enhanced radiography for aircraft materials and components

Enhanced radiography for aircraft materials and components

John Thornton

Airframes and Engines Division, DSTO, Fishermens Bend, Victoria 3207, Australia

Received 1 July 2002; accepted 21 May 2003

Abstract

The sharpest radiographic images are usually obtained with the film adjacent to the component. In this geometry the

contrast is due only to absorption. Greater sensitivity can sometimes be obtained by moving the film away from the com-ponent because this allows refraction and diffraction effects to provide additional contrast. This study quantifies theseeffects and determines the limits of their applicability to the major aerospace materials aluminium, titanium and nickel

based alloys. It shows how the additional contrast is dependent on the use of long wavelengths (50.1 nm). Thus theuse of enhanced radiography with X-rays is limited to the study of thin aluminium and composite structures because ofabsorption. However, it is shown that long wavelengths neutrons can produce radiographs of most metals withenhanced contrast. A comparison of conventional and enhanced radiography techniques applied to a typical aircraft

problem is presented and includes results from a previous study with neutrons.Crown Copyright # 2003 Published by Elsevier Ltd. All rights reserved.

Keywords: Non-destructive inspection; X-ray analysis; Cracks; Defects; Turbine blade

1. Introduction

Conventional radiography has found widespread use in the detection of defects in aircraft components.The sharpest images are usually obtained with the film adjacent to the component. In this geometry thecontrast is due only to absorption. Greater sensitivity can sometimes be obtained by moving the film awayfrom the component because this allows refraction, small angle scattering and diffraction effects to provideadditional contrast [1–3]. This technique is usually referred to as phase contrast; however, this label may beinappropriate when small angle scattering is responsible for enhancing contrast.Spatially coherent radiation is required for the diffraction, refraction and small angle scattering effects to

occur. Thus parallel radiation from a synchrotron or spherical radiation from a micro-focus source orpinhole are usually employed. Also the effects become clearer when the radiation is monochromatic, this isparticularly true for diffraction and small angle scattering.Conventional X-ray radiography is used extensively for inspecting a wide range of aircraft components.

Conventional neutron radiography has also been used to inspect dense gas turbine components and search

1350-6307/$ - see front matter Crown Copyright # 2003 Published by Elsevier Ltd. All rights reserved.

doi:10.1016/j.engfailanal.2003.05.008

Engineering Failure Analysis 11 (2004) 207–220

www.elsevier.com/locate/engfailanal

E-mail address: [email protected] (J. Thornton).

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for corrosion in aircraft structures. However, conventional radiography is relatively insensitive to cracksunless the cracks are aligned parallel to the incident rays because contrast relies solely on absorption. Intheory, phase contrast should be sensitive to cracks that are not parallel to the incident rays because suchcracks can introduce significant (>0.1 p) phase differences. If such phase differences occur abruptly overthe surface of the object Fresnel diffraction fringes can be produced at the edges of the cracks’ images.Furthermore, even if the phase difference occurs less abruptly refraction can cause a significant redis-tribution of intensity in the image. However, the angle through which the incident radiation is ‘‘scattered’’by diffraction, refraction and small angle scattering increases with wavelength. Thus for practical object toimage distances (metres) long wavelength radiation is required. This may limit the application of X-rayphase contrast techniques to low density materials such as aluminium or polymer matrix composites (orbiological objects) because of absorption. However, long wavelength neutrons are very penetrating com-pared to X-rays and are therefore ideally suited to phase contrast techniques. This is of particular relevanceto Australia because a replacement research reactor will soon be constructed at Lucas Heights in NSW.This paper outlines the optical theory of the diffraction and refraction based mechanisms, and uses the

theory to quantify the practical limitations of phase contrast. Small angle scattering is not considered. Thelimitations are illustrated by comparing X-ray and neutron radiography studies of a damaged turbineblade.

