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31. Green, D.R. 11 Experimental
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486 Infrared and Thermal Testing

44. Okamoto, Y. and T. Inagaki. Remote
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japan: Corona Publisher (1995):
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45. lnagaki, T., C. Liu, I. Miyata and Y.
Okamoto. "Detection of Flashing
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by h,feans of Infrared Radiometer."
Qua11titalh'e lnfimed Thennography:
QIRT '96. Eurotherm Seminar No. SO
)Stuttgart, Germany, September 1996).
Pisa, Jtaly: Edizione ETS (1997):
p 324-330.

46. Sato, (vf., K. Kato, 'J: \'\1aterai, K.
Miyata, T. Inagaki andY. Okamoto.
"Detection of High-Temperature Spots
on Reciprocating Interface under Dry
Friction by Means of an Infrared
Radiometer." Thermosense X\1111: An
lntematimwl Cunfi.'reHce on '111emwl
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Applimtions [Orlando, FL, April 1996).
SPIE Proceedings, Vol. 2766.
Bellingham, V·lA: International Society
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and Lubrication of Solids, Part 1.
Oxford, United Kingdom: Oxford
University Press (1950): p 31-50.

48. Inagaki, T., M. Sato and Y. Okamoto.
"Thermal Visualization Study of
Friction and Its Interface." Proceedings
of tile 3rd lVorld Conference on
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[Honolulu, HI, November 1993].
Vol. 1. New York, NY: Elsevier Science
(1993): p 739-744.

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Okamoto. 11 Heat Transfer and
Temperature Field of Rotating Friction
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Inte1pretati011 ofTribological PlleiW111CIW.
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NY: Elsevier Science (1996): p 1-8.

SO. Parker, B. and L. Baughman. "Metal
Assembly Thermography." ASNTs
Infrared Tllermograplly Topical
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OH: American Society for
Nondestructive Testing (June 1997):
p 23-27.

51. Rhinehart, D. and K. Henderson.
"Press Room Thermography." ASNTs
Infrared 11wrmograplly Topical
Conference !Cleveland, OH). Columbus,
OH: American Society for
Nondestructive Testing (June 1997):
p 31-33.

Infrared and Thermal Testing of Metals 487



CHAPTER

Aerospace Applications
of Infrared and
Thermal Testing

Robert L. Crane, Air Force Research Laboratory, Wright
Patterson Air Force Base, Ohio
Tommaso Astarita, Universita degli studi di Napoli
"Federico II/' Naples, Italy (Part 5)
Harold Berger, Industrial Quality, Incorporated,
Gaithersburg, Maryland (Part 4)
Gennaro Cardone, Universita degli studi di Napoli
"Federico II," Naples, Italy (Part 5)
Giovanni M. Carlomagno, Universita degli studi di
Napoli "Federico II," Naples, Italy (Part 5)
Thomas S. jones, Industrial Quality, Incorporated,
Gaithersburg, Maryland (Part 4)
Matthew D. Lansing, National Aeronautics and Space
Administration, Marshall Space Flight Center, Alabama
(Parts 1 and 2)
Samuel S. Russell, National Aeronautics and Space
Administration, Marshall Space Flight Center, Alabama
(Parts 1 and 2)
james L. Walker, National Aeronautics and Space
Administration, Marshall Space Flight Center, Alabama
(Parts 1 and 2)
Gary L. Workman, University of Alabama, Huntsville,
Alabama (Parts 1 and 2)

PART 1. Infrared Thermography of Space Shuttle
and Related Aerospace Structures

The nondestructive testing of aerospace bonded together with a resin or braze.
structures creates many challenges These types of structures require a
because of the exceptional demands on particularly difficult test in that the skins
the materials and processes used. The have a high or anisotropic in·plime
structures are generally large and complex thermal conductivity that can distort the
in design and have limited accessibility. thermal image of the discontinuity below
Infrared thermal imaging provides a it. Another problem is that metal face
useful test method for these types of sheets often have a specular (mirrorlike or
structures because it is a noncontact highly reflective) surface that greatly
method, can detect discontinuities at inhibits the flow of radiant heat into the
moderate depths and can image structure as well as being an inefficient
reasonably large coverage areas. radiator that yklds poor infrared images.
Techniques are described which1 when
Aerospace examples covered below permitted, increase surface emissivity and
include leak testing of the space shuttle absorptivity by dulling surface reflections
main engine nozzle and tubing sections, and increase the ability of the surface to
composite nose cone structures, radiate heat to a usable leveL
composite propellant fuel tanks on launch
vehicles and metal honeycomb core and The results of thermographic tests are
skin assemblies. often very subjective and strongly affected
by surface conditions, material properties
Composite materials are used in large and the experience of the test conductor.
primary aerospace structures in launch Test panels or calibmtiou standards built
vehicles, such as spars1 aeroshells and fuel ·with artificial discontinuities that
tanks after many years serving only as resemble true critical sized discontinuities
structurally noncritical components. can be inspected along with the actual
These composite structures offer a unique structure under test to ensure that a given
set of characteristics that make the testing process will find all critical sized
detection of structural and material discontinuities. The true measure of a
discontinuities very important. calibration reference standard, ·with its
Composites are designed to be highly built~in discontinuities, is that it be
efficient in highly stressed situations. It is thermally similar to that of the actual
often very difficult to predict exactly what structure and that its artificial
influence a discontinuity will have on discontinuities react as real discontinuities
structural performance. The most would in the article under investigation.
prominent types of discontinuities
occurring in composite aerospace All these tools, when used together,
structures include porosity, delaminations, allow infrared thermal imaging to provide
disbands, fiber breakage and foreign test capability where more traditional
material inclusion. Infrared thermography nondestructive methods cannot provjde
is an ideal inspection tool for large acceptable levels of performance. The
composite structures because these following case studies illustrate how the
discontinuities show up as anomalous method can be used with several types of
temperature observations across the aerospace structures. However, there are
surfaces of the structures. Special many more types of aerospace structures
considerations must be made to account that have not been inspected using
for the variety of thermal properties of the infrared thermal imaging. Applying these
materials used in the composite structure, approaches for testing of other aerospace
which can make it hard to induce a systems may provide interesting benefits.
uniform thermal load. \Vithout a uniform
reference the identification of critical leak Testing with
discontinuities in complex composite Thermography
aerospace structures- such as those
including honeycomb, syntactic foam or Detecting leaks in pressure vessels or
aerogel cores as ·well as exotic matrix fiber piping can be simple or extremely
combinations of metal and ceramic ~ can challenging. If the vessel is geometrically
be very difficult. simple and unrestricted in access, a low
pressme leak check using a liquid bubble
Many applications exist in the solution applied to the vessel with a brush
aerospace industry for structures that
feature metal skins and honeycomb core

490 Infrared and Thermal Testing

or bottle is usually adequate. Bubbles in diverging nozzie and compressible gasse\
supersonic flmv is possible if the internal
the leak check solution appear as a result pressure of the contained gas is 1.893
of escaping gas and can even be used to 1 times the absolute outside pressure. Under
normal atmospheric conditions this is a
estimate the leak rate. tank pressure of about 90 kPa (13 lbrin.-2
launch vehicle pressure vessels include gage). If the flow at the opening of the
pressure vessel is sonic then for air the
fuel or oxidizer tanks, feed lines, temperature drop may be as low as 244 K
(~29 °C = -20 oJ;) for (I tank temperature of
tmbopump chambers, valves, combustion 293 K (20 oc = 68 of). If the geometry act>
as a converging or diverging nozzle the
chamber coOling cavities and nozzles. tempewture drop away from the leak if
Frequently these and other aerospace supersonic will be even larger. Clearly
pressure vessels are so complicated that using modern infrared cameras, if the
leaks cannot he located in inaccessible flow is supersonic this effect ·wiJJ be a
major factor in the detected temperature
areas. But if the vessel has a complicated change.

geometry, restricted access, chemical There may be good reasons not to use a
sensitivity or a large area, then an infrared steady state condition during testing. An
example of a transient test will be
video camera may be a good tool for presented later. But for complex
identifying, locating and ranking leaks. geometries during rapid pressurization,
gasses contained in confined regions of
The temperature change caused by a leak the vessel may be compressed by the
in a hidden area of a complex structure pressure increase. \'\'ark is then done on
may be detected thermographically on an this confined slug of gas by the incoming
outer surface. Also, infrared video cameras gas. This causes a decrease in the volume
of the slug and a corresponding
can test large areas very rapidly and temperature increase.
without contacting the component.
For organic compound gasses or
Several physical phenomena may be saturated mixtures a phase change may
active at a leak and leak geometry is contribute to the complexity of this
usually unknown and unpredictable. The problem. Examples of these gasses include
leak may act like a throttle valve or a refrigerant 134A, butane, propane and
moist air. The temperature change because
nozzle or a combination of both. These of one of the previous mentioned
may cause either heating or cooling. Also, phenomena may cause a phase change.
This phase change may absorb or release
transient effects including the energy and cause a detectable temperature
pressurization of gas already in the vessel change.

causes work on the trapped gas. For the As an illustration of how leaks behave
detection of a leak, it is not always thermally, a series of tests were performed
on a single tube ·with a 0.13 mm
necessary to understand the leak process. (0.005 in.) hole. The tube was painted flat
A local temperature change is simply black, giving it an emissivity of about 0.85
detected. However, for leak location or and imaged with a high resolution
(256 x 256 pixels), high sensitivity
leakage rate approximation, it may be (0.025 K [0.025 oc; 0.045 OF] per level)
infrared camera. The tube was connected
necessary to understand the process. A to a K bottle containing either nitrogen,
discussion of some of these and argon, helium or carbon dioxide gas. The
pressure regulator on the K bottle was set
competing effects follows. to deliver 276 kPa (40 lbrin.-2) gage and
The joule-thompson effect is classically was valved to provide that pressure to the
tube in less than 2 s. To ensure that the
exhibited in flow through a throttling gas in the tube ·was pure, the tube was
restriction. The flmv through the purged for 5 minutes with a low pressure
flow of the test gas. The infrared imager
restriction causes no work to be done and was set to acquire images from before
is considered adiabatic. For real gasses, pressurization to about 8 to 10 s after
pressure initiation.
either the temperature can increase or
decrease depending on the type of gas, Figure 1 presents a time-temperature
temperature and pressure. For each given plot depicting the transient behavior of
gas there is a temperature (inversion each gas. Each gas has its mvn unique
thermal response (Tabk 1). First and most
temperature) at which no temperature obvious, note how the nitrogen, argon
change occurs for the throttling process. and carbon dioxide all increase in

This inversion temperature is a weak

function of temperature. Belnw the
inversion temperature the gas heats and
above the gas cools on throttling. This
critical temperature is below standard
conditions for hydrogen (202 K =- ~71 °C. =-

-96 "F) and helium (25 K =-248 "C =

~415 °F) and above standard conditions

for nitrogen (621 K =348 °C = 658 °F), air
(603 K =330 "C = 626 °F), argon (723 K =
450 oc = 842 °F) and carbon dioxide

(1500 K = 1227 oc = 2240 °F).
A leak may occur in a region with a

flow cross section that geometrically

resembks a converging or diverging
nozzle. An example of this is a cooling
tube that has a crack that opens into a

confined region. For the converging or

Aerospace Applications of Infrared and Thermal Testing 491

temperature when pressure is first applied be produced as a result of manufacturing
as opposed to the helium, which begins to anomalies and the extreme service
cool immediately. Secondly, notice that
the nitrogen, argon and helium all show environment. These discontinuities may
some cooling after the pressure is applied lead to poor or incomplete bonding of the
for a long period of time, whereas the
carbon dioxide returned to room tubes to the structural jacket, interstitial
temperature. leaks and hot wall leaks.

Interstitial leak Detection1 Through the application of a bubbling
leak check solution and visual testing hot
The National Aeronautics and Space
Administration space shuttle main engine wall leaks are easily detected. Tests for
nozzles consist of over 1000 tapered
coolant tubes brazed to a structural steel determining braze line integrity can he
jacket (Fig. 2). The cold wall of the nozzle performed by standard X-ray practices
is defined as its outer jacket surface
whereas the inner, flame bearing, tube and1 when the jacket is accessible, by flash
surface is known as the llot wall. The thermography. Here1 thermographic
somewhat triangular gaps between the techniques are used to address the
round tubes and the flat jacket are called
the interslitials. The diameter of the detection, location and characterization of
individual cooling tubes decreases from interstitial leaks (J:ig. 2).
the aft end to the forward end1 causing
the interstitials to become smaller also. The standard test for locating
Discontinuities in the coolant tubes may interstitial leakage is the application of

leak check solution in the openings where
the interstitials vent at the aft end of the

nozzle ·while the tubes are pressurized to
172.3 kl'a (25 lb1·in.-2) with gaseous
helium. \Vhen a leak is found1 it is
classified based on the bubbles formed/
per Table 21 and if the leakage is severe

FIGURE 1. Plot of time versus temperature shows transient behavior of leaks: (a) nitrogen; (b) argon; (c) helium; (d) carbon
dioxide.
(a)
(c)
l- ri _ , I .E 294.55 (21.40) [70.52]
G:'
G 294.45 (21.30) [70.341

L.. 294.35 (21.20) (70.16]
'-.. 295.65 (ZZ.50) [72.50} ,. -,- . ! ---I·,·--~~
G ··--+·----~- ··.. -
295.55 (22.40) {72.32)
" 295.45 (ZZ.30) [72.14}
II 1I -
8ack~round
~ 294.25 (21.1 O) (69.98] 1\ ::.:: 295.35 (22.20) (71.96} . ·- ' - -.··- -t -- -
~i~n~;'_ E F- ---, . -ltiT 295.25 (22.10) [71.78} .
~' 294.15 (21.00) [69.80] .\ .""· ., ... ___j__ _j Backgroumi i
i:.l'? 294.05 (20.90) [69.62] 8 10 12 ~ 295.15 (22.00) [71.60] ... : .~:. ~a.,~~-- ~~-.,_..,_"'··.:·'.t.. \{~-t·,t-N-<'·
·"'!" c 6

25_ 293.95 (20.80) (69.44] w 295.05 (21.90) [71.42] ,...~--1-- Signal ·

E 293.85 (20.70) (69.26] ~ 294.95 (21.80) {71.241 --~ -----r---r~·~···......w;.,_o..w...~
294.85 (21.70) (71.06]
{" 0 z 4 - ·- - - - L _ _~- -L __ _j _ _ •

0 2 4 6 8 10 12

,_295.75 (22.60) [72.68] Time (s) (d) Time (s)
(b) E JK j '-296.15
295.55 (22.40) [72.32] --Background LL 296.35 (23.20) (73.76] ~···-.-· -1, - - T- ·--~-;---.--' -, ---,~
G:' ·-~. L 296.25 (23.10) {73.58] ----:-·-
-295.35 (22.20) [71.96} .·
2:e.__. Signal r ·· - t:·--t·· -i:::0:::: 296.05 (22.90) [73.22}
295.95 (22.80) [73.04]
~ I (23.00J [73.401 1 - • LBac-kg-~io-un-d-::I
~'
6 8 10 lZ 1 -rr-··.-·-~~;1;-:~~-~-··-·-±-~·,~:,.~'·~·~~',.: ..r v'~.;~~·t..;...-=.-'-1--~:1
.3
295.15 (22.00) (71.60) 1\ l::-~ ;~~:~~ ~~;:~~~ ~;~::~~
~ 294.95 (21.80) {71.24) r"'·2£ 295.65 (22.50) {n.so1
~ 294.75 (21.60) [70.88] ·~

E 294.55 (21.40) (70.52] E 295.55 (22.40) [72.32} - -
0 24 ~ 0 2 4 6 8 10 12
{"

Time (s) Time (s)

TABLE 1. Thermal behavior of leaks (see Fig. 1). Conversion factor: 1 K = 1 oc = 1.8 oF.

