NONLINEAR LASER ULTRASONIC MEASUREMENTS OF LOCALIZED FATIGUE DAMAGE

摘要

A nonlinear laser ultrasonic system was developed and used to characterize the fatigue state of a fractured Ti-6Al-4V sample with high spatial-resolution and sensitivity. The measurement system is built around a scanning heterodyne interferometer, which allows detailed displacement field images to be created and visualized for propagating surface and bulk acoustic fields on a material surface. An assessment of the local fatigue damage of the material was made using nonlinear ultrasonic interaction principles, where the local amplitudes of the fundamental and second harmonic displacement fields are monitored simultaneously. This provides a means for evaluating the local acoustical nonlinearity parameter, p, which can be related to the accumulation of fatigue damage in a material. A large increase in (5 was observed between the unfatigued area (near the grip section) and the heavily fatigued area (gauge section) for a fractured dogbone specimen. The measurements show the potential for spatially-resolving the local fatigue state of a material using laser ultrasonics.