GB1095562A - - Google Patents

Abstract

1,095,562. Detecting strains and flaws electro-magnetically. GARRETT CORPORATION. March 23, 1965 [March 24, 1964], No.12332/65. Heading G1N. Irregularities, e.g. cracks, stresses, strains or flaws, in magnetic and/or conductive bodies are detected by placing the body in the air gap of a magnetic core to complete the magnetic circuit. Flux through the circuit, produced by an A.C. energized primary coil, is detected by a secondary coil, whose output signal varies both in phase and amplitude with the irregularities in the body. In one embodiment a planar or bent E-shaped magnetic core 2 or (85), Figs. 1 and 3 (latter not shown), is employed, the primary coil, energized from oscillator 45, being wound round the central leg 2A and two secondary coils 4, 5 being wound round the outer legs 2B, 2C to detect the flux in two magnetic circuits 6, 7, completed by the specimen 8. The outputs of the two coils 4, 5 are connected in opposition, to give a signal indicative of differences in magnetic properties between the two parts 8A, 8B of the specimen, across a potentiometer 11 and voltage divider 12. The wiper arm 11A of the potentiometer is connected to the junction 17 between the coils and is adjusted initially until zero output voltage is obtained for a specimen having uniform magnetic characteristics. In operation the output from the voltage divider is amplified in amplifier 20, tuned to the frequency of the oscillator 45, and transmitted both to an audio amplifier 40 and phase sensitive rectifier 30. In the rectifier 30, the signal is compared in phase with a reference signal from the oscillator 45, rectified and fed to an ammeter 60 by which its magnitude and polarity is indicated. It is found that irregularities in ferromagnetic materials produce a more noticeable phase change than amplitude change in the signal from the secondary coil while irregularities in conductive materials produce more noticeable amplitude changes. In a second embodiment, an H-shaped core (95), Fig.4 (not shown), is provided, the primary coil (96) being wound round its cross-member (95A) and two secondary coils being wound round opposite ends (95B, 95C) of one leg to provide two magnetic circuits which are completed by the test specimen 8 and a reference specimen (8C) devoid of irregularities, for quantitative tests or having maximum acceptable irregularities for "accept", "reject" tests. In the former case the potentiometer is adjusted to give zero output for a test specimen devoid of irregularities while in the latter case zero output is arranged for a test specimen having maximum acceptable irregularities. In a third embodiment in which a C-shaped core (110), Fig. 5 (not shown), is employed, having primary and secondary coils (111, 112) wound on its two arms, the output signal from the secondary coil is algebraically added to a reference signal, supplied from the oscillator 45, whose amplitude is arranged either to correspond to that due to a reference specimen devoid of irregularities or with maximum acceptable irregularities, the potentiometer being adjusted as in the previous embodiment.