Nitrogen Argon Helium Carbon Dioxide

Source Background ---· Source Background Source Background
l>K l>K l>K l>K 6K M
Source Background
l>K l>K

Peak 0.413 0.089 0.409 0.074 -0.822 -0.024 0.556 0.049
End -0.148 0.069 -0.207 0.020 -0.034 0.158

492 Infrared and Thermal Testing

enough then the tubes in the suspected of the geometry and loCation of the
region are cut open and examined with a discontinuities, eddy current and visual
borescope. Because the borescopc can
only cover a finite tube length and 1
because it is impossible to identify which
tube (to the right or left of the identified tests are not possible.
interstitial} is leaking, many superfluous Thermographic techniques, on the
tube cuts and repairs must be made to fix
just one leak. Also, the size of the other hand, are we11 suited to locate
discontinuity creating the leak may make interstitial leaks by measuring, from the
it impossible to see the source point. In hot wall or cold wall side of the nozzle,
certain instances when the interstitials are the localized cooling created !Jy an
interlinked by poor braze bonding, many escaping gas at the loe<ttion of an
interstitials will show indications of interstitial discontinuity. Hot wall tests are
leakage although there is a single source. preferred because 100 percent of the
interstices can be inspected. Cold wall
The small size of the tube wall cracks tests are restricted to the open bays
causing these leaks and the large size of between the hat bands and away from the
the nozzle inhibit the ability to locate liquid hydrogen feed lines. \.Yhen the test
them using X-radiography, ultrasonic is performed from the inside hot wall of
testing or computed tomography. Because the nozzle a personnel lift supporting a
pan/tilt apparatus placed on a tripod is
FIGURE 2. Space shuttle main engine nozzle. used (Fig. 3). This technique keeps
personnel outside the confined space of
l~..,.. I the nozzle during pressurization,
eliminating the risk of suffocation.
'"~~"""'""
In general, the as-fired finish on the
Hot wall side Coolant tubes inside of the nozzle has a sufficiently high
and uniform emissivity for the thermal
tests. In regions that have prior repairs, a
nearly mirror finish greatly inhibits the
thermal test. To overcome this problem, a
water soluble flat black paint or chalk
powder (such as liquid penetrant
developer) must be applied. The desired
result is a surface emissivity of about 0.85.

tvlany experiments were performed to
assess the ability to identify the location
and severity of a interstitial leaking from
the hot wall side of the nozzle.
Preliminary experiments have indicated,
as expected, that the peak pressure
achieved durjng testing influences the
cletectability of a given discontinuity. This
influence is characterized as a near linear
increase in discontinuity thermal contrast
with increased peak pressure. The
discontinuity thermal contrast is
determined by computing the
temperature differenti,:ll between a
discontinuity region and an acreage
region. The results arc shown in I:ig. 4 for

FIGURE 3. Thermal imager inside nozzle.

TABlE 2. Flow rate of detected leakage.

leak Flow Rate Description
Class
~---------~--~

std cm3·s-1 (std in.3·mln-1)

>0.3 >2 foaming

2 0.3 to 0.5 2 to 50 bubbling

3 14 50to100 strong bubbling that
bursts and reforms

4 >27 >100 blowing (bubbles that
do not persist)

Aerospace Applications of Infrared and Thermal Testing 493

typical class 2 and class 3 interstitial leaks could produce a false indication of a leak.
in the lower (•nd of the nozzle. Calculations show that the largest flow
rate expected for a nozzle leak check is
The thermograms in Fig. 5 clearly below 12.2 m·s- 1 (2400 ft·minutes- 1). Two
demonstrate the need for higher gas weld repair restrictions were fabricated by

pressures to be able to uniquely identify opening the tubing, applying braze to the
the leak source. Jn the figure, the class 3 bottom of the hole and then welding the
leak appears as a large cool (dark) region
just above the upper repair patch. The tubes closed. Nitrogen gas was flowed
class 2 leak is not visible at 172 kPa (25 through the tubes and its velocity
lbrin.-2 gage) yet is clearly visible above
measured with a hot wire anemometer at
the lmver repair patch at 310 kPa (45 a distance of 3.18 mm (0.125 in.) from
lbrin.-2 gage).
the tube opening. The tests demonstrated
To investigate the effect of gas type on that the flow rate threshold for thermal

the ability to resolve the discontinuity detection of a 50 percent area restriction
size along the length of the nozzle, a is above 12.2 m·s-1 (2400 ft·minutes- 1).
series of tests were performed on a panel That is, below 12.2 m-s~1 (2400
representative of the forward (small tube)
end of the nozzle and a panel ft-minutes-1) the signature of the
representative of the aft (larger tube) end.
restriction is too small for repeatable
The results (Fig. 6) indicate that in the
forward end of the nozzle, helium gas detection. Based on these findings it is

works the best for identifying leaks. highly unlikely that a restriction alone
However, in the aft end of the nozzle, would create a false positive signature
when helium is used a small class 2 leak
cannot be resolved. Instead the best during a thermal test of the nozzle.
Flow rate has also been found to affect
results are attained when nitrogen or
argon gas was used. how much of a thermal signature is
produced for each gas. In general a
Tests were also conducted to determine
if the blockage associated with a repair pressure transient requiring less than 8 s
to reach 265 kPa {40 lbrin.-2 gage), that is,
35 kPa (,) lbf'in.-2 gage), produces the best

FIGURE 4. Thermal response as function of pressure. fiGuRE 6. Forward end of nozzle.

0.12 (0.22) ~----------_.. i 1.0 (1.80)
0.8 (1.44)
30.1 o 3

0.08 ~
(0.18) 11---------~---=c:-;1-;.,-:,-~-:-:--:-_--_-~___.._d'
(0.14) ~G:'
2E'p-- 0.6 (1.08)
1 2o.o6 (O.ll) 1---.--/-~"""=---c--,-,---__.--~"'
~II 0.4 (0.72)
0.04 (0.07) ,j,...C::_ _ _____.o--___'=~"""'==--
""'~ 0.2 (0.36)
0.02 (0.04) ~~--~,c-:::__ _ _ __
w
172 241 310 Nitrogen Argon Helium
(45) "' 0
(25) (35)
Pressure, kPa (lbrin.-2) 0

legend

R=Ciass3
0"'-Class2

FIGURE 5. Thermograms at three pressure levels: (a) 172 kPa (25 Jb,-in.-2 gage); (b) 241 kPa
(35Jb,.in.-2 gage); (c) 310 kPa (45Jb1-in.-2 gage).
(a) (b) (c)

494 Infrared and Thermal Testing

thermal gradient. The exception to the from the cold wall side. Here, just as with
rule is found for carbon dioxide, which the hot wall tests the nozzle is pressurized
was unaffected by pressurization rate and to 276 kPa {40 lbrin.-2 gage) in less than
testing with carbon dioxide could be
performed with ramp times in excess of 10 s. As seen in Fig. 7, a better thermal
40 sat 7 kPa·s· 1 (I Ib,.in.-2 gage per signature is given ·when the cold wall side

second). The only factor limiting the is inspected. The primary reason for this is
ramp time for carbon dioxide is the that the cold wall (metal jacket) side is
in-plane conductivity of the nozzle itself.
much smoother than the hot wall side
An investigation into the ability to test (exposed tubing) side and as such gives
tlle interstitials from the outer jacket side
of the nozzle was also performed. \".'hen less background thermal aberrations. The
accessible the jacket of the nozzle primary problems with performing all
provides an excel1ent surface to view the
effect of an interstitial leak. Instead of the cold wall side tests are the inability to
thermal effects having to flow around the
tubing to the hot waH side, by viewing easily transfer indication locations to an
the jacket a more direct thermal path is individual tube and the preseilce of the
attained.
hat bands, a structural reinforcement, and
Tests performed on a forward and aft miscellaneous tubing that block some
section of the nozzle clearly demonstrate
the ability of thermography to locate leaks inspection sites.
In summary it was determined that for
FIGURE 7. Cold wall inspection: (a) aft end of
nozzle; (b) forward end of nozzle. Conversion a test to be successful the operating
pressure must be greater than 276 kPa
factor (M): 1.0 K ~ 1.0 oc ~ 1.8 °F. (40 lbrin.-2 gage) and the pressure rate in
excess of SO kPa (8 lbrin.·2 gage) per
(a)
second. Carbon dioxide gas was found to
I!T = 0.06 K = 0.06 "C (0.11 ~F) give the best thermal indications of a

>- ,,,,.,- ·::..:t- ~, ~. ·,,_,.:;,;:! > """'~ class 2 or greater leak. A gas temperature
slightly beJo·w ambient was required to get
"-·- the desired thermal gradient and an
ambient temperature of 293 to 304 K

(20 oc to 31 °C; 68 op to 88 °F) was

required. It was determined that the

spatial resolution must permit
identification of individual tubes and the

infrared imager needs to have a thermal
resolution of at least ±0.025 K

(0.025 oc ~ 0.045 °F) per level. Finally, a

surface emissivity of 0.8 or greater was
required to allow detection of the thermal
gradient produced by the leak.

Composite Aerospace
Structures

After many years serving only structurally
noncritical ·applications in launch
vehicles, composite materials are now in
primary structures such as spars,
aeroshells and fuel tanks. Composites
(b) offer a unique set of characteristics that
make the detection of structural and
material discontinuities very important.
By their very nature, composites are used
in highly efficient, highly stressed
situationS. It is often very difficult to
determine exactly what influence a
discontinuity will have on structuml
performance. The most prominent types
of discontinuities seen in aerospace
composites include porosity,
delamination, disbond, fiber breakage and
foreign material inclusion.

Excess porosity reduces the compressive
load carrying capability of most
composite systems by leaving fibers
unsupported. Jt can also serve as a
location for moisture to collect, which
can degrade the mechanical properties of

Aerospace Applications of Infrared and Thermal Testing 495

somt: resins or lead to freeze or thaw the material thickness, it may he
related problems. Disbands are of necessary to test the inner and outt:r
particular concern in regions precured bands of overlaid material separately. In
and tl1en bonded together. An improperly the case of a closed structure, .'IUch as a
designed joint or one with surface fuel tank, access to the interior band must
contamination can lead to a disband. be provided for personnel and equipment
Delamination, a phenomenon similar to a (Fig. 8). Sufficient access ports must he
disband, occurs when the plies of a included in the structural design.
composite separate. Typically,
delaminations are the result of low Because disband deLectability by
strength in the resin, contamination thermography decreases with depth,
between plies, impact loading or localized discontinuity standards may be required
imbalances in the stresses between plies. to demonstrate detection capability for
Inclusions, foreign matter trapped within a design critical disband sizes through the
composite, can reduce strength by kinking entire thickness of the overlaid material
the fibers around the inserted material or with the infrared thermographic
creating a disband between the plies. Fiber technique to be used. Figure 9 is a
breakasc as a result of an impact or stress thermogram or thermographic image of
concentration is normally considered the such a discontinuity standard having four
worst form of damage because the load overlaid layers, stepped up from the
carrying capability of the structure is substrate material at the left and right
directly affected. edges to a maximum thickness in the
center. Simulated disbands of various sizes
Postcure Bondline Thermal are included between the substrate and
Testing
FIGURE 9. Splice joint discontinuity reference
Large composite structures may he standard thermographic result:
fabricated in smaller pieces that arc then (a) thermogram; (b) diagram.
bonded together. This manufacturing
technique provides the capability for a (a)
larger final structure than that which the
capacity of curing facilities (ovens, (b)
autoclaves and others) \\'Ould othenvise
allow. For example, a large composite fuel 14.0 rnrn (0.55 in.)
tank may be fabricated similar to a pill
capsule by bonding two bullet shaped - 1 1 . 0 mm (0.43 in.)~
halves together. Curing facilities thus
need only accommodate one half of the -+-------9.0 mm (0.35 i n . ) -
tank at a time. A butt joint where the two
halves meet may be overlaid with ~~-7 0 mm (0.28 ;n.)+l
additional material on the inner and !J
outer surfaces, stepping up in thickness to
facilitate load transfer across the joint -
without excessive stress concentration. (50..200mirnn.)~

Infrared thermography may be used in
such structures to detect many types of
discontinuities, including disbanded
regions in tlw overlaid joints. Because of

FIGURE 8. Thermographic evaluation of inner
splice joint on inside of large composite
tank.

0 00 0 ) l

00 00 E
E
00 0 0
0
00 00
"
00 0 0

00 00

496 Infrared and Thermal Testing

the first (deepest) overlaid layer (Fig. 9). Thermal Testing of Impact
Flash heating was used for a one~sidcd test Damage
in this example. The thermogram
demonstrates the detectability of all Impact from dropped tools, mishandling
programmed discontinuity sizes at all or flying debris can produce critical levels
material depths. For image interpretation, of damage in composite structures with
the inspector also wants to know {1) the little or no visible surface indications.
material and its diffusivity, {2) the Although the impact may not even visibly
thickness of each layer and possibly scratch the surface of the structure,
(3) the recording time after each pulse. subsurface damage such as delamination,
matrix cracking and even fiber breakage
In-Situ or Wet Bond line Thermal may be present. Low velocity, low to
Testing2 medium energy blunt impacts are
normally the cause of this kind of
It is often advantageous to identify the damage} High velocity, high energy
presence of voids in a wet bond line impacts tend to produce a noticeable
before final cure. At that stage, the joint crater at the impact site and spalling on
may be worked mechanically to vent the back side. Low energy impacts are
trapped air. \·Vhere that approach fails, the simply absorbed by the composite and
joint may be disassembled and actions their energy dissipated with little or no
may he taken to eliminate any geometric influence on the integrity of the structure.
mismatch. In certain applications it is
possible to identify the presence of voids In a study conducted on 6.35 mm
in a wet bond line with infrared (0.25 in.) thick graphite phenolic samples
thermography before the two components cut from a section of a test nose cone for
are finally cured together.
FIGURE 10. Bondline thermograms: (a) before
The thennograms in Fig. 10 illustrate working; (b) after working.
the former procedure in the case of a (a)
composite cap as initially wet bonded and
after being worked to remove entrapped ==-=I~ mr--c·:rJ'C'+1ncw~
air. The caps were flash heated and viewed
during cool down. The images were taken (b)
through the vacuum bag and a release ply.
The voids appear at higher temperatures
than the surrounding region,
demonstrating the lower heat conduction
of the void.

Note in the images (l:ig. 10) the
presence of a fibrous bleeder material. The
purpose of the bleeder is to let air flow
out from under the vacuum bag as the
bag is puHed down against the cap.
\'\7ithout the bleeder material the vacuum
bag would form a partial seal around the
edges of the cap and restrict the vacuum
level in the center of the cap. The regions
·where the bleeder is present and where
the vacuum bag is held off of the cap
cannot be inspected thermographically
because of the lack of a heat path for
absorbing the heat of the thermal flash.
The bleeder cloth is an insulating layer. In
other words, any region where the
vacuum bag does not make direct contact
with the cap will not be able to be
inspected because there exists no direct
path for heat to travel to the surface of
the cap.

It should also be noted that certain
bagging materials behave differently
thermographically and should be tested
before a bond is attempted. Because of the
inherent emissivity of some materials and
their natural insulating characteristics
certain materials commonly used for
composite manufacture may inhibit the
thermographic testing.

