Comparison of longitudinal critically refracted ultrasonic waves and magnetic Barkhausen noise for tensile stress measurement

Abstract

Longitudinal critically refracted (LCR) ultrasonic waves and magnetic Barkhausen noise (MBN) are two commonly used methods for measuring the residual stress of steel. Both of these methods have distinct advantages and disadvantages that need to be taken into consideration when selecting the most appropriate technique for measuring residual stress under specific circumstances. This issue is experimentally investigated in this paper by comparing the linearity, sensitivity and repeatability of these two measurement techniques for evaluating the stress within Q345 steel samples subjected to different levels of applied tensile stress. The time-of-flight (TOF) of LCR waves is demonstrated to conform almost perfectly with the applied stress in a linear relationship, while a similarly perfect confirmation is obtained for the root mean square (RMS) voltage of MBN signals to a quadratic curve. The results demonstrate that the RMS voltage of MBN signals is much more sensitive than the TOF of LCR waves to variations in surface stress, where the application of a 240 MPa tensile stress increases the RMS voltage of MBN signals by about 140% relative to that of the unstressed sample, while the TOF of LCR waves increases by less than 0.15% under corresponding conditions. Multiple testing of the same specimen demonstrates that MBN measurements more closely coincide than those of LCR waves. Therefore, MBN exhibits superior repeatability to LCR waves.