2. Theory

2.1. Penumbra

When the film is moved away from the object, blurring of the image will occur because the radiationsource is always larger than a mathematical point. The blurring is simply due to parallax and the blurredregion at edges of the image is the penumbra. To avoid the penumbra becoming significant, micro-focusradiation sources or pinholes can be used. Geometric magnification is the another factor that increases thesize of the penumbra, that is, the size of the penumbra is proportional to the ratio of the object to filmdistances and source to object. Thus unlimited magnification is not possible and small source sizes arenecessary to avoid the penumbra obscuring the diffraction, refraction and small angle scattering effects.The width, b, of the penumbra is given by:

b ¼ sr=L ð1Þ

e s is the diameter of the radiation source, and r is the object to image plane (film) distance, and L is
wherthe source to object distance.Today, position sensitive detectors are often used instead of film. The term image plane is used here to be
more general and indicates the position of the detector or film.To prevent the penumbra from limiting the spatial resolution, the geometry needs to be optimised for

the space available. If the available source size is larger than the spatial resolution of the film or position-sensitive radiation detector then r and L are to be equal. If the source size is smaller r can be expanded atthe expense of L.

2.2. Phase shift

In a similar fashion to visible optics the X-ray [4,5] and neutron [6] refractive indices of materials can beused to quantify phase and refraction effects. As in visible optics the real part of the refractive indexaccounts for the phase shift, and the imaginary part accounts for absorption.

208 J. Thornton / Engineering Failure Analysis 11 (2004) 207–220

Fig. 1 shows what happens to a spherical wave front as it passes through an object with an internal flaw(crack or void). As we are concerned with X-rays and neutrons, the refractive index of most materials isactually slightly less than in air and therefore the wave front moves faster through the object than throughthe crack. The result is that the portion of the wavefront that passes through the crack is then behind thebulk of the wavefront. The phase shift �� is given by:

D� ¼ �2� n1 � n2ð Þd=l ð2Þ

where d is the thickness of the crack in the y-direction, n1 and n2 are the real part of the refractive indices ofthe air in the crack and the matrix respectively, and l is the wavelength of the radiation.The minus sign denotes that it is a phase lag. As the refractive indices are usually just less than unity the

real part can be written as 1�". For air or the inside the crack we can assume " is zero. Thus, Eq. (2)becomes:

D� ¼ �2�"d=l ð3Þ

Away from absorption edges the relationship between wavelength and " for X-rays and neutrons is simple.For neutrons [6]:

" ¼ �bl2=2� ð4Þ

l is the neutron wavelength, b is the bound coherent scattering length—this can be obtained frompublished tables [7], and n is the average number of atoms per unit volume.Similarly for X-rays [4]:

" ¼ �re Zþ Df0

a

� �l2=2� ð5Þ

Fig. 1. The CSIRO phase contrast geometry. The effect of an internal flaw (void or crack) (a) in a nickel plate (Object) on the trans-

mitted wavefront (b) and the observed intensity profile (c) at the image plane. Dark bands are produced at the edges of the crack’s

image. Further oscillations of the intensity profile may also be present depending on the wavelength purity of the incident radiation.

The coordinate system employed in this paper is also illustrated.

J. Thornton / Engineering Failure Analysis 11 (2004) 207–220 209

e re is the classical radius of the electron (2.817�10�15 m), Z is the atomic number (the number of
wherelectrons per atom), and Df
0

a is the real part of the dispersion correction for the atomic scattering factor—which can again be obtained from published tables [8].For the short wavelength X-rays used in radiography Df

0

a is usually negligible.The variation of " with wavelength for the elemental bases of the major aerospace alloys (Ni, Ti, Al) is

plotted in Fig. 2. Absorption edges aside " is proportional to l2. Notice that with neutrons Ti has a nega-tive " and thus, in contrast with all the other cases, a slightly positive refractive index. The phase shift inFig. 1 will therefore be an advance not a lag for this case.The combination of Eqs. (3) and (4), and (3) and (5) show that the phase shifts are proportional to the

wavelength. The phase shift for voids is similar to that for cracks in that Eqs. (3), (4) and (5) can be appliedunchanged. Other flaws such as inclusions may not produce such a significant phase shift. In these case thedifference in the refractive indices between the matrix and the substance of the flaw must be used in place of" in Eq. (3).

