Experimental and numerical investigation of wood fracture mechanisms at different humidity levels

Abstract

Differences in fracture patterns and properties at various moisture levels are experimentally and numerically evaluated and discussed. Experiments were performed on spruce, pine, oak and beech. The influence of moisture at 98%, 80%, 65% and 30% RH and the mechanisms involved were investigated for softwoods and hardwoods subjected to opening mode I fracture using in situ and ex situ real-time environmental scanning electron microscopy (ESEM). The wedge-splitting technique was employed. To quantify the effect of humidity, fracture toughness values were obtained from ex situ tests and finite element analysis of the contact problem in wedge-splitting. In addition, lattice fracture model simulations were performed for numerical investigation of the fracture mechanisms. Distinct changes in wood fracture behaviour were observed as a function of moisture content. Fracture toughness was highest at 30% RH for all species except for oak, and showed higher values in the radial-longitudinal than in the tangential-radial direction. In green wood, water droplets moved away from the cell lumens around the crack tip. Drying of wood promotes microcracking and crack bridging as toughening mechanisms. The findings reported may be useful for further research into the interaction between moisture transfer and stress gradients in wood accompanied by moisture-crack phenomena.

Influence of the moisture content on the fracture characteristics of welded wood joint. Part 1: Mode I fracture

“…During crack propagation, the softening behavior is reported and fiber bridging occurs behind the crack tip. Like the other mechanical properties of wood, the fracture characteristics are also influenced by moisture, that is, they tend to decrease with increasing moisture content (MC) (Pluvinage 1992;Vasic and Stanzl-Tschegg 2007;Majano-Majano et al 2012). The maximal value of the stress intensity factor (i.e., fracture toughness K Ic ) is reached at 16% MC (Liyu et al 2003) or between 7% and 13% MC (Kretschmann and Green 1996).…”

Synchrotron-based tomographic microscopy (SbTM) of wood: development of a testing device and observation of plastic deformation of uniaxially compressed Norway spruce samples

“…The structure-property relationships of wood was investigated by EM and ESEM ex situ and in situ (Tabarsa and Chui 2000 ;Sippola and Fr ü hmann 2002 ;Fr ü hmann et al 2003 ;M ü ller et al 2003 ;Vasic and Stanzl -Tschegg 2006 ). While optical microscopy was mainly used for ex situ observations, EM and especially ESEM are very suitable for various in situ experiments to evaluate crack propagation and reaction to load (Sippola and Fr ü hmann 2002 ;Fr ü hmann et al 2003 ;M ü ller et al 2003 ;Vasic and Stanzl -Tschegg 2006 ).…”

“…While optical microscopy was mainly used for ex situ observations, EM and especially ESEM are very suitable for various in situ experiments to evaluate crack propagation and reaction to load (Sippola and Fr ü hmann 2002 ;Fr ü hmann et al 2003 ;M ü ller et al 2003 ;Vasic and Stanzl -Tschegg 2006 ). Both methods have certain disadvantages for in situ tests under mechanical load: during sample preparation, defects can be induced (Jansen et al 2008 ).…”

Fracture properties of green wood formed within the forks of hazel (Corylus avellana L.)

“…For instance, wood is usually far more ductile across the grain (LR and LT fracture systems), but brittle along the grain (RL, RT, TL, and TR fracture systems). Results of the previous research has also showed that wood is generally tougher radially than tangentially (Atack et al 1961;Ashby et al 1985;Stanzl-Tschegg et al 1995;Reiterer et al 2002a, b;Smith and Vasic 2003;Marki et al 2005;Vasic and Stanzl-Tschegg 2007;Yoshihara and Nobusue 2007;Majano-Majano et al 2012;Ozden and Ennos 2014;Özden et al 2016). …”