Dynamic crack propagation in wood fibre composites analysed by high speed photography and a dynamic phase field model

Using an experimental setup, with a high-speed camera to track crack tip velocity, dynamic fracture is studied in wood fibre polylactic acid (PLA) composite and pure PLA. The experiments are analysed quantitatively in terms of the relation between energy release rate and crack tip velocity, and qualitatively in terms of branching occurrence and fracture surface appearance. Branching occurs frequently in PLA specimens but not in wood fibre composite specimens, in spite of high energy release rates. Scanning electron microscopy images of the fracture surfaces show that the fracture surfaces in wood fibre composite materials are rugged and uneven compared to PLA, whose surfaces are smoother. The experimental results are compared to numerical results, obtained using a dynamic phase field finite element model. Simulations correlate well with experiments with respect to the relation between energy release rate and crack tip velocity. For PLA, the simulations also predict branching correctly, but for wood fibre composites, the simulations slightly over-predict the amount of branching and point to a need for further development of fracture models in order to better capture the constitutive behaviour of these heterogeneous materials. (C) 2018 The Author(s). Published by Elsevier Ltd.