ND T Abstracts

measurement set biases the obtained s i ~ . The second one is daivnd from the power specUum Of the rndio fnMuoncy (RP) b e c k ~ sismd and gives a non biased s l~-- . ,~ for red signal and gives a non biased signature for highly b e t e ~ media. Data are presonted for highly heterogeneous media like concrete.

40081 Stockman, A.; Nichoison, P.S. Ultrasonic cha _r~t_erization of microsphericol indus ions in zlrconia and crystallized glass Review of Progress in Quantitative Nondestructive Evaluation, Williamsburg, Virginia (United States), 22-26 Jun. 1987. Vol. 713, pp. 1273-1284. Edited by D.D. Thompson and D.E. Chimenti, Plenum Press, 1988.

In this work a ~ c u l model has been developed which calculates the waveforms and frequency spectra for sound waves scattered from various sized inbomogeueities as a function of the host and inclusion material parameters and the spectrum of the incoming sound waves. The model uses the technique of ultrasound for defect detection, it is necessary to modify the analysis to i ~ the use of a focused sound beam to increase the power and a pulsed system for locating of the defect. To test the model, spherical and near spherical voids in a glass matrix were first examined. Next spherical particles of oxides were embedded in the glass and the model compared with experiment. The glass was subsequently crystalfized to approximate a real ceramic. Recently, spherical panicles of platinum (50 and 200 micrometer diame~) have been embedded in dense zircoula. This work is reported. These inclusions were located ultmaonically by immersion scanning with a pulsed, focused 25 MHz Uansdueer. The beckscattered signals were compmed with the model predictions.

40080 Nagy, P.B.; Adler, L. Scattering induced a t tenuat ion of ultrasonic backscatterin 8 Review of Progress in Quantitative Nondestructive Evaluation, Williamsburg, Virginia (United States), 22-26 Jun. 1987. Vol. 7B, pp. 1263-1272. Edited by D.D. Thompson and D.E. Chimenti, Plenum Press, 1988.

Scattering induced ulWasonic attenuation offers a simple way to characterize material inbumngeneitles. Due to the inherent technical difficulties associated with backscattering measurements, it is rather hard to experimentally verify or contradict the presumption that the beckscattered wave is attenuated in the same way as a coherent plane wave. Whenever adsorption domina~s the total attenuation, as in biological tissues, the attenuation is the same rogerdless of whether it is measured from the backscatte~ signal or from the transmitted one by a phaso-semitive (or even by a phase-insensitive) deU~:mr. On the other hand, expedm~tal results show that the I ~ r e d sisnal is much less attenuated by scattering than the transmitted coherent wave. It will be shown that this is due to the coherent nature of the backscJ_.nered waves and that the effect is present from the very beginning of the back~J~_~ered signal.

40069 Roberts, RA. Character izat ion of porosity in cont inuous fiber.reinforced composites with ul trasonic backseatter Review of Progress in Quantitative Nondestructive Evaluation, Williamsburg, Virginia (United States), 22-26 Jun. 1987. Vol. 7B, pp. 1053-1062. Edited by D.D. Thompson and D.E. Chimenti, Plenum Press, 1988.

This paper discusses the application and extension of experimen~ techniques to multldirectioually reinforced composite laminates, i.e., those in which the reinforcing directions in successive pries are rotated. The layered anisotropic structure of the rnultidirectionally reinforced laminates has significant consequences for the application of the previously developed angular scanning and spectral analysis techniques.

40068 Ohyoshi, T.; Achenbach, J.D. Effect of bottom-surface reflections on backscatter from porosity in a composite layer Review of Progress in Quantitative Nondestructive Evaluation, Williamsburg, Virginia (United States), 22-26 Jun. 1987. VoL 7B, pp. 1045-1052. Edited by D.D. Thompson and D.E. Chimenti, Plenum Press, 1988.

Polar backscutwxing from a fiber-reinforced composite which contains regions of porosity is investigated. Several scattering mechanisms appear to contribute to the t~:kscatterod signal. One effect, concerns the influence of reflection from the bottom surface of a specimen. Fiber-reinforced composites display a great deal of attenuation of propagating disturbances due to the structuring of the material, due to porosity and due to the constitutive behavior of the fiber and matrix materials. Hence in some composites the ultrasonic wave motion may not have a significant amplitude when it reaches the bottom face of a composite specimen.

