Delamination Resistance and Size Effect in Discontinuous Fiber Composites

In this paper, we investigate Mode-I and Mode-II delamination behavior of Discontinuous Fiber Composites (DFCs). Owing to the complex heterogeneous mesostructure in DFCs, conventional testing methodologies such as the double cantilever beam (DCB) and end-notched flexure (ENF) tests used to characterize Mode-I and Mode-II interlaminar failure may fail to characterize the non-linear behavior during delamination. This is because DCB and ENF tests based on Linear Elastic Fracture Mechanics (LEFM) models, fails to account for the quasi-brittleness of DFCs. As a result, this approach may not be able to capture the variation in the Fracture Process Zone (FPZ) which becomes large due the distributed damage in the platelets. Hence, there is a need to account for this non-linear behavior of the FPZ to effectively estimate the delamination fracture energy. This paper proposes an experimental investigation on the effects of the FPZ on the inter-laminar delamination of DFCs. To shed light on the role of the FPZ size versus the structure size and geometry, geometrically-scaled DCB and ENF specimens were tested. The results show a significant size effect. While for small sizes the specimens exhibit a limited strength reduction by the presence of the crack (which indicates a pseudo-ductile behaviour), the failure becomes more and more brittle for larger sizes. Future work will focus on the understanding of this phenomenon leveraging stochastic Finite Element modelling and quasi-brittle fracture mechanics.