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Pictorial ultrasonic methods in non-destructive testing
of goods. This involved extensive collaboration with the suppliers. It goes a long way towards improving quality assurance in a company and it is thus a money-earning activity. S. Weinberg, a consultant, in the last paper of the conference emphasized that quality assurance does earn money. The answer to those who held inspection to be a marginal activity is that profit itself is marginal. The company's earnings is the margin between costs and sales. Quality
assurance can make a large contribution to this margin. He recommended that a company should try to quantify the cost of poor quality. This might be crude at first but it is a guide to the correct level of inspection. The conference ended with a discussion on the need for accountants to be aware of the need for inspection. S. Weinberg commented that accountants must be quality minded.
D. Brocklesby
Pictorial ultrasonic methods in non-destructive testing Wallingford, UK, 10 April 1973 If the NASA space programme had needed an efficient Sokoloff tube in its efforts to put a man on the moon then it would probably have had one by now. This interesting thought was made by Professor J. W. R. Griffiths of Loughborough University who opened this meeting on ultrasonic visualization with a brief 'philosophical' talk on acoustical holography and visualization systems in general. His talk also brought home the message that it is now relatively easy to visualize ultrasound, but much harder to use the techniques in practical test situations. Organized by the Non-destructive Testing Society of Great Britain, the meeting was attended by a relatively small number of people. However, all those that did attend were enthusiasts and it is a tribute to the NDTSGB for bringing together so many leading workers in the field. Credit must also go to Dr G. J. Curtis of the NDT Centre, HarweU, who ably chaired the meeting throughout. Mr A. B. Clare described recent work on acoustical holography at the NDT Centre, Harwell. The work has largely been motivated by the thought that it should be possible to augment and possibly improve the information available from established ultrasonic inspection techniques in certain areas. The form in which the holographic results are presented to an observer is a pictorial one which, though still requiring interpretation, has immediate appeal to the untrained eye. It could be the case, therefore, that a metallurgist wishing to perform a fracture mechanics analysis could make his own assessment of the defect structure from the pictures he sees himself. The information from established techniques would still be to hand if required. Results showing the amount of spatial detail were given for a defect structure contained within a test block of forged steel.
objects illustrating the astigmatic nature of the scan were shown. Talking in more general terms, Mr Clare said that methods which set out to produce an image in the true sense of the word have to contend with the fact that, in ultrasonics, devices which in optics are taken for granted (ie, lenses and photographic plates) are not quite so straightforward, if indeed they exist at all. The methods stand or fall depending upon how they get around this particular bottleneck. Holograms made with mechanical scanning offer one solution. He went on to say that the future of holography is best viewed against a background presented by the wide variety of problems which crop up in ndt and in this context it will probably take its place as one amongst a number of techniques to be exploited where particular circumstances warrant its use. Its particular advantage is that it is capable of yielding high-quality images in a format which can be fairly readily appreciated by ones own visual senses. But, as with all ultrasonic images, some degree of interpretation is essential. Another speaker on mechanically-scanned acoustic holography systems was M J-M. Clement from the University of Loughborough. He described a sensitive but fast system using a translated circular scanner. The properties of the two-dimensional Fourier transform of the translated circular sampling function show that the system can tolerate coarser sampling than a rectilinear system. A single 2 MHz transmitter/receiver is used, sampling every 3.3 wavelengths. The hologram is recorded by means of light emitting diodes. This raised the problem of the need for the operator to work in the dark. However, the speaker said that this was not a serious disadvantage in a laboratory situation if fast cameras are used.
Recently Harwell was commissioned by Moorfields Eye Hospital, London, to construct a fast Cartesian scan so that they could evaluate the usefulness of ultrasonic holography in ophthalmic diagnostics. A 40 x 40 mm aperture is scanned with 160 lines in approximately 23 seconds, and pictures of simple test objects obtained with such a high-speed scan were shown. An additional facility is a B-scan display mode, the refresh rate being the line period of 145 ms.
The two-step process of obtaining the reconstruction in both the Harwell and Loughborough systems was further discussed when the disadvantages of holography systems were raised. Although it might be felt that the two-step process is too severe a constraint to render the system attractive, there is little doubt that in the near future advances in technology will alleviate the problem. Electrooptic light modulators are currently under development and such components could eliminate the photographic development lag inherent in present systems.