2.3. Diffraction

The fringes produced by the step in phase difference introduced by the edge of a crack or similar compactfeature are due to Fresnel diffraction. In textbooks of optics [9,10] diffraction at the edge of an opaque halfscreen is usually analysed; however, phase differences are also predicted to produce such fringes. The rele-vant equation is the Fresnel–Kirchoff diffraction formula (p. 623 [9] or p. 382 [10]) which is an extension ofHuygen’s principle (p. 71 [9]) which treats each point on the wavefront as a separate source. The fringesoccur because the distance to a point on the screen from one of these virtual sources changes abruptly asthe step in wavefront is traversed. Thus interference is possible. With monochromatic radiation, diffractionwill produce a number of fringes, but with a spread of wavelengths only the major first order fringe may bevisible.Solving the Fresnel–Kirchoff diffraction formula for general cases of Fresnel diffraction can be difficult;

therefore, in this paper, a numerical method based on Huygen’s principle was used to calculate the fringepatterns. The actual source of the incident radiation was also treated as a point. At each point on the imageplane the contributions to the complex amplitude from 5000 equally spaced points across the wavefrontwere summed. The amplitude obtained was then squared to give the intensity at this point on the imageplane. By repeating this process point by point over the image plane the intensity profiles shown wereconstructed. The contribution to the amplitude from a point on the wavefront was assumed to fall as 1/r,

Fig. 2. Variation of ", 1�Re(n), with wavelength. Where Re(n) is the real part of the refractive index.


where r is the distance from the point. This gives the expected 1/r2 reduction in intensity. With no region ofphase lag in the wavefront a flat intensity profile would be expected. This was used as a basic test for thealgorithm, a test it passed.

2.4. Refraction

The step in the wavefront can be a smooth transition if the flaw has rough edges. This will reduce thestrength of the diffraction fringes but may still produce significant addition contrast when the film is distantfrom the object. This can be understood by asking what direction a ray from the wavefront will be headinggiven that it will be normal to the local wavefront as shown in Fig. 3.With X-rays and neutrons � is small and therefore the following equation can be derived from Fig. 3.

� ¼df yð Þ

dyð6Þ

Fig. 4 is a schematic diagram of how an ellipsoidal void will produce a smooth dip or phase lag in thetransmitted wavefront. Here the rays will come to a focus and then diverge. Thus image of the flaw will besmall and bright at the focal distance and dimmer for large object to film distances. For the case of a denseinclusion the wavefront will have a hump and the rays from the hump will diverge and the correspondingregion on the film will become fainter if the film is moved further away from the object. The case of a voidof height t (z direction), full width 2w (y direction) and a matrix of refractive index 1�", is illustrated inFig. 3. The median � for the dip in the wavefront is seen to be:

� ¼ t"=w ð7Þ

Fig. 2 shows that " is very small (2�10�5) for wavelengths less than 0.1 nm. Thus, � is also very small forflaws with modest aspect ratios (t/w). Even with the film 5m from the object the shift on the film would be0.1 mm or less.As noted in Section 2.1, to generalise from voids or cracks to flaws such as inclusions, " in Eq. (7)

becomes the difference of the refractive indices. Note even with monochromatic radiation, refraction is notexpected to produce fringes in the image, like diffraction. Instead the radiation is re-directed (shifted,focused or de-focused) to brighten some areas of the image at the expense of others.

Fig. 3. The geometric relationship between the divergence, �, of a refracted ray from the z direction and df(y)/dy. Where f(y) is the

displacement of the emerging wavefront in the z direction or in other words the phase profile.


