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Evaluation of the dynamic stress intensity factor KI(t) is especially important in impact fracture toughness testing, since KI(t) is influenced by the inertia force of a specimen and quasi-static estimation of KI(t) usually leads to erroneous results.In the present paper, a procedure is presented for measuring KI(t) in the impact three-point bend testing. The contact force between the impactor and the specimen is computed first by using the output signal of a strain gauge mounted on the impactor and the transfer function of impactor. The FFT (Fast Fourier Transform) analyzer is utilized for measuring the transfer function in the pre-experiment in which the impactor is struck by a steel sphere and the impact force is measured by a piezoelectric accelerometer pick-up attached to the sphere. The time variation of KI(t) is then determined by the computed contact force and a simple formula which has been derived previously by the present authors using Timoshenko's beam theory.A program to run on a microcomputer was developed and assessment tests were conducted for the case that a steel or aluminum alloy specimen was impacted by a falling cylinder. Good agreement was obtained between the computed KI(t) and the value determined from the strain gauge mounted near the crack-tip.

A measuring system for dynamic stress intensity factors using a FFT analyzer - Application to drop weight impact three point bend testing.