Soutenance de thèse : Hugo GIRARD

Characterization of the fiber-matrix interface fracture properties in long fiber composites

Doctorant : Hugo GIRARD

Laboratoire INSA : MATEIS

École doctorale : ED34 : Matériaux de Lyon

Fiber-matrix interface in long fiber composite is a key aspect of global composite mechanical properties since it drives damage initiation and load transfer. Fiber-matrix interface debonding is usually the first type of damage that occurs when the composite is subjected to transverse loading. After initiation, the interface debonding propagates and often kinks into the matrix, leading to further critical defects for the structure. As a result, it is crucial to accurately characterize the fiber-matrix interface in order to prevent or control damage in composites. Going beyond existing experimental methods currently focused on interface shear fracture properties, single-fiber microcomposite tensile sample loaded transversely are developed to simultaneously characterize opening and shear fracture properties. An accurate experimental characterization of the fiber-matrix debonding process allowed the identification of the interface fracture properties using adequate 2D and 3D numerical approaches and related fracture models such as the Coupled Criterion (CC) and Cohesive Zone Models (CZM). Both the CC and the CZM are able to reproduce the experimentally observed debonding process in 2D, the 3D model being able to describe the free surface singularity. In 3D, the fracture property identification yields tensile strengths and critical energy release rates respectively slightly higher and in the same order of magnitude than those identified in 2D. The 3D model does not enable identifying the shear fracture properties, unlike in 2D. In 2D the optimal initiation crack shapes correspond to i) the stress isocontours for small brittleness numbers, ii) the energy-based shapes for large brittleness numbers and iii) neither of them for intermediate brittleness numbers. The 2D stress isocontours-based debonding shapes provide an accurate estimate of the initiation loading. In 3D, the optimum initiation crack always corresponds to energy-based debonding shapes and the 3D stress isocontours-based debonding shapes may thus overestimate the initiation loading by up to 30%.