2.5. Absorption

The absorption of radiation increases exponentially as the thickness of the object increases. The reci-procal of the linear attenuation coefficient, �l, is the distance over which the intensity of radiation drops by1/e (0.3679).Fig. 5 shows this distance as a function of wavelength for X-rays in the major aerospace alloy bases Al,

Ti and Ni [11]. Fig. 6 is the equivalent plot for neutrons [7]. The exponential nature of X-ray and neutronabsorption means that thickness limits are essentially arbitrary. There is always a non-zero probability thatsome radiation will penetrate any thickness. However, from established guidelines (p. 207 [12]) a practical

Fig. 4. The refraction of X-rays or neutrons seen as a consequence of the distortion of the wavefront after it passes through a uniform

plate containing a void—a typical flaw. Note that the diagram is not to scale. In particular, the object to image plane distance is

several meters while the object is only around a millimetre thick.

Fig. 5. Characteristic absorption length (1/�l) for X-rays as a function of wavelength. The dashed line is given by combining a

transmission limit of 1% and requiring that the components be at least 1 mm thick.


transmission limit of 1% was derived. As components are unlikely to be thinner than 1 mm, this was usedas a practical lower limit for thickness. The combination of the thickness and transmission limits and theuse of the exponential absorption equation (p. 161 [12]) produces the horizontal dashed lines on Figs. 5 and6. The intersection of these lines with the absorption curves gives the upper wavelength limits (arrows inFig. 5); longer wavelengths are absorbed too strongly. Even for aluminium based components the wave-length must be less than 0.13 nm. Looked at another way only 1% of 0.13 nm radiation will penetrate 1mm of aluminium. Figs. 5 and 6 show that neutrons are much more penetrating than X-rays. The upperwavelength limit for neutrons is greater than 1 nm.

3. Feasibility study

Given the theory presented in Section 2, a feasibility study was performed to ascertain the practicallimitations of the diffraction and refraction mechanisms.

3.1. Fresnel diffraction

As the phase shift for both X-rays and neutrons is proportional to the wavelength (Section 2.2) thenusing longer wavelength radiation can increase the sensitivity to small cracks or voids. However, attenua-tion imposes a 0.13 nm upper limit on the wavelength of X-rays that can be used (Section 2.5) and thislimits the sensitivity that can be obtained. Neutrons do not suffer this limitation.Aside from wavelength, the other limitations are the space available, the size of the source, and the

spatial resolution of the film or detector. For this study we assume the arbitrary but reasonable upper limitto the source to film distance of 6 m. This allows a magnification of �5 with L=1 m and r=5 m. Micro-focus X-ray sources can produce circular sources with diameters <10 mm. For this study a diameter of 5mm will be assumed.Film can have a spatial resolution of 1 mm [12], although examination by an optical microscope is

required to inspect the film. Image plates and CCD can have spatial resolutions of 25 [13,14] to 14 mm [15],respectively. As a feature must usually span many pixels to be identified the criteria for resolving a featureis that its image must be >0.1 mm. The geometric magnification must not cause the size of the penumbra

Fig. 6. Characteristic absorption length (1/�l) for neutrons as a function of wavelength. The dashed line is given by combining a

transmission limit of 1% and requiring that the components be at least 1 mm thick.


to degrade the resolution. Eq. (1) shows that a magnification of �5 is acceptable (L=1 m, r=5 m ands=5 mm gives b=25 mm).A numerical simulation of the Fresnel diffraction from the edge of a transparent plate was performed

using the geometry outlined above. The phase change introduced by the plate was �/2, and the radiationwas assumed to be monochromatic. This simple example is the basis of many more complex features suchas cracks. The fringe pattern predicted for 0.0559 nm radiation (AgKa) is shown in Fig. 7.As shown in Fig. 7 the distance between the main peaks is taken as the fringe spacing. A plot of this

fringe spacing as a function of wavelength is shown in Fig. 8. The function fringe spacing, fs=0.092l1/2

