Noncontact evaluation of steel mechanical properties using nonlinear laser ultrasonics

Abstract

Quantitative nondestructive evaluation (NDE) of mechanical properties, such as hardness, yield strength (YS), and ultimate tensile strength (UTS), is critical for industrial applications. Nonlinear ultrasonic (NLU) techniques have shown promise in linking microstructure evolution to mechanical performance through higher harmonics generation. However, existing methods relying on piezoelectric transducers face limitations, including coupling requirements, and restricted accessibility. This study proposes a noncontact, all-optical approach using grating laser-induced narrowband Rayleigh waves to address these challenges. By generating frequency-specific surface waves and analyzing second-harmonic amplitudes, nonlinear parameters (β) are extracted to evaluate steel samples under varied annealing conditions. Mechanical properties are concurrently measured via micro-Vickers hardness and uniaxial tensile tests, while dislocation density evolution was quantified via X-ray diffraction (XRD). Results reveal a unified exponential decay relationship between normalized β and mechanical properties (YS, UTS, and hardness), attributed to dislocation density reduction during annealing. Extended annealing diminished dislocation interactions, thereby lowering both nonlinearity and strength. The established acousto-mechanical model (goodness-of-fit >0.97) demonstrates the feasibility of in-situ, single-sided mechanical property assessment, bridging acoustics with macroscopic performance. The work provides a noncontact framework for quantitative NDE, offering insights into dislocation-mediated nonlinear mechanisms and scalable industrial applications.