Fatigue of structural plywood under cyclic shear through thickness III: energy dissipation performance

Wood and wood composites have viscoelasticity, and show a hysteresis loop in the stress-strain relationship during cyclic loading such that part of the mechanical work applied is dissipated in the materials. In this study, the energy dissipation performance of plywood specimens under cyclic shear through thickness was investigated. Fatigue testing was conducted under three loading conditions: a square waveform at a loading frequency of 0.5 Hz, a triangular waveform at 0.5 Hz, and a triangular waveform at 5.0 Hz. The stress level was determined to be 0.5, 0.7, and 0.9 of the static strength in shear through thickness. The energy dissipation ratio was defined as the ratio of energy loss per cycle to the strain energy per cycle, and was evaluated throughout the fatigue test. It was found that the energy dissipation ratio of a plywood specimen was kept constant during most of the fatigue process for a given stress level and loading condition. The energy dissipation performance was significantly dependent on stress level and loading condition, and became higher according to the damage intensity of cyclic load even if the same strain energy was applied.     

Fatigue of structural plywood under cyclic shear through thickness II: a new method for fatigue life prediction

For plywood specimens under shear through the thickness, a fatigue life prediction method based on strain energy has been newly developed with the fatigue process and failure criterion applicable to various loading conditions. Once the fatigue process and failure criterion of the plywood specimen were determined by the fatigue data measured under a loading condition other than the square loading waveform, the fatigue life of a specimen under various loading conditions could be predicted easily and accurately by the first cycle loading test. The relationship between stress level and the predicted fatigue life was also similar to that between stress level and the experimentally determined fatigue life. The fatigue life prediction method proposed may be widely applicable to the prediction of the fatigue life of solid wood and wood composites.     

Fatigue life of structural plywood under two-stage panel shear load: a new cumulative fatigue damage theory

The fatigue life of structural plywood under two-stage panel shear load was experimentally examined. Two experimental conditions were determined for two-stage fatigue of plywood specimen: one used variable applied stress and the other used variable stress, loading waveform, and loading frequency, because fatigue life of wood composite under constant load depended on loading waveform and loading frequency as well as stress level. The most famous cumulative fatigue damage theory is the Palmgren-Miner rule, which is the summation of the ratio of the applied loading cycle to the fatigue life under each loading stage. However, the applicability of this rule to the two-stage fatigue of wood composites has not been investigated. It was first demonstrated in this study that the fatigue life of the plywood specimen reached in the two-stage fatigue test did not obey the Palmgren-Miner rule. Here, we propose the new cumulative fatigue damage model by modification of the Palmgren-Miner rule on the basis of the assumption that fatigue damage accumulates with loading cycle on a logarithmic scale. The newly proposed model was in good agreement with the fatigue life reached in the two-stage fatigue test.     

Effect of loading frequency on fatigue life and dissipated energy of structural plywood under panel shear load

Wood-based panels are subjected to cyclic panel shear load caused by wind and seismic forces in such an application as the sheathing of bearing walls. The fatigue behavior of structural plywood under panel shear load with two different loading frequencies was examined. Pulsating panel shear load with a triangular waveform and loading frequency of 0.5 or 5 Hz was applied to the plywood specimens. Stress−strain hysteresis loops were measured throughout the fatigue tests. Fatigue life was highly dependent on loading frequency at more than 0.5 stress level. The deterioration of mechanical property and damage accumulation in plywood specimen was observed to be slower at higher loading frequency at more than 0.5 stress level. Analyses based on energy loss suggest that panel shear load with higher loading frequency causes less damage to the plywood specimen during one loading cycle at higher stress level, and that the fatigue damage accumulation causing failure might be dependent on stress level although it seems to be unaffected by loading frequency. Based on these results, a new fatigue failure model for plywood specimen was qualitatively developed by combining Weibull’s weakest link model and Daniels’ fiber bundle model.