The effect of microstructure and fatigue on the acoustoelastic response of aerospace materials

This thesis presents work relating to the measurement of the nonlinear acoustoelastic response of several materials with respect to microstructure and fatigue life. The nonlinear acoustoelastic response measures the acoustoelastic coefficient of a material. During normal usage components are subject to stresses that while not sufficient to cause fracture cause fatigue, gradually weakening the component. Linear ultrasonic methods have been shown to be poor at detecting fatigue. However, there is evidence that the accumulation of fatigue damage gives the material a nonlinear elastic response that can be probed by ultrasound. A potential technique to monitor fatigue is produced by monitoring changes in the response of nonlinear ultrasonic techniques. Several methods of detecting material nonlinearity using acoustic waves have been proposed. In this thesis a system using the collinear mixing of ultrasonic waves is developed. By measuring the velocity change of a probe wave due to the induced stress from a pump wave, a measure of the nonlinearity is obtained. By using laser ultrasound techniques we gain the benefits of high spatial and temporal resolution. This is important when investigating the nonlinear response of a material as there is evidence that the microstructure affects the nonlinear response of a material. Single point measurements of the acoustoelastic coefficient are taken. In polycrystalline materials the measurement is found to vary with location. A technique is developed to measure the spatial variation of the acoustoelastic coefficient. A relationship between the acoustoelastic coefficient of a material and the underlying microstructure is found. The technique to monitor the spatial variation of the acoustoelastic coefficient is used to monitor the change in samples as they are fatigued. The acoustoelastic coefficient is found to change with fatigue by a larger proportion (9-30%) than linear velocity measurements (0.5-0.6%). Spatial variation of the acoustoelastic coefficient indicated a complex relationship between fatigue and the acoustoelastic coefficient of a material. The implications and further work required from the observed changes in the acoustoelastic coefficient with microstructure and fatigue are discussed