Ultrasonic Measurement of Residual Stress and Microstructure: From LCR Acoustoelasticity to Phased Arrays, Robotics and Digital Twins

Abstract

Residual stress influences dimensional stability, fatigue, fracture and structural integrity in welded and additively manufactured components. Ultrasonic methods offer a non-destructive and deployable alternative to diffraction and strain-relief techniques because stress changes elastic-wave velocity through acoustoelasticity. This review traces ultrasonic residual stress measurement from nonlinear elasticity and single-element longitudinal critically refracted (LCR) waves to finite-element-assisted LCR, robotic phased-array inspection and Phased-Array Ultrasonics for Residual Stress Measurement (PAURS). It critically examines time-of-flight precision, depth and path averaging, reference-state uncertainty, temperature, coupling, geometry, Type I residual stress and its interaction with phase transformation, microstructure and texture. PAURS improves multi-path redundancy and diagnostic capability, while quantitative accuracy remains dependent on calibration and material-state effects. PAURS+ is proposed as the convergence of PAURS with phased-array microstructure characterisation; unlike experimentally demonstrated PAURS, it remains a conceptual multi-observable framework requiring quantitative validation. The review concludes that integrated in-process material-state measurement requires PAURS+ for multi-observable sensing, robotic high-temperature inspection for deployment, and coupled process, material and wave models for interpretation. Physics-informed AI, independent ground truth, uncertainty assessment and digital-twin updating are required to connect these capabilities to traceable manufacturing decisions.