Abstract

The conceptual simplicity and the ability of cohesive finite element models to describe complex fracture phenomena makes them often the approach of choice to study dynamic fracture. These models have proven to reproduce some experimental features, but to this point, no systematic study has validated their predictive ability; the difficulty in producing a sufficiently complete experimental record, and the intensive computational requirements needed to obtain converged simulations are possible causes. Here, we present a systematic integrated numerical–experimental approach to the verification and validation (V&V) of simulations of dynamic fracture along weak planes. We describe the intertwined computational and the experimental sides of the work, present the V&V results, and extract general conclusions about this kind of integrative approach.

Abstract

A well-controlled and minimal experimental scheme for dynamic fracture along weak planes is specifically designed for the validation of large-scale simulations using cohesive finite elements. The role of the experiments in the integrated approach is two-fold. On the one hand, careful measurements provide accurate boundary conditions and material parameters for a complete setup of the simulations without free parameters. On the other hand, quantitative performance metrics are provided by the experiments, which are compared a posteriori with the results of the simulations. A modified Hopkinson bar setup in association with notch-face loading is used to obtain controlled loading of the fracture specimens. An inverse problem of cohesive zone modeling is performed to obtain accurate mode-I cohesive zone laws from experimentally measured deformation fields. The speckle interferometry technique is employed to obtain the experimentally measured deformation field. Dynamic photoelasticity in conjunction with high-speed photography is used to capture experimental records of crack propagation. The comparison shows that both the experiments and the numerical simulations result in very similar crack initiation times and produce crack tip velocities which differ by less than 6%. The results also confirm that the detailed shape of the non-linear cohesive zone law has no significant influence on the numerical results.

Abstract

Impact is one of most critical stress over a structure, so as it induces a high strain rate on the material. Under low speed transverse impacts in composites, the secuence of material degradation is: intralaminar fracture of the matrix that does not mean a load bearing capacity decrease but makes it easier the starting of the seconf failure mode: the interlaminar failure of the matrix or delamination and, finally, the intralaminar failure of fibre.

Delamination is one of the most critical failure mechanisms in composites because it means an important bearing capacity loss, It is quite difficult to detect visually and it's evolution determines the subsequent failure of the component. That behaviour depends on it's interlaminar fracture toughness. Nowadays there is not a general agreement over the dependency between interlaminar fracture toughness, in Modes I&II, and the strain rate. Nevertheless several biaxial equi energy impacts show that delaminated area increases as same as load rate application, so it is clear a dependency between fracture energy and load rate.

This work shows the set-up and first results of a Mode I test using a drop weigth impact tester.

Abstract