In addition to Cartesian scans Harwell is also investigating scans with cylindrical geometry and pictures of test
Mr Clare, commenting on the Loughborough system, said that holograms made with a scanning system have within
NON-DESTRUCTIVE TESTING. FEBRUARY 1974
41
them a degree of sampling because every time the transducer crosses the aperture it is sampling the ultrasonic wavefront. The analysis of the sampling problem involves the trajectory of the transducer, and in the case of a translated circular scan it was interesting to see the effect on the spatial frequency distribution. One result of the analysis was that the arc over which a single transducer is allowed to 'see' the object should be restricted to a fraction (1/6) of a complete revolution. In the case of Cartesian scans the sampling geometry is a number of orthogonal lines whose spacing must be chosen so as to be consistent with the resolution required in the final image. Such results, even though rather theoretical, have important practical consequences since they indicate the pitfalls which must be avoided if image quality is to be maintained. For many years B-scan equipment has been available for medical diagnosis. Unfortunately, this has not been the case in ndt, for reasons no one seems sure of. At last however, this is no longer the case. Dr H. Harper and Mr A. H. Spinks from the Central Electricity Generating Board, NW region, have developed a B-scan flaw detector designed for use in power station inspections but illustrating principles of much wider application. Their presentation consisted mainly of a film - 'Imaging flaws by ultrasound' - illustrating the development and use of the new detector. Its main features are a B-scan display on a direct-view bistable storage screen giving an image which is visable as it builds up and which can be held for as long as necessary. The screen is calibrated to give quantitative plotting of the component surfaces and defects in their correct spatial relationship. A threshold level for echoes which are selected for plotting is available, allowing a definite sensitivity level to be associated with the B-scan image. An A-scan display is also available for calibration, measurement of pulse amplitudes, and monitoring coupling during scanning. The scan is indexed automatically either by hand or machine using a simple mechanism for flat surfaces and a more complex mechanism for non-planar components. Finally, the equipment is lightweight and robust enough for site use. Interpretation of the display uses those factors presently used in A-scan flaw detection: position of a flaw in relation to surfaces; the indicated size of the flaw indication; behaviour of the ultrasonic reflector at various angles of approach; and amplitude of the echo. The equipment represents an advance on previous attempts to develop B-scan flaw detectors which were only usable on flat surfaces, in some cases used photographic displays which could not be viewed during scanning, provided no means of assessing coupling, and did not allow a unique sensitivity level to be ascribed to the image. With equipment such as this, the way must now be open to producing automatic B-scan ndt systems. Improvement in photoelastic visualization was the title of the paper given by Dr R. C. Wyatt from the CEGB, SW Region. He described the use of stroboscopic photoelasticity for rendering repetitively-pulsed ultrasound visible in transparent solids. The use of solid media makes the information obtained especially relevant to ultrasonic ndt, and a particularly suitable visualizing medium is goodquality fused quartz which exhibits ultrasonic velocities
42
similar to those in steel. An application being actively pursued is the study of the acoustic outputs from ultrasonic test probes. The apparatus is compact and inexpensive, and a large field is easily obtainable. For maximum information from a stroboscopically frozen image of an ultrasonic pulse, the light source must provide repetitively-triggered flashes of sufficiently short duration and of low enough time jitter to provide resolution of wave structure. A simple spark light source which readily resolves 5 MHz ultrasound, giving flashes of about 20 ns duration with negligible jitter, has been developed by the speaker. The method produces a bright image against a dark background, the brightness at any point in the image being directly related to the instantaneous stress at that point. This dependance on stress makes image interpretation somewhat easier than with the schlieren technique which responds to stress gradient. Measurements of the equipment's sensitivity show that a longitudinal acoustic pulse of peak power as low as 0.1 W can be readily visualized with good image contrast, while an image of poorer contrast is retained for acoustic powers down to about 30 mW. Sensitivity to a shear wave depends upon the wave's polarization but can be as much as four times better (in acoustic power terms). The method can be rendered quantitative by the use of a simple compensation technique which provides a measure of the instantaneous stress at any point in an image. Thus, means are available for examining the structure of a pulse, for comparing the intensities of different pulses, and for measuring the divergence of an ultrasonic beam. Various illustrations of visualization were shown which demonstrated the