Minimizing inspection dead zones in pipeline stress evaluation via acoustoelastic local resonance of zero-group-velocity guided waves

Small-diameter thin-walled metal pipelines are critical components in industrial infrastructure, where localized axial stress concentrations at joints and extremities frequently serve as precursors to structural failure. Conventional ultrasonic guided wave techniques rely on propagating modes and time-of-flight (ToF) measurements, which inherently suffer from boundary interference and significant physical “dead zones” near pipe ends, severely limiting their applicability in near-boundary stress evaluation. To address these limitations, this study proposes a non-contact stress characterization methodology utilizing Zero-Group-Velocity (ZGV) guided wave modes excited by an optimized Electromagnetic Acoustic Transducer (EMAT). A nonlinear acoustoelastic theoretical framework is first established to quantify the interaction between the axial stress field and the localized standing wave resonance of the L(0,4)-ZGV mode, revealing a linear stress-frequency dependence. A wavenumber-filtered periodic permanent magnet EMAT is then designed to selectively excite the target ZGV mode, confining acoustic energy predominantly beneath the transducer footprint. Experimental validations on 6061-T6 aluminum alloy pipes demonstrate a highly linear stress-induced frequency redshift (R² > 0.99) with an experimental sensitivity of 15.63 Hz/MPa. Owing to this energy localization mechanism, the measurement exhibits strong resistance to boundary reflections, substantially reducing the practical inspection blind zone and enabling stress characterization as close as 20 mm from the boundary fixture. Furthermore, the frequency-domain measurement approach provides good spatial measurement stability, with a spatial repeatability of 1.03 MPa obtained from the stress evaluated at successive axial positions under constant load. This work presents a robust paradigm for non-contact, in-situ monitoring of pipeline axial stress distributions in spatially restricted critical regions where conventional acoustic methods are compromised.