Effect of Axial Load on Torsion Fatigue Behavior of Wood

The torsion fatigue behavior of solid wood under cyclic torsion-axial combined loading was investigated. The test specimens used were air-dried Japanese cypress that were cut into sections of 17.5 mm (tangential) x 17.5 mm (radial), with their major axis lying along the fiber direction of 300 mm (longitudinal). A pulsating torsion with a triangular waveform was applied along the longitudinal axis of the specimens at 1 Hz, while the specimen was also simultaneously subjected to an axial (tension or compression) load at the same phase along the longitudinal direction, at stress levels corresponding to 50-100% of each static strength. The results obtained are summarized as follows: When tension was added to pure torsion, the inclination of the S-N curve tended to decrease as the tensile stress component increased. On the other hand, when compression was added to pure torsion, the S-N curve generally moved to the long-life side. The strength of the wood was different between dynamic and static modes, under not only pure loading but also axial-torsion combined loading. The torsion deformation under compression-torsion combined loading kept its initial value small until the increase before final failure, and this was considered to cause the lengthening of the fatigue life under the stress state, in which shear and compression stress components were almost equal