Softwood fracture characterization at the mesoscopic scale Loane Bigorgne*a, Michel Bruneta, Hubert Maigrea, Pierre Simona, Philippe Chaudeta a LaMCoS,

Université de Lyon, CNRS INSA-Lyon UMR5259 20 avenue Albert Einstein, F69621 Villeurbanne Cedex, France @insa-lyon.fr

Motivation Softwood material is still not well understood. To prevent crack apparition, specific reference rules have been recorded into some reference manual such has Eurocode 5 and leads to oversized structures. A more precise analysis of wood is needed. Wood complex and heterogeneous structure explain these rough dimensioning rules. In heartwood, at the macroscopic scale, wood is orthotropic and cylindrical. At the mesoscopic scale, wood is like a multilayer material from earlywood to latewood with gradual local density and mechanical properties [1]. Fracture criteria are often based from observations and measurements at the macroscopic scale. Given the annual ring regular variation, the mesoscopic scale seems to be convenient to describe and explain local fracture phenomena.

 Improve wood fracture knowledge  Understand and prevent crack propagation  Define new fracture criteria at the mesoscopic scale

Mesoscopic fracture criteria

Validation and application Mesoscopic fracture criteria are implemented in the source code of the MPM software used. Gc (r ,0)  Gc (r ,90) Gc (r ,0)  Gc (r ,90) Gc (r , )  cos(2 )  2 2

Each sample is digitized and imported to the numerical tool. Gray level map input helps to define local density and so local material properties. Angular map input helps to precise orthotropic directions.

The correlation between experimental and fracture behavior is good. This new MPM program can be used as a tool for crack path prediction.

Gc(r,Thêta) [J/m²]

Macroscopic phenomena related to continuous cracks can thus be explained by this process. Local phenomena cannot be represented. r

Thêta [°]

J  lim   ij d  ij n  ij n j  0



 ui d  xm

J c  Gc

Acknowledgment Technical help provided by Dr. Julien Subercaze is gratefully acknowledged.

References [1] Simon P. Approche multiéchelle du comportement mécanique du bois dans le plan transverse. Thèse de doctorat. Université de Lyon. France 2009 [2] Nairn J.A. Material Point Method Calculations with Explicit Cracks. Computer Modeling in Engineering & Sciences 2003; 4:649-664 [3] http://code.google.com/p/nairn-mpm-fea/