Crack characterisation in a thin plate with unknown acoustic properties based on phase difference of Lamb wave propagation

A method is presented to characterise a crack in a thin plate with unknown acoustic properties when the position of the acoustic source is approximately known. Due to the unknown source generation waveform, a method based on the phase difference of Lamb wave propagation is proposed to determine the propagation distance difference between two different direct Lamb waves and extract the propagation distances of crack-scattered Lamb waves. The acoustic properties are inverted by establishing their relationship with the dispersion degree of intersections between any two hyperbolas formed by direct Lamb waves, and these intersections determine the source position. The difference in amplitude of crack-scattered Lamb waves is used to distinguish reflections from diffractions, and both edge points and endpoints of the crack are identified using the ellipse method to estimate the crack location, size, and shape. Linear, arc-shaped, and kinked cracks in aluminium plates are experimentally verified using a line-scanning laser vibrometer to collect Lamb wave signals from an acoustic source that is generated by a PZT transducer installed on the plate. The results show that the obtained cracks agree well with the actual cracks in terms of location, size, and shape, with mean relative errors of less than 1.59%.