Thermo-mechanical behaviour of flax/green epoxy composites: Evaluation of thermal expansion coefficients and application to internal stress calculation

This work aims at evaluating the coefficients of linear thermal expansion (CLTE) of flax/green epoxy unidirectional composites and the CLTE of flax fibre. This required using high precision measuring instrument to experimentally evaluate the CLTE of unidirectional composites with various fibre contents. The flax fibre CLTE were first estimated using an inverse approach with two micromechanical models. From the longitudinal and transverse CLTE, the internal stresses of various symmetric and antisymmetric laminates, due to temperature variation, were then predicted by a 2D analytical model based on classical laminate theory. The inverse approach results showed that the transverse CLTE of flax fibre was positive and estimated at 75 ± 5 × 10−6 /K whereas the longitudinal CLTE was negative and equal to -1.2 ± 0.1 × 10−6 /K, highlighting the high anisotropy of flax fibres. The internal stress analysis in flax fibre laminates showed that the stacking sequence had a significant effect on the internal stresses, whatever the temperature variation. Regarding stacking the layers, choosing symmetric cross-ply laminate was more interesting than antisymmetric one for minimising the internal stresses. The normal stresses reached their maximum absolute values for the cross-ply laminates, whereas the maximum shear stress occurred in the [0/60]s and [0/60/0/60] stacking. This study highlighted the importance of choosing an optimised stacking sequence, such as the 0/30° oriented laminates, and a relevant curing cycle prior to the manufacturing process, in order to obtain flax fibre laminates with low internal stresses.