use of the equipment for studying ultrasonic test probes. In particular, several photographs demonstrated the ability of the method to reveal faults in a probe's output. The output from a commercial 45 degree shear wave probe, for example, was seen to include several longitudinal wave pulses, as well as a shear wave pulse propagating in a backward direction relative to the main shear wave pulse. In another example, the output from a longitudinal wave probe was seen to be markedly asymmetric. Also demonstrated was the effect of a probe nose-piece on the shape of the output pulse. Two speakers dealt with schlieren visualization techniques. Mr I. Glover from Automation Industries (UK) presented a film which commenced with a brief description of the schlieren method of viewing ultrasound in various media. It showed the normal laboratory set-up with a high-energy light source and television viewing system coupled to a high-power ultrasonic flaw detector. The capabilities of such a system were explored in detail by replacing the television camera with the optical camera to give direct real-time visualization of the ultrasonic waves. First, the advantages of the pulsed real-time system were illustrated by adjusting the synchronization to slow down a pulse as it passed from probe to specimen, enabling the viewer to get a slow-motion assessment of the ultrasonic process. A series of examples were shown to illustrate simple phenomena such as Snell's Law and refraction at an interface. The effect of artificial defects was shown in simple samples. Finally, a series of sequences were shown illustrating more complex effects taking place within tubes
NON-DESTRUCTIVE TESTING . FEBRUARY
1974
of various sizes. In these sequences it was easy to see the quite complicated patterns produced from relatively simple shaped sections.
pies of visualization of shear, longitudinal, surface and Lamb waves with illustrations of the complementary nature of all three types of output.
In answer to a question, Mr Glover explained that the purpose of building the equipment was to enable the already advanced programme on probe design and performance assessment to be continued and expanded in a new area. Also, rather more as an incidental, to enable application studies to help in solving customer problems. In conclusion, Mr Glover said that the film was available to any suitable company or organisation free of charge for training or similar purposes.
The last contribution came from Mr P. D. Hanstead from the CEGB, SW Region, who described a new method for imaging discontinuities in materials. The method is termed direct ultrasonic visualization of defects (DUVD); it offers the attractions of ultrasonic holography but with several advantages, not the lease of which are that imaging is inherently instantaneous, mechanical scanning is not needed, and no laser is required.
The second paper on schlieren visualization techniques, given by Mr D. M. Marsh of Tube Investments, was originally scheduled to discuss visualization in solids, but in fact turned into a review of photoelastic, schlieren and computer methods for both liquids and solids. The basic schlieren and photoelastic systems were described and their relative advantages and disadvantages discussed. It was pointed out that, although the basic simplicity of photoelastic methods were attractive, the complete systems with their pulsed high-intensity light sources and other ancillaries were comparable in cost at least for small diameter systems. Since each had exclusive features to offer, these two methods were complementary rather than competitive and a case was put forward for use of a combined photoelastic-schlieren apparatus such as that in use at the TI Research Laboratories. The various differences were then discussed. The photoelastic system gives an output depending directly on the stresses not their spatial rate of change, and can be made quantitative by use of a compensator although the 'extinction voltage' method, common to both techniques, is more useful in practice. Photoelastic techniques are of course only useful in solids. Schlieren methods on the other hand work in all media and have the advantage that, unlike the photoelastic methods, they have equal sensitivity to both shear and longitudinal waves. The major area of difference, however, lies in the model requirements. Photoelastic models need not be particularly fiat but must not contain long-range stresses which swamp the contrast. Schlieren methods are insensitive to these stresses but need very fiat models. Surprisingly, the elimination of stresses after, say, making a cut in a photoelastic model, is very much more troublesome than obtaining a flat surface for a schlieren model. Selected sheets of plate glass provide the latter very readily, while stress relief requires days of annealing. A final difference lies in the sensitivity of the two methods. For 2 MHz ultrasound the two techniques have comparable sensitivities, the photoelastic method giving better sensitivity below this frequency and vice versa. In practice these effects are not very important and both methods are competitive for the whole of the ndt ultrasonic spectrum. The speaker then stressed the importance of computer simulation of ultrasound, the point being that agreement between computed and actual outputs provides verification of the physical model incorporated in the computer programme and hence an understanding of the processes observed. Examples of a wave theory solution for the complicated interaction between ultrasound and a defect were shown to be in close agreement with the schlieren output. Finally the speaker presented a number of exam-
