GB627543A - Apparatus for fatigue testing specimens or test pieces - Google Patents

Abstract

627,543. Fatigue testing of materials. DE HAVILLAND AIRCRAFT CO., Ltd., HADWIN, R. N., and POLLARD, J. R. June 20, 1946, No. 27930/48. Divided out of 627,501. [Class 106 (ii)] In apparatus for testing specimens for fatigue, particularly those of metal or wood, and having welded or resin-bonded joints, one or two specimen strips being fixed at one end to a base and maintained in vibration by a magnetic exciter fed by an amplifier, and the input to the amplifier being supplied by a pick-up device in close proximity to the strips, the output of the amplifier is limited to a constant amplitude independently of the amplitude of the input to the amplifier from the pick-up device. The apparatus, apart from the ampli tude limiter, is described in Specification 627,501, a pick-up feeding a degenerative amplifier which passes the signals to a phasecorrector, an amplitude limiter, a filter, a power amplifier, a condenser, and then to one coil of an E-shaped electromagnetic exciter arranged to vibrate the specimens at their natural frequency. A further coil on the exciter is' supplied with direct current to polarize the exciter, or a permanent magnet may be used for this purpose; this avoids frequency-doubling and ensures that the specimens vibrate at their fundamental frequency. The amplitude limiter comprises two diodes 18, 19 biassed by voltages derived from potential divider resistances 20, 21, 22 so that the amplitude of the signal in either direction is short-circuited when it exceeds the value of the bias on one or the other diode. Bias is supplied from a high-tension supply 23. Resistances 21, 22 are equal in value, and the mid-point is connected through resistances 13, 24 to the cathode of diode 18 and anode of diode 19, the anode of diode 18 being earthed at 31 and the cathode of diode 19 being connected to the positive end of resistance 21. Condensers 25, 26 prevent A.C. voltage at signal frequency being developed across resistances 21, 22. The signal is fed from the amplifier to the input 27 of a resistance-coupled triode amplifier 28, the amplified signal is then fed through a blocking condenser 29. The signal is changed by the action of the diodes 18, 19 to a square-topped wave-form of amplitude equal to the voltage developed across either of resistances 21, 22, and is passed from the amplitude-limiter to a filter and power amplifier.