Modeling of Thermo-Mechanical Degradation of Polymer Matrix Composites at High Temperatures

Abstract

Carbon fiber reinforced polymer (CFRP) composites are used extensively inaerospace structures due to their high stiffness, strength and low weight. However, these materials have low operational temperatures as their properties are adversely affected by heating. At temperatures above glass transition chemical pyrolysis reac- tions occur in the polymer leading to the formation of a carbonaceous char residue, and pores filled with pyrolysis gasses. This process, also known as thermal decom- position, leads to an irreversible loss of mass and deterioration of mechanical and thermal properties of the material. Characterizing the behavior of these materials at high temperatures enables the design of structures for severe loading conditions such as lightning strike, laser ablation and fire. In this work micromechanics-based approach is used to predict the effective ther-mal and mechanical properties of AS4/3501-6 CFRP composite while thermal de- composition occurs. Arrhenius decay law from chemical kinetics is used to create a physics based model to account for the formation of new phases (char and pores) and mass loss in the material. This model enables to create temperature depen- dent multi-phase Representative Volume Elements (RVE) of CFRP composite at any stage of thermal decomposition. The RVE generated are densely packed with particles randomly distributed and oriented without any directional dependencies. Finite element analysis (FEA) based numerical homogenization procedures for calculating the effective thermal conductivity, specific heat, elastic moduli and co- efficient of thermal expansion are developed. The developed procedures are used to calculate the effective properties of AS4/3501-6 composite as a function of tempera- ture and heating rate until the thermal decomposition of the polymer is completed. The calculated effective properties are compared with experimental data from liter- ature and theoretical bounds for multiphase materials. The effects of the decomposition reactions, material phase changes, mass loss andthe pressure exerted by the pyrolysis gases trapped in the pores of the material on the effective properties of AS4/3501-6 composite are investigated. Decomposition reactions in 3501-6 polymer are exothermic causing additional heat generation in the material during phase change. A nonlinear transient heat transfer problem is solved at the micro-scale to account for this additional heat generation in the calculation of the effective specific heat. The developed micromechanics based material model is implemented into anFEA analysis of a lightning strike on a CFRP composite. The multi-physic inter- action of a lightning current channel with a conductive CFRP laminate is modeled. The model features the effect of Joule heating, spatial and temporal evolution of the lightning current channel, temperature and heating rate dependent material prop- erties, fiber sublimation and material removal. The surface recession and thermal damage of AS4/3501-6 composite subjected to a lightning strike using the devel- oped micromechanics material model and empirical material models from literature is compared with experimental data.