produces a curve that is a good fit to the simulation data. Basically, the fringe spacing increased as thewavelength increases although the phase shift was kept constant. Thus the diffraction effects will be easierto resolve with long wavelength radiation.The resolution limit of 0.1 mm is shown as a dashed horizontal line in Fig. 8. The intersection of this line

and the fringe spacing curve give an upper wavelength limit of 0.12nm. It will be difficult to resolve thediffraction effects using shorter wavelengths. A comparison of Fig. 8 to the X-ray absorption plots of Fig. 5shows that the observation of X-ray diffraction effects from Ni and Ti alloy components is not feasible.Absorption is too strong for the long wavelength X-rays required. For thin (1 mm) Al alloy components itmay just be feasible.To estimate the size of feature that can be detected using diffraction the data of Fig. 2 and Eq. (3) can be

used to find the phase shift as a function of the length of the defect in the z direction, d. For a wavelengthof 0.12 nm the real part of the refractive index is 1�10�5 (Fig. 2). Eq. (3) becomes��=1.7�105 d, where d isin metres, and�� in units of �. Thus a 100 mm long crack will give a phase change of 17 �. This is a large phasechange especially when one considers the cyclic nature of the interference as a function of phase and that theperiod is just 2�. Such large phase shifts will cause the fringe pattern to average out unless the radiation isstrongly coherent. However, phase shifts of a tenth of a period will be detected easily thus the diffractiontechnique will be able to detect flaws with dimensions in the z-direction down to 6 mm. As the width of cracksare of this order then it should be feasible to detect cracks running perpendicular to the rays. The proviso isthat the flaws must have abrupt edges. The question of how abrupt will be dealt with in the discussionsection.For neutrons micro-focus sources are not available. However, a pinhole in a sheet of neutron absorbing

material such as cadmium or gadolinium can also be used in front of a neutron source. In deriving thecurve of Fig. 8 no assumption was made about the type of radiation therefore the figure applies to neutrons

Fig. 7. Predicted fringe pattern for AgKa X-rays and s=5 m, r=1 m, and a �/2 phase shift.


as well as X-rays. As spatial resolution of the detectors and space limitations are also similar for X-raysand neutrons the horizontal limit in Fig. 8 also applies. Thus the neutron wavelength must be greater than0.12 nm for diffraction effects to be detected.Fortunately, the production of neutrons at a suitable wavelength is not a problem. The cold neutron

sources of nuclear reactors produce neutrons with wavelengths in the 0.1–1 nm region [16]. Furthermore,as can be seen from Fig. 6, absorption is not significant for all but the most dense and bulky components.Therefore neutrons are ideally suited to enhancing radiographic images using Fresnel diffraction.

3.2. Refraction

To determine whether refraction effects will be visible, Eq. (7) must be used together with the refractiveindex data calculated using Eqs. (4) and (5). These imply the use of longer wavelengths but absorptionlimits this for X-rays. From Fig. 5 the longest wavelength X-rays that can be used was determined. Section2.4 showed that the divergence of the rays increases with increasing aspect ratio (t/w) of the flaw [Eq. (7)].Thus flaws that are long in the direction of propagation will be easier to detect. The aspect ratio could alsobe viewed as a measure of abruptness of the edge of a large flaw.Table 1 shows the smallest detectable aspect ratio for voids using X-rays. In summary, refraction of X-

rays will provide additional contrast in X-ray radiography. However, the aspect ratio of the flaws that canbe detected vary. For all these materials the flaws need to be longer in the direction of propagation. For Al-based alloys this requirement is modest and it may be possible to detect small cracks or cracks at obliqueangles. For Ti and Ni based alloys, however, this requirement is similar to that for conventional radio-graphy where the long axis of a crack has to be aligned with the direction of propagation. Refraction

Fig. 8. The increasing visibility of the diffraction pattern with increasing wavelength.

