Damage assessment in structural steel subjected to tensile load using nonlinear and linear ultrasonic techniques

Abstract

This paper reports a novel use of combining nonlinear ultrasonic technique with traditional linear ultrasonic technique to assess the tensile failure in civil mild steel. Two parts are consisted in this research. Firstly, we investigated the tensile damage of the mild steel within its tensile strength, the SEM images reveal that the void initiation and growth are the main damage characteristic here. The nonlinear parameters show a general upward trend at this stage, while the longitudinal wave velocities and attenuation coefficient did not show an obvious regularity. This means that, compared with the traditional linear ultrasonic methods, the nonlinear ultrasonic technique is sensitive to the earlier damage in mild steel. Secondly, we used both ultrasonic techniques to investigate the damage evolution in mild steel when tensile stress exceeds its tensile strength till the specimen fracture. In this process, the main damage characteristic of the specimen are the voids coalescence and micro-cracks propagation. The nonlinear parameters show a decreasing tendency instead of continuing rise, while the attenuation coefficients increase significantly. This result indicates that the nonlinear ultrasonic technique is not appropriate to estimate the severe damage in mild steel such as crack formation and expansion, while we could integrate the traditional linear ultrasonic technique to detected the damage evolution at the late stage.