Dynamic delamination testing of a composite material

Delamination is one of the main damage mechanisms in composite materials, and a material’s resistance to it is characterised by interlaminar fracture toughness. This property is used in finite element material models and delamination simulation methods, such as the virtual crack closure technique. By definition, delamination takes place in the inter-ply region composed mostly of resin, which can notoriously have strain rate-dependent mechanical properties. Rate dependence of interlaminar fracture toughness needs therefore to be studied in order to develop material models usable in highly dynamic simulations, e.g. those of automotive crash absorbing structures. To date, no standardised dynamic delamination test exists. In this work, a test method to determine Mode I interlaminar fracture toughness was developed starting from the quasi-static ISO standard and existing literature. The commonly used double cantilever beam specimen geometry and a hydraulic test machine were employed. The material that has been characterised is a non-crimp fabric unidirectional carbon fibre-epoxy composite, manufactured via HP-RTM. Specimens were tested at opening rates of 20 mm/s and 1300 mm/s. Quasi-static tests were performed as well to provide baseline results, on specimens of the same geometry and at a rate of 5 mm/min. High-speed imaging and direct image correlation were used to measure the relevant quantities. This was paired with a custom-developed MATLAB script for tracking crack length, the most critical measurement in this sort of tests. Interlaminar fracture toughness for this material was found to be around 0.5 kJ/m^2, with a slight tendency to decrease as rate rises. As a conclusion, it was determined that it may be interesting to further characterise the material at even higher rates with a purpose-developed Hopkinson bar apparatus.