GB774675A - Improvements in or relating to apparatus for flaw detection and velocity measurementby ultrasonic echo methods - Google Patents

Abstract

774,675. Ultrasonic inspection devices. SPROULE, D. O., and GLASS DEVELOPMENTS, Ltd. Dec. 24, 1953 [Sept. 26, 1952], No. 24233/52. Class 118 (2). [Also in Group XXXIX] In an ultrasonic pulse-echo flaw-detecting or testing system comprising separate transmitting and receiving crystal transducers 2 and 4, the transmitting transducer 2 is coupled by a metallic member 3 to the receiving transducer 4 which is coupled to the workpiece 5 so that the transducer 4 receives both the transmitted and echo pulses which may be displayed on a cathode-ray oscilloscope. A metallic damping block 1 is mounted on top of transducer 2, absorbing material being provided at the top of the block or distributed throughout it. The elements 1 . . . 5 are preferably matched to each other such that p1v1=p2v2 ... =p3v3 where p is the density of the element and v is the wave velocity in the element. The transmitting transducer 2 is energized by applying to the block 1 a D.C. voltage step or squarewave voltage to produce a corresponding non- oscillatory pressure wave pulse, Figs. 2 and 3; when the Fig. 3 waveform is employed the signal/noise ratio may be improved by employing a resonant receiving transducer or amplifier. The receiving transducer 4 is preferably of " sandwich " construction with earthed outer electrodes and a live inner electrode, the waveform of the voltage output from the transducer 4 for an applied pressure wave of the type shown in Fig. 2 being as shown in Figs. 4 and 5, depending on whether the thickness x4 of the transducer 4 is such that t4=x4/v4 is less than or equal to the duration t2 of the pressure wave pulse. The Fig. 5 waveform can be converted to that shown in Fig. 8 by differentiating the output from transducer 4. If the output from transducer 4 is as shown in Figs. 4 or 8, then a flaw near the surface of the workpiece can be located if it is such that the echo pulse E, Fig. 6, is reversed in polarity relative to the transmitted pulse P since the transmitted and echo pulses will overlap to give a composite output pulse, Fig. 7, having an identifiable portion corresponding to the leading edge of the echo pulse; the effect of the direct pulse P can be diminished by suitable gating or attenuating circuits. Near flaws may also be detected by introducing a long column of liquid couplant between the transducer 4 and workpiece 5. Reference has been directed by the Comptroller to Specification 715,143.