Stress Quantification of Complexly Loaded Structural Components using Acoustoelasticity

The ultrasonic monitoring of steels and other structural materials relies on measuring the velocity of elastic waves, which varies with the existing stress state in the material—a phenomenon known as acoustoelasticity. This research is focused on applying the acoustoelastic method for stress analysis to thick steel plates (9-12 mm) of the type commonly used for gusset plates of steel truss bridges, where shear stress is significant. The acoustoelastic coefficients are determined in thick steel plates in normal, orthogonal, and angled directions using an array of ultrasonic sensors. A three-dimensional material model is developed which includes Murnaghan hyper-elasticity and can determine the effects of plate thickness and excitation frequency on the acoustoelastic coefficients. This model is experimentally validated by tensile loading of a thick steel plate by measuring the ultrasonic signals in three directions. Numerical and experimental results agree within the measurement uncertainties of each method. The 1.0 MHz ultrasonic frequency has the highest resolution for measuring normal and shear stresses in plates typically used in highway bridges. Additionally, the ultrasonic velocity and stress equation is modified to add the shear effect in addition to normal stresses. As a result, the theory of acoustoelasticity in a homogenous and isotropic plate is developed for the presence of multi-directional stress in a cross section considering normal and shear stresses. The resulting analytical equations are solved to understand the influence of shear stress on the acoustoelastic coefficients.