Aerospace Applications of Infrared and Thermal Testing 497

the space shuttle external tank the ability in-plane conductivity of the composite
to thermally image impact damage was material dominates the heat transfer in
demonstrated. In this study1 impacts at the structure blending away their details
various energy levels and with three slzf'~ that were seen when the part was flash
of impact tips were used to determine heated. Because of the in-plane heat loss,
whether thermography could measure the identification of the true discontinuity
extent of damage in a flight structure. A boundary is very difficult. (Misleading
75 mm (3 in.) square test piece, cut from a cool zones can be caused by in-plane
section of the nose cone that was conduction also with the one-sided
trimmed off during production, was approach.)
struck by a 2.27 kg (5 Ibm) impactor in a
single bluw. Images were taken using both Detection of Porosity4
flash and the more traditional long
duration heat lamp thermography. The One part of the qualification of an
sizes of the discontinuities in the resulting aerospace structure for service invOlves
thermograms were then measured
(Table 3). The maximum and minimum FIGURE 11. Typical thermograms of high energy impact
discontinuity dimensions, as measured damage: (a) flash, impact side; (b) flash, back side;
from the impact side using flash heating, (c) through-heat. Conversion factor: 1.00 in. = 25.4 mm.
are given in the max and minutes columns (a) (b) (c)
respectively. The discontinuity
dimensions measured from flash heating FIGURE 12. Typical thermograms of low energy impact
the back (opposite to impact) side of damage: (a) flash, impact side; (b) flash, back side;
coupon are given in the column labeled (c) through-heat. Conversion factor: 1.00 in.= 25.4 rnm.
back. As a point of reference these results (a) (b) (c)
were compared to those obtained with the
more traditional long duration heating
thermography. Here1 a 1000 \-\1 infrared
lamp was used to heat the rear surface of
the specimen while images ·were collected
from the front (impact) side during the

cool down.
The damage appears to follow the fiber

direction though the thickness of the
laminate (Figs. 11 and 12). From the front
surface the large delamination zones can
be seen to spiral a1;vay from the impact
site. \Vhen the side opposite the impact is
viewed a circular area of damage is seen.
These images illustrate the conical damage
region extending away from the impact
site. The damage is seen to be hotter
(whiter) than the surrounding material as
a result of heat being trapped on the front
surface by the subsurface delamination.

The same damage zones show up as
cool zones when heated from the back
side and vie·wed from the impact side. The

TABLE 3. Impact damage area measurements.

Impact Energy - -T-up-Si-ze- - Maximum Minimum Back lamp
mm (in.) mm (in.) mm (ln.) mm (in.) mm (in.)
Part N·m (ft·lb,)

5.95 (4.39) 6.35 (0.25) 15.24 (0.60) 15.24 (0.60) 23.37 (0.92) 26.67 (1.05)
2 17.06 (12.58) 6.35 (0.25) 66.55 (2.62) 37.59 (1.48) 54.10 (2.13) 47.50 (1.87)
3 13.78 (10.16) 6.35 (0.25) 40.64 (1.60) 27.43 (1.08) 42.16 (1.66) 36.83 (1.45)
4 4.53 (3.34) 1.59 (0.0625) 11.68 (0.46) 11.68 (0.46) 21.59 (0.85) 15.24 (0.60)
5 11.88 (8.76) 1.59 (0.0625) 23.11 (0.91) 23.11 (0.91) 35.56 (1.40) 22.61 (0.89)
6 7.03 (5.18) 1.59 (0.0625) 17.53 (0.69) 15.24 (0.60) 25.65 (1.01) 20.07 (0.79)
7 8.87 (6.54) 6.35 (0.25) 27.18 (1.07) 23.11 (0.91) 35.05 (1.38) 24.38 (0.96)
8 12.51 (9.22) 12.70 (0.50) 36.83 (1.45) 29.46 (1.16) 46.74 (1.84) 30.99 (1.22)
9 25.47 (18.78) 6.35 (0.25) 76.20 (3.00') 47.75 (1.88) 76.20 (3.00') 76.20 (3.00')

a. Damage extended to the edge of the sample.

498 Infrared and Thermal Testing

verifying that no discontinuities were As previously discussed, one of the
created during the manufacturing process. principle advantages of infrared
During manufacture it is possible that thermography over conventional
regions \'·lithin the laminate may contaih ultrasonic testing is that with
tiny entrapped air bubbles or porosity. thermography large regions are viewed in
The porosity can come from many each test operation as opposed to the
sources: degassing of contaminates (such point-by-point coverage of ultrasonic
as oils and silicones); improper debulking, testing. Also, no direct contact with the
leaving air trapped between the plies; or structure is required with thermography,
poor ventilation restricting the removal of such as the water coupling used with
any degassing of the panel, to name a few. ultrasonic testing, therefore little
No matter what the source, porosity can disassembly of the structure is required.
have a detrimental effect on the One of the main disadvantages of
performance of the structure by leaving thermography is that for a discontinuity
regions of unsupported fibers and points to be detected it must interfere with an
of stress concentration. If the regions of externally applied heat pulse in such a
porosity are large enough they may begin way as to develop a change in the normal
to link to each other under stress and surface temperature profile. A very tight
drastically weaken the load carrying discontinuity or one that is thermally
capability of the fibers, especially when similar to the surrounding material has
put under compressive forces. The little chance of being detected. Moreover,
porosity can also fill with water over time detection with the one-sided approach is
and if subjected to freeze/thaw conditions difficult ·when the discontinuity is deeper
may begin to delaminate the composite than half the total thickness.
laminate.
One potential application for
The damage tolerance models and thermography has been the test of the
geometry of the component under test instrumentation rack shell for the
dictate the size of discontinuity that the International Space Station. The racks
evaluation technique can identify. under investigation are constructed from
Typically, discontinuities as small as an autoclaved graphite epoxy ·with 18-ply
161 mm2 (0.25 in. 2) in area; over large acreage regions building up to 22 plies in
acreage regions, tight compound radii and the corners. A protective layer of fiberglass
thickness transition zones must be provides the finish layer and serves as a
demonstrated to be detectable for a indicator of inservice damage. The
evaluation technique to be considered. fiberglass shows as white against the black
graphite background ·when its surface is
A part of the certification of a given struck hard enough to produce significant
technique for inspecting a structure internal damage. The discontinuities of
involves demonstrating that it can find interest include voids, inclusions,
discontinuities of a given type and delaminations and porosity with a critical
geometry at a given depth into a area threshold of 161 nun2 (0.25 in.2)
laminate. The testing process is typically
validated by evaluating discontinuity Two tests were performed to determine
standard reference panels built to the the limits of thermographic
same specifications as the component nondestructive testing for detecting
under test, except for the insertion of porosity. First, a section of a rack with
artificially fabricated discontinuities. The known porosity documented through an
artificial discontinuities are designed to ultrasonic map was thermographically
closely match those most prevalent in the examined. Second, a 24 ply monolithic
actual structure. test panel was created with simulated
porosity of known size and depth into the
The conventional method for laminate. The panel was constructed from
inspecting monolithic composite shells is bidirectional graphite epoxy (22 plies) and
to perform a series of ultrasonic tests after E-glass {two cover plies) using a
fabrication. Ultrasonic testing {UT) of symmetric quasi isotropic layup schedule
100 percent of the structure typically can similar to that of the actual racks. The
locate all critically sized discontinuities. simulated discontinuities were sized
The disadvantage of ultrasonic testing is around the critical area by using 6.4, 12.7,
that it is time consuming and requires 19.1 mm (0.25, 0.50, 0.75 in.) square
submersion of the stmcture in a water regions yielding areas of 0.40, 1.61 and
tank during the scanning operation unless 3.63 cm2 (0.0625, 0.25 and 0.56 in. 2). The
laser ultrasonic testing is used. Therefore, pattern of planned discontinuities is
the method is not suited for postflight or shown in Fig. 13.
inservice inspections ~ that is, a flight
ready piece of hardware. Infrared The light gray regions in Fig. 13 depict
thermographic techniques on the other where the simulated porosity microballum
hand have proven to be well suited for (7.0 nm diameter h1sed silica Si02
the inspection of monolithic shell spheres) were placed in the panel. The
structures in situ. dark gray regions in Fig. 13 are folded
plastic backing material, 25 pm (0.001 in.)

Aerospace Applications of Infrared and Thermal Testing 499

thick polypropylene, used to simulate ultrasonic plot can be seen on the
inclusions and disband areas. \·Vedge thermography image.
shaped stainless steel shims, 150 Jllll
(0.006 in.) thick, were also inserted into Using the discontinuity standard panel
the laminate to create void regions when previously described tests ·were corh!uctcd
they were removed after cure and are to determine the depth detection limits
visible at the bottom of the panel. The on the thermal testing system. The
discontinuities were placed in five resulting thermograms indicate that the
columns so that the depth of limits of the system are somewhere
inspectability could he investigated. The between the second and third column of
discontinuities ·were placed between plies discontinuities, that is, between the sixth
2-3, S-6, 11-12, 16-17 and 21-22. and eleventh ply (out of 22 plies). Figure
15 shows the discontinuities from the first
Porosity Tests column of the panel.

A section of a rack panel with known The primary drivers behind a successful
porosity was inspected thermographically thermal test lie in the ability to get heat
and compared against ultrasonic test
results. Hgure 14 shows that the FIGURE 14. Comparison of porosity detected:
thermogram closely matches the pattern (a) ultrasonic image; (b) thermogram.
of porosity in the ultrasonic image. In
Fig. 14 the porosity shows up as a dark (a) (b)
patch for the ultrasonic image and as a
white (hotter} zone on the thermogram.
The single thermogram does not show all
the details of the ultrasonic image because
the porosity actually resides throughout
the thickness. The ultrasonic image
measures the complete thickness of the
panel whereas the thermogram shows
only the effects from a single slice of the
cross section. When several thennograms
are viewed over time, by rastering through
the scan history, most of the details in the

fiGURE 13. Fabricated test panel.

38 mm 75 mm 75 mnl 75 mm 69.9 mm 69.9 mm

(1.5 in.) (3.0 ln.) (3.0 in.) (3.0 in.) (2.75 in.) (2.75 in.)

1---1--1---------------1 1~1

1 2 3 4 5 ] 38 mm (1.5 in.)

II

• J 50 mm (2.0 ;n.)

"" • • "" ""~ ~ ~
Jso mm (2.0 ;n.)
J 50 mm (2.0 ln.)
fJ J_j _j f-.-1 f-.-1
48 mm (1.9 in.)

153cJ cJ cJ f-J f-J mm (2.1 ;n.)

163 mm (2.5 ;n.)

}so mm (2.0 ;n.)

1--1-------------+11~
50 mm 190 mm 63 mm
(2.0 in.) (7.5 in.) (2.5 ln)

500 Infrared and Thermal Testing

into a structure uniformly and with porosity and inclusions in this particular
sufficient intensity to create a temporary graphite epoxy part. Porosity close to the
thermal imbalance around an anomaly as surface was found to also he a good
well as to resolve those temperature candidate for locadou with thermography.
variations. Although this graphite epoxy '"'hen the porosity reached depths of six
structure· appeared dark and was thus or more plies it is difficult to say with
expected to be a good absorber of light confidence that it could he detected
and thermal energy, the surface was glossy without some surface preparation of tht'
as manufactured and highly reflective. racks. The application of a flat black,
Because of the high reflectivity1 even with water wasl1able, paint made the porosity
a large heat pulse most of the energy is at the midplane of the panel detectable.
reflected away from the panel. Possible Finally, embedded inclusions and voids
solutions to this problem include spraying were detectable down to about six plies
the surface with a flat black water soluble into the laminate and deeper with the
paint, using peal ply during manufacture addition of a dulling agent to the panel's
to dull the surface or increasing the surface.
number of flash heat lamps (input heat
energy). Of these solutions, increasing the FIGURE 16. Midply porosity viewed when test
number of heat lamps would have the panel is painted flat black.
least effect on production time but would
require doubling the support hardware for
the thermographic testing. Dulling the
surface during manufacture with peal ply
would not be a likely choice because it
would mean reevaluating the structural
performance of the rack. The application
of a water washable paint appears to be
the best choice for increasing the surface
conductivity of the panel.

As shown in Hg. 16, it is possible to
thermally penetrate to the midply of the
laminate by spraying the surface with a
water washable flat black paint. The
160 mm2 {0.25 in.2) square region of
porosity at the edge of detectability but
can be seen when the thermograms are
viewed in a series over time. Although
this technique would require that the
panel be inspected from both sides and
the rack be spayed with paint and cleaned
up after the inspection, it does
demonstrate the potential to test the racks
with thermography.

Infrared thermography has been
demonstrated to be capable of detecting

FIGURE 15. Results from test panel.

Aerospace Applications of Infrared and Thermal Testing SOl

PART 2. Applications to Metal Aerospace
Structures

:~v1any applications exist in the aerospace discontinuity is on the heated side of the
industry where structures are built that panel, the heat dissipates before it reaches
feature metal skins and core bonded the core and is not conducted to the
together with a resin or braze. These types viewing side of the panel, producing a
of structures present a particularly cold region. Because the heat travels faster
problematic situation in that the skins through the metal core than it does
have a high in-plane thermal conductivity through the air, differentiating the
that can distort the thermal image of the location of the source (discontinuity) is
discontinuity below it. Another problem difficult.
is that the face sheets often have a
specular, mirrorlike, or highly reflective, Metal Honeycomb Core
surface that greatly inhibits the flow of Skin Assemblies
radiant heat into the structure as well as
obscuring portions of the image with the A containment vessel featuring stainless
camera's reflection. steel skins over an aluminum core was
inspected thermographically for
The effects of the high in-plane delaminations. The outer and inner
conductivity can be partially overcome by surface of the vessel was painted with
using flash heating- that is, by applying three light coats of a water
a brief heat pulse and by recording the soluble/washable flat black paint. The
temperature field as soon as the heat front inside of the vessel was inspected by using
has reached the tested interface. Flash a thermography periscope (Fig. 17). In
heating generaHy produces the dearest this configuration the vessel was heated
images with the sharpest contrast \Vhen from the outside using a hand held
the discontinuities are viewed soon after 1000 \V infrared heat lamp. A grid pattern
the flash. Hmvever, the camera may still was selected that enabled the vessel to be
be blinded by the flash and, by the time it scanned in three axial rings of nine radial
recovers, the transient related to the segments. A portable electric fan was used
location of the source (discontinuity) may to cool the vessel back to ambient
be lost. temperatures between each test.

The problem with the highly reflective The outside of the vessel \\'as scannt'd
surface finish of most metals can be often without the periscope while the inside
be overcome by the application of a was heated with the 1000 \V lamp. A
emissivity enhancing paint. For example, typical thermogram from the structure
a water washable flat black paint may shows a region with low bond integrity
often be applied to increase surface (Fig. 18). The delamination appears cooler
emissivity. Two to three light coats of (darker) than the surrounding good
paint will often provide enough dulling to material and the core is not visible in the
block the reflection of the camera and discontinuity region.
increase the surface emissivity to a usable
level. To validate the test process, test panels
were built with programmed
Discontinuities in these types of
structures are usually of the form of FIGURE 17. Inspection using infrared
missing cobond tape or braze integrity, periscope.
which tends to lower the ability of the
core to transmit heat to the opposite side
face sheet, producing a hot (white) region
in the image. These types of
discontinuities may not be detectable to
flash heating if they are on the side
opposite the flash. In these situations,
through~thickness heating with a hand
held lamp may permit discontinuities
anywhere between the two faet·sheets to
be detected. \·Vhen the discontinuity is
nearest the camera side of the panel, the
missing cobond tape limits the flow of
heat to the viewing surface, producing a
cold (dark) region. \·Vhen the

502 Infrared and Thermal Testing

delaminations formed by removing by the flame resulting in a catastrophic
sections of the cobond adhesive. The failure. It is thus necessary that the braze
planned discontinuitles could be sef:'n line integrity be verified.
from either side of the panel and
differentiation between which side was Traditional methods1 such as
closest to the discontinuity was difficult if radiogwphy, often can detect the presence
not impossible from the images and time or absence of braze material along the
temperature plot histories. bond line but it is often difficult to
distinguish regions in which the braze
Flash heating produced the clearest material is present but disbanded. Flash
images of the scheduled discontinuities heating infrared thermography has been
but missed several of the edge indic.:ttions. demol1strated to he capable of locating
The primary reason that the flash system and characterizing brazeline disbands
missed some of the edge discontinuities between the space shuttle main engine
·was that the discontinuities were on the nozzle jacket and cooling tubes over most
edge of the field of view (which blocked of the nozzle acreage.
part of the heat input) and were covered
by tape (which restricted the flow of heat As shown in Fig. 19, the test process
into the part). The through-transmission consists of placing the camera and flash
heating did not have these problems unit in contact with the outer cold wall
because the whole back side of the panel side of the nozzle and recording the cool
(side opposite the camera) was heated down of the nozzle after the flash. The
uniformly and the amount of heat was braze line shows up as dark (cold) Jines in
much greater than with the flash system. the image and heat is pulled away from
the structural jacket. The interstitial voids
Brazeline Tests between the tubes appear hot (\Vhite) as
does any braze void.
As previously discussed, the space shuttle
main engine nozzles are constructed of Regions of the nozzle jacket obscured
many cooling tubes brazed to an outer by plumbing, stiffener dngs and other
structural jacket. Braze line discontinuities obstructions are not inspectab1e1 for they
may be introduced by manufacturing
processes (that is, omission of braze fiGURE 19. Detection of braze line disbonds:
material) or service loads (that is, fatigue). (a) camera unit in contact with outer cold
Missing braze material and fracture wall of nozzle; (b) interstitial voids between
disbanded braze regions are among the tubes appear white (hot) as does braze
most critical braze line discontinuities. void.