Table 1

Minimum aspect ratios of flaws based on Eq. 7 and a � of 2�10�5 rad and the upper wavelength limits from Fig. 5

Material
" (pure number) Ratio, t/w
(pure number)

Ni
1.44�10�6 13.9
Ti
10.63�10�6 10.63
Al
3.48�10�6 3.48

enhanced X-ray radiography of thin Al alloy based components is therefore feasible. It will also be feasiblefor epoxy based composites as they are also weak X-ray absorbers. However, for the X-ray radiography ofTi and Ni based alloys refraction offers little or no improvement on conventional absorption.For a given wavelength, the " for neutrons is usually much smaller than that for X-rays. However, as

absorption is not significant, longer wavelengths can be used. For 0.4 nm neutrons, the " for Ni, Ti and Alare given in Table 2 along with the minimum aspect ratio of the flaws that can be detected. The technique isshown to be insensitive to flat flaws, those with an aspect ratio much less than unity. However, the detec-tion of all other flaw geometries appears to be feasible provided the difference in the refractive indicesbetween the matrix and the flaw is large enough. Also even better sensitivity can be obtained by using evenlonger wavelengths. For example, 1nm neutrons have a " in nickel of 1.5�10�4 and therefore even thedetection of flat flaws will be possible.

4. Experimental

Preliminary experiments were performed on the damaged tip of a nickel superalloy turbine blade to testthe feasibility study. The sources to object and object to detector (or film) distances were both 1.8 m. Thisgives a penumbra equal to the source size. Exposures were also taken in the conventional contact radio-graphic configuration with the detector or film next to the object.For the X-ray experiments, a range of tube voltages and exposure times were tried, the exposure that

showed the best contrast and the greatest amount of detail was used. The X-ray machine was a Balteau 225keV with a W target. An acceleration voltage of 80 kV and a 0.4 mm diameter source size were used forthese measurements. This source size was the smallest available on this machine. Kodak M film wasemployed. The peak wavelength for the bremsstrahlung radiation was expected to be 0.03 nm. WKa lineradiation at 0.021 nm was also expected to be significant.The neutron results have been reported previously [3]. The neutron radiography was performed at the

NIST center for Neutron Research in Gaithersburg, MD, USA in collaboration involving the University ofMelbourne, NIST and DSTO. The neutrons had a thermal wavelength distribution with a peak wavelengthof 0.432 nm. The diameter of the pinhole used to define the source was 0. 2mm. A further mask limited thedivergence of the neutron beam to 6 mrad. The images were captured by a neutron camera consisting of aneutron scintillator screen, the scintillations from which were recorded in an optical charge coupled devicewith 512�512 50 mm pixels.

5. Results

The two X-ray radiographs are shown in Figs. 9 and 10, and the two neutron radiographs in Figs. 11and 12.

Table 2

Minimum aspect ratios of flaws based on Eq. (5) and a � of 2�10�5 rad and a neutron wavelength of 0.4 nm

Material
E (pure number) Ratio, t/w
(pure number)

Ni
2.45�10�5 0.82
Ti
�5.0�10�6 4
Al
5.7�10�6 3.5

6. Discussion

No difference between the distant and contact X-ray radiographs can be identified. Thus only absorptioncontrast was apparent; the deviations due to refraction and diffraction were insignificant. This is in agree-ment with the feasibility study. In comparison the two neutron radiographs show a large difference incontrast. The contact image shows little detail because there was little absorption and thus negligible dif-ference in absorption. However, the distant image shows much more detail because of the long wavelength

Fig. 9. Contact (conventional) X-ray radiograph.

Fig. 10. Distant X-ray radiograph with an object to film distance of 1.8 m.


(0.4 nm) that could be used. Thus in this case the refraction and diffraction deviations are significant andare able to produce significant intensity variations on the distant detector.The use of radiation with a range of wavelengths prevented the differentiation between the refraction and

the diffraction mechanisms. With monochromatic radiation, diffraction will produce a number of fringes,while the refraction image will still be without fringes. However, the definition of refraction contrast mayimprove with monochromatic radiation, as this will eliminate chromatic aberrations. An experiment withmonochromatic X-rays is feasible for thin (1 mm) aluminium or epoxy based composite materials using

Fig. 11. Contact (conventional) neutron radiograph—after Ref. [3].

