GB866570A - Improvements in or relating to apparatus for material testing by magnetic means - Google Patents

Abstract

866,570. Electric tests. CENTRAL ELECTRICITY GENERATING BOARD, WISEMAN, J. E. T., and HANSTEAD, P. D. Feb. 10,1959 [Dec. 27, 1957], No. 40223/57. Class 37. An apparatus for testing ferromagnetic material comprises a magnetizing winding adapted to be positioned near the material to be tested so that when a unidirectional current is passed through the winding the material is magnetized, and a core, also adapted to be positioned near the material, the core carrying an input winding to which an alternating current is supplied and an output winding connected to means for determining the magnitude of the voltage developed therein and thus the properties of the material, the core and the magnetizing winding being mounted in a probe unit connected by flexible leads to the remainder of the apparatus. In Fig. 2 (Fig. 3, not shown) the probe includes two multi-turn windings represented by 20 and 21 arranged with parts facing each other and parts extending in opposite directions in a common plane for location adjacent the material to be tested, so that when a continuous or pulsed direct current is passed through the windings the material is subjected to a field which is in opposite directions in the parts opposite the two windings. Two cores 22, 23 are located adjacent the windings and are provided with input windings to which an A.C. signal is applied and output windings connected to a detector. The current is passed through the magnetizing windings 21, 22 until the material being tested is magnetically saturated, and the voltage induced in the output winding is then measured. The windings are so arranged that only the second harmonic of the input frequency appears in the output windings the amplitude of this harmonic representing the remanent magnetism and thus the hardness or composition of the material. If a continuous current is applied to the magnetizing coils 21 and 22 the apparatus will measure the permeability of the material while if a magnetizing field is applied to the material, as by a permanent magnet, the apparatus will detect discontinuities in the magnetic field and thus flaws and cracks. The current supply and measuring circuits are connected to the probe by flexible leads. D.C. magnetizing current pulses for the coils 21 and 22 are generated in a circuit comprising a capacitor 32 which charges through a relay coil 30 or a gas discharge tube, so that the charging current operates the relay and opens the contact 31. When the capacitor is charged the relay releases or the gas discharge tube extinguishes and the capacitor discharges through resistor 40, a closed manually-operated switch 39 and the magnetizing winding 20, 21, so magnetizing the material. The discharge current flows until the capacitor voltage drops to a value at which the relay reoperates, whereupon the cycle repeats itself until the remanent magnetism reaches its maximum value and the manually-operated switch 39 is released to terminate the magnetizing period. The input windings 24 are energized from an oscillator circuit 45, 46 and the output windings are connected via an amplifier 47 tuned to the second harmonic of the oscillator frequency and a further amplifier 49 to a detector 50. This detector may be calibrated to represent the hardness of the specimen, e.g. of a turbine blade. It is stated that a single core only need be used for the input and output windings in which case the indicator must be highly selective to the second harmonic voltage. The correct operation of the apparatus may be tested by applying the output of the oscillator directly or through a suitable harmonic producing circuit to the amplifier and adjusting the controls to produce a predetermined output value.