Discontinuities in the braze line (a)
between the tubes and jacket locally
weaken the nozzle rigidity and increase
susceptibility to buckling. ;\ buckle in the
cooling tubes may cause them to protrude
into the flame flow along the inner nozzle
surface. Exposed to excessive heat, the
liquid hydrogen carrying tubes may be cut

FIGURE 18. Disbands between core and face
sheet of test structure.

(b)

Aerospace Applications of Infrared and Thermal Testing 503

cannot he viewed directly by the thermal Assembly Process For
camera. Other heating techniques, such as Delamination Simulation
steady lamp heating or hot air guns, ;ue
less capable in this aiJplication because of A standard technique for creating
the high conductivity of the jacket delamination discontinuities in laminated
material, thickness of the jacket and the composite structures is to place a material
large mass of the nozzle that acts as a heat between the plies to prevent bonding
sink. when the panel is cured. Fluorocarbon
resin tape, backing plastic and bagging
Fabrication of Reference film are the most common materials used
Discontinuities for when making simulated disbands.
Calibration Typically, the laminate is assembled from
the discontinuity side up so that the
The results of thermographic tests are inserts would be trapped under as many
often very subjective and strongly affected plies as possible during cure. In this
by surface conditions, cross sectional manner the signal component from the
properties and even to some extent the additional thickness of the inserts would
experience of the test conductor.· As seen be minimized; that is, the plies above the
in previous discussions of specific inserts would each compress slightly so as
applications, test panels or standard to take up the additional thickness of the
discontinuities can be referred to during inserts. Although this technique is easy to
calibration and testing at the same time as accomplish, questions arise as the validity
the test objects to ensure that the test will of the thermal indication - that is,
find all critically sized discontinuities. A whether the thermal signature resulting
properly designed calibration reference from an insert indicates the presence of
standard will enable the user to gain the disband or the thermal difference
created by the insert material itself. An
confidence that once an the scheduled alternate approach is to create thin voids
in the panel during the curing process.
discontinuities are found in the
calibration component, then any One such alternate technique for
potential discontinuity will be found in creating delamination and disband
the actual structure under examination. discontinuities is through forming an
This therefore requires that not only docs indentation in a precured section of the
the calibration article with its built in panel by means of a thin shim. The
discontinuities be thermally similar to assembly process for making voids
that which can be expected in the actual through shimming techniques requires
structure but that also the discontinuities that several subassemblies be fabricated
behave thermally as real discontinuities and finally joined to form the resulting
would. panel. To keep residual stresseS to a
tolerable level and to prevent each
Calibration panels containing subassembly from warping, care was taken
programmed discontinuities are fabricated to try to maintain symmetric and
to determine the limits of an infrared balanced laminates. Also, because the
thermographic nondestructive testing subassembly with the shimmed
process. Artificial discontinuities indentation must be compressed by a
resembling delaminations and porosity small amount such as 75 pm (0.003 in.) at
are fabricated for the purpose of testing the points where voids are to be created
the ability to locate and size real sufficient material must be present to
discontinuity by using standard absorb the shim's thickness. The
thermographic techniques. depressions formed by trapping the shim
stock between the prepreg and tooling
Construction Techniques plate during the precure process would
then become voids in the finished panel.
The test panels should always be No peal ply is typically placed under the
fabricated from materials that will yield faceplate with the shim stock and a
similar thermal properties to those tooling plate cover to ensure the flatness
featured by the tested article. \'\'henever of the indented panel. Final assembly of
possible material from the same batch and the panel consists of placing the
lot should be used to ensure consistent appropriate uncured ply between each
mechanical and thermal properties. The subassembly, rebagging and postcuring
panel should also be cured as closely to the entire laminate.
the test article as possible. Through
thickness and in-plane conductivities, A very thin shim, less than 75 pm
surface emissivities and textun: all play a (0.003 in.), minimizes the effect on local
vital role in establishing the limits for a density. ln fact the amount of disband
given test. can be tailored from nearly touching to
loose disbonds.

Another technique that has shown
promise in creating tight disbands is to

504 Infrared and Thermal Testing

place a high temperature wax between the FIGURE 20. Delamination in panel:
plies durlng assembly. The problem with (a) thermogram; (b) defect map.
this method, however, that it is difficult
to control the shape of the disbond. (a)

Assembly Process for Porosity
Simulation

The same general procedure used to make
panels with inserts can be used to produce
panels ·with simulated porosity. The only
difference between the two processes is
that instead of inserts, a thin layer of
7.0 pm (0.0003 in.) diameter glass
microballons are placed between the plies.
To control the size and thickness of the
porosity regions the microballons are
pressed into templates made from 75 pm
(0.003 in.) stock.

Test Thresholding (b) 460 mm (18 in.} =nA:j
~
As an example of these discontinuity tA A .~
manufacturing techniques a series of 8 00
graphite epoxy panels were constructed of • ••
different thicknesses featuring E
discontinuities of various sizes, " c" E
orientations and depths into the •• B0
laminates (Table 4). The thennograms
produced by flash heating the test panels cD0 i l""'
were analyzed for the ability to detect the
planned discontinuities. A typical legend
discontinuity map and thermogram from
a discontinuity standard panel is shown A= 92 mm {3.62 in.)
in Fig. 20. Each discontinuity was rated as B = 77 mm (3.03 in.)
visible, barely visible or not visible. The C = 75 mm (2.95 in.)
rating system was set so that for a
discontinuity to be visible it must be
easily detected from a standard laser print
or single image on the monitor. Barely
\'isible is defined as a discontinuity that
requires many sequenced images to be
able to be seen and may or may not
produce a good hard copy. Nut \'isible is

TABLE 4. Descriptions of tested panels,

Panel Dimension Thickness Flaw Flaw -·-~--···_ _S!~ze Rang_~
Shape Type mrn (in.)
Number mmxmm (in. x in.) Plies mm (in.)

310x310 (12x12) 10 1.6 (0.06) square delamination 38.0, 25.0, 19.0, 12.5 (1.5, 1.0, 0.75, 0.5)
2 310x310 (12x12) 10 1.6 (0.06) circle porosity 38.0, 25.0, 19.0, 12.5 (1.5, 1.0, 0.75, 0.5)
3 460 X 460 (18 X 18) 10 1.6 (0.06) circle porosity 12.5, 10.2, 7.6, 5.1 (0.5, 0.4, 0.3, 0.2)
4 460 X 460 (18x18) 10 1.6 (0.06) square delamination 12.5, 10.2, 7.6, 5.1 (0.5, 0.4, 0.3, 0.2)
5 310x310 (12 X 12) 20 3.2 (0.13) square delamination 38.0, 25.0, 19.0, 12.5 (1.5, 1.0, 0.75, 0.5)
6 310x310 (12x12) 20 3.2 (0.13) circle porosity 38,0, 25.0, 19.0, 12.5 (1.5, 1.0, 0.75, 0.5)
7 460 X 460 (18 X 18) 20 3.2 (0.13) circle porosity 12.5, 10.2, 7.6, 5.1 (0.5, 0.4, 0.3, 0.2)
8 460 X 460 (18x18) 20 3.2 (0.13) square delamination 12.5, 10.2, 7.6, 5.1 (0.5, 0.4, 0.3, 0.2)
9 310x310 (12x12) 40 6.4 (0.25) square delamination 38.0, 2.5, 19.0, 12.5 (1.5, 1.0, 0.75, 0.5)
10 310 X 310 (12x12) 40 6.4 (0.25) circle porosity 38.0, 2.5, 19.0, 12.5 (1.5, 1.0, 0.75, 0.5)
11 460 X 460 (18x18) 40 6.4 (0.25) circle porosity 12.5, 10.2, 7.6, 5.1 (0.5, 0.4, 0.3, 0.2)
12 460 X 460 (18x18) 40 6.4 (0.25) square delamination 12.5, 10.2, 7.6, 5.1 (0.5, 0.4, 0.3, 0.2)

Aerospace Applications of Infrared and Thermal Testing 505

defined as a discontinuity that shows no The results of this study provide a
contrast to the background on a benchmark for discontinuity size and
thermogram. depth that can be used for future tests of
graphite epoxy structures fabricated using
A useful image processing procedure is vacuum bagging operations. The panels
to subtract the cold image (image before provide a common set of discontinuities
flash heating). See discussion of data and conditions that permit the
processing elsewhere in this volume. detectability of the thermographic test
process to be determined.
All thermal tests were made with an
infrared thermal camera with a ±0.025 K The tests performed on fabricated
(±0.025 'C = ±0.045 'F) level thermal panels indicate that the ability to locate a
resolution. The heat source for the discontinuity of a particular size is a
thermal analysis was a high energy flash function not only of the depth into the
system capable of yielding a 16 ~ laminate but also of the entire laminate
(15.2 BlUrd energy pulse. The camera to thickness. As expected, the type of
panel distance was set at 813 mm (32 in.) discontinuity makes a difference in its
providing a 775 mm (30.5 in.) square field ability to be seen as demonstrated by tlle
of view with the 25 mm lens. lower detectability of the porosity over
that of the delaminations.
The delectability chart (Fig. 21)
demonstrates the limits of the The importance of having good
thermographic process to locate discontinuity standards to work from has
discontinuities in the delamination panels been illustrated in this work. As
constructed. From the chart the limits for demonstrated in the two detectability
detecting delaminations can be charts (Figs. 21 and 22) a good
established by a boundary defined by a discontinuity reference standard panel or
line over the range of discontinuities series of panels can quickly take the
tested: guesswork out of knowing the limit of the
test process. Each ne'i\' structural system
(1) depth (plies) 0.143 x size (mm) featuring a unique fiber, resin, cure or
even geometry should have a new set of
+ 3.33 curves developed that determine the
limits of the thermographic or other
Below a discontinuity size of 5.0 mm nondestructive tests.
(0.2 in.) it is expected that the
discontinuity detection line would move In-Plane Measurement of Material
toward a depth of zero. For the porosity Properties
class of discontinuities a similar chart was
made (Fig. 22). Here, the decision line was A technique has been investigated for
determined over the range of measurement of the through-thickness
discontinuity sizes tested: diffusivity component.s A similar
technique offers potential for
(2) depth (plies) 0.213 x size (mm) measurement of the in-plane diffusivity

+ 3.33

FIGURE 21. Detectability chart for delaminations. FIGURE 22. Detectability chart for porosity.

10 10

9 9

8 8 2:

7 1-- 7 /

~6 ~6 /
b b5
-- -
a.~ 5 "K 4
1--;: ~ v
~4
03 ' 03 /
2
2' p
'
v
0'
0
0 4 8 12 16 20 24 28 32 36 40 0 4 8 12 16 20 24 28 32 36 40
(16) (32) (47} (66) (79) (95) (llO) (126) (142) (157)
(16) (32) (47) (66) (79) (95) (110) (126) (142) (1S7)

Flaw 5ize, mm (1 Q-2 in.) Flaw size, mm (1 0-2 in.)

legend legend

(> "" visible (> "' visible
D "' barely visible 0 = barely visible
A = not visible
.4. "" not visible

506 Infrared and Thermal Testing

components.6•7 This technique again uses
a two+sided, or through+transmission,
apparatus arrangement, where the
specimen is continuously heated on one
side (the back surface) and an infrared
camera is used to image tile response of
the opposite side Of the specimen (the
front surface).

Point heating is provided by an
unexpanded laser beam at normal
incidence to the center of the back
surface. Flash heating with a corner mark~>
or a grid rnark9 could also be used.

Aerospace Applications of Infrared and Thermal Testing 507

PART 3. Pulsed Thermal Inspection of Aging
Aircraft10

Thermal imaging uses pulses of heat to Depending on the frequency, waves
interrogate subsurface features in solid penetrate to distances of the order of a
objects. These pulses propagate into the few millimeters beneath the surface of the
object as· waves of temperature variation aircraft skin. Discontinuities that occur in
(thermal waves), are reflected from such depths can be imaged over wide
subsurface discontinuities and return to areas in short times and the resulting
images have intuitively obvious
the surface as thermal wave echoes. The interpretations.

echoes can be detected by infrared video Pulsed Thermal System for
cameras coupled to appropriate electronic Composite Materials
hardware and software. The pattern of
these echoes on the surface of the object A representative pulse echo thermal wave
can be used to image subsurface corrosion imaging system consists of a pulsed heat
and disbands in aircraft skins. (See source {typically high power photographic
Figs. 23 and 24.) Thermal wave imaging flash lamps), an infrared video camera
finds application in testing of aging and image processing hardware and
aircraft. software, all of which are controlled by a
personal computer.
Thermal ·wave imaging was pioneered
in the 1960s.l 1•12 The imaging process was In the following example, the area of
theoretically described in terms of thermal the aircraft to be examined is a lap splice
waves in 1980 and excellent agreement region. Because the bare aluminum skin
was obtained between theory and of the aircraft is an extremely good
experimental results.B Thermal waves are reflector of visible light and also a poor
heavily damped propagating variations in emitter of infrared radiation, the lap splio.'
temperature, analogous in many ways to region is covered with a water soluble
other electromagnetic radiations paint.
propagating in a very good conductor. As
such, they have a relatively shallm\' skin \'\1hen the energy from the thermal
depth; nevertheless, it is clear that system is absorbed at the surface of the
thermal waves undergo all the usual wave aircraft, a thermal wave pulse is launched
phenomena, such as diffraction, into its skin. \'\1hen this pulse is reflected
interference and others, and can be used from a locally corroded or disbanded

for imaging. The technology possesses a

number of advantages for aircraft
nondestructive testing applications.

FIGURE 23. Thermogram of section of FIGURE 24. Thermogram of experimental
validation library test sample taken from boron patch on aircraft, showing both
near crown of aircraft fuselage. Image shows intentional and nonintentional defects.
firmly bonded tear straps on rear side of
sample.

