ASM Metals HandBook Vol. 17 - Nondestructive Evaluation and Quality Control
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The transfer method is especially valuable for inspecting a radioactive specimen. Although the radiation emitted by the
specimen (especially -rays) causes heavy film fogging during conventional radiography or direct-exposure neutron
radiography, the same radiation will not induce radioactivity in a transfer screen. Therefore, a clear image of the specimen
can be obtained even when there is a high level of background radiation.
In comparing the two primary detection methods, the direct-exposure method offers high speed, unlimited image-
integration time, and the best spatial resolution. The transfer method offers insensitivity to the -rays emitted by the
specimen and greater contrast because of lower amounts of scattered and secondary radiation.
Real-time imaging, in which light from a scintillator is observed by a television camera, can also be used for neutron
radiography. Because of low brightness, most real-time neutron radiographic images are enhanced by an image-intensifier
tube, which may be separate or integral with the scintillator screen. This method can be used for such applications as the
study of fluid flow in a closed system or the study of metal flow in a mold during casting. The lubricants moving in an
operating engine have been observed with the real-time neutron imaging method.
Neutron Radiography
Harold Berger, Industrial Quality, Inc.
Applications
Various applications concerning the inspection of ordnance, explosive, aerospace, and nuclear components are discussed
in Ref 1, 2, 3, 4, 5, 6, 7, 8, 9. The presence, absence, or correct placement of explosives, adhesives, O-rings, plastic
components, and similar materials can be verified. Nuclear fuel and control materials can be inspected to determine the
distribution of isotopes and to detect foreign or imperfect material. Ceramic residual core in investment cast turbine
blades can be detected. Observations of corrosion in metal assemblies are possible because of the excellent neutron
sensitivity to the hydrogenous corrosion product. Hydride deposition in metals and diffusion of boron in heat treated
boron-fiber composites can be observed. The following examples illustrate the application of neutron radiography to the
inspection of radioactive materials and several assemblies of metallic and nonmetallic components.
Example 1: Thermal-Neutron Radiography Used to Determine Size of Highly
Radioactive Nuclear Fuel Elements.
Highly radioactive nuclear fuel elements required size measurements to determine the extent of dimensional changes that
may have occurred during irradiation. Generally, inspection is done in a hot cell, but because hot-cell inspection is a
relatively long, tedious, and costly procedure, neutron radiography was selected.
The fuel elements to be inspected consisted of 6.4 mm ( in.) diam cylindrical pellets of UO
2
-PuO
2
; the plutonium
content was 20%, and the uranium was enriched in
235
U. The pellets had been irradiated to 10% burnup, which resulted in
a level of radioactivity of 3 × 10
-2
C/kg · h (10 KR/h) at 0.3 m (1 ft).
Five elements were selected for inspection. A neutron radiograph was taken by activating 0.25 mm (0.010 in.) thick
dysprosium foil with a transmitted beam of thermal neutrons. An autoradiograph of the activated-dysprosium transfer
screen on a medium-speed x-ray film yielded the result shown in the positive print in Fig. 6.
Fig. 6 Positive print of a thermal-neutron radiograph of five irradiated nuclear fuel elements, t
aken to
determine if dimensional changes occurred during irradiation. Radiograph was made using a dysprosium
transfer-screen method. Dark squares in middle element are voids.
Both
235
U and plutonium have high attenuation coefficients for thermal neutrons. The high contrast of the fuel pellets
made it possible to measure pellet diameter directly from the neutron radiographs. These measurements were both
repeatable and statistically significant within 0.013 mm (0.0005 in.). Later, radiographic measurements were compared
with physical measurements made in a hot cell. The two sets of values corresponded within 0.038 mm (0.0015 in.).
Example 2: Indium-Resonance Technique for Determining Internal Details of
Highly Radioactive Nuclear Fuel Elements.
The five nuclear fuel elements inspected for dimensional changes in Example 1 were further inspected for internal details.
This was necessary because the thermal-neutron inspection procedure did not reveal any internal details; it only shadowed
the pellets, as shown by the positive print in Fig. 6.
To inspect for internal details, an indium-resonance technique, which utilizes epithermal neutrons, was used. In this
technique, a collimated neutron beam was filtered by 0.5 mm (0.02 in.) of cadmium to remove most of the thermal
neutrons. Filtering produced a neutron beam with a nominal average energy somewhat above thermal. The epithermal-
neutron beam passed through the fuel elements and activated a sheet of indium foil. Neutrons with an energy of about
2.34 × 10
-19
J (1.46 eV), which is the resonance-absorption energy for indium, were primarily involved in activation. The
positive print of a radiograph made with epithermal neutrons shown in Fig. 7 reveals considerable internal details, in
contrast to the lack of internal details in Fig. 6.
