In a previous paper, we suggested that the shear moduli of the LT-(longitudinal-tangential)plane or LR-(longitudinal-radial) plane can be measured approximately from one torsional moment-shear strain relationship. This measurement method, however, cannot be applied for the specimen with a strong orthotropy between the planes along the torsional axis, and we thought that the shear modulus of the RT-(radial-tangential) plane cannot be given properly by this approximation method. In this paper, hence, we examined the feasibility in measuring the shear modulus of the RT-plane only from the torsional momentshear strain relationship of the RT-plane.
Sitka spruce (Picea sitchensis Carr.) and Konara (Japanese oak, Quercus serrata Murray) were used for the specimens. These specimens were twisted along the radial direction. Shear moduli on the side planes were calculated by substituting the torsional moment-shear strain relationships at the side planes into the solution of differential equations for the torsion of orthotropic rectangular bar. On the other hand, an approximating equation for deriving the shear modulus of the RT-plane was proposed from the hypothesis that the ratio of the shear modulus of the RT-plane to that of the LR-plane is in a limited range, and the shear modulus given by this equation was compared with that derived from the rigorous solution.
We thought that the shear modulus of RT-plane can be obtained properly by the approximating equation proposed here when the aspect ratio of the specimen rs determined properly.