Effect of Waveform and Loading Sequence on Low-Cycle Compressive Fatigue Life of Spruce

The effect of waveform and loading sequence on the low-cycle fatigue life of spruce was investigated using parallel-to-grain compressive tests. Triangular, sinusoidal, and square waveforms were used, with a peak stress level of 90% of static compressive strength and a load frequency of 0.5 Hz. Duty ratios ranging from 0.10 to 0.90 were adopted for square waveforms. Two types of stepped-cyclic-loading sequences with a duty ratio of 0.50 were also used. It was found that fatigue life is a function of the work done by the load and the shape of the waveform over a cycle. Damage accumulated most rapidly under square waveforms with a high duty ratio. A triangular waveform is the least damaging of those considered. For waveforms having the same work, the maximum loading rate is the strongest influence on the damage accumulation in wood. High-to-low stepped-cyclic-loading sequences are more damaging than low-to-high sequences, suggesting that wood has “memory.” This study has practical significance to the behavior of compression members and connections within structural systems subjected to cyclic loads.