A COMSOL Model of Damage Evolution Due to High Energy Laser Irradiation of Partially Absorptive Materials J.J. Radice * , P.J. Joyce, A.C. Tresansky, R.J.Watkins Mechanical Engineering Department, United States Naval Academy *Corresponding author: 590 Holloway Road, Annapolis, MD, 21402. radice@usna.edu Abstract: In this paper we present a numerical model of the transient heat transfer and thermochemical damage evolution in an IR opaque material using COMSOL Multiphysics. The model is applied to a carbon black loaded PMMA (polymethyl-methacrylate), using material properties available in the literature supplemented with experiments. This variant of PMMA was chosen because it is homogeneous, isotropic, and the decomposition from solid to gas is relatively straightforward to characterize. The incident high energy laser beam is modeled as a Gaussian heat flux. The beam parameters used for this study are selected to mimic that of a Nd:YAG high energy laser with a wavelength of 1070nm, an output power of 110W, and a spot size with a 5.5mm beam diameter. At this wavelength, NIRS/FTIR experiments demonstrate that PMMA absorbs slightly more than half of the incident laser energy. PMMA was experimentally observed to vaporize from 310°C to 475°C via differential scanning calorimetry. The temperature range and endothermic phase change from solid to gas is incorporated in the COMSOL model. Rather than use an adaptive mesh , material removal (or ablation) is captured using temperature and phase dependent material properties. The model captures vaporization material removal by appropriately varying the thermal conductivity and density of the PMMA at the vaporization temperature. The model results are compared to experiments on carbon-laden PMMA. Keywords: Heat Transfer, Phase Change, High Energy Laser, Material Damage, Ablation 1. Introduction Composite materials have shown a marked increase in their use in aerospace and military applications in recent years. Concurrent maturation of the technology in near Infrared High Energy Lasers (HEL) has shown promise for their use in directed energy weaponry. The intersection of these two technologies will likely occur in the near future. A numerical method for predicting the resultant transient temperature field from the interaction of a high energy laser and structural materials could aid in the prediction of damage caused by the HEL. In the present work, we propose a numerical model for the transient temperature field and damage evolution resulting from high energy near infrared laser irradiation incident on a monolithic polymeric material. The matrix phase of many common aerospace composite materials are made of polymeric materials, therefore modeling pure polymer ablation provides an avenue to begin capturing composite material response to such insults. The investigation begins with a transient heat transfer model of a linearly ablating polymer. For the present purposes, we have chosen to model polymethyl-methacrylate, or PMMA, in the COMSOL Multiphysics environment. The model developed here incorporates material ablation and the concomitant simulated change of boundary conditions/damage evolution. We have accomplished this by simulating phase change to account for the latent heat of vaporization and by artificially varying material properties to simulate the moving boundary condition of the incident laser energy boring through the material. 2. Background Many studies have been published on the laser ablation of polymers and polymeric materials. Cozzens studied the infrared ablation of polymers using a 10.6µm laser and related ablation energy to irradiance as well as investigating the method of ablation [1]. Lloyd and Myers studied the use of ablative polymeric materials for near IR (1.06µm) High Energy Laser beam diagnostics, and suggested the use of