Meso-structure ageing mechanism of 3-D braided composite's compressive behaviors under accelerated thermo-oxidative ageing environment

The present work constructs a meso-structure finite element model to characterize the effect of thermo-oxidative ageing on compressive behaviors of 3-D carbon fiber/epoxy braided composites. Nanoindentation tests were carried out to investigate the thermo-oxidation effect on localized property of aged resin. Based on the experimental results obtained from both macro-compression tests and nanoindentation tests, two different models referred to as 'homogeneous model' and 'multilayer model' were proposed, respectively. Both of the two models gave satisfactory predictions. We found that the post-curing stiffening and thermal degradation of matrix are the main ageing mechanisms at lower temperatures (below glass-transition temperature). At higher temperatures above the glass-transition point, interfacial damage also contributes a lot to the property degradation of the composite. The combination of matrix ageing and interfacial weakening leads to a significant reduction in compressive behaviors of the 3-D braided composite. We also found that the presence of oxidized layer has little effect on the compressive properties of aged braided composites. Simulation results have confirmed that ageing mechanism strongly depends on the ageing conditions.