Influence of Anisotropy Generated by Rolling on the Stress Measurement by Ultrasound in 7050 T7451 Aluminum

Abstract

Stress applied to a material can be evaluated using ultrasonic waves. This practice is based on acoustoelastic theory, which relates the stress to the velocity of a wave traveling through the body. How the stress affects the wave velocity is determined by the material’s acoustoelastic constant. This constant can be experimentally measured or calculated from the material’s elastic constants. However, ultrasonic techniques have yet to be adopted as an inspection tool in the field. A factor contributing to this fact is the non-uniformity of materials, mostly associated with grain alignment or texture. As researchers consider this factor, they should take into account the anisotropy generated by rolling. The common practice, however, when relating strain and wave velocity is to ignore anisotropy and to simply utilize isotropic models. No studies have been performed to evaluate the effect of anisotropy on the stress measurement by ultrasound, especially for methods using critically refracted longitudinal waves. The aim of this study is to evaluate how the anisotropy generated by rolling affects the acoustoelastic effect for 7050 T7451 aluminum alloy. We compare the value of the acoustoelastic constant obtained experimentally for rolled samples to the constant calculated with measured elastic constants when the material is assumed to be isotropic. The results show that the methods yield different results, suggesting that the simplified isotropic model should be applied with caution. Since no true known value for elastic constants exists, the results can be used to approach the uncertainty when employing the isotropic model to evaluate stresses in aluminum alloys.