Assessment of Thermal Expansion Coefficients and Prediction of Internal Stresses of Flax/Green Epoxy Laminates

This work evaluated the coefficients of thermal expansion (CTE) in the axial and transversal directions of unidirectional (UD) composites composed of green epoxy and flax fibers. The determination was made by taking length measurements with a caliper and a three-dimensional measuring machine at three discrete temperatures. An inverse approach estimated the flax fiber’s axial and transversal CTE using two micromechanical models. It was required to use an optimization program based on minimizing the quadratic error. In addition, the thermally generated residual stresses were predicted in a symmetric cross-ply laminate ([0/90]s) using equations derived from classical lamination theory. This was defined firstly without and secondly with considering the layer thermal strain in the thickness of the laminate (case 2D and 3D, respectively). The highly anisotropic nature of flax fibers is reflected in the results since the CTEs are very different and of opposite sign (75 × 10–6/°C in the transversal direction compared with −1.2 × 10–6/°C in the axial direction). The microstructure and anisotropic properties of the wall components of the flax fiber can explain this. For a temperature variation of −50 ℃, the results show that, in the 2D case, external 0° plies are subject only to normal compressive stress along the x-axis (−14.8 MPa) and tensile stress along the y-axis (14.8 MPa). For the 90° internal plies, the contrary is assumed and is proved right since the laminate is both symmetrical and cross-ply. In the 3D case, considering the thermal strain in the laminate thickness in the 3D case explains that the normal stresses are higher than the 2D case (+67%).