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1.
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.  相似文献   

2.
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.  相似文献   

3.
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.  相似文献   

4.
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.  相似文献   

5.
Dynamic tests of nailed plywood–timber joints are conducted under harmonic vibrations from 2 to 7 Hz. The principal results are as follows: under dynamic loading, nailed plywood–timber joints may break in low-cyclic bending fatigue failure of nails besides the other failure modes typical under static loading. The dynamic response of nailed plywood–timber joints is clearly dependent upon both the input frequency and the acceleration. These responsive characteristics arise from the nonlinear load–slip relationships and the characteristic cyclic stiffness degradation of nailed joints; that is, the cyclic degradation of the equivalent linear stiffness decreases the resonant frequencies of the same joints, which results in a transition of dynamic responses. It indicates that frequency components of seismic waves resonant to the frequencies corresponding to safety-limit stiffness of nailed joints may lead them to critical failures, even if the accelerations do not exceed the accelerations equivalent to the static damage-limit resistance.  相似文献   

6.
Failure mechanisms of small clear specimens (6×6×24 mm) of air-dried black spruce (Picea mariana) under parallel-to-grain compression were investigated by polarised-light microscopy. Fatigue load was used with a peak stress level of 90% static strength, a load frequency of 0.5 Hz, and a square waveform with a duty ratio of 0.50. Matched pure creep and static load tests were carried out. Damage was quantified in terms of the permanent microstructural changes (kinks) in tracheid walls. In static load tests, kinks develop quickly with any increase in stress beyond the limit of proportionality. In creep tests, damage develops mainly from kinks formed during the initial load application. In fatigue tests, damage develops both from kinks formed during the initial load cycle, and kinks formed during subsequent cycles. The number of kinks exhibits a strong relationship with relative cyclic creep or relative creep.  相似文献   

7.
Structural grade chipboard was subjected to fatigue and creep loads in four-point bending, the peak fatigue stress being equal to the constant creep stress. Peak fatigue stresses of 50, 60, 70 and 80% of the static bend strength were selected and an S-N (stress versus log10 (cycles)) curve was generated. Stress versus strain hysteresis loops were captured automatically throughout fatigue tests so that underlying creep strain, dynamic modulus and energy dissipated per cycle were continuously monitored. The possibility of superimposing creep and fatigue data was investigated.The S-N curve generated at R = 0.01 demonstrates that for lives of less than 107 cycles chipboard does not show a fatigue endurance limit. The 70 and 80% samples experience a gradual decrease in dynamic modulus and an increase in the area of the hysteresis loop during fatigue tests. Samples at the 50 and 60% levels show an initial increase in dynamic modulus before a decline to failure is observed.Creep samples never failed before fatigue samples at the same peak stress level, but until close to the point of failure, creep strains were nearly always greater than fatigue strains on elapsed time. It is concluded that the mechanism of fatigue damage accumulation differs from the mechanism of creep deformation.  相似文献   

8.
Several plywood materials made from spruce wood and, for comparison, solid spruce wood were investigated focusing on the sub-macroscopic damage evolution during tensile loading of the specimens. The destructive tests were simultaneously monitored by the acoustic emission (AE) method and strain field deformation measurement using digital image correlation (DIC). The bilinear interpretation of exponential defect growth identified the start of significant nonlinear behavior at 70 % of ultimate strength for all plywood materials. However, already the preceding and more stable damage evolution at lower stress levels has indicated a variation in intensity of the source mechanisms evaluated by AE energy of the detected events. Additional information on the formation of strain field concentration, which correlates with discrete accumulation in AE events and increased spreading in the distribution of AE energy, reveals the complexity of pre-damage due to the variation in cracks’ magnitude and timescales involved. The correlation between ultimate tensile strength and damage accumulation below 70 % of ultimate strength is determined, as well as the influence of layered structures on damage size shown by the percentage distribution of AE energy.  相似文献   

9.
Summary  An investigation of fatigue failure in wood subjected to load cycles in compression parallel to grain is presented. Small clear specimens of spruce are taken to failure in square wave formed fatigue loading at a stress excitation level corresponding to 80% of the short term strength. Four frequencies ranging from 0.01 Hz to 10 Hz are used. The number of cycles to failure is found to be a poor measure of the fatigue performance of wood. Creep, maximum strain, stiffness and work are monitored throughout the fatigue tests. Accumulated creep is suggested identified with damage and a correlation is observed between stiffness reduction and accumulated creep. A failure model based on the total work during the fatigue life is rejected, and a modified work model based on elastic, viscous and non-recovered viscoelastic work is experimentally supported, and an explanation at a microstructural level is attempted. The outline of a model explaining the interaction of the effect of load duration and the effect of the loading sequences is presented. Received 8 December 1997  相似文献   

10.
This study investigated the relationship between the cellulose crystal lattice strain (crystalline region) and the macroscopic surface strain in specimens of Chamaecyparis obtusa wood under repeated uniaxial tension stress in the fiber direction. Changes in the strain of the crystal lattice were measured from the peak of (004) reflection using the transit X-ray method. The macroscopic surface strain of each specimen was measured with a strain gauge. In both loading and unloading, the surface strain changed linearly with changes in stress. However, crystal lattice strain was not linear but exhibited changes along a curve with changing stress. Under stressed conditions, the crystal lattice strain was always less than the surface strain, regardless of the frequency of repetition in the loading and unloading cycle. The ratio of the crystal lattice strain to the surface strain showed a negative correlation for stress in both loading and unloading. That is, the ratio decreased with increasing stress, and finally tend to converge to a specific value. The ratio (I/I 0) between the diffracted intensity (I 0) in the (004) plane in the unloaded condition and the diffracted intensity (I) in the (004) plane in the loaded condition tend to converge on a specific value with increasing frequency of repetition. When the substantial tension Young’s modulus of the wood in the longitudinal direction decreased, the ratio of the strain of the crystal lattice to the surface strain also decreased. Moreover, the ratio decreased with increasing microfibril angle of the specimen.  相似文献   

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