Low-frequency acoustoelastic-based stress state characterization: Theory and experimental validation

The acoustoelastic theory has been widely utilized for nondestructive stress measurements in structural components. Most of the currently available techniques operate at the high-frequency, weakly-dispersive portions of the dispersion curves and rely on time-of-flight measurements to quantify the effects of stress state on wave speed. High-frequency elastic waves are known to be less sensitive to the state-of-stress of the structure. As a result of such low sensitivity, calibration at a known stress state is required to compensate for material uncertainties, texture effects, and geometry variations of the structure under test. In this work, a new model-based stress measurement technique is developed. The technique integrates the acoustoelastic theory with numerical optimization and allows the utilization of the highly-stress-sensitive, strongly-dispersive, low-frequency flexural waves for reference-free stress measurements. The technique is experimentally validated on a long, rectangular aluminum beam, where accurate stress measurements have been achieved at low excitation frequencies. For instance, with a 500 Hz excitation signal, the error in the measured state-of-stress is found to be in the order of 1 MPa for the different loading scenarios considered in this study. Experimental results show that the developed technique is capable of measuring the state-of-stress without the need for calibration at a known stress state, which makes it ideal for in-service structures.