over the 300 kHz to IS MHz frequency range. Quantitative characterization of ultrasonic probe performance is critical for proper interpretation of ultrasonic signals using advanced signal digitization and signal processing instrumentation that has become available for this work. The laser-optical probe, although limited in signal intensity/sensitivity, provides large tolerance of geometry variations and very accurate ultrasonic waveforms without limitations of mechanical frequency resonance or phase-cancellation effects.

In summary, large structure NDE requires rapid, practical and quantitative ultrasonic scanning probes. With newly designed laser-optical or low-noise. liquid-jet probes. high fidelity. quantitative ultrasonic testing can be performed.

Kinra, Vikram K; Zhu, Changyi Time-domain ultrasonic NDE of layered media Texas A&M University, Aerospace Engineering Department, College Station, TX 77843, USA The integrity of adhesively bonded structures is of paramount importance in the safe and reliable operation of aging aircraft; these structures can be modelled as layered media. The ultrasonic nondestructive evaluation (NDE) of layered media has, in the past, been conducted solely in the frequency-domain. The commonly used methods are the resonance techniquellJ , leaky Lamb wave approach l "'] and ultrasonic spectroscopy!<). As far as we know a time-domain solution for NDE of layered media has not been reported in the literature.

For an extremely thin single layer, the authors reported the first time-domain ultrasonic NDE technique for the measurement of the thickness l» or the wave velocity(6): the ratio of the plate thickness (h) to the wavelength (A) is of the order of 10 - 2 Here an infinite series was employed in the theoretical simulation of the transmitted and the reHected fields. More recently, a retrieve function approach was proposed by the authors for the reconstruction of the incident (reHected) field by a simple two-term (three-term) summation (7 ). This resulted in at least one order of magnitude of saving in the computational effort.

The objective of the present work is to extend the retrieve function approach to the time-domain NDE of an N-layered plate. A modified retrieve function is proposed for the reconstruction of the incident (reHected) field through a convolution of a retrieve function and the transmitted (incident) field. For an N-Iayered medium, the incident (reHected) field can be reconstructed by a 2N (2N

- 1) term summation. The acoustic properties of a layered medium can be determined through a comparison of the theory and the experiment.

To keep the discussion simple, we will here use the case of a two-layered plate to illustrate the more general theory. Consider a two-layered plate Immersed 10

an elastic Huid (water) occupying the space a ,,;; x ,,;; b, b = a + hI + h,. Let ui"'(x, t) = fIt - sox) be the incident wave travelling along the positive x direction. Let u'''"'(x, I) = gl(t - sox) be the transmitted wave and u"'(x, I) =

g'(1 - sox) be the reHected wave, where t is time, and So is the slowness of the water. Through the use of a retrieve function the relationship between the reHected and the incident fields for a two-layered medium is obtained as:

g'(1) = RI,[(t) + R,,[f(t - t l ) - Rug'(1 - II)]

+ R 23 R,.[R I,[(t - t,) - g'(t - t,)]

+ R,.[f(t" - t,) - Rug'(1 - tl - t,)] (I)

where II = slh l , t, = s2h" Rij is the reHection coefficient for a wave reHected in medium i at its interface with medium j. Similarly, the relationship between the incident and the transmitted fields is obtained as:

f(t) = 1 [g'( -10 + II + I,) + ROI Ro2 g'(t - to - tl + I,) TOI Tl,T,o

+ RuR,og'(t - to + II - t,) + R OI R 20g'(1 - to - II - t 2)] (2)

where Tij is the transmission coefficient for a wave transmitted from medium i to mediumj.

Similar relations between the incident and reHected (transmitted) fields can be obtained for N = 3,4, .... In an N -layered medium there are (N + I) independent reHection coefficients: ROl> R I2 ,· .. , R,N-IJ.N' RNO (observe that R(j+I)j = -Rj(j+IJ)' It can be shown that for an N:layered plate the reHectedNfield can be constructed by a summatIOn of 2N terms 10 the mCldent field and (2 - I) terms in the reHected field. The incident field can always be constructed by a 2N term summation over the transmitted field. Specifically, for N = 2, the reHected field can be obtained by the summation of four terms in the incident field and three terms in the reHected field, while the incident field can be calculated by a four-term summation in the transmitted field.

A systematic sensitivity analysis of this technique has been carried out. Experiments have been conducted on a two-layered plate. An excellent agreement between the theory and the experiments was observed.

