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.
State of the Art/Maturity
In manufacturing, knowledge and understanding of residual stresses is a key information for the control and management of stresses and distortion throughout the production route. The non-destructive evaluation (NDE) of a part’s internal stress state can enable machinists to account for the variability of stresses within a part or between parts, batches, and material suppliers and assist the design of machining strategies. Ultrasonic systems can be integrated into milling/turning machines to implement adaptive machining strategies based on a combined criterion of the measured stress state and distortion. Such techniques can be applied to smaller length scales and assess the cutting conditions and finish quality (i.e. machining induced effects/RS), particularly for difficult to cut materials, like aerospace grade Titanium and Nickel alloys. Laser-based systems offer the advantage of non-contact measurements and can provide significant flexibility and allow the application of the method in complex shapes where access is limited. The technology is cost effective and can be integrated with existing equipment towards a fully automated production, with in-situ, in-line, real-time measurement options. Ultrasonic NDE can be an efficient tool in the advancement of manufacturing and can offer a step change towards digital twins and industry 4.0.
Practical applications for Machining
- Non-destructive evaluation of Residual Stresses pre, during and post machining;
- Real time, in-/ex-situ, off-/in-line, automation and integration with legacy equipment;
- Preliminary assessment of the stress state of parts to verify parts, design and optimize machining strategies and re-design the entire method of manufacture.