Investigation of the fatigue process using nonlinear ultrasound

During normal usage components are subject to stresses that while not sufficient to cause fracture cause fatigue, which gradually weakens the component. Linear ultrasonic methods have been shown to be poor at detecting fatigue. However, there is evidence that the accumulation of damage gives the material a nonlinear elastic response that can be probed by ultrasound. By measuring the change in a material's nonlinear properties a measure of the fatigue can be obtained. Several methods of detecting material nonlinearity using acoustic waves have been proposed. The collinear mixing technique is used here. By measuring the velocity change of a probe wave due to the induced stress from a second pump wave, a measure of the nonlinearity is obtained. By generating the probe wave and detecting both waves 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. The change in nonlinearity over a region of a specimen (aluminium) has been monitored over several fatigue levels to investigate any relation. Early stage results are given with a discussion on the development of the technique.