Carbon fiber reinforced polymer (CFRP) composites are materials of choice in the advanced aerospace structures due to high stiffness, strength, and low weight. At the same time, it is well known that the operating temperature of the polymer matrix composites is quite limited, and properties are adversely affected by the heating. At temperatures above glass transition, a rapid degradation of the polymer matrix occurs, which leads to deterioration in composite thermal and mechanical properties. Characterization and prediction of the properties at elevated and high temperatures are critical for many applications including lightning strike, laser ablation, fire, etc. The elevated temperatures range is defined as temperatures above glass transition temperature but below thermal decomposition. A high temperature range is defined by temperatures above the elevated temperature range.The present work is a continuation of our efforts to develop models for prediction of thermal and mechanical properties of polymer matrix composites at high temperatures. The ultimate goal is to have a general framework enabling to investigate the effects of the mass loss and material phase changes (i.e. formation of the secondary char and gas phases), heating rate, decomposition reaction, and pyrolysis pore pressure on the overall material properties. The present paper is focused on the effects of the decomposition reaction.The first-order Arrhenius kinetics is used to model thermal degradation in the AS4/3501-6 CFRP composite material. This gives temperature-dependent volume fractions of the constituent phases. We assume that all phases (polymer, char, pores filled with pyrolysis gas, and fibers) coexist at a given temperature and microstructure evolves with temperature. Two-step numerical homogenization of representative volume elements (RVEs) is performed to determine overall material properties. Macroscale and microscale analyses are bridged to investigate the effect of the endothermic reactions occurring during pyrolysis on the overall material properties.&nbsp; At the macroscale, volumetric heat flux due to endothermic pyrolysis reaction is introduced in the heat transfer problem and “apparent” specific heat as well as the temperature field in the bulk of the homogenized material is calculated across a wide temperature range. This temperature field is compared to the homogenized temperature field obtained from the analysis of the corresponding RVEs.&nbsp;Our analysis shows that the effect of the endothermic reactions on the overall specific heat of the AS4/3501-6 composites can be significant and needs to be incorporated in the micromechanics-based analysis.&nbsp;It was also determined that overall material properties exhibit strong dependence not only on the temperature but also on the heating rate. An increase in the heating rate shifts initiation of thermal decomposition to the higher temperature. As a result, composite properties are retained at the higher temperatures. The heating rate dependence is important in many problems including assessment and prediction of the lightning strike and laser induced damage in polymer matrix composites.

Capturing Effects of Thermal Decomposition Reactions in Micromechanical Modeling of Polymer Matrix Composites at High Temperatures

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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