Location of impact in composite plates using waveform-based acoustic emission and Gaussian cross-correlation techniques

This paper investigates the use of waveform-based acoustic emission (AE) techniques for location of low velocity impact in composite plates. An eight-ply [0 degrees/90 degrees/0 degrees/90 degrees], laminated glass/epoxy plate is investigated. The plate is impacted with a steel ball using a drop tower. Three broadband AE sensors are mounted on the surface of the composite plate. The response signals of the AE transducers are amplified by broadband AE preamplifiers and fed into a fracture wave detector made by Digital Wave Corporation. The signals are instantaneously sampled and stored in a pentium computer. The digitized AE signals are processed in the time and frequency domains. The raw AE signals were pre- processed to remove reflections from the plate boundaries that cause location error. The Gaussian cross-correlation method and Hilbert transformation are used to obtain arrival time differences among sensors and to overcome dispersion problems. Theoretical flexural wave velocities are calculated using the first order shear deformation plate theory. The calculated arrival time differences among sensors and the flexural velocities are used to determine impact location. The experimental results show that the flexural wave is the dominant mode for low velocity impact of the composite plate. Little or no extensional waves were observed. The locations of impact points are successfully determined. The location error varied from 0.00984 to 0.4807 radial inch for the 11.5' by 11.5' composite plate investigated. The method of using the Gaussian cross-correlation technique to determine AE arrival time differences among AE sensors is suitable for AE wave propagation detection due to low velocity impact.