# Internal Pressure–Temperature Coupling Analysis Method for Thermal Decomposition of GFRP Composites Based on the Overlapping Elements Method

## TL;DR

This paper introduces a new method to analyze how heat and pressure interact during the thermal breakdown of GFRP composites using numerical simulations and user subroutines.

## Contribution

A novel internal pressure–temperature coupling analysis method for GFRP composites using overlapping mesh and user subroutines is proposed.

## Key findings

- The predicted temperature and pressure values align well with experimental data for two types of GFRP composites.
- Internal pressure peaks due to gas accumulation during thermal decomposition and then decreases with location-dependent factors.
- Pressure behavior varies based on proximity to the heating surface, influenced by decomposition rate, permeability, and porosity.

## Abstract

A method of internal pressure–temperature coupling analysis for the thermal decomposition of GFRP composites under high-temperature conditions was established, which incorporates coupled calculations of heat transfer equations, the Arrhenius equation, Darcy’s law, and the ideal gas state equation. Using the overlapping mesh method, the coupling calculation of temperature and internal pressure is realized based on the UMATHT and USDFLD user subroutines developed. Specifically, two user subroutines, UMATHT-1 and UMATHT-2, are used to define the heat transfer equation and gas diffusion equation separately. Numerical simulations are conducted to simulate the polymers’ thermal decomposition in high-temperature environments. For glass fiber/vinyl ester composites and glass fiber/phenolic composites, the predicted temperature and pressure values are in good agreement with experimental measurements, and porosity and permeability are then analyzed. Due to the accumulation of thermal decomposition gases, inter-pressure within the material surged and reached a peak value. After that, it began to decrease, but the factors affecting the pressure decrease vary at different positions. Specifically, the pressure closest to the heating surface is influenced by the combined effects of decomposition rate, permeability, and porosity, while the pressure far away from the heating surface is only affected by the initial permeability. The pressure in the intermediate region may be influenced by both increased porosity and initial permeability.