GB856692A - Flaw detection - Google Patents

Abstract

856,692. Electric tests. IMPERIAL CHEMICAL INDUSTRIES Ltd., BRYANT, J. D. W., and PERRIAM, L. T. March 7, 1957 [March 7, 1956], No. 7229/56. Class 37 In a probe device for flaw detection of the type comprising a core having 'three flanges with a coil wound round a core between each pair of flanges, the flanges are provided with pole pieces the distance between the edge of each pair being less than the distance between the flanges. Since the gaps between the two sets of pole pieces are widely spaced, when the device is traversed along an object under test, e.g. inside a condenser tube there is time for the magnetisation caused by one coil to decay before the other gap passes the flow. As shown, a device intended to be traversed up the centre of a tube under test comprises a guiding nose 5, a magnetic core 8 provided with flanges 2, 3, 4, a pair of coils 1, 7 situated respectively between flanges 2, 4 and 3, 4. Pole pieces, also of magnetic material, are attached to flanges 2, 3, 4 respectively by pegs P. Gaps 15, 16 are provided between the coil and centre pole piece and are shorter than a coil; they may be fitted with a synthetic resin. An insulated handle 6 in which are connections 9, 10, 11 is provided. Fig. 2 shows the circuit. Coils 7, 8 are mounted in adjacent arms of an inductance bridge circuit the output of which is developed across resistor 23 and may be fed to amplifier 24, the bridge is supplied from oscillator 32. Initially the device is placed in a standard tube known to be free of flaws and resistors 21, 22 are adjusted to balance the bridge as indicated by meter 25 switch-connected to amplifier 24. The device is then put in a tube under test and amplifier 24 is connected to phase discriminator 26 fed with a reference signal from the oscillator. As the device passes a flaw a signal is produced first of one phase sense and then of the other and the phase discriminator feeds this signal through a D.C. amplifier to mark a recorder with a straight line which deviates first in one direction and then in the other. In order that changes in thickness may be detected, gaps 15, 16, may be of unequal length so that the line may deflect and stay deflected (Fig. 3 not shown). The sensitivity of the device may be increased by inclining the gaps (Fig. 4, not shown). Fig. 5 shows a device intended to be guided by flared opening 82 as the outside of a bar under test, flanges 71, 72, 73 are of a non-magnetic material e.g. "Lufnol" (Registered Trade Mark) pole pieces 79, 80, 81 are secured internally.