Fig. 7 Positive print of a neutron radiograph of the same five nuclear fuel elements shown in Fig. 6
. Radiograph
was made with epithermal neutrons and an indium-resonance technique, and it r
eveals internal details not
shown in the thermal-neutron radiograph in Fig. 6
With epithermal neutrons, there was less attenuation by the fuel elements than with thermal neutrons. Therefore, internal
details that were not revealed by thermal-neutron radiography--such as cracking or chipping of fuel pellets, and
dimensional features of the central void in the fuel pellets (including changes in size and accumulation of fission
products)--were revealed with the indium-resonance technique.
Example 3: Use of Conventional and Neutron Radiography to Inspect an
Explosive Device for Correct Assembly.
Small explosive devices assembled from both metallic and nonmetallic components required inspection to ensure correct
assembly. The explosive and the components made of paper, plastic, or other low atomic number materials, which are less
transparent to thermal neutrons than to x-rays, could be readily observed with thermal-neutron radiography. Metallic
components were inspected by conventional x-ray radiography.
A positive print of a thermal-neutron, direct-exposure radiograph of a 50 mm (2 in.) long explosive device is shown in
Fig. 8(a). The radiograph was made on Industrex R film (Eastman Kodak), using a gadolinium-foil screen. Total exposure
was 3 × 10
9
n/cm
2
, which was achieved with an exposure time of 4 to 5 min. At the top in Fig. 8(a), just inside the
stainless steel cap, can be seen a line image that corresponds to a moisture absorbent made of chemically treated paper.
Below the paper is a mottled image, which is the explosive charge. Below the explosive charge are plastic components
and, at the very bottom, epoxy.
Fig. 8 Comparison of positive prints of a thermal-
neutron radiograph (a) and a conventional radiograph (b) of a
50 mm (2
in.) long explosive device. Neutron radiograph reveals details of paper, explosive compound, and
plastic components not revealed by x-rays.
A conventional radiograph of the same device is shown in Fig. 8(b). The metallic components, which were poorly
delineated in the thermal-neutron radiograph, are more clearly seen in Fig. 8(b). Together, the two radiographs verified
that both metallic and nonmetallic components were correctly assembled.
Example 4: Use of Neutron Radiography to Detect Corrosion in Aircraft
Components.
Aluminum honeycomb components are extensively used for aircraft construction. The aluminum material is subject to
corrosion if exposed to water or humid environments. Thermal-neutron radiography is an excellent method of detecting
hidden corrosion in these assemblies. The corrosion products are typically hydroxides or water-containing oxides; these
corrosion products contain hydrogen, a material that strongly attenuates thermal neutrons. The aluminum metal, on the
other hand, is essentially transparent to the neutrons. Therefore, a thermal-neutron radiograph of a corroded aluminum
honeycomb assembly shows the corrosion product and other attenuating components such as adhesives and sealants.
Figure 9 depicts a thermal-neutron radiograph of an aluminum honeycomb assembly showing the beginnings of
corrosion. The white line image across the middle of the radiograph represents the adhesive coupling together two core
sections. The faint white smears in the upper half of the image and the double dot in the lower left area are images of
corrosion as disclosed by the thermal-neutron radiograph. Developmental work has shown that thermal-neutron imaging
techniques are capable of detecting the corrosion product buildup represented by an aluminum metal loss of 25 m (1000
in.). The neutron method, therefore, is a very sensitive technique for the detection of corrosion.
Fig. 9 Thermal-
neutron radiograph of aluminum honeycomb aircraft component showing early evidence of
hydrogen corrosion. See text for discussion. Courtesy of D. Froom, U.S. Air Force, McClellan Air Force Base
Example 5: Use of Neutron Radiography to Detect Corrosion in Adhesive-
Bonded Aluminum Honeycomb Structures.
Aluminum corrosion of aircraft surfaces has plagued both military and civilian aircraft. Identification of this corrosion has
been difficult, at best, usually being detected after the corrosion has caused the part to fail. Of the nondestructive testing
methods used to detect aluminum corrosion, thermal neutron radiography has proved the most sensitive method to date.
The detection of aluminum corrosion is based on the attenuation properties of hydrogen associated with the corrosion
products rather than aluminum and aluminum oxide with their low attenuation coefficients. Depending on the
environment, the corrosion products include aluminum trihydrates, monohydrates, and various other aluminum salts.
