Analysis of the effect of impact loadings on the gas permeability properties of CF-PA6 composites

This study investigates the damage-dependent permeability properties of polyamide 6 based carbon fiber-reinforced thermoplastic composites (CF-PA6) by analyzing the helium gas permeability differences before and after low-velocity impact loading. For this, a high-pressure permeability test rig was developed and is presented which allows quantitative measurements of gas leakage through composites. Two types of composites, braided (BR) and unidirectional (UD) cross-plies, were analyzed. As the textile architecture affects the damage structure, this allows a larger variety of damages and thus deeper understanding of the damage dependent permeability properties. Prior to impact loading, both braided and unidirectional cross-ply specimens exhibited near- Fick ian gas diffusion behavior. However, permeability measurements revealed a significant difference between both, with unidirectional cross-ply specimens demonstrating 10 times higher permeability compared to the braided specimens. Microstructural investigations attributed this disparity to the higher crack density observed in unidirectional cross-ply specimens. Post impact loading, the permeability of both specimen types increased significantly by several orders of magnitude and showed no agreement with Fick ’s law. Microstructural analysis revealed the formation of interconnected crack networks throughout the impacted specimens, which facilitated large gas flows through the damaged regions. The study shows that the presented test rig is capable of measuring gas permeability through composites in a large range, from diffusion driven processes ( ∼ 1 0 − 14 m 2 s ) up to gas flow driven processes through damage networks (up to ∼ 1 0 − 9 m 2 s ). Additionally, the study shows the complex relationship between damage and permeability, e.g. how internal damage influences permeability and how the gas transport changes as soon as cross-linked crack networks are present. • Introduction of permeability test rig with large measurement range. • Reproducible leakage measurement method for fiber composites before and after impact. • Leakage rate increases significant after impact load through developed crack network. • Specimens with larger density of transverse cracks show an increased permeability.