Acoustic Nonlinearity as an In-Situ Indicator of Yield Onset in S235 Structural Steel Under Monotonic Tensile Loading

The residual life of metallic structures is dictated by microstructural plastic damage, which conventional linear ultrasonics often fail to detect early. This study investigates the continuous, in-situ evolution of the second-order acoustic nonlinearity parameter (β2) throughout the monotonic tensile response of S235 structural steel, from the elastic regime to fracture. Bulk longitudinal waves were excited by a contact piezoelectric transducer and detected through-thickness using a non-contact Laser Doppler Vibrometer on a notched specimen. This allowed the normalized acoustic parameters to be mapped directly onto the macroscopic stress-strain curve. Three distinct acoustic regimes were identified. Up to the onset of the yield plateau, the nonlinearity parameter and the linear attenuation remained at unity within the specimen-to-specimen scatter. Beyond the yield point, β2/β2(0) rose steeply to approximately 4.5, whereas the normalised attenuation increased by at most approximately 20%. The rise extended over the yield plateau of the stress–strain curves and the parameter saturated when work hardening resumed (nominal strain ≈ 0.031–0.033), ahead of the maximum load (≈0.043); a lower value was recorded only at the last acquisition, at which the signal quality was degraded. The departure of β2 from unity, i.e., the first increase of the second-harmonic amplitude, occurred between nominal strains of 0.015 and 0.021, coinciding with the onset of the yield plateau (≈0.017) and slightly above the 0.2% offset yield strain (0.0128 ± 0.0004), as expected for a notched specimen in which plasticity reaches the interrogated volume once it spreads across the ligament; this supports its potential as a marker of yield onset. The increasing trend provides a continuously acquired ferritic-pearlitic reference data set and is consistent with, although not by itself proof of, a governing role of the initial dislocation substructure in harmonic generation.