Micromechanical properties of longitudinally compressed wood

Longitudinal compression induces changes in wood tissue resulting in better bendability. The main purpose of the present study is to identify the changes occurring at the microscopic level. Air-dried oak wood samples (Quercus petraea (Matt.) Liebl.) exposed to a thermo-hydro-mechanical treatmentcombined with different longitudinal compression and relaxation treatmentswere subjected to micromechanical (nanoindentation) and ultrastructural (scanning electron microscopy, atomic force microscopy) analyses at the wood cell wall level. The comparison of the various methods of treatments shows that the secondary cell wall S2 hardness decreased by 6% as a result of the compression and by 12%, when combined with a long relaxation period. Cell wall indentation modulus decreased by 25% and 51%, respectively. Relative elastic deformation work increased at the expense of plastic deformation work, while the relative proportion of viscoelastic work remained unchanged. Atomic force microscopic images revealed distinct disorientation of the normally straight microfibrils as a result of longitudinal compression, which was attributed to cause the weakening of the cell wall. The results indicate that changes at the ultra-structural level of the wood cell wall contributed to the changes in the macromechanical properties (for example modulus of elasticity and bendability) already reported previously, of the treated wood samples.