Quantification of the thermal expansion of carbon fibres in CFRP at low temperatures using X-ray diffraction

This study presents the first demonstration of the use of X-ray diffraction (XRD) to quantify the radial or transverse deformation in Hexcel IM7 PolyAcryloNitrile (PAN)-based carbon fibres at temperatures as low as 200 K (-70 °C). The Coefficient of Thermal Expansion (CTE) is a critical design parameter that needs to be precisely quantified for the next generation of carbon fibre-based Liquid Hydrogen (LH₂) storage tanks for net-zero aviation. This variable quantitatively describes the thermal mismatch between the fibre and the resin that is the driver for microcracking and tank leakage. However, quantification of the CTE of the fibres is experimentally challenging. The results provide unique insights, indicating that the microscopic transverse CTE of the fibre (α₂₂) is equal to 26.2 × 10^-6 K^-1 and is governed by van der Waals forces, similar to those in the basal c-axis (out-of-plane) direction of graphite and the radial direction of multi-wall carbon nanotubes. Taking into account the microcrack-induced relaxation effect reported in polycrystalline graphite, the macroscopic fibre transverse CTE was determined to be 7.86 × 10^-6 K^-1. XRD data were also collected on Hexcel IM7/8552 Uni-directional (UD) and Quasi-isotropic (QI) composite laminates to investigate the influence of the interaction of the resin matrix with the fibre lattice and the stacking sequence on the development of thermal fibre lattice strain. In the UD laminate, the presence of resin induces an additional transverse strain in the fibres as a result of resin contraction during cooling, leading to the development of a compressive strain in the fibre direction. This behaviour was found to be in good agreement with numerical simulations, with a 13 % error at the lowest measured temperature. In contrast, the fibres in the QI configuration were reinforced in the transverse direction, effectively mitigating the influence of resin contraction. These CTE values, insights, and resulting models are essential for multi-scale modelling, design and certification of carbon fibre composite LH₂ tanks that are required to achieve net-zero aviation.