508 Infrared and Thermal Testing

subsurface region, the reflected portion course of this development had been
propagates back to the surface, where it developed for surveillance applications
modifies the time dependence of the and hence were designed to be used at
surface temperature. This occurs with a relatively long distances from the tmget.
delay determined by the thermal wave In a hangar the inspector must operate at
transit time for the pulse to propagate short distances between the camera and
down to the boundary and back to the the airplane of 0.5 m (20 in.} or less,
surface. The return of the thermal wave compatible with operation from typical
signal is monitored by means of the scaffolding, under the belly of the plane
infrared video camera, which follows the or even in cramped interior spaces.
time dependent surface distribution of the
infrared emb.S:ion from the surface. The The desktop computer and 355 mm
signal from the camera is fed to a real (14 in.) monitor used in the original
time processing board in a computer. This system had to be carried on a separate cart
board carries out fast pipeline processing connected to the power supply cart, flash
of the signal under the control of lamps and camera by a number of cables.
dedicated software. This second cart had to be moved around
the aircraft in unison with the movement
Case History of System of the power supply cart, making the
Development nmvement of the total system a
multiperson operation.
In the 1990s, a series of tests provided
valuable information about human factors The problems described above were
in inspecting aircraft in a hangar encountered in the development period of
environment. It became evident that the the system, with iterative testing of the
original system, which was functionally modifications. Below are described several
acceptable for thermal wave imaging in of the modifications.
the laboratory, had a number of practical
drawbacks. A discussion of the drawbacks The second version of the system used
may be useful to persons designing a separate cart to carry the power supplies,
custom test installations or assembling to make a major reduction in the weight
systems from preexisting components. and a major improvement in ease of
manipulation of the cart that carries the
The cart with the six power supplies camera and flash lamps. In this form, it
mounted on it was too heavy for manual was possible for two people to lift the
lifting onto the stair lifts, scaffolding and flash lamp cart up onto the lowered stair
elsewhere and, because of its weight, was lift. It was necessary to add 8 m (26 ft)
clumsy .to manipulate around the aircraft. extensions to the flash lamp cables,
thereby partially offsetting the reduction
The lamps used in this original system, to the overall weight achieved by
purchased off the shelf from a reducing the number of power supplies
photographic supply vendor, also had and the removal of the power supplies
deficiencies. The primary problem was from the cart carrying the camera.
that they consisted of U shaped flash Another improvement in this second
tubes with circular reflectors whereas the version of the cart was the addition of an
area being imaged ·was rectangular. It electromech<mical actuator for raising and
proved to be nearly impossible for the lowering the shroud containing the flash
operator to aim these lamps in such a way lamps and camera, so that more precise
that the area to be imaged ·was both and convenient positioning could be
uniformly and intensely illuminated achieved. Using a cmnbination of this cart
(although post processing can overcome and the stair lift available at the center, it
this problem). The problem became more was possible to reach all of the aircraft
severe when an attempt to reduce the fuselage, from directly under the belly to
weight of the system by using a new and about 1 m (40 in.) above the windows
more sensitive infrared camera included a (with the exception of the area directly
reduction in the number of lamps and over the wings). It was also possible to
power supplies from six to four. The total image the underside of the wings with
intensity with four lamps was more than this cart. This still left inaccessible the
adequate but experiments at the crown of the fuselage, the area of the
validation center have proven that the fuselage directly above the wings and
circular lamps were not able to provide a most of the top surface of the wings.
satisfactor}' final solution to the heating
problem. Also, each of these commercially Input from aircraft manufacturers and
available lamps carried two, very heavy, airlines led to development of a system
multiconductor cables, which increased that was light and portable for
the weight of the system still further and manipulation by one person. This version
made the manipulation of the camera and combines the major elements of the
flash lamps very awkward. previous version (camera, flash lamps and
power supplies, shroud, computer) all on
Lenses commonly available for the a single, two-wheeled cart that an
various infrared cameras used during the inspector can wheel about with one hand
and that can easily be tm·en up and down

Aerospace Applications of lnfrare i and Thermal Testing 509

scaffolding stairs by one person. This
radical decrease in weight and increase in
portability was achieved by reducing the
size of the shroud and flash lamp system,
thus reducing to two the power supplies
needed, by moving from a desktOp
computer and monitor to a laptop
computer with a miniature card cage, by a
camera weighing about 1.5 kg (3 lb) and
by integrating all of these into a single
unit, mounted on a two-wheeled cart with
large pneumatic tires.

Unlike the previous tvm versions, in
which the shroud, flash l'dmps and camera
system were manipulated on an arm
attached to the cart, in this portable
system the shroud is designed for hand
held use. It can be separated from the cart
and carried onto the crown of the plane,
the top of the \Ving, inside baggage
compartments and elsewhere. This
version can !Je used on any support
structure that can be accessed by one
person, from elaborate maintenance
hangar scaffolding to something as simple
as a step ladder. Additionally, it would
require only simple modification to be
mounted as a sensor on a robotically
manipulated scanner, as developed for
other test techniques.

To overcome the problems of
nonuniform heating, the commercial
flash lamps and circular reflectors were
replaced with linear flash lamps and
reflectors tailored specifically for the
redesigned shrouds. Linear flash lamps
and reflectors have been fabricated for
several different sizes of shroud. In
connection with the change from tile
commercial flash lamps to the custom
design, the cabling has also been
modified. Each lamp has only one cable
and that cable is roughly one third the
weight of one of the original cables, two
of which were used with each of the
original lamps.

Figures 23 and 24 show thermograms
produced by the system. \•Vide angle
lenses can be used at short working
distances.

Closing

Developed in the 1990s, the pulsed
thermal wave technique finds application
in aircraft maintenance hangars. The case
history above is ·well documented.'4-20
Many other examples are discussed in the
literature21-26 and in conferences devoted
to this topic.

510 Infrared and Thermal Testing

PART 4. Thermographic Detection of Impact
Damage in Graphite Epoxy Composites26

Pulsed thermal imaging has been and the aerospace industry accounts for
investigated for the detection of impact more than half of that marketP
damage in graphite epoxy materiaL The
Investigation was part of a program to Production testing of composite
determine the feasibility of infrared structures has been addressed by a
thermographic techniques as a field test number of companies and an assortment
system for composite structures. of nondestructive testing tools are
available for the production quality
Graphite epoxy composite samples assurance of these materials and
were prepared and subjected to a wide structmes. These systems include
range of impact damage, resulting in automated ultrasonic scanning systems
damage levels that varied from barely and automated radiographic and
visible damage to severe delamination. radioscopic test systemsP' Several
Comparative tests of the damaged samples nondestructive test systems have also
were obtained by ultrasonic and eddy been proposed for use in the evaluation of
current testing. The test results showed inservice composite structures.29,30
that all three techniques were capable of However, these systems have often been
detecting the onset of damage in the either too slow, difficult to use in a field
graphite epoxy at a point where visible environment or not completely effective
damage to either surface was barely for the testing of inservice composites.
detectable.
Impact damage represents one of the
for the samples, 4.8 to 6.4 mm (0.19 to common in service problems with
0.25 in.) thick impact levels on the order composite structures. In this program,
of 13 j (9.5 ft·lb1) produced this barely infrared techniques have been
detectable damage in edge clamped investigated as a field testing technique
samples. This level of barely detectable for the detection and evaluation of this
damage has been independently reported type of inservice degradation of
by others for ultrasonic testing. The composites. The work emphasized
thermal flow analysis presented shows graphite epoxy composite materiaL
that infrared testing has limitations in
terms of detecting deep lying damage. The basis for infrared testing relies on
the fact that an application (or removal)
Nevertheless, thermographic testing of heat on the surface of a test material
offers excellent potential for field will result in surface temperature changes.
inspection of composite structures. The material surface temperature will
Degradation of composites in service will show variations as the heat diffuses and
include near surface damage, which can encounters material heterogeneities such
readily be detected by infrared imaging. as delaminations, cracks and foreign
The infrared and thermal test method material. The difference !lT in surface
offers many advantages for field temperature at a discontinuity is a
inspection app1ications in addition to the dynamic process. Immediately after
excellent demonstrated sensitivity; these heating, there is no D.T (assuming uniform
include ready portability for noncontact, heating). ·within a short time, D.T will
one·sided, large area testing. show a maximum value and then decrease
as the heat diffuses around and through
Introduction the discontinuity. The times for peak
response and the surface temperature
Advanced fiber reinforced composites equalization vary depending on the heat
offer many advantages in high excitation and the thermal properties of
performance applications. The high lJoth the test materia) and the
strength and stiffness of the fibers, light discontinuity.
weight, designable properties and net
shape manufacturing features of these Infrared and thermal testing can he
materials make them very attractive. As a done in several ways. The most useful
result of these advantages, composites are approach is a one-sided test in which the
being used for more and more surface of the object is subjected to a
applications, particularly in the defense temperature change and the inspection is
and aerospace markets. Advanced done hy observing temperature variations
composites represent a large world market on the same surface of the object. Assume
that the test surface is heated
momentarily by a heat gun or radiant

Aerospace Applications of Infrared and Thermal Testing 511

heat source, for example. In a sound multilevel impact damage readily detected
material, the heat will diffuse uniformly. hy infrared testing. These samples show a
However, if there is a thermal barrier (for mild surface indentation, some front
example1 caused by a delamination, crack surface cracking and back surface
or foreign material), the area over such a blistering. Several new samples were
discontinuity will retain its surface heat acquired for this program. The'new
longer than the surrounding uniform samples were 100 x 150 mm (4 x 6 in.) in
region. A typical infrared test system is area and 4.8 mm (0.19 in.) thick with a
shown in diagram form in Fig. 25. A quasi isotropic layup. These samples were
radiant heater is shown as the thermal impacted hy using a fixture in V\'hich the
excitation source; other types, such as hot sample was clamped around all the edges
air guns and contact heaters, can be used. over a 75 x 125 mm (3.0 x 5.0 in.)
window. The impactor was a 5 kg (1.1 lb1)
Another approach for infrared testing mass with a 16 mm (0.625 in.) diameter
involves heat excitation on one side and impact tup.
infrared imaging on the opposite side-
basically a through~transmission test. In The impact damaged samples covered a
this case, a thermal barrier in the test
material would appear cooler than the range from 3.4 to 13.6 J (2.5 to 10 ft.Jb,).
surrounding region on the test side. This
technique for infrared testing offers some The samples impacted at energies of 3.4
improved response to deep lying
discontinuities in thick material. The and 6.8 J (2.5 and 5 ft·lbr) showed no
detection of deep lying discontinuities
can be a problem because of lateral surface damage from the impact. Samples
thermal diffusion. A general rule of
thumb for this type of test is that impacted at energies of 10.2 and 13.6 J
detection of deep discontinuities is
limited to a minimum size comparable to (7.5 and 10 ft·lb1) showed only mild
the depth in the material. surface indentations.

Infrared thermography can be used to In addition, several other impact
image surface temperature patterns at full damaged graphite epoxy samples were
video frame rates. Through study of a available. These samples were impacted by
thermal perturbation on the surface of the dropping a 39.4 kg (86.9 lb) mass through
sample, these surface temperature patterns a smaller distance to produce similar
can be understood to represent the nominal values of impact energy. These
internal structure and discontinuities samples were edge supported hut not edge
within the component. The physical hasis clamped. Although the impact damage
for this test technique is well levels for these samples did not correlate
understood31-36 and the technique has with those that were edge clamped, the
been used for a wide range of test samples provided some useful
applicationsY-B comparisons and helped fill in the gaps in
the impact energy values of the samples.
Test Samples
Test Results
The test samples were graphite epoxy
composite laminates. Two 150 by 150 mm A variety of infrared testing systems and
(6.0 by 6.0 in.) by 6.4 mm (0.25 in.) thick heat application systems were evaluated
impact damaged samples were available on the test samples. !vfost of the testing
from prior work. These samples had been was accomplished with the same infrared
camera. This camera uses a dual-element
impacted at energies of 27 and 30.5 J mercury cadmium tellurium detector with
a 2 mrad instantaneous field of view
(20 and 22.5 ft-Jb1) and contained sensitive in the 8 to 12 pm .wavelength
band. lt produces 445 lines of infrared
FIGURE 25. Block diagram of typical thermographic testing data. Several other infrared imaging
system. systems were used to examine a smaller
group of the impacted samples.
Video cassette recorder Infrared /
camera The other thermographic camera uses a
I I IFrame grabber ---------- Radiant single-element mercury cadmium
tellurium detector sensitive in the 8 to
heal Test 12 pm band. It uses a mirror scanning
system similar to 445 line model produce
............... source panel 200 distinct lines of infrared data with
250 resolvable elements per line. lt
', produces seven-hit intensity resolution. A
chopped pyroelectric vidicon produces a
Computer image Df--- Image rectangular image field and 300 lines of
processing system monitor infrared image. \1\/ith germanium optics,
the vidicon is sensitive in the 8 to 14 Jlill
\\'avelength range. A different thermal
imager u.ses a 512 x 512 element platinum
silicide infrared charge sweep device
sensitive in the 3 to S pm wavelength

512 Infrared and Thermal Testing

band. The camera produces a very high nearly all of the images presented.
spatial resolution image with an eight·bit Contrast manipulations were generally
intensity resolution. The system was effective at boosting the detail in the
evaluated with the standard 50 mm lens smaller discontinuities. The amount of
contrast enhancement needed varied with
with a focal number of 2. Acomparison the imaging system used and with the
intensity of the heat source. In many
of these infrared imagers is given in cases where the more sensitive infrared
Table 5. cameras were used with a relatively high
intensity heat source, no contrast
Several heat sources were also enhancement was required. 1\•fatrix filters,
evaluated. These included a heat gun with including a variety of edge enhancement
a fishtail nozzle, a 1000 VV quartz halogen and high and low pass filters, provided
lamp in a reflector, an array of four 600 \•V very limited benefit for these images. Line
quartz halogen lamps in reflectors, a set of scans were shown to be useful for
three 910 mm (36 in.) long quartz evaluating the relative intensity of various
halogen lamps in a custom elliptical areas of the image using a graphical
reflector and a single·lamp 4800 \fl.'·s display but this feature generally did not
xenon strobe. All of these systems improve the detectability of image details.
produced good results with the impact Image subtraction was evaluated as both a
damaged samples hut the quartz halogen real time and a postprocessing tool and
lamps generally produced the most showed a limited benefit with these
uniform results and provided the most images.
attractive portable design opportunities.
A real time processing system44 was
The 13.6J (10.0 ft·lbr) impact sample evaluated using a video tape of the
showed a level of damage that could just infrared tests on the impact damaged
be detected by the infrared testing. The samples. The processing system provides a
damage is somewhat smaller than but number of features, the most important of
which is a locally adaptive contrast
similar in form to that in the 3.4 J enhancement. This process provides a real
time compensation for regions tha~ vary
(2.5 ft·lbr) sample impacted with the in background intensity, providing in
effect a simultaneous field flattening and
larger mass and without edge clamping. contrast enhancement. This appwach is
Hguie 26 shows this damage indication very effective where uniform heating is
for the edge damped sample. In Figs. 26c not achieved or ·where differing ambient
and 26d the contrast has been enhanced temperatures (sunlight and shadow) or
to about 180 percent of normal. The small surface emissivity (paint color change)
delaminations are slightly more visible in produce nonuniform thermal
the enhanced image. Figure 27 shows backgrounds. The system effectively
infrared results for several other samples removes these background variances while
that were impact damaged without edge enhancing local detail.
clamping. The similarity in the damage
indication in Figs. 26a and 26b to that in The resolution and contrast of infrared
Figs. 27a and 27b is apparent. images correlates with the number of scan
lines and dynamic range.
A wide variety of image processing
tools were evaluated for use with the Marginal indications of impact damage
infrared images on this program. These are shown in Figs. 26. A pyroelectric
tools have included frame averaging, camera has a resolution comparable to
contrast manipulations, various matrix that of an infrared scanner but produces
filters, line scans, image subtraction and images with lower contrast. With a
time sensitive processing. Many of these moderate amount of contrast
tools were evnluated in both real time and enhancement, the im;:~ges were roughly
postprocessing modes. In general terms,
frame averaging was almost universally
effective at reducing the noise of the
detector signal. Frame averaging on the
order of four to eight frames was used for

TABLE 5. Comparison of infrared camera performance parameters.

Field Instantaneous Sensitivity Infrared Pixels Resolvable Dynamic
of View Field of View Range Lines per
Imager. (degree) per Elements Range
(mrad) (~m) Frame
Detector line per line (bits)

21 X 28 2.0 8 lo 12 HgCdTeJ 445 400 7
2 15 X 20 2.0 8 to 12 HgCdTe 3 200 256 175 8
3 11 X 14 0.5 3 to 5 CSD0 512 512
4 50 mm lens 8 to 14 250 256
crvc

'· Sing!e-dement mercury cadmium tellurium detector.

b. 512 >< 512 platinum silicon infrared charge 5weep device array.
c. Chopped pyroelectric vidicon.

