Ultrasonic through-transmission characterization of thick fibre-reinforced composites

Abstract

Thick composites pose a significant challenge to the ultrasonic nondestructive evaluation process due to the increased attenuation as well as the influence of material anisotropy. The physics of ultrasonic wave propagation in anisotropic material involves understanding such phenomena as beam skewing, material focusing/defocusing, unsymmetrical beam profiles, etc. This ultrasonic behaviour can be considered to be relatively insignificant in the nondestructive evaluation of thin anisotropic composite structures, but cannot be neglected in thick composites. In this paper, the ultrasonic characterization of thick glass-epoxy composites using an immersion through-transmission method employing a standard ultrasonic robot scanner system is discussed in detail. An inverse technique for computing material elastic constants from the acquired data is discussed. The experiment measures group velocity is a function of energy propagation angle (group angle), from which phase velocity is numerically computed as a function of phase angle. Then, the material constants are determined from phase velocity profiles using commercially available parameter identification software. Both uni-directional (five independent elastic constants) as well as cross-ply (nine independent elastic constants) were considered. Through-transmitted ultrasonic beam characteristics in thick composites are then analysed using feature extraction procedures and compared with the isotropic plexiglass structure.