Baseline-free local stress measurement based on the multi-directional polarization characteristics of zero-group velocity Lamb waves

Localized stress concentrations in critical structural regions pose serious risks to the integrity and safety of components in aerospace, transportation, and wind power applications. Conventional ultrasonic stress measurement techniques, such as time-of-flight (TOF) methods, provide only averaged stress information along the wave path and suffer from limited spatial resolution. Importantly, these methods often rely on baseline data, which can be difficult to obtain or affected by environmental variations. To address these limitations, this study proposes a baseline-free localized stress monitoring method based on stress-induced multi-directional frequency shifts of zero-group-velocity (ZGV) Lamb wave modes, ensuring greater flexibility and robustness in real-world applications. Based on the anisotropy caused by stress, a piezoelectric (PZT) transducer array is employed to capture ZGV responses in multiple directions, and the frequency differences among these directions are fused to estimate localized stress. Experimental validation demonstrates that the proposed method achieves a maximum absolute error of 3.3 MPa within a stress range of 20–100 MPa, over a distance of 7.5 mm, under the condition of eliminating reference signal dependence. Compared with conventional TOF-based approaches, the method offers significantly enhanced spatial resolution and measurement accuracy, highlighting its potential for high-precision local stress monitoring in critical structural components.