Residual Stress Measurement with Ultrasonic Method

Manufacturing operations introduce Residual Stresses (RS) during sequential thermo-mechanical processes, like forming, heat-treatment and machining. These Stresses can lead to cracking, distortion and lower fatigue life and they need to be properly assessed and managed. Standard RS measurement techniques are often destructive, time consuming, and lab-based, increasing the cost of quality inspection and process optimization. The ultrasonic method is an alternative, non-destructive, portable technique designed for the assessment of sub-surface (0.5-5.0mm) or bulk (i.e. through-thickness) stresses. Standard commercial applications are based on the pulse echo technique, where a probe is placed on the surface of a part and a depth-averaged stress is estimated from the change in the Time of Flight, as per the acoustoelastic theory. The method is calibrated for each material under uniform (applied) stress conditions by extracting the Acoustoelastic coefficients. The ultrasonic technique has good penetration depth, good spatial resolution, fast measurement time, and can also provide the Young modulus, Poison ratio, and thickness of parts.