Excitation of S1-ZGV Lamb wave with a compact wedged transducer and its application in local stress measurement

Residual stress and its distribution are the key indicators to ensure structural integrity. However, current ultrasonic methods are not appropriate for complex stress measurement. Considering the proximity of zero-group-velocity (ZGV) mode with the cut-off frequency and its sensitivity to local material property, this paper proposed a local stress measurement method based on S1-ZGV mode. The stress field reconstruction was demonstrated numerically with a nonuniform plate. Then, a FE model was built to investigate the excitation of S1-ZGV mode by oblique incidence approach, proving that S1-ZGV mode can be effectively excited and received with a small distance. Thus, a contact ultrasonic system along with a tailor-made compact wedged transducer was built for experimental validation. Parametric studies were performed to obtain the optimal excitation conditions for S1-ZGV mode. At last, experimental validation was carried out by standard tensile testing. The stress coefficient between the frequency shift and the applied stress was obtained with a uniform plate. The stresses of the nonuniform plate at three different positions were measured, proving that S1-ZGV method is effective for local stress measurement in thin plates.