Subsurface Waves in Solids with Curved Surface and Acoustical Impedance on It

This paper presents the results of a study of the peculiarities of subsurface longitudinal (head) waves (SLW) and transverse waves (STW) propagation in solids with wavequide surface of different geometry. Studying an acoustical path of SLW propagation along axis of metallic cylindrical solid of radius R we established, that dependence of pulse amplitude vs. distance r can be approximated by function ∼r, where n≈1,8÷1,9 for dimensionless curvature ε=0÷0,36 and n≈1,8÷2,1 for ε=-(0÷0,48). Much more amplitude wave decrease vs. r is when ε<0 and wave vector is perpendicular to cylindrical axis. As follows from laboratory finding, the fluid layer of length l, contacting with a solid (waveguide) surface between two probes, causes PA increase by 2–4 dB and more. Function ∆PA∼logPA is nonlinear one and increasing vs. l: δ=d(∆PA)/dl has absolute maximum at l={0, lmax}; but minimum at l=0,5lmax. This effect is more significant for STW than for SLW mode. If a solid waveguide surface has rectangular projection, we observe substantial amplitude growth by 10–15 dB and more. As a result of diffraction effect, the top part of the directivity lobe is increasing one. If STW used, the directivity lobe has two maximums, caused by base mode and secondary one (surface waves). It is shown that the former effects must be taken into account when methods using STW and SLW are applied. Introduction: Subsurface longitudinal (SLW) ore head waves and vertically polarized transverse waves (STW) are excited at critical angles of incidence βi of ultrasonic beam, and are propagating in solids along its surface (waveguide). Here βi=arcsin(C1/C2p); C1 and C2p are ultrasonic velocities in the layers of contacting materials; index p≡{l, t} refer to the used wave mode [1]. Group of scientists led by I. Ermolov has obtained significant experimental and theoretical data on excitation and propagation SLW in steel, which are important for development methods and instruments of ultrasonic flaw detection [2–4]. It should be noted, that the former and known laboratory finding [5–7], as a rule, have been obtained in simplified experimental conditions when SLW mode excitation and propagation been in solids with free and plane waveguide surface. Figure 1. Schematic diagram of ultrasound generation and receiving. k r