Analysis of shear creep behaviors of hardwood and softwood using creep recovery curves

This study was aimed at exploring the sheer creep behavior of wood through off-axis tensile creep and creep recovery tests. Using the creep recovery data, the shear creep properties of softwood (Japanese Hinoki cypress, Chamaecyparis obtusa) and hardwood (Japanese Buna beech, Fagus crenata) were compared. The trends of three components of strain, i.e., instantaneous elastic, delayed elastic, and permanent strains, during shear creep were predicted by decomposing the total strain during creep recovery, assuming that the rate of increase in delayed elastic strain is the same as the recovery rate during creep recovery. Fitting a Burger model to each predicted strain yielded more reliable material parameters compared with those obtained by simply mathematically fitting the Burger model to the total creep strain. The Burger model demonstrated excellent accuracy in fitting the measured creep curves of hardwood. However, it could not explain the shear creep behavior of softwood. This discrepancy in the fitting results was attributable to the differences in the behavior of permanent strain: The permanent strain of cypress exhibited a curvilinear trend, while that of beech displayed a more linear trend. To explain the curvilinear behavior of permanent strain, a modified Burger model, which assumes that the apparent viscosity of permanent strain changes in a strain-rate-dependent manner, was proposed. The modified Burger model yielded better fitting results than the conventional Burger model, suggesting that the viscous component of wood exhibits an apparent viscosity that depends on the strain rate rather than a constant value, as assumed in the conventional Burger model.