A novel laser light absorption model for composite materials in the context of laser ultrasonic testing

Abstract

When a laser pulse impinges on a composite material, a portion of the light is absorbed by the matrix, while the remainder is absorbed by the embedded fibers. Conventional models overlook the substantial differences in optical properties between these two constituents. This paper, however, introduces a novel analytical model for laser light absorption in fiber-reinforced composite materials. This macroscopic model incorporates distinct optical absorptivity for both the matrix and embedded fibers, offering adaptability to various scenarios without introducing additional computational cost to simulations. The dynamic thermal expansion and elastic wave generation inside the material is considered to study the effects of the presented model on the generated waves. Investigations are conducted on a Carbon Fiber Reinforced Plastic (CFRP) plate with variable epoxy layer thickness using Nd:YAG and mid-IR lasers. Results demonstrate a significant enhancement in temperature distribution within the material, elucidating experimental observations previously unexplained by conventional models. Under consistent laser energy, pulse duration, and beam radius, the Nd:YAG laser induces notably higher temperatures in the CFRP-epoxy interface compared to the mid-IR laser. Furthermore, thicker epoxy layers result in lower temperatures and higher displacement amplitudes, aligning with the desirable characteristics for laser ultrasonic testing applications.