A multi-surface plasticity model for clear wood and its application to the finite element analysis of structural details

Abstract

Recent biaxial experiments on spruce wood show that consideration of an elliptic failure surface according to Tsai and Wu, and of an elastic model for stress states within this envelope, gives an insufficient description of the mechanical behavior. As compression perpendicular to grain occurs, a nonlinear stress path results from a proportional biaxial strain path. Moreover, a phenomenological single-surface model does not permit easy identification of failure modes and thus renders the description of different post-failure mechanisms very difficult. Investigation of characteristic samples for various biaxial loading conditions enables the identification of four basic mechanisms covering the behavior of wood under plane stress conditions. The experimentally observed mechanical behavior will be described by means of a multi-surface plasticity model. It consists of four surfaces representing four basic failure modes. The first is a modified tension cut-off for the description of fiber rupture. The second is a mixed mode radial tension-shear model by Weihe applied to the perpendicular to grain direction. The third is an extension of the authors' prior model for perpendicular to grain compression, and the fourth surface covers the compressive failure parallel to grain. The model represents the orthotropic multi-surface elasto-plastic material clear wood. The aim of this paper is to present and discuss selected experimental data from biaxial tests with respect to distinct failure modes, and to develop an orthotropic plasticity model for its mathematical description. Since available experimental data cover only plane stress in the LR-plane, 1 both orthotropic failure and yield surfaces, respectively, are restricted to this case. IntroductionFrom the mechanical point of view, wood exhibits a pronounced orthotropic behavior with large ratios of mechanical properties such as Young's modulus or strength between the respective values parallel and transverse to the grain direction. Furthermore, it is well known that strength differs in tension and compression.Though a good amount of testing has been done for uniaxial states of stress, surprisingly little was known on the general effect of multiaxial states of stress on strength until the mid 1980s. The first remarkable experiments under mixed loading conditions were performed and reported by Spengler [26], Ehlbeck and Hemmer [10], and Hemmer [16]. Their experiments were restricted to small test specimens and to a restricted number of stress states. Spengler [26] investigated loading zones of beams. The specimen used was a small plate subjected to shear stress under superposed lateral compression. The results were restricted to the failure locations for this particular set of load cases. No deformation measurement has been performed.Ehlbeck and Hemmer [10], and Hemmer [16] used cylindrical pipes subjected to tension, torque, internal pressure, and combinations thereof. These experiments cover strength characteristics in the LT-plane. The deformation behavior has been reported...

Moisture-induced stresses in spruce cross-laminates

“…However, under drying, crosslaminates failed in terms of cracking. Appropriate models were introduced by [20][21][22][23][24], among others.…”

Models for numerical failure analysis of wooden structures

“…This ductile behaviour can be described phenomenologically as plasticity, although metal plasticity and wood pressure failure are motivated micro-mechanically differently. A 2D multi-surface plasticity model for clear wood is presented in [12], which introduces this kind of material approach into wood science. A further development of this formulation is the moisture-dependent, three-dimensional elastoplastic material model for spruce wood, which is presented and evaluated on the basis of experimental investigations in [16].…”

Compressive behaviour of axially loaded spruce wood under large deformations at different strain rates

“…The underlying fracture criteria have been investigated by Daudeville (1999), Eberhardsteiner (2002 and Mackenzie-Helnwein et al (2003). Different angles to the grain have been investigated by Reiterer and Stanzl-Tschegg (2001).…”