Experimental Study on Stress Measurement Based on Zero-Group-Velocity (ZGV) Lamb Waves

The stress state of engineering structures is the key information for the reliability evaluation. However, conventional ultrasonic method is not suitable for local stress measurement. The dispersion characteristics of Lamb wave acoustoelasticity show that higher-order Lamb waves near cut-off frequency possess higher sensitivity to stress. Considering that the proximity of zero-group-velocity (ZGV) mode and the cut-off frequency and its sensitivity to local material property, a local stress measurement method based on ZGV mode is proposed for thin plates. An oblique incident ultrasonic system was built for experimental validation. The wedges were tailored made based on the Snell’s law to efficiently excite and receive the signal of S1-ZGV mode. The frequency of S1-ZGV was obtained by spectra analysis after filtering out the direct wave part. Tensile experiments were carried out on a uniform 6061 aluminum plate with 3mm to obtain the calibration curve between S1-ZGV frequency and stress level. Then a step-wise plate was designed to generate different stress at two positions on a tensile testing machine. The stresses were then evaluated by the calibrated frequency-stress coefficient. The measured stresses agreed well with the calculated stress, proving that S1-ZGV method is effective for local stress measurement in thin plates.