An Improved Method for Threshold Fatigue Crack Propagation Testing on an Electromagnetic Resonance Type Machine

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Abstract
An improved technique of fatigue crack propagation testing is proposed for the determination of threshold stress intensity range ΔKth using an electromagnetic resonance type testing system. A special device is used to compensate for the change of specimen compliance during the test. It is composed of a set of traverses mutually hinged at one end between which an elastic body and a specimen are mounted in parallel, respectively, at predetermined distances from the hinge, so that the resultant composite compliance of the device is designed to be practically insensitive to the change of specimen compliance. A series of ΔKth measurements was performed at various load ratios for six steels of various strength levels, using small compact type (CT) specimens of 25 mm width and 5 mm thickness, under combined control modes of the applied load and the backface strain of the specimen. The use of the compliance stabilizing device improved markedly the controlling stability of the testing machine and facilitated the determination of ΔKth value under autodecreasing ΔK conditions at high load ratios. The obtained ΔKth values from the experiments were in excellent agreement with those found in the literature.