NON-DESTRUCTIVE T E S T I N G . FEBRUARY 1974
Mr Hanstead related how his DUVD system is based on a little-known property of optical systems, whereby an assembly of two lenses can be made to form a true-to-scale three-dimensional image of an array of objects. In its optical manifestation, the phenomenon is of little more than curiosity value, but if the same principle is applied to focussing ultrasound certain extra features become possible. By using pulsed ultrasonic waves, and by visualizing the focussed reflections stroboscopically with pulsed light, the images of defects become 'frozen', presenting to the eye the illusion of a continuous picture which preserves the original spatial relationships; no process analogous to this can be applied to the optical form of the phenomenon. The progress of DUVD from the original idea to practical demonstrations was described in detail. The required focussing properties were first shown to be practicable by algebraic analysis, and it was shown to be possible to cause all defect images in a three-dimensional field to come to their foci at the same instant in time. An ingenious graphical approach to design was then developed to avoid tedious substitutions in equations. For the first practical demonstration the main transmission medium used was mercury, with fused quartz for the ultrasonic lenses. Using this apparatus imaging was successfully obtained. Further development culminated in a system using water as the transmission medium and plastics lenses (originally made for optical purposes) for the focussing components. Impressive demonstrations using this equipment were illustrated, some of them representative of practical inspection problems; these included clear imaging of a small fatigue crack and display of a simulated crack in a bolt, access being to the head end only. Apart from the application of DUVD to engineering problems, Mr Hanstead illustrated the possibilities of applying it to medical diagnosis. Since biological tissue is acoustically similar to water, a system has been designed to focus images of discontinuities in a water-filled vessel. Upon inserting an assembly of steel reflecting pins into the water their pattern was clearly imaged. The scope for further development of DUVD was described, particularly in relation to improving sensitivity and removing acoustic focussing aberrations. Concerning the latter, a computer-aided ray plotting study was illustrated which showed that aberrations may be controlled using methods analogous to those used in optics. It is difficult to sum up such a stimulating meeting as this, but perhaps it was best done by one speaker who said that research into ultrasonic visualization has given us the solutions, what we needed now were the problems! L. Starnes
43
assurance can make a large contribution to this margin. He recommended that a company should try to quantify the cost of poor quality. This might be crude at first but it is a guide to the correct level of inspection. The conference ended with a discussion on the need for accountants to be aware of the need for inspection. S. Weinberg commented that accountants must be quality minded.
D. Brocklesby
Pictorial ultrasonic methods in non-destructive testing Wallingford, UK, 10 April 1973 If the NASA space programme had needed an efficient Sokoloff tube in its efforts to put a man on the moon then it would probably have had one by now. This interesting thought was made by Professor J. W. R. Griffiths of Loughborough University who opened this meeting on ultrasonic visualization with a brief 'philosophical' talk on acoustical holography and visualization systems in general. His talk also brought home the message that it is now relatively easy to visualize ultrasound, but much harder to use the techniques in practical test situations. Organized by the Non-destructive Testing Society of Great Britain, the meeting was attended by a relatively small number of people. However, all those that did attend were enthusiasts and it is a tribute to the NDTSGB for bringing together so many leading workers in the field. Credit must also go to Dr G. J. Curtis of the NDT Centre, HarweU, who ably chaired the meeting throughout. Mr A. B. Clare described recent work on acoustical holography at the NDT Centre, Harwell. The work has largely been motivated by the thought that it should be possible to augment and possibly improve the information available from established ultrasonic inspection techniques in certain areas. The form in which the holographic results are presented to an observer is a pictorial one which, though still requiring interpretation, has immediate appeal to the untrained eye. It could be the case, therefore, that a metallurgist wishing to perform a fracture mechanics analysis could make his own assessment of the defect structure from the pictures he sees himself. The information from established techniques would still be to hand if required. Results showing the amount of spatial detail were given for a defect structure contained within a test block of forged steel.