Because the linear attenuation coefficient for aluminum is similar to that of water and about 28 times greater than that for
aluminum, a 0.13 mm (0.005 in.) corrosion layer should be detectable under optimum conditions.
The sensitivity standard plate for aluminum corrosion fabricated by the Aeronautical Research Laboratories (Australia)
contains corrosion products varying from 0.13 to 0.61 mm (0.005 to 0.024 in.) thick (Fig. 10).
Fig. 10
Standard plate for aluminum corrosion detection contains 0.13 to 0.61 mm (0.005 to 0.024 in.) thick
corrosion products. Courtesy of R. Tsukimura, Aerotest Operations Inc.
Aluminum corrosion of honeycomb structures is complicated by the bonding adhesives that may appear similar in a
neutron radiograph (Fig. 11a) (see the article "Adhesive-Bonded Joints" in this Volume). Tilting of the honeycomb
structure will alleviate this problem by allowing adhesive found along the bond lines to be distinguished from the
randomly distributed corrosion products (Fig. 11b).
Fig. 11 Effect of bonding adhesives on the quality of neutron
radiographs obtained when checking for
aluminum corrosion in honeycomb structures. Radiograph taken (a) normal to specimen surface and (b) tilted
at any angle other than 90° to specimen surface. Courtesy of R. Tsukimura, Aerotest Operations Inc.
Example 6: Use of Neutron Radiography to Verify Welding of Dissimilar
Materials (Titanium and Niobium).
Exotic metal welded joints are a product of the extremely cold environment of space and man's desire to explore the vast
emptiness of space. For space vehicles, attitude control rockets provide the fine touch for proper vehicle alignment.
For one application, a titanium-niobium welded joint was required between the light-weight propellant tank and the
nozzle section. Attempts to verify weld integrity using conventional radiography were not productive.
Thermal neutron radiography provided the image required to ensure quality welds. This defect standard weld shows the
porosity at the seam and the similar thermal neutron attenuation for both titanium (Ti) and niobium (formerly known as
columbium, Cb) (Fig. 12a). For comparison, the x-ray radiograph image is also shown (Fig. 12b).
Fig. 12 Comparison of thermal neutron (a) and x-ray (b) radiographs of a titanium-
niobium welded joint.
Courtesy of R. Tsukimura, Aerotest Operations Inc.
Example 7: Use of Neutron Radiography to Detect Core Material Still Remaining
in the Interior Cooling Passages of Air-Cooled Turbine Blades.
Investment casting of turbine blades using the lost wax process results in relatively clean castings. As the demand for
higher-powered turbine engines has increased, the interior cooling passages for air-cooled turbine blades have become
more and more complex. Concurrently, the removal of the core material has become increasingly more difficult.
Incomplete removal of the core results in restricted flow through the cooling passages and possible failure of the
overheated blade.
Previously, visual inspection was the nondestructive inspection method of choice for residual core detection. However,
current designs preclude the use of borescopes and other visual means for the interior passages. X-radiography has proved
rather ineffective in detecting residual core material. Thermal neutron radiography is the nondestructive testing method of
choice, especially when gadolinium oxide (Gd
2
O
3
) is used to dope the core material (1 to 3% by weight) (Fig. 13) prior to
casting the blade.
Fig. 13 Residual core material in a gas-cooled aircraft-
engine turbine blade as detected by thermal neutron
radiography. The excess core material, tagged with 1.5% Gd
2
O
3
, is shown circled in the second photo from the
right. Courtesy of R. Tsukimura, Aerotest Operations Inc.
When concerns about the possible detrimental effects of Gd
2
O
3
during the casting process prevents its use in the core
material, a procedure was developed to tag the residual core material after the core removal process. The castings are
dipped in a gadolinium solution [Gd(NO
3
)
2
in solution] to impregnate any residual core, which is then imaged and
subsequently detected by neutron radiography.
The blades shown in Fig. 14 have been tagged. The neutron radiograph shows any residual core material greater than 0.38
mm (0.015 in.) in diameter. Figure 15 is a schematic of typical core fragments in investment cast turbine blades detected
by thermal neutron radiography. Image clarity of gadolinium tagged or doped cores is much greater than that of normal
cores.
Fig. 14 Thermal neutron radiograph of 12 turbine blades tagged with Gd
2
O
3
solution. One of the 12 blades
(located in the top row and second from the left) contains residual core material in its upper right-
hand corner
cooling passage. Courtesy of R. Tsukimura, Aerotest Operations Inc.
