Continuum micromechanics estimation of wood strength

Wood strength is highly anisotropic and tissue-specific. We herein show how it can be predicted from local failure of the nanoscaled wood component lignin and from the microstructure and the composition of the wood tissue, by means of continuum micromechanics. The suitability of the model is confirmed by the good agreement between model-predicted biaxial strength values of wood and corresponding experimental results. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

“Assessing the RAFT Equilibrium Constant via Model Systems: An EPR Study”—Response to a Comment

We have presented an EPR‐based approach for deducing the RAFT equilibrium constant, K_eq, of a dithiobenzoate‐mediated system [Meiser, W. and Buback M. Macromol. Rapid Commun. 2011 , 32, 1490]. Our value is by four orders of magnitude below K_eq from ab initio calculations for the identical monomer‐free system. Junkers et al. [Macromol. Rapid Commun. 2011 , 32, 1891] claim that our EPR approach would be model dependent and our data could be equally well fitted by assuming slow addition of radicals to the RAFT agent and slow fragmentation of the so‐obtained intermediate radical as well as high cross‐termination rate. By identification of all side products, our EPR‐based method is shown to be model independent and to provide reliable K_eq values, which demonstrate the validity of the intermediate radical termination model.     

Homogenization of bone elasticity based on tissue‐independent (‘universal’) phase properties

As candidates for tissue‐independent phase properties of cortical and trabecular bone we consider (i) hydroxyapatite, (ii) collagen, (iii) ultrastructural water and non‐collagenous proteins, and (iv) marrow (water) filling the Haversian canals and the intertrabecular space. From experiments reported in the literature, we assign stiffness properties to these phases (experimental set I). On the basis of these phase definitions, we develop, within the framework of continuum micromechanics, a two step homogenization procedure: (i) At a length scale of 100 – 200 nm, hydroxyapatite (HA) crystals build up a crystal foam ('polycrystal'), and water and non‐collagenous organic matter fill the intercrystalline space (homogenization step I); (ii) At the ultrastructural scale of mineralized tissues, i.e. 5 to 10 microns, collagen assemblies composed of collagen molecules are embedded into the crystal foam, acting mechanically as cylindrical templates. At an enlarged material scale of 5 to 10 mm, the second homogenization step also accommodates the micropore space as cylindrical pore inclusions (Haversian and Volkmann canals, inter‐trabecular space), that are suitable for both trabecular and cortical bone. The input of this micromechanical model are tissue‐specific volume fractions of HA, collagen, and of the micropore space. The output are tissue‐specific ultrastructural and microstructural (=macroscopic=apparent) elasticity tensors. A second independent experimental set (composition data and experimental stiffness values) is employed to validate the proposed model. We report a a good agreement between model predictions and experimentally determined macroscopic stiffness values. The validation suggests that hydroxyapatite, collagen, and water are tissue‐independent phases, which define, through their mechanical interaction, the elasticity of all bones, whether cortical or trabecular.