Patent GB-569874-A (GB)
Title: Improvements in or relating to supersonic inspection devices
IPC: G01N29/07
Filed: 1943-07-13
Abstract: 569,874. Supersonic inspection devices. UNITED AIRCRAFT CORPORATION. July 13, 1943, No. 14926/44. 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, means for limiting said train to a predetermined number of oscillations, an electromechanical transducer actuated by said voltage train producing means and adapted to be 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 wave trains reflected within said part, an amplifier actuated by said transducer, and indicating 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 refiected back from the opposite boundary and from any flaws in the object, and react in 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 R. The horizontal deflecting plates are fed from a time-base 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, Fig. 4, comprises a large condenser 11 which is charged to about one thousand volts through a resistance 12. Shunted across condenser 11 is an oscillatory circuit 13, 14 and the desired voltage trains are taken across 14 (or 13) and applied to the electrodes of crystal 3. The condenser 11 is also shunted by a grid-controlled gas-filled triode 15 normally biassed to cut-off by a source 16. On the application of a 60-cycle voltage through transformer 17, the triode 15 periodically conducts to discharge condenser 11 suddenly and generate 60 trains per sec. of voltage waves of a frequency (about 5 megacycles) determined by the combination 11, 13, 14. A lead 19 is connected to the synchronising terminal of the time base unit 9, Fig. 1. Means for shortening the wave trains comprises a gas-filled triode 20 connected in series with a resistance 21 across the crystal 3. The triode is normally biassed to cut-off by a source 22. The grid of triode 20 is connected through a time-delay network 25, 26 to the anode of a one-stage amplifier 24 whose input is connected to one terminal of condenser 11. When condenser 11 discharges the consequent negative impulse drives amplifier 24 to cut-off and allows the upper terminal of condenser 26 to go less negative until after a time interval, at an instant when the anode of 20 is positive, valve 20 conducts and the oscillation is completely damped within a half-cycle. This 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 reflection pattern. In modifications, the means, for shortening the wave trains are omitted (Figs. 3, 3a, not shown), the condenser 11 may be discharged through a triode in series with a resistance, and the voltage across the resistance is used to start the oscillatory circuit into oscillation. Separate sending and receiving crystals may be used as described in U.S.A. Specification 2,280,226. Specification 569,858, [Group XL], also is referred to.