Non-destructive evaluation of non-uniform stress fields and subsurface stress gradients using ultrasonic waves

Abstract

The principle of acousto-elasticity related to ultrasonic measurements for stress is introduced briefly. Longitudinal critically refracted (Lcr) ultrasonic waves are used to determine non-uniform stress fields in a high strength 7050-T7451 aluminium alloy. The stress gradient in the same material is also studied using these waves. Special transducers with frequencies of 1·0 and 5·0 MHz are used to transmit and receive ultrasonic waves with different wavelengths. Non-uniform stress fields and subsurface stress gradients are introduced in special specimens using axial tension. A specimen with a residual stress induced by interference fitting is also tested. The results indicate that coupling is the main test error source. Errors induced by coupling and other variable are tolerable, less than 40 MPa, and reduce with increasing stress. Ultrasonic waves with different frequencies represent the mean internal stress within a certain depth range and show a measurable change in time-of-flight values. The non-uniform stress field mapped using ultrasound has a good coincidence with the actual stress field. The tested stress gradient is consistent with the artificial stress distribution. This technique will not only provide a non-destructive method to measure distribution of stress in a plane, but it will also predict the stress gradient in the thickness with a maximum depth of 8 mm, which is 100 times deeper than the detection depth of an X-ray method. It has high potential to show subsurface residual stress in three dimensions.