objects illustrating the astigmatic nature of the scan were shown. Talking in more general terms, Mr Clare said that methods which set out to produce an image in the true sense of the word have to contend with the fact that, in ultrasonics, devices which in optics are taken for granted (ie, lenses and photographic plates) are not quite so straightforward, if indeed they exist at all. The methods stand or fall depending upon how they get around this particular bottleneck. Holograms made with mechanical scanning offer one solution. He went on to say that the future of holography is best viewed against a background presented by the wide variety of problems which crop up in ndt and in this context it will probably take its place as one amongst a number of techniques to be exploited where particular circumstances warrant its use. Its particular advantage is that it is capable of yielding high-quality images in a format which can be fairly readily appreciated by ones own visual senses. But, as with all ultrasonic images, some degree of interpretation is essential. Another speaker on mechanically-scanned acoustic holography systems was M J-M. Clement from the University of Loughborough. He described a sensitive but fast system using a translated circular scanner. The properties of the two-dimensional Fourier transform of the translated circular sampling function show that the system can tolerate coarser sampling than a rectilinear system. A single 2 MHz transmitter/receiver is used, sampling every 3.3 wavelengths. The hologram is recorded by means of light emitting diodes. This raised the problem of the need for the operator to work in the dark. However, the speaker said that this was not a serious disadvantage in a laboratory situation if fast cameras are used.
Recently Harwell was commissioned by Moorfields Eye Hospital, London, to construct a fast Cartesian scan so that they could evaluate the usefulness of ultrasonic holography in ophthalmic diagnostics. A 40 x 40 mm aperture is scanned with 160 lines in approximately 23 seconds, and pictures of simple test objects obtained with such a high-speed scan were shown. An additional facility is a B-scan display mode, the refresh rate being the line period of 145 ms.
The two-step process of obtaining the reconstruction in both the Harwell and Loughborough systems was further discussed when the disadvantages of holography systems were raised. Although it might be felt that the two-step process is too severe a constraint to render the system attractive, there is little doubt that in the near future advances in technology will alleviate the problem. Electrooptic light modulators are currently under development and such components could eliminate the photographic development lag inherent in present systems.
In addition to Cartesian scans Harwell is also investigating scans with cylindrical geometry and pictures of test
Mr Clare, commenting on the Loughborough system, said that holograms made with a scanning system have within
NON-DESTRUCTIVE TESTING. FEBRUARY 1974
41
them a degree of sampling because every time the transducer crosses the aperture it is sampling the ultrasonic wavefront. The analysis of the sampling problem involves the trajectory of the transducer, and in the case of a translated circular scan it was interesting to see the effect on the spatial frequency distribution. One result of the analysis was that the arc over which a single transducer is allowed to 'see' the object should be restricted to a fraction (1/6) of a complete revolution. In the case of Cartesian scans the sampling geometry is a number of orthogonal lines whose spacing must be chosen so as to be consistent with the resolution required in the final image. Such results, even though rather theoretical, have important practical consequences since they indicate the pitfalls which must be avoided if image quality is to be maintained. For many years B-scan equipment has been available for medical diagnosis. Unfortunately, this has not been the case in ndt, for reasons no one seems sure of. At last however, this is no longer the case. Dr H. Harper and Mr A. H. Spinks from the Central Electricity Generating Board, NW region, have developed a B-scan flaw detector designed for use in power station inspections but illustrating principles of much wider application. Their presentation consisted mainly of a film - 'Imaging flaws by ultrasound' - illustrating the development and use of the new detector. Its main features are a B-scan display on a direct-view bistable storage screen giving an image which is visable as it builds up and which can be held for as long as necessary. The screen is calibrated to give quantitative plotting of the component surfaces and defects in their correct spatial relationship. A threshold level for echoes which are selected for plotting is available, allowing a definite sensitivity level to be associated with the B-scan image. An A-scan display is also available for calibration, measurement of pulse amplitudes, and monitoring coupling during scanning. The scan is indexed automatically either by hand or machine using a simple mechanism for flat surfaces and a more complex mechanism for non-planar components. Finally, the equipment is lightweight and robust enough for site use. Interpretation of the display uses those factors presently used in A-scan flaw detection: position of a flaw in relation to surfaces; the indicated size of the flaw indication; behaviour of the ultrasonic reflector at various angles