Dynamics and Precursor of Delayed Fracture Studied by the Waveform Simulation of 7MHz Acoustic Emission Signals and by the Laser Excited Longitudinal Wave Velocity

Abstract

With the aim of studying the precursor and dynamics of delayed fracture of high tension steel, we developed two advanced elastic wave monitoring systems. One is a broad-band digital AE monitoring equipment consisting of a multi-resonant 7MHz AE sensor, a 10MHz pre-amplifier and a 200MHz A/D converter. The overall transfer function was determined as a response to a pulse YAG laser break-down of silicon placed in slit (or crack). The break down was found to simulate the Mode-I fracture with effective source rise time of 0.035μs and successfully utilized to determine the overall transfer function of the system. The waveform simulation of the monitored AE signals revealed the succession of fast and small (<10μm) cleavage cracks with source rise time of 0.07μs to 0.24μs in high hydrostatic stress field. In the 20MHz laser-ultrasonic system developed, a velocity change of spherical longitudinal (P-) wave was continuously monitored to detect the precursor of delayed fracture. Here, the P-wave was excited by pulse YAG laser so that it propagates through the hydrostatic stress field. The velocity decreased by 100m/s at 2.8ks before the first AE signal. The velocity decrease appears to be due to hydrogen induced voids. The method has potential for monitoring the precursor of delayed fracture.