The object of this project has been to investigate the applicability of an alternating-current potential difference method of crack length estimation to rapid loading fracture mechanics tests in ferromagnetic materials. The more commonly used direct-current method has been demonstrated to be very sensitive to induced magnetization under rapid loadings. The resulting direct-current output signal is complex and not repeatable, and the component related to crack extension cannot be separated from the total response. The tests reported here show calibrations of a 10 kHz alternating-current system on an A533B material and application of the calibration to a static unloading compliance test and to rapid servohydraulic tests. A d-c component dependent on stress-induced magnetization is still present but can now be separated from the high frequency component using Fourier series methods.
The major conclusion is that an alternating-current technique with a properly chosen excitation frequency can be used to detect crack growth in rapidly loaded specimens. Additional comments on developing an improved system are presented.
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