GB643146A - Improvements in or relating to supersonic testing apparatus - Google Patents

Abstract

643,146. Supersonic inspection devices. SPERRY PRODUCTS, Inc. Dec. 10, 1946, No. 36423. Convention date, Dec. 19, 1945. [Class 118 (ii)] In a method of supersonic testing, a fundamental frequency is generated which is strong in harmonics and transmitted to the test object through a piezo-electric crystal having a natural frequency which either corresponds to or is a harmonic of the said fundamental frequency, and thereafter the piezo-electric crystal is substituted by another such crystal having a different natural frequency which is also either the fundamental frequency or a harmonic thereof. Providing these specified crystals are used, the oscillator circuit of the apparatus can be fixed. As shown, the pulse impressed on a crystal 11 in engagement with an object 12 is also impressed on an amplifier 15 associated with an oscilloscope 16. The pulse generator comprises an oscillator 30 having a control grid 31 on which is placed a relatively large negative bias by a trigger valve 32. A square wave generator 40 feeds through an amplifier 34 and inverter 35 to bias the grid 33 of the trigger valve 32 sufficiently to render the latter non-conducting, thus reducing the negative bias on the grid 31 of the oscillator 30 to allow the production of the desired pulse. The duration of the pulse is a function of the square wave which can be adjusted by potentiometer 37. The oscillator includes a tuned circuit having a coil 45 and a condenser 46 which is fixed at the frequency of the crystal having the lowest frequency in a set to be employed in testing. The characteristics of the oscillator circuit are chosen to yield the maximum number of harmonic frequencies. Specifications 637,164, [Group XL (c)], and 641,095 are referred to.