Aerospace Applications of Infrared and Thermal Testing 513

comparable. The camera showed excellent superposition of the infrared results for
spatial resolution and thermal sensitivity the front and back surface examinations.
roughly comparablE' to those of the
infrared scanner. Three of the impact damaged samples
were inspected with an imaging eddy
The impact damaged samples were current test system. Eddy current testing
subjected to a variety of other has been shown to be very effective in
nondestructive tests, including ultrasonic mapping the damage associated with
and eddy current tests. A focused 10 MHz impact in the relatively conductive
transducer was used to perform the graphite epoxy composites.45Ar. The eddy
reflector plate examinations. The current scans tend to map damage
ultrasonic results correlate quite ·well with associated with the conductive fibers.
the infrared test results. The ultrasonic However, it follows that the impact
results for the minimal impact sample damage that produces the infrared and
(Figs. 27) correspond with the nearly ultrasonic results would have some effect
circular perimeter of damage seen in the on the eddy current impedance because of
infrared image. The reflector plate the separation of the layers of fibers at a
examination does not retain any depth delamination (a liftoff effect) and because
information. Thus, the ultrasonic results of some broken fibers at the impact site.
shO\\' the extent of internal damage Frequency scans were run for the eddy
projected onto the surface of the sample current coil in air and in contact with the
and are roughly equal to the laminate. The frequency was swept

FIGURE 26. Infrared results for sample impacted with edge clamping at level of 1 3.6 J
(1 0.0 ft·lb,): (a) front surface unenhanced; (b) back surface unenhanced; (c) front surface
contrast enhanced; (d) back surface contrast enhanced.

(a) (c)

(b) (d)

514 Infrared and Thermal Testing

through the range of 500kHz to 5 MHz frequency for separation of the composite
and the reactance and resistance were signals from those of air. This frequency
recorded throughout the sweeps. The ·was then selected to produce the eddy
results of the two sweeps were then
subtracted to determine the optimal current scans. The eddy current scan of
the front side of the minimal impact

FiGURE 27. Infrared results for samples impacted without edge clamping: (a) front surface at

3.4 I (2.5 ft·lb,); (b) back surface at 3.4 I (2.5 ft·lb,); (c) front surface at 4.1 I (3.0 ft-lb,);
(d) back surface at 4.1 I (3.0 ft·lb,); (e) front surface at 5.4 I (4.0 ft.Jb,); (f) back surface at

5.4 I (4.0 ft·lb,).

w 00

(b)

Aerospace Applications of Infrared and Thermal Testing 515

sample showed an image with a damage Because quasiisotropic layup is assunwd,
region comparable to that of the infrared the explicit ply layup configuration does
not need to be known. The material
re~ults. properties within the plane of a lamina
are assumed to be the average of the
Discussion properties along the fibers and the
properties transverse to the fibers. The
The infrared signal provides a reasonable thickness direction properties are assumed
estimate of discontinuity size and, in to be those predicted for the direction
some cases, discontinuity depth (subject transverse to the fibers.
to limitations discussed in the theoretical
analyses to follow). Conclusions can also The material properties given in
be drawn about discontinuity type, at Table 6 were assumed for the constituent
least in terms of the discontinuity's and composite materials, assuming
relative thermal properties compared to graphite fibers in an epoxy matrix.
those of the composite material itself. For Calculations were performed for a fiber
example, in a one*sided test a wet pocket volume fraction of 65 percent, resulting in
would act as a heat sink, giving a cool the lamina properties shown in Table 6
area on the test surface. A delamination, and used in the heat flow model. The two
on the other hand, would act as a thermal values listed for conductivity of the fiber~
harrier, resulting in a warmer surface area. are for the longitudinal and transverse
There will often be other clues available conductivity, which for graphite fibers
(for example, recent service history) that differ by an order of magnitude. The
may help determine the cause of a micromechanics equations are for
thermographic test indication. Additional determining lamina thermal properties:J?
insight can be gained from a thermal flow
analysis. Heat flow analyses were performed for
a solid graphite epoxy laminate S mm
A computer simulation has been (0.2 in.) thick. Models were run in ·which
developed to model some of the effects of the discontinuities ·were placed at depths
a thermal pulse traveling through a solid for each 10 percent of the laminate
graphite epoxy laminate. The computer thickness. A large discontinuity, 36 by
simulation assumes the uniform 36 mm (1.4 x 1.4 in.) in area and located
deposition of a thermal load on one 0.5 mm (0.02 in.) deep, 10 percent below
surface at time zero. The laminate is the surface, produces significant contrast.
assumed to have a discontinuity in its The peak contrast can he seen to diminish
central region (away from any edges). The quickly with discontinuities deeper in the
size and depth of the discontinuity can laminate. Discontinuities deeper than
vary. The program then monitors the about halfway through the laminate
thermal contrast on the front and back produce insignificant contrast and
surfaces between the temperature at the generally are only marginally detectable
discontinuity and the surface temperature in a one-sided test.
away from the discontinuity. Timing,
sensitivity, size*to-depth relationships and Similar calculations ·were obtained for
material property effects can be seen from smaller discontinuities of 12 by 12 mm
this model. (0.5 x 0.5 in.). Three differences between
these figures are evident.
The model uses an explicit finite
difference solution of the heat conduction
equation given as the partial differential.

TABLE 6. Assumed properties of carbon epoxy specimens.

Property Sl Unit Value Used
English Unit

Fiber density 1.80 g·cm-3 0.065 lbm·in.-3
Matrix density 1.25 g·cm-3
Fiber heat capacity 837.0 J·kg-1.K-1 0.045 lbm·in.-3
Matrix heat capacity 1.05 kJ·kg-l.K- 1 0.20 ca11cg-1.K-1
Fiber conductivity 83.68/8.368 W·m 1.K 1 0.25 calrc9-1·K·l
Matrix conductivity 0.1803 W·m·l.K 1 0.200/0.020 ca11cs··l.cm-l.K-1
lamina density 1.61 g·cm-~ 4.31 x 10-4 calws-l·cm-l.K-1
lamina heat capacity
longitudinal conductivity 895.376 J·kg-'·K-1 0.058 lb01·in.-3
Transverse conductivity 54.3920 W·m-1-K-1 0.214 calrcg-l.K-1
In-plane conductivity 0.7238 W·m-1-K-1 0.130 ca11cs··l.crn·l.K-1
27.5307 W·m-l.K-1 1.73 x 1Q-3 Gllrcs-1·crn-1-K-1

0.0658 ca11cs-l·cm-l.K-1

516 Infrared and Thermal Testing

1. The contrast generated for all of the Analysis indie<1tes that it takes about
discontinuity depth curves is lower for 2.3 s for the thermal pulse to diffuse
smaller discontinuities than for larger through the thickness of the laminate and
discontinuities. generate a barely perceptible temperature

2. The rate of contrast decay_as the difference of ±0.05 K (0.05 'C "' 0.09 '!').
discontinuities move deeper through This is seen as a flat section at the
the thickness is higher for the smaller beginning of the curves. The increase in
discontinuities. thermal contrast (the curve goes negative
because the temperature over the
3. The widths of the contrast peaks are discontinuity is cooler than the
narrower for the smaller temperature in other areas) increases in a
discontinuities. The narrow peaks very similar fashion for all of the
indicate that the thermal indications discontinuity depths in this case.
will not persist as long in the thermal However, the contrast for the far side
images. discontinuity decays more rapidly as the
lateral thermal flow within the thickness
All of these effects can be explained by of the laminate begins to eliminate the
the in~plane conduction of heat. in-plane temperature differences. For the
near surface discontinuity, the thermal
The graphite fibers conduct heat much shadow caused hy the discontinuity exists
more rapidly than the matrix material in only the closest 10 percent of the
and as a result, the in~plane conductivity laminate thickness (compared to 90
is high relative to the thickness direction percent for the far side discontinuity). The
conductivity. As a thermal indication lateral thermal flow therefore has less
develops because of the blockage of chance to remove this temperature
thermal flow through the thickness, the difference and the image of the
in~plane temperature difference generates
an in~plane thermal flow. For the larger discontinuity wm stay sharper and persist
discontinuities, this initially results in
only a blurring of the edges of the longer than for the far side discontinuity.
indication. For the smaller discontinuities, The cmves for smaller discontinuities
the edge blurring quickly converges to
produce a reduction in the maximum are very similar. The principal differences
contrast produced. In fact, the model are exactly analogous to those seen for
indicates that compared to the larger the one-sided test. Note that the
discontinuities, the in~plane conductivity maximum contrast generated for the
quenches the development of the thermal smaller discontinuity on the near side is
contrast and not only reduces the peak only about 0.17 compared to almost 0.9
contrast but shifts the peak to an earlier for the larger discontinuity. Also, note
time because the in-plane heat flow that the peak occurs at about 5 s for the
quenches the contrast before the near side small discontinuity, compared to
anticipated peak is reached. (The about 8 s for the larger near side
temperature for the discontinuity discontinuity. It should be pointed out
represents an average value computed for that the absolute magnitudes of the
the central 25 percent of the discontinuity thermal contrasts are relatively
area.) The fact that the material is meaningless because they depend on the
anisotropic simply shifts this intensity of an arbitrary thermal pulse;
phenomenon to larger discontinuities by however, because the same thermal pulse
a factor "(kin-plane·ko~lt-of-pl.me).
fiGURE 28. Thermal contrast computations for
These analyses correlate well with the through~transmission test in graphite epoxy.
experimental results. Infrared indirations
of small heterogeneities tend to be of less 0.9
contrast than those of larger
discontinuities, as shown in Figs. 26 and 0.8
27. In addition, the peak contrast for
small discontinuities occurs earlier in ·0co 0.7
time.
-"'- 0.6
In addition to these one~sided thermal "'b 0.5
flow analyses, through transmission
thermal models were run for c 0.4
discontinuities 10 percent of the depth u0 0.3
from the far side, at the midplane and ~
about 10 percent of the laminate mw 0.2
thickness below the imaged surface. (The ~~
terms uear and far are in reference to the ~
infrared camera, not the heat source.) ~ Srnallllow
Again, large discontinuities of :i6 by 0.1
36 mm (1.4 x 1.4 in.) and small 0 L_~~--L_~-~
discontinuities of 12 by 12 mm (0.5 x
0.5 in.) were considered in the models. 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

(2) (4) (6) (8) (10) (12) (14) (16) (18)

Discontinuity depth, mm (1 0-2 in.)

Aerospace Applications of Infrared and Thermal Testing 517

was used for all of the analyses, (visually or by ultrasonics) at an impact
comparisons are valid.
level of 9 J (6.6 ft·lb1). The Canadian test
The thermal contrast as a function of
discontinuity depth belmv the monitored results showed the heginning of ultrasonic
surface is summarized in Fig. 28 for the detection of damage at impact levels in
through-transmission case and in Fig. 29
for the one-sided examination case. Note the range of 12 to 14 J (8.9 to 10.3 ft·lb1).
that J:ig. 28 implies that large
discontinuities may be easily seen at any The results presented here showed initial
depth for the through-transmission detection of the sample impacted at an
examinationi however, smaller
discontinuities near the far side may not energy of 13.6 J (10 ft·lb,), as shown in
generate sufficient contrast. to be seen in
all cases. Not surprisingly, the decay in Fig. 26. Therefore, these independent
thermal contrast with depth is much results tend to confirm the results, which
more dramatic for the one-sided tests, as indicate that the threshold of detecfion of
shown in Fig. 29. Note that for the small composite impact damage \'\'as similar for
discontinuities, the contrast is both infrared and ultrasonic testing, given
approaching zero even before the mid damage induced at these energy levels.
plane of the laminate is reached.
Conclusions
Overall, the one-sided test shows much
more sensitivity to discontinuity depth The infrared tests of the impact damaged
indication than the through-transmission samples reveal delamination damage
tests. The effect of discontinuity size on varying with impact energy. The results of
the time to peak can also be seen to be a the infrared tests are quite consistent
nontrivial effect. among the various infrared imaging
systems used and correlate well \Vith the
These analyses correlate 'Nell with the results of other nondestructive testing
infrared results presented here and those methods, including ultrasonic and eddy
presented in more detail in the final current testing. The results clearly show
report~8 The correlations include the that infrared testing can reveal barely
relative sizes, contrasts and timing of the visible impact damage to a degree
infrared indications. Other studies have comparable to that achieved by other
refined the technique.49 nondestructive testing methods.

In terms of comparative sensitivity for Based on these results, it appears that
the detection of composite impact infrared imaging is a practical method for
damage, recently published work on the field testing of composite structures. The
detection and characterization of sensitivity to a major field degradation
composite impact damage tends to mechanism, impact damage to graphite
confirm the results obtained in this epoxy, has been shown to be comparable
programf'0 The Canadian composite to that of ultrasonic and eddy current
samples were edge clamped during methods. Infrared and thermal testing
impact, so their reported values should be offers the additional advantages in terms
comparable to the results obtained with of noncontact, one-sided, large area
the samples described above. In the tests testing with reasonably straightforward
reported here, no damage was detected in interpretation capability and ready
a sample impacted at 10.2J (7.5 ft.Jb,). availability of lest result recording.
This compares weB to the Canadian 'ivork,
which indicated no detectable damage It is interesting that ultrasonic testing
can detect delaminations induced by

impacts of far lower energy, at least 2.0 J

(1.4 ft·lb,).

FIGURE 29. Thermal contrast numerical simulation for
one-sided tests in graphite epoxy.

%! 16
c
14
~

.t "'~ 12 large flow
10
-f' 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
(2) (4) (6) (8) (10) (12) (14) (16) (18)
"b""' 8
6

c 4

0
v
~2
ro
~
~ 0

0

Discontinuity depth, mm (1 Q-2 in.)

518 Infrared and Thermal Testing

PART 5. Infrared Scanning Radiometry of
Convective Heat Transfer

The following discussion of radiometry of convective heat fluxes in both steady and
convective heat transfer has implications transient techniques56-.'iH \'\'ithin this
for developmental research in wind context, an infrared scanning radiometer
tunnel technology. can he considered <IS a thin film sensor54
hecause it generally measures skin
Nondestructive testing ·with infrared temperatures. In other contexts, infrared
thermography is mostly achieved by detectors are thiu film detectors insofar as
heating or cooling the body to be tested they are fabricated using vapor
and by following its surface temperature deposition. The thermal map obtained by
evolution as a function of time. ,,Vhilc means of currently available computerized
processing the temperature history, it is thermographic systems is formed through
very important to have a precise a large amount of pixels (20 000 to more
knowledge of the net heat flux at the than 60 000 kilobytes) so that infrared
wall, that Is, the imposed heat flux minus scanning radiometry can be practically
the total losses. In most applications of regarded as providing a two-dimensional
infrared thermography, the thermal losses array of thin films. Howe\'er, unlike
are essentially due to radiation and standard thin films, which have a
convection. The evaluation of the response time of the order of
radiative heat flux is straightforward if microsecond, the typical response time of
emissivity is known but the convective infrared scanning radiometry is of the
heat flux is generally more difficult to order of 0.1 to 0.01 s.
measure.
Infrared scanning radiometry as a
It has to be also pointed out that temperature transducer in convective heat
sometimes nondestructive testing can be transfer measurement appears, from
performed by either heating or cooling several points of view, advantageous if
the tested body with a gas stream. For a compared to standard transducers. In fact,
quantitative evaluation of the results, also as already mentioned, infrared scanning
in this case a detailed knowledge of the radiometry is fully two-dimensional. It
heat transfer distribution over the tested permits the evaluation of errors due to
body is compulsory. This section reviews tangential conduction ·and radiation and
some techniques useful to measure is nonintrusive. For example1 the last
convective heat fluxes with the aid of characteristic makes it possible to get rid
infrared thermography. of the conduction errors through the
thermocouple's or resistance temperature
Usually, measuring convective heat detector's wires.
fluxes requires both a sensor (with its
corresponding thermal model) and some Heat Flux Sensors
temperature measurements. In ordinary
techniques,51•55 where temperature is Heat·flux sensors generally consist of
measured by thermocouples, thermistors, plane slabs with a known thermal
resistance temperature detectors or behavior, whose temperature is to be
pyrometers, each transducer yields the measured at fixed points.51 -55 The
heat flux at a single point or at a space equation for heat conduction in solids
averaged point. Hence, in terms of spatial applied to the proper sensor model yields
resolution, the sensor itself can be the relationship by ·which measured
considered as zero dimensional. This temperature is correlated to the heat
constraint makes experimental transfer rate.
measurements particularly troublesome
whenever temperature, or heat flux fields The most commonly used heat flux
exhibit high spatial gradients. sensors are the so·called one-dimensional
ones, where the heat flux to be measured
Infrared scanning radiometry is assumed to be normal to the sensing
constitutes a true two-dimensional element surface. That is, the temperature
temperature transducer because it allows gradient components parallel to the slab
the performance of accurate measurement plane are neglected. In practice, the slab
of surface temperature maps even in the surfaces can also he curved, hut their
presence of relatively high thermal curvature may be ignored if the layer
gradients. Correspondingly, the heat flux affected by the input heat flux is much
sensor may become two-dimensional as
·well. In particular, infrared thermography
can be fruitfully employed to measure

Aerospace Applications of Infrared and Thermal Testing 519

smaller than the local radius of curvature normal to the sensor surface is related to a
of the slab. radial temperature difference, in the
direction parallel to the plane of the
In the following, ideal one~dimensional gage.S1 This sensor is practically of no use
sensors are considered. Some of them will in infrared thermography.
be extended to the multidimensional
case.s7,SS The term ideal means that Application of infrared scanning
thennophysical properties of the sensor radiometry to both the thick film and the
material are assumed to be independent gradic11t sensors is not easily performed.
of temperature and that the influence of
the actual temperature sensing element is Theoretical treatment of the relevant
not considered. steady state techniques56·tl0 and transient
techniques0H>3 lies outside the scope of
The most commonly used the present discussion.
one~dimensional sensor models are:
Applications
Thin Film Sensor. A very thin resistance
thermometer (film) classically measures Two different fluid flow configurations are
the surface temperature of a thermal/}' analyzed by means of infrared
thicker slab to which it is bonded. Heat thermography: a jet of fluid impinging on
flux is inferred from the theory of heat a flat plate and a circular cylinder in a
conduction in a semiinfinite wind tunnel. In the first configuration
one~dimensional wall. The surface film average heat transfer data will be
must be so thin that it has a negligible discussed whereas for the second one a
heat capacity and thermal resistance as detailed description of the convective heat
compared to the slab's. To use this sensor transfer coefficient will be given. For both
with infrared thermography, the heat experiments, the convective heat transfer
exchanging surface must be necessarily coefficient is measured by means of the
viewed by infrared scanning radiometry. heated thin foil technique.

