Micromechanics of an epoxy matrix for fiber reinforced composites : experiments and physics-based modelling

Given the multi-scale nature of the damage and deformation mechanisms in fiber reinforced composites, bottom-up approaches are believed to be the more efficient strategy to develop generic models for the predictions of the mechanical behaviour of large structures. These analyses start at the microscopic scale, where reliable models for the constituents are essential input ingredients. This thesis is dedicated to the characterization, understanding and modelling of the fracture and deformation mechanisms of the RTM6 epoxy resin. A first part of the thesis aimed at unravelling the origin of crack initiation in epoxy resins. It provided a physical explanation of the pressure dependence of the fracture stress and strain in highly cross-linked epoxy resins, unifying the occurrence of fracture under a wide range of stress triaxialities based on a single mechanism. A second part of the thesis was devoted to the microscale characterization of RTM6 in a unidirectional composite, based on in situ experimental tests providing quantitative pictures of the strain field at the microscale. The comparison between experimental results and finite element analyses both at the macro- and microscale enabled a critical assessment of the validity of macroscopic properties at the microscopic scale. Finally, a novel modelling approach based on the shear transformation zones framework was developed for epoxy resins. The proposed approach offers an alternative to complex phenomenological constitutive models, reducing the number of parameters from more than twenty to seven, while bridging the gap between atomistic and continuum mechanics approaches by providing the missing ingredients for seamless scale transition.