Application of alternating current potential difference to crack length measurement during rapid loading

The objective 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 done 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 dc 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