40066 (2u, J.; Achenbach, JD. Backseatter from pormity in crms-ply compmites Review of Progress in Quantitative Nondestructive Evaluation, Williamsburg, Virginia (United States), 22-26 Jun. 1987. Vol. 7B, pp. 1209-1036. Edited by D.D. Thompson and D.E. Chimenti, Plenum Press, 1988.

The wesence of porosity in a fiber-reinforced composite may have a critical effect on the composite's mechanical properties and service perfonnance. For the purpose of the present analysis, it may be assumed that the fihers are of the order of 8 micro metem in diameter, and that tbeY are combined with a matrix rnat~ial into plies of approximately 100 micro meters thickness. The plies are stacked to form a plate. In this paper, the pores are modeled as penny-shaped cracks of dimneters much larger than a single fiber diameter. It is assumed thst the wavelength is sufficiently larger than the ply thickness. Hence, scattering by individual fibers and by the layered structure can be ignon~L It is also assum~ that reflection from the buttom surface can be ge~l out and tl~ plate cun be tre.aW~l as a half- spuce. Hence, the composite is considered as a homogeneous but anisotropic semi-infinite solid. Inspection of a cross-soctional area of a fiber reinforced composite tends to show that the fibers are not uniformly spaced. There appear to be regions where the fibers are closely packed, while in other areas there is greater spacing.

40021 Nagy, P.B.; Adler, L. The role of coherent backscattering in quant i ta t ive NDE Review of Progress in Quantitative Nondestructive Evaluation, Williamsburg, Virginia (United States), 22-26 Jun. 1987. Vol. 7A, pp. 113-122. Edited by D.D. Thompson and D.E. Chimenti, Plenum Press, 1988.

Random signals scattered from weak material inhomogeueities are usually assumed to be completely incoherent and are evaluated accordingly. On the other hand, the ~ J o d signal often exhibits some kind of weak coherency induced by different factors. In the pru~ce this partial coherency appears as some p.nnnant modulation on the spectrum of the signal, which does not diminish as extensive spatial averaging is used to get rid of the random modulation due to interference between many incoherent ~ e r e r s . As it will be demonstrated by the following examples, the weak spurious coherency can be ~ _ ~ I simply by the particular measuring equipment. The induced coherent term yields no useful information on the scattering inhomogeueity, and the aim is to get rid of it. The coherency can be induced by material propafies of the sample itself, and may be enhanced rather than eliminated so that some additional information can be obtained on the inhomogeueous material. Both cases will be demonstrated via simple examples often encountered in nondestructive evaluation.

39889 BUgutay, N.M.; Li, X.; Saniie, J. Spectral analysis of randomly dis t r ibuted scatterers for ultrasonic grain size estimation Ultrasonics, Vol. 27, No. 1, pp. 19-25 (Jan. 1989)

Since the average grain size is an important parameter in material characterization, the non-destructive evaluation of the mierostructure in poly~-ystalline materials using ultrasonic beckscane~d echoes has significant practical implications. However, the complexity of the ultrasonic backscattered signal, which consists of interfering multiple echoes with random positions and amplitudes, makes accurate grain size evaluation difficult with conventional signal processing teclmiques. This paper introduces an alternative approach which examines the spectrum of an ensemble of randomly distributed scatterers with known distribution to establish a relationship between the spectral and temporal sta;istical properties of the scatterers.

39557 Roberts, RA. Focused ultrasonic backscatter technique for near-surface flaw detection in s t ructural ceramics Proceed~ss of the 16th Symposium on Nondestructive Evaluation, San Antonio, Texas (United States), 21-23 Apr. 1987. pp. 224-230. Nondestructive Testing Information Analysis Center, Texas, USA.

The stre.~,th of ceramic components that are subject to bending stresses is primarily controlled by the presence of near-surface flaws, because elevated tensile stresses exist at the component surface. A nondestructive evaluation (NDE) technique that is sensitive primarily to near-surface flaws may, therefore, be desirable. This paper describes a focused ultrasonic backsca~r technique that is currently being used at Argonne for the detection of uear-sorface flaws in ceramic modulus-of-rupture (MOR) bars. Because the technique exploits the generation of leaky surface waves in such a way that precise lransducer positioning is not critical, it is a potential candidate for the inspection of curved surfaces. Included in the presentstion of results are the detection of near-surface flaws in SiN/sub4/MOR bets, and a correlation of NDE and fractography results obtained after failure in four-point bending.

N D T & E Internat ional Vo lume 26 Number 5 1993 269