Fig. 15 Schematic of turbine blade core standards: gadolinium [Gd(NO
3
)
3
in solution] tagged core, normal core
(no gadolinium tagging or doping), and Gd
2
O
3
doped co
re. Typical core fragments of various thicknesses are
shown. Source: R. Tsukimura, Aerotest Operations Inc.
Example 8: Use of Neutron Radiography to Verify Position of Explosive Charges
and Seating of O-Ring Seals in Explosive Bolt Assemblies.
There are many critical applications of explosive release devices in aircraft, space, and missile systems. Nondestructive
testing is an important step in the quality control portion of the production cycle for these units. Thermal neutron
radiography has proved an indispensable tool in the nondestructive testing arsenal, particularly for thick-walled, metal
devices, such as explosive bolts (Fig. 16).
Fig. 16 Schematic showing location of critical components that comprise an explosive bolt.
Source: R.
Tsukimura, Aerotest Operations Inc.
The inner details of explosive bolts can be imaged only by thermal neutron radiography methods (Fig. 17). This particular
type of bolt from a missile system is activated from the bottom by actuating the firing pin onto the primer. The short
section of mild detonating cord carries the energy to the output charge, which fractures the bolt and allows the bolt to be
severed.
Fig. 17 Thermal neutron radiograph showing two sample bolts identical to the workpiece shown sch
ematically
in Fig. 16. Courtesy of R. Tsukimura, Aerotest Operations Inc.
In addition to the explosive charges, the internal O-ring seals, including the concentric pair around the firing pin and for
the body are readily visible. For safety's sake, determining the presence of the shear pin can also be accomplished through
the use of thermal neutron radiography.
Example 9: Application of Neutron Radiography to Determine Potting Fill Levels
in Encapsulated Electronic Filters.
Electronic filters are an integral component of all space and satellite systems. Because the cost of these satellites is very
high and the cost to repair them even more prohibitive, high reliability filters are necessary.
A common mode of filter failure is that caused by inadequate potting of the internal components and the subsequent
physical breakdown of the filter during periods of high vibration, such as that encountered during vehicle launch. Thermal
neutron radiography is the method of choice for determining potting fill levels in encapsulated filters.
The potting material attenuates the thermal neutrons and appears as the light density area. Voids in the potting material,
the fill level, and the distribution can readily be detected with neutron radiography.
References cited in this section
1.
H. Berger, Ed., Practical Applications of Neutron Radiography and Gaging,
STP 586, American Society for
Testing and Materials, 1976
2.
Neutron Radiography Issue, At. Energy Rev., Vol 15 (No. 2), 1977, p 123-364
3.
N.D. Tyufyakov and A.S. Shtan, Principles of Neutron Radiography,
Amerind Publishing, 1979 (translated
from the Russian)
4.
P. Von der Hardt and H. Rottger, Ed., Neutron Radiography Handbook, D. Reidel Publishing, 1981
5.
J.P. Barton and P. Von der Hardt, Ed., Neutron Radiography, D. Reidel Publishing, 1983
6.
L.E. Bryant and P. McIntire, Ed., Radiography and Radiation Testing, in Nondestructive Testing Handbook,
Vol 3, American Society for Nondestructive Testing, 1985
7.
"Standard Practices for Thermal Neutron Radiography of Materials," E 748, Annual Book of ASTM
Standards, American Society for Testing and Materials
8.
J.P. Barton, G. Farny, J.L. Person, and H. Rottger, Ed., Neutron Radiography, D. Reidel Publishing, 1987
9.
H. Berger, Some Recent Developments in X-Ray and Neutron Imaging Methods, in Nondestructive Testing,
Vol 1, J.M. Farley and R.W. Nichols, Ed., Pergamon Press, 1988, p 155-162
Neutron Radiography
Harold Berger, Industrial Quality, Inc.
References
1. H. Berger, Ed., Practical Applications of Neutron Radiography and Gaging,
STP 586, American Society
for Testing and Materials, 1976
2. Neutron Radiography Issue, At. Energy Rev., Vol 15 (No. 2), 1977, p 123-364
3. N.D. Tyufyakov and A.S. Shtan, Principles of Neutron Radiography,
Amerind Publishing, 1979
(translated from the Russian)
4. P. Von der Hardt and H. Rottger, Ed., Neutron Radiography Handbook, D. Reidel Publishing, 1981
5. J.P. Barton and P. Von der Hardt, Ed., Neutron Radiography, D. Reidel Publishing, 1983
6. L.E. Bryant and P. McIntire, Ed., Radiography and Radiation Testing, in
Nondestructive Testing
Handbook, Vol 3, American Society for Nondestructive Testing, 1985
7.
