GB841511A - Improvements in and relating to apparatus for launching mechanical vibrations - Google Patents

Abstract

841,511. Ultrasonic wave couplings; piezoelectric instruments. NATIONAL RESEARCH DEVELOPMENT CORPORATION. June 6, 1955 [March 4, 1954], No. 6372/54. Class 40 (4). [Also in Groups XL (b) and XL (c)] Relates to means for testing the efficiency of ultrasonic wave coupling at the interface between a solid work-piece and a piezo-electric transmitting transducer or between the workpiece and a mode-changing wedge to which the transducer is coupled. In one embodiment, Fig. 2, and Fig. 1 (not shown), the transducer comprises a disc 11 excited by an annular electrode 14 backed by damping material 16 and a central subsidiary electrode 13 which produces a signal proportional to any mismatch between the crystal 11 and the workpiece; alternatively the subsidiary electrode may comprise a portion of the annular electrode 14. During normal operation the subsidiary electrode may be connected to the main electrode and damping material may be applied to the subsidiary electrode, said material being retracted when testing the coupling. In a further embodiment, Fig. 4 and Fig. 3 (not shown), the main transducer 19 is attached to the front surface of a damping block 21 and the mismatch signal is derived from a subsidiary bar-like transducer 22 mounted on the sloping rear surface of the block 21. When shear waves 32, Fig. 6, are produced by mounting the main transducer 27 on one surface of a mode-changing wedge 28 of low-wave velocity material such as polymethyl methacrylate or the composite material disclosed in Specification 766,983, a signal indicating the coupling efficiency at the interface 29 between the wedge and the workpiece is derived from a subsidiary bar-shaped transducer 33 mounted on a face of the wedge whose normal is inclined to the direction of the longitudinal waves 31 reflected at the interface 29. The main transducer is backed by damping material 35 and a saw cut 34 reduces clutter. The direction of the shear waves in the workpiece may be varied as disclosed in Specification 766,984 by mounting the main transducer 38, Fig. 7, on a shoe 37 which is movable over a curved surface 43 of the wedge 36; in this case the surface 44 of the wedge is so shaped that the reflected longitudinal waves 39 are always re-reflected thereby along a path 45 to the subsidiary transducer 46 mounted on another surface of the wedge. In the embodiments of Figs. 3, 4, 6 and 7 the subsidiary transducer is preferably tuned to the fundamental frequency of the main transducer by means of an inductance, Fig. 9 (not shown), and the monitoring signal from the subsidiary transducer may be compared with a reference signal derived from an auxiliary receiving transducer 52, Fig. 5, coupled to an auxiliary transmitting transducer 51 by means of a pad 53 of the same material as the damping block 21, Fig. 4, or the wedge in Figs. 6 and 7, the transducer 51 being energized in parallel with the main transducer; a loading block 54 simulating the workpiece may be applied to the other side of the transducer 51. When used in pulse testing systems with a C.R.T. display, Fig. 8 (not shown), the pad 53 is such that the wave transit time therein is less than in the damping block or wedge coupled to the main transducer and the monitoring and reference pulse outputs from the subsidiary and auxiliary transducers are displayed on an additional trace on the C.R.T.; alternatively the relative strengths of the two signals may be determined as a ratio and displayed on a meter. If the main and auxiliary transducers are identical and if the loading 54 is absorptive, the two transducers can be mutually balanced in a bridge circuit of the type disclosed in Specification 766,982. Also in the embodiments of Figs. 4, 6 and 7 a series of obstacles or holes may be arranged just below the subsidiary transducer in order to produce a monitoring signal dependent on the coupling over the whole of the interface. Alternatively the subsidiary transducer may comprise a slotted barium titanate crystal, the out-ofphase outputs from alternate sections thereof being applied to the opposite ends of a centretapped transformer. In a further embodiment, Fig. 10, a modechanging wedge 81 is so shaped that the wave b reflected from the interface is returned to the main transducer 83 by reflection at surfaces 82, 85 and 82, the surface 82 being stepped so that at the frequency f employed for flaw detection components of the wave reflected from adjacent steps are out-of-phase. When monitoring the coupling, a frequency 2f is employed so that the components are in phase and produce a monitoring signal at the main transducer; alternatively the steps may be replaced by an array of parallel scatterers. A suitable transducer capable of operating over a frequency range of 2: 1 is illustrated in Fig. 11 and forms the subject-matter of Specification 841,512. As shown, it comprises four alternatively-poled crystal layers 91 ... 94 having metal film electrodes a ... e and a dissipative backing. In the monitoring mode electrodes a, c and e are earthed, the transmitter pulse is applied to electrode b and the monitoring signal is derived from electrode d; in the detection mode electrodes a, b, d and e are earthed and the transmitter pulse is applied to electrode c. In a modification the crystal layer 94 is omitted and in the monitoring mode electrodes a and c are earthed, the transmitter pulse is applied to electrode b and the monitoring signal is derived from electrode a. Each crystal layer may comprise a grooved plate or array of elements as in Specifications 703,508 and 772,083 and the crystal material may comprise barium titanate with admixtures of calcium titanate and/or lead titanate or of zirconates or niobates. The electrode a is extended to form a diaphragm and may be of an alloy comprising 29% nickel, 17% cobalt and the rest iron. The dissipative backings shown in Figs. 2, 4, 6, 7 and 11 may comprise a dense powder such as tungsten embedded in a plastic material also containing fibrous material such as asbestos.