Thick Film Sensor. The slab is used as a Impinging jets
calorimeter; heat flux is obtained from the
time rate of change of the mean slab Heat transfer between impinging jets and
temperature. This temperature is usuaiJy a plate has been the subject matter of
measured by using the slab as a resistance several studiesM·67 because of their wide
thermometer. use for heating, cooling or drying surfaces
in many industrial applications.
Wall Calorimeter or Thin Skin Sensor. The
slab is made thermally thin (so that its The target plate is a thin constantan
temperature can be assumed to be foil, 50 [1111 (0.002 in.) thick, heated by
constant across its thickness) and, as in the joule effect. The jet is obtained by
the previous sensor, is used as a flowing air, coming from a compressor,
calorimeter. Heat flux is typically inferred trough a truncated cone nozzle 80 mm
from the time rate of change of the slab (3.2 in.) long and with an exit section of
temperature usually measured by a diameter D::::: 10 mm (0.4 in.). 1\\•o
thermocouple. To use this sensor ·with different nozzle plate configurations are
infrared thermography, either one of the considered: an open one with the nozzle
slab's surfaces generally can be viewed by externally joined to a slender stagnation
infrared scanning radiometry. chamber and a closed one with the nozzte
submerged into the stagnation chamber
Gradient Sensor. In this sensor the that ends with a 300 x 300 mm (12 x
temperature difference across the slab 12 in.) flat plate that is parallel to the
thickness is measured. By considering a target plate having the same dimensions.
steady state heat transfer process, heat In the second case the spent air is
flux is computed by means of the exhausted through the gap between the
temperature gradient across the slab. The two parallel plates: one that is flush
temperature difference is usually mounted at the exit section of the nozzle
measured by thermopiles made of (top plate) and the other one that is the
very-thin~ribbon thermocouples or by two target plate. In particular, the top plate
thin film resistance thermometers. embodies a serpentine passage through
which a thermostatic liquid flows that
Heated Thin Foil Sensor. This technique allows majntaining its surface temperature
consists of steadily heating a thermally equal to the air stagnation temperature.
thin metalHc foil or a printed circuit
board by the joule effect and by Tests are carried out by varying the
measuring the heat transfer coefficient reynolds number (based on the nozzle
from an overall energy balance. Also in exit diameter) and the impingement
this case, due the thinness of the foil, distance Z (bet\veen nozzle exit and target
either one of the slab's surfaces can be plate) for the two different exhau~t areas
vie\ved by infrared scanning radiometry. (open or closed). Data, generally averaged
over each circumference of given radius, is
Strictly speaking, there is another type
of one~dimensional sensor, the circular
gardon gage, in \Vhich the heat flux

520 Infrared and Thermal Testing

reduced in dimensionless form in terms of radiometer takes into account its
the nusselt number based on the presence.
diameter D).
To measme temperatures in the whole
For short impingement distances, the heated zone and to account for the
small gap between the nozzle exit section directional emissivity coefficient, three
and the target plate does not allow the jet thermal images in the azimuthal direction
air, after impingement, to fluw directly to are taken and patched up. In particular to
ambient; a part of this air tends to come reduce the measurement noise, each
back towards the nozzle exit giving rise to image is obtained by averaging 32
recirculation patterns. Separation over the thermograms in a time sequence. Because
target plate occurs at about 1.2 D from the of the end _conduction effects near the
jet center where the vertical component forebody, the portion of the cylinder for
of the jet velocity is decelerated and which the infrared camera gives reliable
transformed into a horizontal one.67 At data actually starts at X·D-1 == 0.2 (x being
small Z values, the flow separates in the the coordinate along the cylinder axis)
open case but it reattaches downstream at and data are reported up to x·D ·1 :::: 5.68
about 2 D. Separation and reattachment This zone is precisely identified by putting
of the jet flow give rise, in terms of heat markers over the cylinder surface. The
transfer, to a local minimum in the markers are useful also to patch up the
nusselt number distribution at about 1.2 various thermal images.
D and to a local maximum at about 2 D.
On the contrary, in the closed case the The flow field around a cylindrical
flow is prevented from a strong body is characterized by separation and
reattachment and tends most likely to he reattachment of the flow, ·which can be
transformed into recirculation patterns. inferred from the distribution of the heat
transfer coefficients. The heat transfer
The recirculating air coming back coefficients are computed in
toward the nozzle exit causes forced nondimensional form in terms of the
mixing and entails a general decrease of nusselt number based on cylinder
the local nusselt number for the closed diameter. It must be noted that the
case and short impingement distances. location of the maximum nusselt number
Recirculation effects are stronger for the does not exactly coincide with that of
closed configuration at short flow reattachmentf'9 However, the
impingement distances (Z ~ 10 D). As position of the maximum nusselt number
Z·D--1 increases (above z.J)-1 :::: 10), the can be considered to determine the length
of the thermal separation bubble.68
exhaust area between the two plates
becomes wider and allows the spent air to Results of the present investigation
freely flow into ambient \Vithout major confirm the fundamental role played by
differences between the two freestream turbulence in the formation of
configurations. the leading edge separation bubble?0•71 In
the sharp edged cylinder the separation
Circular Cylinder bubble appears shorter on the windward
side than on the leeward side, and there
The employed longitudinal cylinder has assumes also higher nusselt number
an outer diameter D = 40 mm (1.6 in.), an values. On the contrary, for the round
overall streamwise length of 300 mm nosed cylinder two thermal reattachment
(12 in.) and its lateral surface is made out points are present on the lee\-vard side. A
of a printed circuit board (bonded to a likely explanation for this is that the
fiberglass layer) so as to generate a separation bubble disappears on the
constant joule heat flux over it. The windward side giving rise to the
copper conducting tracks of the printed formation of two vortices, which can he
circuit are 35 pm (1.4 x 10-3 in.) thick, assumed to coincide with saddle points 72
3 mm (0.12 in.) wide, placed at 4 mm on either side of the nodal separation
(0.16 in.) pitch and aligned point on the leeward side.
perpendicularly to the cylinder axis. Two
different configurations of the cylinder
leading edge (nose) arc tested: a sharp
edge nose and a hemispherical blunt one.

Tests are performed in an open circuit
wind tunnel having a 300 x 400 mm
(12 x 16 in.) rectangular test section that
is 1.1 m (43 in.) long. The free stream
turbulence intensity of the tunnel is quite
low and lies in the range 0.08 to
0.12 percent depending on the testing
conditions. The access window for the
infrared e<unera to the test sertion of tlw
wind tunnel is made of bidirectional
polyethylene; calibration of the

Aerospace Applications of Infrared and Thermal Testing 521

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526 Infrared and Thermal Testing

16

CH APT ER

Elect ric Pow er Applicat ions

of Infrared and Thermal

Test ing

Blair R. Bosworth, Foseco, Conneaut, Ohio (Part 5)
Motokuni Eto, Japan Atomic Energy Research Institute,
Ibaraki, Japan (Part 4)
Toshimitsu Ishii, Japan Atomic Energy Research
Institute, Ibaraki, Japan (Part 4)
David L. Mader, Ontario Hydro Technologies, Toronto,
Ontario, Canada (Part 6)
Yoshizo Okamoto, East Asia University, Shemonoseki,
Japan (Part 3)
Lars Persson, Västerås, Sweden (Part 7)
Raymond R. Rayl, Consumers Energy, Jackson,
Michigan (Part 6)
R. James Seffrin, Infraspection Institute, Burlington,
New Jersey (Part 5)
Steven M. Shepard, Thermal Wave Imaging, Ferndale,
Michigan (Part 5)
John R. Snell, Jr., Snell Infrared, Montpelier, Vermont
(Part 2)
Andrew C. Teich, FLIR Systems, Portland, Oregon
(Part 1)
Stig-Björn Westberg, Vattenfall Utveckling AB,
Älvkarleby, Sweden (Part 7)
Paul A. Zayicek, Electric Power Research Institute,
Charlotte, North Carolina (Part 5)

PART 1. Thermographic Systems for Power
Generation and Distribution1

All utilities and their customers depend Range of Applications
on system reliability. When outages occur,
utilities are forced to produce and Thermography is cost effective within the
distribute power that is more expensive. power generation and distribution
In the past, the best means of reducing industries because of its multitude of
the number of forced outages was through applications, ranging from the monitoring
preventive maintenance, which required of electrical connections and switching
the replacement of specific components equipment to the evaluation of
after a certain life span, whether the mechanical equipment and fluid transfer
component needed replacement or not. systems.2-8
Although better than breakdown
maintenance, preventive maintenance One of the most common applications
often leaves components untouched that involves monitoring electrical distribution
should be replaced. systems, from transmission lines to motor
control centers. Electrical systems
Many utilities have discovered that the typically suffer from problems such as
best way to reduce outages is to find and loose connections, load imbalances and
correct failing components before they corrosion. These problems cause an
become major problems that cause increase in impedance to current,
interruption in service. One of the easiest resulting in resistive heating. If left
and most cost effective methods is unchecked, this heat can build to a point
predictive maintenance with infrared at which connections melt — breaking
thermography. the circuit and in some cases creating
fires. Thermography is well suited to this
All equipment that conducts, application because thermography quickly
consumes or generates power will also locates hot spots and determines the
emit heat as a result of energy loss in the severity of the problem and how soon the
system. Typically, when these components equipment should be repaired.
become less efficient at doing their job,
the heat they emit will increase. Often, Some microprocessor based
the temperature of a faulty component quantitative infrared test systems can
will increase rapidly before failure. perform trend analysis, letting the
Thermography lets plant engineers maintenance team set up periodic
evaluate the thermal condition of a wide intervals for equipment inspections. The
variety of plant and substation equipment data are then stored in a database of
through the use of infrared cameras that thermal images, which logs images and
produce high resolution thermal video corresponding test data. The program
images in color or black and white. These then displays multiple images of the same
images can quickly and easily uncover component surveyed over a period of
potential problems in electrical and several months. The temperature rise of
mechanical systems. the object is plotted to determine the
temperature increase trend. When the
An increasing number of plant slope of the temperature rise curve
maintenance teams now use thermal changes suddenly, this change typically
imaging systems to evaluate and predict indicates an impending failure (Fig. 1).
the failure of many types of equipment
within power generation and distribution Thermographic trend analysis can be
systems. Today’s portable thermal imaging applied to indoor components such as
measurement systems can provide high motor control centers, breaker panels and
resolution live images that can be stored disconnect switches and transformers, as
on conventional videotape recorders or well as on outdoor components in
on built-in floppy disk drives. These substations — switchgear, transformers
images can be retrieved for later analysis and output current boosters. Utilities
in personal computer based image including Pacific Gas and Electric (San
processing programs. Some systems even Francisco, California), Florida Power and
allow detailed playback analysis within Light (Miami, Florida), Northern States
the basic system electronics. Power (Minneapolis, Minnesota) and
Texas Utility Systems (Dallas, Texas), have
adopted programs for regularly surveying
their substations with thermal imaging

528 Infrared and Thermal Testing

equipment in an effort to maximize popup periscopes housing infrared
efficiency and reliability. systems and video cameras. The cameras
are controlled by a motorized pan-and-tilt
Thermography is also highly effective device operated by a technician within
in evaluating a utility’s many miles of the van. Also, built into the rear of the
transmission and distribution lines. van is a workstation with two monitors
Modular thermographic systems where for display of the visual and thermal
the camera is separate from the images, a laptop computer for storing
electronics are ideal for surveys conducted statistical data and a video printer for
from either a mobile land or airborne generating repair reports. The high scan
vehicle. In these cases, the camera is speed and the long wave detector of the
mounted outside the vehicle and the infrared systems allow the technicians to
controller and display are used inside the travel at reasonable road speeds and, for
vehicle. Images can be stored on some applications, to work day or night.
videotape or on disk for analysis after the
survey has been conducted. Longwave Scanning powerlines and equipment
(8 to 12 µm) systems are typically over long distances can be expedited by
preferred for these applications because using a technique called thresholding.
they can be used during the day without Thresholding allows the instrument to
picking up false readings as a result of display an easily recognizable
reflections from the sun. black-and-white image with the hottest
regions of the image colorized in red.
Northeast Utilities, Berlin, Connecticut, With this feature, operators can set a
pioneered a technique of mobile power threshold temperature; any objects
line scanning. They have developed exceeding this temperature will be readily
several customized vans incorporating noticed by the operator when red appears
in the display. Systems incorporating this
FIGURE 1. Absolute temperature measurements of loose feature along with high speed scanning at
connection over six months: (a) thermograms; (b) graph. television rates increase the number of
Component should be repaired at point where curve rises miles that can be covered in a day with a
rapidly, that is, when temperature rises abruptly. thermographic system.
(a)
Telescopes and electrooptical zoom
Temperature, K (°C) [°F](b) November January March features are frequently required when
scanning small connections over long
333 (60) [140] distances. It is important to understand
322 (49) [120] the spatial resolution limitations of the
311 (38) [100] instrument in use. Systems with high
300 (27) [80] spatial resolution have the benefit of
289 (16) [60] being able to view small objects at long
278 (4) [40] distances.
266 (–7) [20]
Mechanical Systems and Rotating
September Equipment Analysis

Month surveyed There are numerous applications for
predictive maintenance with
Legend thermography on mechanical systems and
= temperature rise over ambient rotating equipment. These systems
= actual target temperature typically fail as a result of excessive
vibration or poor lubrication, resulting in
an increase in temperature.

Thermography is frequently used as a
screening tool for vibration equipment.
Electrical motors, pumps and solenoids
can be evaluated quickly for abnormal
conditions. These studies can also be
conducted over time to look for
temperature increase trends.

