ECCOMAS Congress 2016
Fracture nucleation and continued crack growth on the cell scale in wood analyzed by a high-resolution finite element model
Jenny Carlsson
*, Per Isaksson Solid Mechanics
Uppsala University
Box 534, SE-751 21, Uppsala, Sweden {jenny.carlsson, per.isaksson}@angstrom.uu.se
ABSTRACT
Abstract: How does wood break? The microstructural influence on the deformation field in wood and especially its practical implications for understanding macroscopic failure is studied here. Specifically, theoretical strain fields on the cell level are computed in discrete high-resolution finite element microstructural models having varying cell sizes and subjected to combined compression and shear load surface tractions induced by moving rigid contacts of different size and shape.
X-ray computational tomography experiments, performed on small wood-specimens (Norway spruce) loaded in-situ, together with subsequent digital image correlation techniques, were performed to compare the numerically approximated microscopic strain fields with estimated real strain fields in fracturing wood on the cellular scale. Thus, deformation fields in the vicinity of slowly growing cracks at the cell level are analyzed, both theoretically and experimentally, Fig. 1.
It is observed, in experiment and finite element models, that the region of high shear stresses, where a crack may grow despite a confining pressure [1], is located significantly deeper down in the material than what is predicted in classical continuum theories (measured from the loading specimen) and is also substantially larger compared to the theoretical field. The high-resolution finite element models produces remarkably similar strain fields as the experiments and is thus seemingly able to capture microstructural size effects.
These important observations on altered strain fields suggest that the loading tip geometry, penetration depth and/or direction can be designed to control e.g. fiber defibration processes in pulping industry or properties for grinding materials.
Figure 1. A crack advances in-between cell walls, in the lamella, in the TR-plane: a) X-ray computational tomography experiment, b) high-resolution FE model.
References
[1] Isaksson, P. Ståhle, P. (2002). Prediction of shear crack growth direction under compressive loading. Int.
J. Fract., 113, 175-194.
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