Determination of acousto-ultrasonics wave propagation in an isotropic Al6082-T6 plate: Experimental and analytical investigations

The ability to guarantee reliable and damage-tolerant operation of structures is critical to their safe operation. Various non-destructive evaluation (NDE) techniques, such as ultrasonics, acoustic emission, thermography, acousto-ultrasonics (AU), among others, provide a means of inspecting cracks, defects and damage responses in structures. AU technique involves active excitation of a structure, using ultrasonic waves when accessing defects, damage and mechanical property variation based on acoustic emission sensing and analysis. The focus of previous works has been on experimental understanding of the relationship between propagating AU wave attenuation parameters and various defects, damage and mechanical property variation without establishing fundamental AU wave propagation problems. Therefore, the objective of this paper is to provide an understanding of the physical problem of AU wave propagation through an isotropic plate and its analytical validation, using wave dispersion concept. The experimental investigation was conducted on Al6082-T6 plate specimen. Wave propagation speed on upper and lower surfaces of the specimen was investigated. The changes of the wave speed and attenuation with frequency were identified. From the results obtained experimentally, the wave dispersion was verified analytically. Also, Lamb waves were discovered to be the dominating AU waves. The wave was appreciably non-dispersive in the frequency range from 150 to 1000 kHz and the attenuation coefficient was increased with the frequency. This finding provides a breakthrough to the implementation of Acousto- ultrasonics in the non- destructive testing applications such as to possibly access the damages in the wind turbine blade composite structures as AU has the active sensing mechanism providing an edge over AE method which is a passive mode of damage sensing.