Elastic Waves Scattering from Corrugated Metal Interfaces

This is a study of elastic waves diffracted by corrugated metallic surfaces. The corrugations consist of triangular grooves with variable parameters. The results of the narrow band experiments show significant diffraction patterns depending on angle and frequency. In addition, a continuous schlieren system is used to visualize the diffracted orders. Measurements were also carried out using a broadband pulse echo system. The behavior of the received spectra is characteristic of the surface profile. Both front and back surfaces have been investigated. INTRODUCTION There is a considerable amount of interest in NDE to study elastic wave interaction with material discontinuities. There are numerous theoretical [1,2,3] and experimental works concentrated on the scattering of elastic waves from single scatterers such as cavities, cracks, etc., in the bulk [4,5] or on the surface [6,7] of materials. The study of elastic wave scattering from multiple structures is not so extensive, :1owever. The scattering of waves from multiple structures, whether they are random or periodic, frequently occurs in materials as the result of grain structure, l~mination, fiber reinforcing, multiple defects, surface preparation, etc. The elastic wave diffraction from periodic surfaces has been treated recently by Fokkema and van den Berg [8] but only numerical results are available and only for a few special cases. More extensive results are given for the scattering of electromagnetic waves from periodic and random surfaces by Bechmann and Spi zzi cliino [9]. On the experimental side, ultrasonic wave scattering from periodic surfaces in water has been studied by Jungman et al [10] using ultrasonic spectroscopy. Using an optical model of a diffraction grating, the angular position of the various diffraction orders are related to the grating constant for a given frequency. In this paper we are addressing the problem of elastic wave scattering from periodically corrugated structures from interfaces and from the bulk of the solid. The following periodic interfaces are considered: (a) solid-air; (b) solid-water; (c) watersolid; and (d) solid-solid. In addition, there are two corrugated structures considered in the bulk of the material: (a) the region of corrugation is much larger than the size of the beam; (b) the region of the corrugation is smaller than the beam size, e.g., a defect with a periodic surface. DIFFRACTION BY PERIODIC ROUGH SURFACES Theoretical Considerations. Elastic wave diffraction by a periodic rough surface has been discussed recently by Fokkema and P. M. van den Berg. By using equations of elastodynamics for linear isotropic solids they obtain numerical solutions for the representation integral for the case of sinusoidal periodicity. Incident longitudinal and transverse waves are considered. For a triangular shaped grating surface (which is used in this investigation) the same result is not available although the numerical analysis may be carried out in the same manner. Electromagnetic wave scattering from a triangular shaped grating has been solved using Kirchoff approximations [9]. The intensity of the mth diffracted order is expressed as: