A Two-Layer Model for the Laser Generation of Ultrasound in Graphite-Epoxy Laminates

Abstract

We previously reported the performances of a numerical simulation model [1] that calculates the mechanical displacements induced within a sample by the absorption of a laser pulse. This model solves the heat diffusion and acoustic wave propagation equations over an orthotropic slab of finite thickness with the help of temporal Laplace and spatial 2D Fourier transformations. The parallel and normal displacements predicted by the model were found to be in generally very good agreement with experimental data obtained on various samples in various excitation conditions. Among these experiments, one consisted in the CO₂ laser excitation of a graphite-epoxy sample. We performed an optical study of the graphite-epoxy composite using FTIR photoacoustic spectroscopy [2] to determine the optical penetration depth spectrum of this material. This study revealed that a thin (≈ 30 μm thick) epoxy layer covered the top graphite fiber sheet of the composite, and that the optical penetration depth of the CO₂ radiation in the epoxy was about 20 μm. Consequently, when a CO₂ laser pulse impinges on the composite, all the radiation is absorbed in the epoxy layer, and it is easy to simulate this situation with the model, using the rigidity-expansion tensor [λ] of the epoxy for the generation and the rigidity tensor [C] of the composite for the propagation (see [1]).