Developing a Heterogeneous Attenuation Model for Fiber Reinforced Thermoplastic Composites in the Kilohertz Region of the Ultrasonic Spectrum

ABSTRACT This study experimentally investigated the discrete phase contributions of fibers and entrapped air on the attenuation spectrum of a fiber‐filled thermoplastic composite and derives a mathematical expression for sound attenuation in this heterogeneous material. The frequency‐domain attenuation spectrum found with a parts inspection system using ultrasonic guided waves showed two notable spectral fingerprint regions, each corresponding to different material features of the composite. The first region (250–310 kHz) displayed significant sensitivity to the porosity and fiber content in the test specimens, with these two material features found to influence the symmetric S1 dispersion mode phase velocity. An expression describing the attenuation spectrum in this region was developed for the heterogeneous material with the aid of genetic programming algorithms to describe its dependencies on entrapped air and fiber content. Attenuation displayed a linear relationship with frequency in the second region (310–500 kHz), resembling the dampening nature seen with homogenous dispersive materials. A simpler mathematical expression was proposed for this region showing only dependency on whether short glass fibers or carbon fibers were present. The two expressions together capture the ultrasound dampening dominated by material features in the studied thermoplastic composite.