GB703480A - Means for launching beams of ultrasonic radiation in a medium - Google Patents

Abstract

703,480. Ultrasonic couplings ; piezoelectric transducers. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Nov. 16, 1951 [Nov. 17, 1950], No. 28227/50. Class 40 (4) [Also in Group XXXVIII] In an ultrasonic flaw detector the means for launching a beam of ultrasonic waves into the work piece comprises a linear array of one set of electro-mechanical transducer elements producing predominantly shear vibrations interspaced with another set of transducer elements producing compressional and possibly shear vibrations, the spacing of the two sets of transducers being different. As shown in Fig. 3 the shear transducers are arranged in pairs a b, d c, and g h, the spacing between the pairs being four times the spacing d between the members of a pair and the other transducers c and fare located in the gaps between the pairs. For a mild steel work piece with d = 0.73 mm. and an excitation frequency of 2.5 Mc/s the shear transducers produce a beam OA, Fig. 4, at 25 degrees to the boundary when a, d and g are excited in anti-phase with b, e and h, and when the other transducers c and f are excited in anti-phase they produce a compressional wave OB and a shear wave OC at angles of 66 degrees and 77 degrees respectively. Due to the gaps in the array a, b, d, e, g, h, a spurious beam at 64 degrees is produced which will be weaker if the shear elements are quartz crystals cut with the XY plane parallel to the boundary of the medium and the X direction normal to the plane of the diagram. When shear crystals are used with the Y direction normal to the boundary and the X direction normal to the plane of the diagram they may be excited with adjacent elements in anti-phase or with two adjacent elements in one phase and the next two elements in the opposite phase. In the latter case both shear and weaker compressional waves will be produced which are propagated in the same direction as the waves generated by the compressional elements c, f, the intensity of the compressional waves being further reduced by suitable choice of the widths of the crystals. The direction of the beam may be varied by varying the excitation frequency and the spacing between the elements may be increased if a coupling material, e.g. magnesium or sintered alumina, is interposed between the array and the work piece such that the wave velocity is greater in the coupling material than in the work piece. Beams corresponding to A, B, C, Fig. 4, will also occur on the other side of the normal and these may be prevented from reaching the work piece by using a coupling as shown in Figs. 5 and 6 in which the unwanted beam Q is propagated into a portion 7 of the coupling having roughened surfaces so that the unwanted beam is scattered and dissipated therein. In Fig. 6 the portion 4 of the coupling is shaped to concentrate the wanted beam, the surface 8 being conical with its axis along the boundary of the work piece and its apex at the point 10. Specification 664,763, [Group XXXVIII], is referred to.