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

Jenny Carlsson,Per Isaksson

doi:10.1016/j.ijsolstr.2018.04.015

Jenny Carlsson, Per Isaksson

Open Access

https://doi.org/10.1016/j.ijsolstr.2018.04.015

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Abstract

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.

Highlights

Studies of dynamic crack propagation have, mainly due to the complexity of the field, primarily been focused on relatively homogenous materials such as steel, glass and amorphous polymers like polymetylmetakrylat (PMMA)
This study aims to provide a link between dynamic crack propagation in homogenous materials and that in heterogeneous – albeit relatively isotropic – materials, by comparing crack propagation in a short fibre composite with that in its pure matrix material
The critical energy release rates for both polylactic acid (PLA) and wood fibre composite, Gc, have been taken to be just below the lowest energy release rate obtained in experiments with long initial cracks

Summary

Introduction

Studies of dynamic crack propagation have, mainly due to the complexity of the field, primarily been focused on relatively homogenous materials such as steel, glass and amorphous polymers like polymetylmetakrylat (PMMA). This study aims to provide a link between dynamic crack propagation in homogenous materials and that in heterogeneous – albeit relatively isotropic – materials, by comparing crack propagation in a short fibre composite with that in its pure matrix material. The materials of particular interest for the present study are wood pulp based bio-composites. Natural fibres such as wood or flax fibres – especially when combined with a bio-degradable matrix material such as polylactic acid (PLA) – have an advantage over conventional short fibres, mostly glass fibres, seen over the entire lifecycle, due to low production costs, renewable origin and bio-

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