Fig. 12. Distant neutron radiograph with an object to film distance of 1.8 m—after Ref. [3].


CuK (0.154 nm) radiation. Selecting the neutron wavelength with a crystal monochromator is possible butmay make collection times impractically large with current source intensities.Although the distant neutron image shows more detail than the contact image it is still only as good as

the X-ray image formed by absorption contrast. There are at least two ways in which the distant neutronimage may be further improved: the use of longer wavelengths from a colder thermal source, and theincrease of object to detector distance.As the resolution of the system was limited by the penumbra of 0.2 mm for the neutron case and 0.4 mm

for the X-ray case then the use of a smaller source will also improve the image quality. However, neutronradiography is already flux limited and hours were needed to collect a good image through the 0.2 mmpinhole. The use of a lens to focus the neutrons onto the pinhole may increase the flux and allow a smallerpinhole to be used.The use of larger object to image distances (>1.8 m) may be possible for neutrons. Modern neutron

facilities usually have large instrument halls and neutrons are not scattered much in air. Larger distancescould also be used for X-rays but an evacuated or He filled pipe will be needed to reduce air scatter andprevent the consequent fogging of the image.In the section on Fresnel diffraction it was acknowledged that for diffraction to occur then the dis-

placement in the wavefront had to be abrupt. This implies that the projection of the flaw within the com-ponent also has to be abrupt. A definition of abrupt is that the transition in the wavefront, yt, must occurover a distance much less than the first Fresnel zone (p. 435 in [9]).

yt <<ffiffiffiffiffirl

pð8Þ

Thus, for l equal to 0.12 nm and r equal to 5 m yt must be much less than 25 mm. This condition may notbe met by real cracks that zig-zag (the crack oscillates around a mean straight line path) and have bran-ches. Thus even if the crack to matrix interface is abrupt the projection of the crack in the z-direction isunlikely to meet the condition. Thus the enhanced contrast of many cracks is likely to be due to refractionrather than diffraction.Although neutrons seem appropriate to image most metals they are likely to produced blurred images of

composite materials that contain hydrogen. This is because hydrogen scatters neutrons strongly.Fig. 4. shows the focusing of rays due to refraction. The degree of focusing and hence the optimum dis-

tance of the image plane, film or detector depends on the steepness of the dip in the wavefront. This in turndepends on the aspect ratio of the flaw and the difference in the refractive indices of the matrix and flaw.Thus different shaped flaws will give different focal lengths. Conversely, comparing images taken at a rangeof distances will provide information on the flaw size by allowing the phase profile of the transmittedwavefront to be determined—phase retrieval [17]. This is an avenue for future work on aircraftcomponents.

7. Conclusion

It is feasible to enhance both X-ray and neutron radiography by utilising diffraction and refraction. Thiscan be accomplished by moving the film away from the object. Micro-focus X-ray sources and pinholes areneeded for two reasons: to prevent the penumbra blurring the image, and to provide spatial coherence. Thediffraction and refraction effects become more visible with increasing object to image distances and longerwavelengths. Practical space limitations of several metres usually apply. Long wavelength neutrons can beused to enhance contrast but absorption prevents the use of long wavelength X-rays in all but thin (1 mm)low density components—aluminium or epoxy based composite sheets.


Acknowledgements

I am grateful to Mr. Peter Noy and Mr. Michael Ryan for performing the X-ray radiography; Dr. S.Burke for many helpful discussions; and Dr. P. J. McMahon, Dr. B. E. Allman, Dr. J. E. Murphy andProf. K. A. Nugent of The University of Melbourne, and Dr. D. L. Jacobson, Dr. M. Arif and Dr. S. A.Werner of the National Institute of Standards and Technology, USA for the neutron radiography andadvice.

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