Ultrasonic Determination of Surface Residual Stresses

Abstract

The velocity of an ultrasonic wave in a material is affected by the stresses due to the Acoustoelastic Effect. The present work utilizes this phenomenon to study surface, residual-stresses that result on machining. Two types of surface wave are used: Rayleigh (Rw) and longitudinal, critically refracted (Lcr). Sound velocity may also change due to grain scattering, surface roughness and grain texture. The present work attempts to determine each contribution to exclusively identify the change of velocity due to stress. The problem is studied systematically by choosing increasingly complex materials i.e. Glass (Coming 9604) - no grains, no texture; Crystallized Glass (Coming 9606) - grains but no texture; Single Crystal Magnesium - all texture; Polycrystalline Titanium - grains and texture. Glass, glass-ceramic and titanium were machined to introduce surface stress and the Rw and Lcr velocities measured before and after stress relief. The velocity changes were converted to stress via appropriate Acoustoelastic Coefficients. The relative values agreed well with the XRD-determined stresses, the former results being consistently lower than the latter. This is expected as ultrasound penetrates deeper and thus samples more material. Annealed specimens identified the surface roughness contribution to the changes of ultrasonic velocity. Experiments illustrated that the change of velocity due to stress (maximum observed 0.52%) is small, relative to the other contributions: surface roughness contributes < 8%, texture < 2.4%, and grain scattering < 0.6%. This result underscores the importance of considering all material conditions when using ultrasound to quantify surface residual stresses. Several techniques to exclusively identify the change of velocity due to stress are presented.