Epoxy and bio-sourced resins for composite applications : mechanical testing and modelling

The growing use of epoxy resins for composite applications requires for accurate prediction of their deformation and failure behaviour. Growing concerns of their environmental impact opened the way for bio-sourced epoxy resins. The mechanics of epoxy resins is well understood, however, the lack of generic constitutive models at the microscale makes the prediction of failure in composites difficult. This work is established around the mechanical behaviour of epoxy resins at different levels and around three main parts. First, the validity of the constitutive model of a highly cross-linked epoxy resin (RTM6) is investigated by in-situ crack propagation tests. Digital image correlation (DIC) is used to evaluate the strain field at the crack tip and is shown to properly capture the strain localisation. A comparison with finite element predictions made with the constitutive model shows a satisfactory agreement with the DIC results, validating the use of the model in crack propagation tests. The failure behaviour of RTM6 appears to differ at the microscale, with failure strains twice as high as in macroscopic tension (0.2-0.25 against 0.1). Second, a cross-comparison of the mechanical properties of different epoxy resins shows that they are essentially determined by the network structure and associated properties such as the glass transition temperature and the cohesive energy density, regardless of the type of epoxy resin. Strong correlations of the upper yield stress and strain and of the lower yield stress with the ratio of the absolute temperature and the glass transition temperature are found. Third, the fracture toughness of a bio-sourced epoxy resin is evaluated with single-edge-notch bending (SENB) tests. The conditional fracture toughness is of 4.261 MPa.m^0.5, but not representative of the intrinsic fracture toughness of the resin due to blunted pre-cracks induced during the specimen preparation.