of approach; and amplitude of the echo. The equipment represents an advance on previous attempts to develop B-scan flaw detectors which were only usable on flat surfaces, in some cases used photographic displays which could not be viewed during scanning, provided no means of assessing coupling, and did not allow a unique sensitivity level to be ascribed to the image. With equipment such as this, the way must now be open to producing automatic B-scan ndt systems. Improvement in photoelastic visualization was the title of the paper given by Dr R. C. Wyatt from the CEGB, SW Region. He described the use of stroboscopic photoelasticity for rendering repetitively-pulsed ultrasound visible in transparent solids. The use of solid media makes the information obtained especially relevant to ultrasonic ndt, and a particularly suitable visualizing medium is goodquality fused quartz which exhibits ultrasonic velocities
42
similar to those in steel. An application being actively pursued is the study of the acoustic outputs from ultrasonic test probes. The apparatus is compact and inexpensive, and a large field is easily obtainable. For maximum information from a stroboscopically frozen image of an ultrasonic pulse, the light source must provide repetitively-triggered flashes of sufficiently short duration and of low enough time jitter to provide resolution of wave structure. A simple spark light source which readily resolves 5 MHz ultrasound, giving flashes of about 20 ns duration with negligible jitter, has been developed by the speaker. The method produces a bright image against a dark background, the brightness at any point in the image being directly related to the instantaneous stress at that point. This dependance on stress makes image interpretation somewhat easier than with the schlieren technique which responds to stress gradient. Measurements of the equipment's sensitivity show that a longitudinal acoustic pulse of peak power as low as 0.1 W can be readily visualized with good image contrast, while an image of poorer contrast is retained for acoustic powers down to about 30 mW. Sensitivity to a shear wave depends upon the wave's polarization but can be as much as four times better (in acoustic power terms). The method can be rendered quantitative by the use of a simple compensation technique which provides a measure of the instantaneous stress at any point in an image. Thus, means are available for examining the structure of a pulse, for comparing the intensities of different pulses, and for measuring the divergence of an ultrasonic beam. Various illustrations of visualization were shown which demonstrated the use of the equipment for studying ultrasonic test probes. In particular, several photographs demonstrated the ability of the method to reveal faults in a probe's output. The output from a commercial 45 degree shear wave probe, for example, was seen to include several longitudinal wave pulses, as well as a shear wave pulse propagating in a backward direction relative to the main shear wave pulse. In another example, the output from a longitudinal wave probe was seen to be markedly asymmetric. Also demonstrated was the effect of a probe nose-piece on the shape of the output pulse. Two speakers dealt with schlieren visualization techniques. Mr I. Glover from Automation Industries (UK) presented a film which commenced with a brief description of the schlieren method of viewing ultrasound in various media. It showed the normal laboratory set-up with a high-energy light source and television viewing system coupled to a high-power ultrasonic flaw detector. The capabilities of such a system were explored in detail by replacing the television camera with the optical camera to give direct real-time visualization of the ultrasonic waves. First, the advantages of the pulsed real-time system were illustrated by adjusting the synchronization to slow down a pulse as it passed from probe to specimen, enabling the viewer to get a slow-motion assessment of the ultrasonic process. A series of examples were shown to illustrate simple phenomena such as Snell's Law and refraction at an interface. The effect of artificial defects was shown in simple samples. Finally, a series of sequences were shown illustrating more complex effects taking place within tubes
NON-DESTRUCTIVE TESTING . FEBRUARY
1974
of various sizes. In these sequences it was easy to see the quite complicated patterns produced from relatively simple shaped sections.
pies of visualization of shear, longitudinal, surface and Lamb waves with illustrations of the complementary nature of all three types of output.
In answer to a question, Mr Glover explained that the purpose of building the equipment was to enable the already advanced programme on probe design and performance assessment to be continued and expanded in a new area. Also, rather more as an incidental, to enable application studies to help in solving customer problems. In conclusion, Mr Glover said that the film was available to any suitable company or organisation free of charge for training or similar purposes.
The last contribution came from Mr P. D. Hanstead from the CEGB, SW Region, who described a new method for imaging discontinuities in materials. The method is termed direct ultrasonic visualization of defects (DUVD); it offers the attractions of ultrasonic holography but with several advantages, not the lease of which are that imaging is inherently instantaneous, mechanical scanning is not needed, and no laser is required.
