Effect of pulsating tension-torsion combined loading on fatigue behavior in wood

Abstract

Abstract The effect of cyclic tension-torsion combined loading on the fatigue behavior and stress-strain properties of wood (Japanese cypress) was investigated experimentally. The specimen used in the experiments was a rectangular bar with its major axis in the fiber (longitudinal) direction of wood. Pulsating tension and torsion loadings were respectively applied along and around the longitudinal axis of the specimen. According to the relationships between stress and strain obtained using fatigue tests, the secant modulus of the stress-strain curve changed with an increase in the number of loading cycles, and the differences between the curves for tension and shear were observed. The obtained results of fatigue tests were found to be influenced by the combined stress ratios and the applied stress levels. Bordering on the combined stress state of TB, where tensile and shear stress components were almost equally applied, the trend toward lower stiffness retention was different between tension and shear, and the tensile or torsion failure mode became dominant in the failure mode. The TB state was recognized as a boundary state for both of the stiffness retention and the failure mode. The stiffness retention in the TB state showed a tendency similar to that in the stress state where torsion was dominant regardless of the stress level. On the other hand, the failure mode tended to be tension failure at higher stress levels. Thus, at higher stress levels in the TB state, the effect of the tensile or shear stress component on the fatigue behavior of wood was different between “during the fatigue process” and “the time of failure”. In addition, torsion was dominant for both during the fatigue process and the time of failure at lower stress levels.