combination of etch-polish-etch intervals accumulated strain before detectable
yields the most favorable results. microstructural changes occur. Strain
replication entails inscribing a grid
To replicate the surface microstructure, pattern onto a previously polished
an area is wetted with acetone and a piece surface. A reference grid pattern is
of acetate tape is laid on the surface. The replicated by using material with a
tape is drawn by capillary action to the shrinkage factor quantified through
metal surface, producing an accurate analysis. This known shrinkage factor is
negative image of the surface
microstructure. Thin acetate tape at FIGURE 3. Comparison of optical microscopy
0.025 mm (0.001 in.) provides excellent to scanning electron microscopy in
results and gives the best resolution at documentation of replicated microstructure.
high magnifications. Thicker tapes must Evidence of creep damage is visible in grain
be pressed onto the test surface and, boundaries. Etchant is aqua regia.
depending on the expertise of the Magnification was 100× before publication;
inspector, smearing can result. Thicker (a) optical microscope image; (b) scanning
tapes are more costly and the resolution electron microscope image.
of microscopic detail does not match
thinner tapes. Studies of carbide (a)
morphology and creep damage
mechanisms have been performed at (b)
magnifications as high as 10 000× with
thin tape replicas.

Before removal of the tape from the
test object, the back is coated with paint
to provide a reflective surface that
enhances microscopic viewing. The
replica is removed and can be stored for
future analysis.

If analysis with the scanning electron
microscope is needed, replicas should be
coated to prevent electron charging. This
is accomplished by evaporating or sputter
coating a thin conductive film onto the
replica surface. Carbon, gold,
gold-palladium and other metals are used
for coating. There are differences in the
sputtering yield from different elements
and this should be remembered when
choosing an element or when attempting
to calculate the thickness of the coating.

The main advantage of sputter coating
over evaporation techniques is that it
provides a continuous coating layer.
Complete coating is accomplished
without rotating or tilting the replica.
With evaporation, only line of sight areas
are coated and certain areas typically are
coated more than others.

Some examples of replicated
microstructures, documented with both a
scanning electron microscope and with
conventional optical microscopy, are

Techniques Allied to Visual Testing 293

included in future numerical analysis of After a predetermined period of
strain. The grid is then coated to prevent operation, the coating is removed and the
surface oxidation during use. area is again replicated. The grid
intersection points on the two replicas are
FIGURE 4. Documentation of creep damage: compared for dimensional changes, and
(a) linked creep voids can be observed in the changes are then correlated to units of
microstructure weld consisting of austenitic strain. This technique does not yield
matrix, precipitated nitrides and carbides, as absolute values of strain but gives
viewed originally at 500× with optical information to help project the
microscope; (b) grain boundary carbides, component’s service life.
creep voids and particles believed to be
nitrides can be observed in matrix of alloy in Strain replication may be attempted for
Fig. 3, as viewed originally at 1000× with materials that do not exhibit creep void
scanning electron microscope. Scale has formation until late in their service life
been modified during publication. (Fig. 7).10 When the relationship deviates
from linearity, the strain rate can become
(a) unstable.

In addition to qualitatively assessing by
replication the degree of creep damage,
whenever creep damage is suspected, an
engineering assessment consistent with
API 579 and ASME FFS11 should be
conducted to determine if the component
is fit for continued operation and for how
long it will be safe to operate.

(b) Silicone Rubber Replicas

Nitride Void Silicone impression materials have been
used extensively in medicine, dentistry
Carbide and in the science of anthropology. In
nondestructive testing, silicone materials
are used as tools for documenting
macroscopic and microscopic material
detail. Quantitative measurements can be
obtained for depth of pitting, wear,
surface finish and fracture surface
evaluation.

Silicone material is made with varying
viscosities, setting times and resolution
capabilities. Compared to an acetate
replica, the resolution characteristics of a
silicone replica is limited. With a medium
viscosity compound, fine features visible

FIGURE 5. Documentation of stress corrosion FIGURE 6. Documentation of heat-affected zone cracking in
cracking found in welds of anhydrous boiler grade steel. Cracking associated with nonrelieved
ammonia sphere. Three percent nital etch at stress. Repair weld was unknown until in-field metallography
200× before modification for publication. and replication were performed. Three percent nital etch at
100× before magnification was altered for publication.

Weld

Heat affected zone

294 Visual Testing