Immersion-based non-collinear shear wave mixing: effects of experimental assembly on scattering directivity and nonlinear characterisation

Non-collinear wave mixing has emerged as a powerful technique for characterising material nonlinearity, offering enhanced sensitivity to localised microstructural damage by effectively isolating the mix frequency harmonics. This study develops a numerical model to investigate non-collinear shear wave mixing in near-isotropic materials and validates it through immersion ultrasonic experiments on aluminium and mild steel samples. The primary objective is to quantify the sensitivity of the sum-harmonic wave’s scattering directivity to experimental uncertainties. A systematic sensitivity analysis quantifies the effects of variations in material velocity, transmitter positioning, and transmission angles. Results show that a 1.0% perturbation in the transmission angle induces an 11.7% deviation in the scattering directivity, identifying it as the most influential parameter. This analysis provides a quantitative foundation for designing robust, high-repeatability immersion-based scanning setups.