# Effects of Stress Level and Elevated Temperature on Transverse Compression Stress Relaxation Behavior and Post-Relaxation Mechanical Performance of UD-CFRP

## TL;DR

This study examines how stress and temperature affect the mechanical behavior of carbon fiber composites under transverse compression, offering insights for long-term structural design.

## Contribution

The paper introduces a temperature-stress-time-dependent predictive model for transverse stress relaxation in UD-CFRP composites.

## Key findings

- UD-CFRP exhibits bi-stage stress relaxation under elevated temperatures and compressive stresses.
- Post-relaxation elastic modulus increased by around 10% compared to baseline specimens.
- Relaxation rates at 60 °C/60% stress level are 1.8 times higher than at 20 °C.

## Abstract

Unidirectional carbon fiber-reinforced polymer (UD-CFRP) composites demonstrate superior tensile creep strain and stress relaxation behavior along fiber orientation. However, prolonged transverse compressive loading in structural connection zones induces significant interfacial stress relaxation and creep deformation, primarily driven by resin matrix degradation and interfacial slippage under thermal-mechanical interactions, and remains poorly understood. This study systematically investigates the transverse stress relaxation characteristics of UD-CFRP through controlled experiments under varying thermal conditions (20–80 °C) and compressive stress levels (30–80% ultimate strength). Post-relaxation mechanical properties were quantitatively evaluated, followed by the development of a temperature-stress-time-dependent predictive model aligned with industry standards. The experimental results reveal bi-stage relaxation behavior under elevated temperatures and compressive stresses, characterized by a rapid primary phase and stabilized secondary phase progression. Notably, residual transverse compressive strength remained almost unchanged, while post-relaxation elastic modulus increased by around 10% compared to baseline specimens. Predictive modeling indicates that million-hour relaxation rates escalate with temperature elevation, reaching 51% at 60 °C/60% stress level—about 1.8 times higher than equivalent 20 °C conditions. These findings provide crucial design insights and predictive tools for ensuring the long-term integrity of CFRP-based structures subjected to transverse compression in various thermal environments.