In electrical motors, thermography can
be used to detect deteriorating insulation,
poor windings and bad brushes. Many
electric motor rework facilities also use
thermography to evaluate motor armature
conditions. In this application, it is
helpful if the thermographic equipment
operates at a multiple of the motor speed,
that is, 60 Hz or 50 Hz, to accommodate
international applications. The motor can
be viewed easily while it is running

Electric Power Applications of Infrared and Thermal Testing 529

because the system provides a strobe Equipment Selection
effect, slowing the apparent rotation so it
can be analyzed. There is a wide range of thermographic
equipment available on the market and it
As bearings lose lubrication, vibration is important to find the best equipment
occurs and heat builds to the surface of for each customer’s set of applications.
the object, where it can be detected with Basically, a thermal imaging system
thermography. In Fig. 2, an air compressor performs two primary functions: imaging
and motor are evaluated. As expected, the and temperature measurement. It is
compressor head is warm as a result of the important to evaluate carefully the quality
compression process. The center of the of the thermal image. This includes
compressor pulley is also abnormally factors such as spatial resolution, scan
warm, indicating a poorly lubricated or speed and thermal sensitivity. The
failing bearing. The visible light and temperature measurement capability
infrared images were both gathered should be accurate and repeatable. It is
during the survey and subsequently important that a system maintain its
transferred to an image processing system. accuracy over a wide range of operating
In the image processor, the images are conditions.
displayed side by side and a temperature
cursor is placed on the hot spot. A repair The wavelength band that the
report can then be generated directly from instrument operates in is an important
the image processor. consideration. Short wave instruments are
more significantly influenced by humid
Fluid and Heat Transfer System air and reflections from the sun.
Analysis Longwave systems are typically not
affected by these factors and can provide
Thermography can also be used to a higher resolution image.
evaluate fluid transfer systems such as
pumps, steam traps and underground Instrument portability and ruggedness
steam lines. Heat transfer systems such as are also considerations. Modular systems
heat exchangers, cooling towers and air generally provide the most flexibility
conditioning systems can be inspected for because they can be used in a wide variety
blockages and poor thermal distribution. of configurations.

Insulated pipes can be assessed for heat Data storage and image processing
loss or leakages. Plant buildings can be capabilities are also important factors. It is
studied for insulation voids and roof leaks best to have the option of both digital
with this type of equipment. and videotape storage and analysis. The
image processing system should be able to
FIGURE 2. Compressor bearing overheating. Abnormal hot handle visual and thermal images
spot on drive pulley indicates poor lubrication on sheave simultaneously. An image archiving
bearing. database capability is useful for
implementing a structured maintenance
program.

Other factors such as availability of
accessories, training and rapid service
should also be considered carefully when
selecting a thermal imaging system.

530 Infrared and Thermal Testing

PART 2. Infrared Thermography in Electrical
Maintenance9

Thermography is one of the most warm component seem cool when it is
powerful tools available for electrical really quite warm.
maintenance. With professional training
and some experience a thermographer can The solution is to be aware of the
quickly locate high resistance thermal background when viewing shiny
connections, load imbalances and surfaces.
overloads while the system is in
operation. This can all be accomplished Shiny Surfaces
without direct contact to the energized
system. A much more serious problem on shiny
surfaces is that they simply give little
Electrical inspections have typically visible indication of their actual
produced remarkable returns, with temperature. Polished aluminum, for
documented returns of 30 to 1 on the part instance, emits so little energy that even
of a major industrial insurer. Prevention at temperatures in excess of 373 K (100 °C
of catastrophic failures and unscheduled = 212 °F) it may look like it is at ambient
outages often results in cost savings far in temperature because of a 0.03 emissivity
excess of the cost of the test equipment value.
and program.
When looking at shiny surfaces,
Today’s economic climate, however, remember that they will not appear as
demands even greater assurances for warm (or cold) as they really are. It is not
reliability from maintenance possible to find hot spots on shiny
thermographers than in the past. surfaces until the surfaces are very hot.
Experience can reveal the inspection
program’s successes and limitations. Measurements on Low Emissivity
Surfaces
Some limitations to thermographic
tests of electrical equipment are quite So little radiation is emitted by shiny
obvious. Some problems are inherent to surfaces that trying to convert the
the laws of physics and must be lived radiometric data into accurate
with or worked around. Others are related temperature information is typically
to environmental or operating conditions. impossible on a repeatable basis in the
The latest infrared test equipment is no field. Many professionals do not
longer a limiting factor — it will do more recommend measuring temperatures with
than usually needed. But inadequate data emissivity values lower than about 0.5.
collection procedures and a poor
understanding of how to use the Although the emissivity value can be
information gathered are very much set for less than 0.5 in most systems,
limiting factors. The following discussion beware of the accuracy of measurement
indicates ways to improve electrical using values that small.
inspections by dealing with these
limitations. Emissivity Determination

Limitations from Physics Emissivity values can be taken from
predetermined tables but such values
Reflected or Background should be used with a great deal of care.
Radiation They are usually generic and may not be
accurate for the waveband or temperature
The radiometer sees the combined range under examination. They are also
radiosity (exitance) of both radiated and each averaged over a waveband. If very
reflected energy. Only the emitted energy accurate temperature data are needed,
indicates the temperature of the calculate the emissivity value of the actual
component. The reflected energy can surface being measured with the system.
make a shiny component appear hot
when it is not. Solar reflections are a Whenever possible, measure
problem especially outside. Reflections of temperatures only on highly emissive
cooler ambient backgrounds may make a surfaces, such as electrical tape applied to
the surface, using known, tested
emissivity correction values. When this is
not possible, measure the value of the
surface.

Electric Power Applications of Infrared and Thermal Testing 531

To determine the emissivity value of a be a significant difference in temperature
surface, follow the procedure between the heat source and the
recommended in the training manual or measured surface.
the equipment operator manual. Note,
however, that these techniques will Many factors influence the rate at
probably not be accurate for shiny which the heat is transferred to the
surfaces. The best bet, whenever possible, surface. For example, on a load break
is to measure temperatures only on elbow, variations in the thickness of the
surfaces of high or known emissivity or to electrical insulation alone, because it is
use contact measurement devices such as also a fairly good thermal insulator, can
thermocouples. cause up to a ±11 K (11 °C = 20 °F)
variation in surface temperature.
Angle of Interrogation
Test the component as close to the
Emissivity also changes with the angle of heating source as possible. Where this is
viewing. For instance cylindrical surfaces, not possible, actual problems will always
such as a tubular bus or the barrel of a be more severe than are indicated.
cylindrical fuse, emit and reflect thermal
radiation over a wide range of values. The effects of the materials acting as
What is seen is not necessarily a true thermal insulation must also be
representation of the problem. considered. The greater these effects are,
the less indication there will be of the
Always try to face the target directly. actual temperature at the heat source.
Try to stay within 45 degrees of normal.
On curved surfaces make several Environmental or
measurements, if necessary moving each Generating Conditions
time to be directly in front of a small
section of the curve. Defining Delta Measurements

Emissivity Variations Delta temperatures are sometimes
reported as a difference between the
Temperatures measured on components component and the ambient temperature.
with several emissivities may be incorrect. Yet the term ambience, which actually
Those parts with high emittance may means surroundings, is typically not well
appear to have a different temperature defined. Does it mean air temperature? Or
than parts with lower emittance. A piece the temperature in an electrical panel? Is
of tape or a sticker on a shiny bus bar, for the panel temperature measured
instance, will almost always appear immediately on opening it or after a
warmer than the bus bar itself, which has while? Inaccuracies can result if ambient is
a low emissivity is reflecting the normally not clearly defined or understood.
cooler surroundings.
Delta measurements, when used,
Be cautious when dealing with a should be the temperature difference from
surface that has several different one phase to another (if loads are equal)
emissivity values. While they may all be or to a similar piece of equipment under
the same temperature, they will probably the same influences.
not appear the same.
Environmental Conditions
Emissivity changes may not be
obvious. Cracks, scratches, bolt threads Environmental conditions can make it
and holes will all typically have higher difficult to see or correctly interpret
emissivities than the surfaces they are in problems, that is, operational
and will appear to be at a different dysfunctions caused by material
temperature. A hole is actually a good anomalies. This is especially true for
place to measure an accurate temperature problems with a low temperature
because it has a very high emissivity. A difference. Despite what people may
hole that is seven times deeper than it is think, these problems can be extremely
wide has an emissivity of 0.98. dangerous.

Cases Where Surface Is Measured, A problem with a low temperature
Not Heat Source difference will tend to be hidden, masked
or understated in the following
Failure of electrical components is usually environmental cases: where components
related to excessive heat, especially that are heated by the sun; in ambient
caused by high resistance. Heat is extremes, either hot or cold; when
transferred from this internal source to components are cooled by the wind or
the outer surface visible to the infrared other convection; when surfaces are wet;
camera. For small electrical parts the and when components are lightly loaded
source and the surface may be in very thermally.
close proximity and of similar
temperatures. But in most cases there can Low temperature problems may be
hidden by the environment — do not

532 Infrared and Thermal Testing

assume they disappear. They will be back (2) cost of preventive maintenance,
when conditions change. (3) criticality of equipment, (4) increases
in electrical load, (5) changing trend of
Whenever possible conduct surveys temperatures, (6) history of the
under optimum conditions. Wet equipment or similar equipment,
conditions, hot or sunny afternoons and (7) availability of spare parts and repair
winds greater than 16 km·h–1 (10 mi·h–1) personnel, (8) cost of an unscheduled
should be avoided. Be aware also that the outage or run to failure, (9) ability to
wind will cool off the abnormal phase. reduce loads until scheduled outage,
For a valid inspection, however, normal (10) ability to monitor the condition of
phases just a few degrees above ambient equipment, (11) accuracy of test
on inspected surfaces should not have data, especially temperature, and
cooled below ambient. The stronger the (12) availability of repair opportunity.
wind, the more misleading are
measurements of temperature differences. These criteria can be incorporated into
a weighted matrix that will suggest an
Inservice Inspection appropriate course of action. Compare the
predictions with the actual results, to
Equipment must be under load, preferably improve the matrix.
normal operating loads. The correction
factors based on the power formulas Thermographer Qualification
commonly used in the industry are not
recommended. Although they will Inaccurate results from infrared and
accurately predict the changing heat thermal testing of electrical systems and
output, they will not predict the components are often caused by a poorly
component temperature. qualified thermographer. Bad inspection
practices, lack of information and
Inspect equipment when load is either misunderstanding of what the data really
40 percent of design10 or the highest indicate are all factors. A thermographer
anticipated load — whichever is greater. If qualified at Level I is not supposed to
loads are less than this, problems may interpret data. That is a duty reserved for
exist that cannot be detected or that will someone with Level II training and
give very little warning before failure. experience.

Variations in Ambient Inspectors must get training,
Temperature experience and support to do the job.

Thermal tests must also take into account
changes in ambient air temperature,
especially from summer to winter
extremes. A problem identified during
winter is more likely to fail during
summer conditions.

Whenever possible, conduct tests under
worst case conditions. This usually means
at peak load during hot weather. If this is
not possible, interpret the results with
care.

Data Interpretation

Criteria for Assembling Data

The greatest mistakes during an infrared
inspection of an electrical circuit are
probably made in interpretation and
handling of the data after the inspection.
Temperature is not always the most
important datum required to understand
the problem.

What other factors should be
considered when determining the
seriousness of a problem? This question is
probably best addressed by a group of
people involved in the various levels of
maintenance operations and management
at the work site. Factors to be considered
may include the following: (1) safety,

Electric Power Applications of Infrared and Thermal Testing 533

PART 3. Predictive Maintenance of Nuclear
Reactor Components

Nuclear reactors are composed of a components of the Japan Materials
number of components to which many Testing Reactor: (1) decay heat monitoring
nondestructive testing methods are of an irradiated capsule, (2) monitoring of
applied for inspection during operation as cooling towers under reactor operation,
well as shutdown. However, it is difficult (3) detection of overheating of main
to apply most of the methods to circulation pumps and (4) maintenance of
components where access is restricted the reactor canal wall.12
because of the radioactive environment
where access by inspectors is restricted. Decay Heat Monitoring of
The infrared and thermal test method is Irradiated Capsule under
an efficient remote sensing device when Operation
used to diagnose and inspect the reactor
components of nuclear reactors. The Japan Materials Testing Reactor,
whose thermal output is 50 MW, is used
Infrared thermography has been widely to irradiate test fuels and materials of a
applied to inspect components of nuclear capsule inserted in the reactor core and
reactors — for example, (1) leakage and reflector zone with the fast and thermal
overheating areas at joints, shaft and neutron flux of 1 × 1014 to
bearings, (2) abnormal parts in piping, 4 × 1014 cm–2·s–1 and neutron fluence of
valve, flange and nozzle, (3) internal 1021 to 1022. The capsule tube is 30 to
discontinuities in the containment vessel 80 mm (1.2 to 3.2 in.) in diameter and
and piping such as corrosion, separation 750 mm (30 in.) in length. Fuels and
and cracks, (4) leakage of cooling fluid in materials are placed in the tube for
reactor core and its components, (5) irradiation. The reactor is operated for 28
deteriorated and anomalous areas in days of each cycle. The volumetric heat
metallic and nonmetallic joints and generation of the capsule in the reactor is
bushings in the highly radioactive 10 W·cm–3 (164 W·in.–3) maximum during
environment, (6) continuous monitoring the operation. The reactor pressure vessel
of cooling towers under reactor operation is submerged in an open water container.
(7) overheated areas of main circulation
pumps and (8) deterioration and The irradiated capsule is transferred
anomalies of electric and electronic from the reactor core via an open water
control components.11 canal of square cross section directly

Described next are a few techniques for
detecting and evaluating the integrity of

FIGURE 3. Thermograms of water surface with irradiated capsule: (a) left; (b) right.
(a) (b)

534 Infrared and Thermal Testing

connected to the pool during the Continuous monitoring on the water
shutdown period and is stored under surface of the canal using the infrared
water near the side of the canal wall. The thermography is useful to monitor the
irradiated fuel and material in the capsule decay heat behavior of the irradiated
generate the decaying gamma radiation capsule. Thermographic data on the
heat. Some capsules emit gamma temperature rise of the capsule make it
radiation. Radioactivity at the capsule possible to determine the time to transfer
surface is so high after irradiation that the irradiated capsule from the canal.
water is circulated in the canal to cool the
radiation generated heat for 1 to 3 mo. Continuous Monitoring of
After the radioactivity of the capsule Cooling Tower under
becomes lower than an allowable value, Reactor Operation13
the capsule is transferred to the hot
laboratory building. A water cooling removes the heat
generated in the nuclear reactor and
A monitoring test is used during transmits it to the air. The cooling tower
thermography to detect a small consists of four cross flow and draft units
temperature rise of the water surface that remove the thermal heat of 12.5 MW
caused by the decay heat from the per unit. The flow rate of the water is
irradiated capsule. 3900 m3·h–1.

Figure 3 shows a thermal image of the The cold ambient air supplied from the
water canal above the capsule after bottom intake of the tower flows upward.
irradiation for four days. In general, the The injected water droplets are cooled by
emissivity of the water surface at nearly evaporation and flow down to the bottom
ambient temperature approaches unity of the pool. Hot water at about 315 K
and the radiation temperature of the (42 °C = 108 °F) in inlet is flashed from
water measured by thermography the upper nozzle and transfers its heat to
becomes equal to the real temperature of the cold air. The outlet temperature of the
the surface. water becomes 306 K (33 °C = 91 °F) as
the evaporation of the water droplets heat
The thermal image of the water surface the air. As fine water droplets cover the
in the canal becomes uniform before the wooden cooling panel, the measured
transfer operation. However, thermal radiation temperature of the outer wall of
images of the canal water near the the tower becomes equal to the real
irradiated capsule show that the surface temperature.
temperature of the water above the
irradiated capsule becomes nonuniform Figure 4 shows thermograms, or
and exceeds water temperature without thermal images, of the cooling tower
the capsule (Fig. 3). A temperature rise of units. Three cooling units (the first, third
+0.1 K (0.1 °C = 0.18 °F) observed on the and fourth from the left) are in operation
water surface above the capsule indicates and the second unit from the left in
the warming of the water because of suspension to control the removable
radiation decay heating of the irradiated power. The surface radiation temperatures
capsule. The decayed heat becomes
negligibly smaller than that generated just
after the irradiation.

FIGURE 4. Cooling towers under reactor operation: (a) left, units 1 and 2; (b) right, units 3
and 4.
(a) (b)

Electric Power Applications of Infrared and Thermal Testing 535