Patent GB-569872-A (GB)
Title: Improvements in or relating to supersonic inspection devices
IPC: G01N29/07
Filed: 1943-07-13
Abstract: 569,872. Supersonic inspection devices. UNITED AIRCRAFT CORPORATION. July 13, 1943, No. 14924/44. Convention date, June 29, 1942. Divided out of 569,858, [Group XL]. [Class 118 (ii)] A device for the inspection or measurement of a part by means of supersonic vibration waves comprises means for producing a voltage train and an electro-mechanical transducer actuated by said voltage train producing means and having a sensitive area placed in effective contact with said part so as to radiate supersonic vibration wave trains into said part and generate voltage trains in response to vibration wave trains reflected within said part, a heterodyne amplifier actuated by said transducer and an indicative means actuated by said amplifier. The invention is described in its application to detecting flaws in solid objects by radiating a supersonic vibration into the object and comparing the time of reflection from a flaw with the time of reflection from a distant boundary of the object as shown generally in Fig. 1. The generator 1 produces voltage trains 32 of a frequency of about 5 megacycles, the voltage trains following each other at a frequency of 60 trains per second. The voltage is applied to the electrode of a mechanically damped quartz crystal 3 having the same frequency which is attached by a film of oil to one surface of the object 4 to be tested. Trains of mechanical vibrations pass into the object and are reflected back from the opposite boundary and from any flaws in the object, and react on the crystal 3 to generate corresponding voltage waves. The primary voltage trains a, d, together with voltage trains b, c due to reflection from the opposite boundary, and voltage train f due to reflection from a flaw are all applied to an amplifier 5 and rectifier 7, the output of which is applied to the vertical deflecting plates of a cathode ray tube 8. The horizontal deflecting plates are fed from a timebase unit 9 synchronised with the pulse frequency of generator 1. Thus the trains a, b, c and f give corresponding traces a, b, c and f on the screen of the cathode ray tube which enable the existence and position of a flaw to be determined. The generator 1 is as described in Specification 569,858, [Group XL]. The arrangement produces wave trains in which the individual waves throughout each train are in the same phase as corresponding waves of other trains, thus giving a stationary refleotion pattern. As the amplifier 5, Fig. 1, must recover its sensitivity quickly after receiving the primary impulse of about one thousand volts so as to receive the small reflected impulses, the normal coupling condensers necessary for a frequency of 5 megacycles are too large. This is overcome by the arrangement of Fig. 5 in which the output at 5 megacycles from crystal 3 is fed to a frequency converter 28 fed from 29 with, a frequency of 15 megacycles to give an output at 10 megacycles. This is fed to a band pass amplifier 31 with a range of 8-12 megacycles which can be built to have a considerably shorter blocking period. Even higher frequencies may be used. After rectification at 7 the pulses are fed to the cathode ray tube 8. Separate sending and receiving crystals may be used as described in U.S.A. Specification 2,280,226.