"Standard Practices for Thermal Neutron Radiography of Materials," E 748, Annual Book of ASTM
Standards, American Society for Testing and Materials
8. J.P. Barton, G. Farny, J.L. Person, and H. Rottger, Ed., Neutron Radiography, D. Reidel Publishing, 1987
9. H. Berger, Some Recent Developments in X-Ray and Neutron Imaging Methods, in
Nondestructive
Testing, Vol 1, J.M. Farley and R.W. Nichols, Ed., Pergamon Press, 1988, p 155-162
10. A. Ridal and N.E. Ryan, in Neutron Radiography, Proceedings of the Second World Conference
(Paris,
June 1986), J.L. Barton et al., Ed., D. Reidel Publishing, 1987, p 463-470
Thermal Inspection
Grover Hardy, Wright Research and Development Center, Wright-Patterson Air Force Base; James Bolen, Northrop Aircraft Division
Introduction
THERMAL INSPECTION comprises all methods in which heat-sensing devices are used to measure temperature
variations in components, structures, systems, or physical processes. Thermal methods can be useful in the detection of
subsurface flaws or voids, provided the depth of the flaw is not large compared to its diameter. Thermal inspection
becomes less effective in the detection of subsurface flaws as the thickness of an object increases, because the possible
depth of the defects increases.
Thermal inspection is applicable to complex shapes or assemblies of similar or dissimilar materials and can be used in the
one-sided inspection of objects. Moreover, because of the availability of infrared sensing systems, thermal inspection can
also provide rapid, noncontact scanning of surfaces, components, or assemblies.
Thermal inspection does not include those methods that use thermal excitation of a test object and a nonthermal sensing
device for inspection. For example, thermally induced strain in holography or the technique of thermal excitation with
ultrasonic or acoustic methods does not constitute thermal inspection.
Thermal Inspection
Grover Hardy, Wright Research and Development Center, Wright-Patterson Air Force Base; James Bolen, Northrop Aircraft Division
Principles of Thermal inspection
The basic principle of thermal inspection involves the measurement or mapping of surface temperatures when heat flows
from, to, or through a test object. Temperature differentials on a surface, or changes in surface temperature with time, are
related to heat flow patterns and can be used to detect flaws or to determine the heat transfer characteristics of a test body.
For example, during the operation of a heating system, a hot spot detected at a joint in a heating duct may be caused by a
hot air leak. Another example would be a hot spot generated when an adhesive-bonded panel is uniformly heated on one
side. A localized debonding between the surface being heated and the substructure would hinder heat flow to the
substructure and thus cause a region of higher temperature when compared to the rest of the surface. Generally, the larger
the imperfection and the closer it is to the surface, the greater the temperature differential.
Heat Transfer Mechanisms. Heat will flow from hot to cold within an object by conduction and between an object
and its surroundings by conduction, convection, and radiation. Within a solid or liquid, conduction results from the
random vibrations of individual atoms or molecules that is transferred via the atomic bonding to neighboring atoms or
molecules. In a gas, the same process occurs but is somewhat impeded by the greater distance between the atoms or
molecules and the lack of bonds, thus requiring collisions to transfer the energy. When a gas or liquid flows over a solid,
heat is transferred by convection. This occurs from the collisions between the atoms or molecules of the gas or liquid with
the surface (conduction) as well as the transport of the gas or liquid to and from the surface. Convection depends upon the
velocity of the gas or liquid, and cooling by convection increases as the velocity of the gas or liquid increases.
Radiation is the remaining mechanism for heat transfer. Although conduction and convection are generally the primary
heat transfer mechanisms in a test object, the nature of thermally induced radiation can be important, particularly when
temperature measurements are made with radiation sensors.
Electromagnetic radiation is emitted from a heated body when electrons within the body change to a lower energy state.
Both the intensity and the wavelength of the radiation depend on the temperature of the surface atoms or molecules. For a
blackbody, the radiation wavelength and spectral emission power vary as a function of temperature, as shown in Fig. 1. At
300 K (27 °C), the temperature of a warm day, the dominant wavelength is 10 m (400 in.), which is in the infrared
region (Fig. 2). A surface would have to be much hotter for the dominant wavelength to fall in the visible region below