Residual stress measurement method of thin composite laminates based on phase velocity of air-coupled ultrasonic guided wave

A non-contact residual stress measurement method for thin composite laminates based on the phase velocity of the air-coupled ultrasonic guided wave has been proposed in the study. In the method, ultrasonic signals of the guided wave in composite have been captured through the variable distances between the transmitter and the receiver of air-coupled ultrasonic. Actual acquired ultrasonic signals at different positions have been processed by Butterworth bandpass filter and the interpolation algorithm using discrete Fourier transform method, followed by the acoustic time extraction. The guided wave acoustic time difference and the phase velocity have been calculated through the successive difference method to characterise residual stresses, which can avoid the effect of unknown difference of air gaps between transducers and composite laminates. Theoretical analysis and finite element models for ultrasonic wave fields of composites have been developed to acquire the whole guided wave propagation process. The effects of the excitation centre frequency of the transducer, incident angle, and in-plane propagation distance on the signal-to-noise ratio of the guided wave have been investigated and optimal parameters have been designed for residual stress measurement of thin-walled composite laminates according to the numerical results. Stress calibration experiments have been conducted by applying several known tensile stresses with tensile testing machine. Experimental results demonstrated strong linear correlations between phase velocities and applied stresses in the range of 0 - 90 MPa for composite specimens with different orientations. The standard deviation of acoustic time is less than 0.078 μs. The mean absolute error of phase velocities versus applied stresses is within 2.7 m/s corresponding to the stress prediction deviation of 14 MPa, validating the feasibility of the proposed residual stress measurement method.