Prediction of the off-axis stress-strain relation of wood under compression loading

Uniaxial compression tests were conducted using specimens of agathis (Agathis sp.) and katsura (Cercidiphyllum japonicum Sieb. et Zucc.) with various values of off-axis angle, defined as the angle between the grain axis and the axis along the loading direction. The obtained stress-strain relation was regressed into Ramberg-Osgood's equation, and it was predicted by approximating the relationship between the parameters contained in the regressed equation and off-axis angle into Hankinson's formula. The comparisons between the stress-strain relations obtained by the experiment and prediction revealed that the off-axis compression stress-strain relation can be predicted effectively using the relations in the orthotropic axes.