References

Lloyd, E.A. 'Non-destructive testing of bonded joints, NDT Intern 7 (1974) pp 331-334 Bar-Cohen, Y. and Chementi, D.E. 'Nondestructive evaluation of composite laminates by leaky Lamb waves' Douglas Paper 7598, McDonnell Douglas Corp., Long Beach, California (August 1985) Bar-Cohen, Y., Mal, A.K. and Yin, c.c. 'Ultrasonic evaluation of adhesive bonding' J Adhesion (1989)

4 Hanneman, S.E. and Kinra, V.K. 'A new technique for ultrasonic nondestructive evaluation of adhesive joints: Part I. Theory' SEM J Exper Mech to be published

212

Zhu, C. and Kinra, V.K. 'Time·domain ultrasonic measurement of the thIck ne" of a sub-half-wavelength elastic layer' J Tesl Eval20 4 (1992) pp 265 274

6 Kinra, V.K. and Zhu, C. Time-domain ultrasonic NDE of the wave velocity of a sub-half-wavelength elastic layer' J Test Eval in press Zhu, C. and Kinra, V.K. 'A new technique for time-domain ultrasonic NDF thin plates' J /l/ondestr El'fJl to be published

Imaging

Gunarathne, G.P.P. A new real-time ultrasonic imaging system Robert Gordon University, School of Electronic and Electrical Engineering, Schoolhill, Aberdeen AB9 IFR, UK A new ultrasonic imaging system featuring high resolution, speed and sensitivity has been developed. It is capable of producing coaxial B-scan and sector scan type images at a speed unmatched by conventional techniques. In contrast to existing methods, the new system is developed by combining the desirable features of a range of techniques including digitally controlled precision phased-array scanning, wide-band analogue processing in multiple channels and acousto-optical image reconstruction. Furthermore, this hybrid system is compact and portable and exhibits potential advantages in real-time NDT and also in medical applications.

The system can be operated in three different modes, namely, coaxial B-scan, sector scan and focused scan modes. The basic operation is as follows. The test object is insonified with short pulses of ultrasound using an array of transducers. The echo signals received are amplified in parallel channels and are re-transmitted via a second transducer array into an acousto-optical focusing arrangement, where the image reconstruction occurs. The resulting acoustic image which is an acoustic replica of the test object field is made visible by ultra-short stroboscopic illumination. This scheme gives the system its remarkable speed, approaching theoretical limits. Thus, in a section of steel 200 mm thick, a complete image is formed within 150 /is.

Two further extremely useful features incorporated into the design are image 'linearity' and field 'isochronicity'. Linearity ensures constant object-to-image spatial relationship while isochronicity ensures that the whole image field is formed at one instant of time. (There is no need for a gradual image build up as in conventional methods.) In addition, all the elements of the array are effectively utilized for each image frame as one complete aperture, giving the maximum lateral resolution corresponding to the size of the aperture used. Furthermore, by careful control of the excitation pulses, matching techniques and high-quality transducer arrays, axial resolution of the order of a wavelength in the test object medium is achieved. (The operating frequency of the experimental prototype is 2 MHz.)

A summary of the work carried out in the design of this new hybrid imaging system including theoretical concepts, modelling and experimentation, together with the results achieved in producing reakime, high resolution images will be presented. Work in progress includes further optimization of the basic system and investigating the possibility of adaptive focusing of ultrasonic energy to target sites as a means of on-line image enhancement; an account of which will also be presented.

Kline, R.A.*; Wang, Y.Q.*; Mignogna, R.B.t; Delsanto, P.t A parallel processing approach to acoustic tomography * U niversit) of Oklahoma, School of AME, Norman, 0 K 73019, USA tNaval Research Laboratory, Washington, DC 20375, USA tDepartment of Physics, University of Turin, Turin 10129, Italy The use of tomographic principles to reconstruct images of material property inhomogeneity has grown rapidly in recent years. While this interest has been motivated principally by radiographic applications, a considerable amount of work has also been devoted to the development of image reconstruction techniques suitable for ultrasonic data. While similar to radiographic reconstruction in many respects, acoustic wave propagation differs from its radiographic counterpart in several significant areas. Most notably, acoustic waves will not travel along straight line ray paths in nonhomogeneous media. Therefore, ultrasonic image reconstruction methods must incorporate some means of accounting for ray bending phenomena in the reconstruction algorithm. Also, many materials of practical importance are anisotropic. Conventional approaches to tomography have no means to address the effect of the directional dependence of material properties.

Recently. we have developed reconstruction algorithms to address these concerns. However, this approach has been found to be computationally intensive. particularly as the number of cells in the domain of the reconstructed image increases. In this work, we explore an approach to increasing the computational efficiency of the process through the use of a computer with a massively parallel architecture. While most of the work in this area has been performed on serial machines, it should be noted that the iterative nature of the image reconstruction process makes it well suited to algorithms based on parallel processing. In this study, we use a parallel approach and let each processor represent a particular cell in the image. Then, through the use of a finite difference code, we can rapidly assess the inHuence of each individual cell on acoustic wave propagation in the material This information provides the basis for adjusting the mechanical properties in each cell (in an iterative fashion) to best match the experimental data. Results will be presented for several cases where material inhomogeneity and/or material anisotropy is present.

NDT&E International Volume 25 Number 4/51992