Experimental assessment of failure criteria for the interaction of normal stress perpendicular to the grain with rolling
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ORIGINAL
Experimental assessment of failure criteria for the interaction of normal stress perpendicular to the grain with rolling shear stress in Norway spruce clear wood Shaheda T. Akter1 · Thomas K. Bader1 Received: 6 December 2019 © The Author(s) 2020
Abstract The anisotropic material behavior of wood, considered as a cylindrically orthotropic material with annual rings, leads to several different failure mechanisms already under uniaxial stresses. Stress interaction becomes important in the engineering design of structural elements and is often predicted by failure criteria based on uniaxial properties. The prediction quality of failure criteria has been assessed with longitudinal shear stress interaction, though less is known on rolling shear stress in interaction with stress perpendicular to the grain. The study aims at investigating the corresponding mechanical behavior of Norway spruce (Picea abies) clear wood by validating failure envelopes for stress combinations in the cross-sectional plane, based on experimental investigations. For this purpose, a test setup that controls the stress interaction and loading of clear wood along pre-defined displacement paths needed to be developed. Experimentally defined failure states could then be compared to failure surfaces predicted by the phenomenological failure criteria. Material behavior was quantified in terms of stiffness, strength, and elastic and post-elastic responses on dog-bone shaped specimens loaded along 12 different displacement paths. A comparison with failure criteria for two nominal compressive strain levels showed that a combination of failure criteria would be required to represent the material behavior and consider the positive effect of compressive stresses on the rolling shear strength. The findings of this work will contribute to studying local stress distribution of structural elements and construction details, where stress interactions with rolling shear develop.
1 Introduction Multiaxial and complex stress states can arise in timber structures depending on the position and direction of the applied force with regards to the wood grain direction. This requires special attention due to the material’s anisotropy as a consequence of its heterogeneous and porous microstructure (Kollmann et al. 2012). In particular on the material scale, where the local material orientations are commonly different from the global specimen geometry and loading, multiaxial stress states occur. The annual ring structure in a radial (R)-tangential (T) cross-section of wood that is loaded by uniform compression perpendicular to the grain leads to a non-uniform stress state and combination of normal stresses in the R and T direction with rolling shear. Compression * Thomas K. Bader [email protected] 1
Department of Building Technology, Linnaeus University, 35295 Växjö, Sweden
perpendicular to the grain, such as a macroscopic phenomenon in timber engineering, and the influence of boundary conditions have been studied extensively by Hall (1980), Hoffmeyer et al
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