Change in ultrasonic parameters with loading/unloading process in cyclic loading of aluminium alloy

Abstract

A local immersion method of the ultrasonic measurement was applied to real-time measurement of the ultrasonic parameters, in aluminium alloy (A2024-T3) specimens with curved sides during cyclic loadings in fatigue testing. The propagation time of the ultrasonic wave from the surface to the bottom of the specimen changed with time corresponding to the measured local strain of the specimen, and the discrepancy of the measured and estimated transverse strains was discussed with the acousto-elastic effect. From the Fourier spectrum of the bottom echo, the peak intensity, the peak frequency and the average gradient of the transfer function defined as an ultrasonic nonlinearity parameter in response to ultrasonic wave distortion were obtained. The peak intensity and the average gradient of the transfer function changed sinusoidally in a loading cycle, but no change in the peak frequency. Using the Granato and Lücke string model of dislocation damping, the sinusoidal changes of ultrasonic parameters in a loading cycle were discussed by the interaction between dislocations and point defects, which is activated by the low frequency cyclic stress. The ultrasonic parameters were also obtained at some stages in the fatigue process to give a systematic change in the average gradient of the transfer function.