The second paper on schlieren visualization techniques, given by Mr D. M. Marsh of Tube Investments, was originally scheduled to discuss visualization in solids, but in fact turned into a review of photoelastic, schlieren and computer methods for both liquids and solids. The basic schlieren and photoelastic systems were described and their relative advantages and disadvantages discussed. It was pointed out that, although the basic simplicity of photoelastic methods were attractive, the complete systems with their pulsed high-intensity light sources and other ancillaries were comparable in cost at least for small diameter systems. Since each had exclusive features to offer, these two methods were complementary rather than competitive and a case was put forward for use of a combined photoelastic-schlieren apparatus such as that in use at the TI Research Laboratories. The various differences were then discussed. The photoelastic system gives an output depending directly on the stresses not their spatial rate of change, and can be made quantitative by use of a compensator although the 'extinction voltage' method, common to both techniques, is more useful in practice. Photoelastic techniques are of course only useful in solids. Schlieren methods on the other hand work in all media and have the advantage that, unlike the photoelastic methods, they have equal sensitivity to both shear and longitudinal waves. The major area of difference, however, lies in the model requirements. Photoelastic models need not be particularly fiat but must not contain long-range stresses which swamp the contrast. Schlieren methods are insensitive to these stresses but need very fiat models. Surprisingly, the elimination of stresses after, say, making a cut in a photoelastic model, is very much more troublesome than obtaining a flat surface for a schlieren model. Selected sheets of plate glass provide the latter very readily, while stress relief requires days of annealing. A final difference lies in the sensitivity of the two methods. For 2 MHz ultrasound the two techniques have comparable sensitivities, the photoelastic method giving better sensitivity below this frequency and vice versa. In practice these effects are not very important and both methods are competitive for the whole of the ndt ultrasonic spectrum. The speaker then stressed the importance of computer simulation of ultrasound, the point being that agreement between computed and actual outputs provides verification of the physical model incorporated in the computer programme and hence an understanding of the processes observed. Examples of a wave theory solution for the complicated interaction between ultrasound and a defect were shown to be in close agreement with the schlieren output. Finally the speaker presented a number of exam-
NON-DESTRUCTIVE T E S T I N G . FEBRUARY 1974
Mr Hanstead related how his DUVD system is based on a little-known property of optical systems, whereby an assembly of two lenses can be made to form a true-to-scale three-dimensional image of an array of objects. In its optical manifestation, the phenomenon is of little more than curiosity value, but if the same principle is applied to focussing ultrasound certain extra features become possible. By using pulsed ultrasonic waves, and by visualizing the focussed reflections stroboscopically with pulsed light, the images of defects become 'frozen', presenting to the eye the illusion of a continuous picture which preserves the original spatial relationships; no process analogous to this can be applied to the optical form of the phenomenon. The progress of DUVD from the original idea to practical demonstrations was described in detail. The required focussing properties were first shown to be practicable by algebraic analysis, and it was shown to be possible to cause all defect images in a three-dimensional field to come to their foci at the same instant in time. An ingenious graphical approach to design was then developed to avoid tedious substitutions in equations. For the first practical demonstration the main transmission medium used was mercury, with fused quartz for the ultrasonic lenses. Using this apparatus imaging was successfully obtained. Further development culminated in a system using water as the transmission medium and plastics lenses (originally made for optical purposes) for the focussing components. Impressive demonstrations using this equipment were illustrated, some of them representative of practical inspection problems; these included clear imaging of a small fatigue crack and display of a simulated crack in a bolt, access being to the head end only. Apart from the application of DUVD to engineering problems, Mr Hanstead illustrated the possibilities of applying it to medical diagnosis. Since biological tissue is acoustically similar to water, a system has been designed to focus images of discontinuities in a water-filled vessel. Upon inserting an assembly of steel reflecting pins into the water their pattern was clearly imaged. The scope for further development of DUVD was described, particularly in relation to improving sensitivity and removing acoustic focussing aberrations. Concerning the latter, a computer-aided ray plotting study was illustrated which showed that aberrations may be controlled using methods analogous to those used in optics. It is difficult to sum up such a stimulating meeting as this, but perhaps it was best done by one speaker who said that research into ultrasonic visualization has given us the solutions, what we needed now were the problems! L. Starnes
43