Wedge splitting test of wood for fracture parameters estimation of Norway Spruce

Katarzyna Ostapska,Kjell Arne Malo

doi:10.1016/j.engfracmech.2020.107024

Abstract

Crack evolution in wood from Norway Spruce during mode I wedge splitting tests was measured using a Digital Image Correlation (DIC) system. The resulting series of deformation fields were post-processed in order to obtain kinematically based crack tip location histories throughout the loading procedure, by a purposely made algorithm. The developed algorithm is based on generic mode patterns which are fitted to the observed deformation fields and optimized. The developed procedure and its application are explained and the resulting crack paths are presented. Subsequently, fracture energies, critical stress intensity factors and experimental resistance curves were derived based on the crack path data. Comparisons of the obtained material parameters for fracture mechanics with literature values show that the proposed method is a powerful alternative to the more traditional methods. Moreover, experimentally derived crack tip location histories give more detailed insight into the behaviour of wood (Norway Spruce) during mode I fracture and may replace indirect methods and crack length assumptions.

Highlights

Wood is a challenging material to describe with the means of material mechanics, primarily due to its natural origin that precludes production control
On average the ratio of A11/A21 is around 10 for the RL series and 9 for the TL series, which indicates that opening related deformations are one order of magnitude larger than the shearing deformation
The optimized value of the Kolosov’s parameter for the fields before damage initiation has a very low coefficient of variation (COV) for the RL series. This corresponds to the Poisson’s ratio of 0.5 on average with 7% COV, with the assumption of plane strain (the result for specimen no 5 (WST-06) were here treated as an outlier due to the first unloading-reloading cycle occurring before the test reached maximum force)

Summary

Introduction

Wood is especially challenging when it comes to fracture mechanics. Due to the highly orthotropic nature of wood, certain modes can be distinguished as a leading cause of material failures. Among those are splitting along the grain: Mode I RL and Mode I TL. Those types of failure are the governing mechanisms in structural collapse occurring especially in notched beams and beams with holes, where the concentration of perpendicular to grain stresses is augmented by the chosen design and may lead to delamination. The fracture behaviour of Norway Spruce and other wood species has been described with the means of methods developed for other materials; mainly steel, concrete, and engineered composites

Methods

Results

Discussion

Conclusion