Fracture Mechanism of Hydrogen Assisted Cracking of a QT-treated Low Alloy Steel Estimated by AE Moment Tensor and Source Wave Analyses

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A new system for the source inversion processing and the moment tensor analysis of elastic waves was developed and utilized to elucidate the fracture kinetics and kinematics of hydrogen assisted cracking (HAC) of a QT-treated low alloy steel. The system consists of 8 channels. Seven channels is utilized to measure the arrival time and the amplitude of the longitudinal waves of the AE signal caused by the microcracking and the other to measure the out-plane displacement. The source location for AE signal and the moment tensor were, respectively, computed from the difference of the arrival times and from the relative amplitude of the longitudinal waves. Fracture kinetics was deduced by the inversion processing of the measured displacement with the Green's function of the second kind. The tensor analysis revealed that most cracks in HAC were caused by the quasi-cleavage fracture whose shear ratio was less than 40%, however, the Mode-II shear fracture connecting the isolated the Mode-I cracks, and the Mode-II fracture of vertical crack were also found to occur during HAC. The crack size and its evolution rate were found to change from place to place and with the testing time. The system can make clear "what kind of fracture occurs, at what rate, and where".
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