共查询到20条相似文献，搜索用时 503 毫秒
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.
Takuro Hirai Takeyoshi Uematsu Yoshihisa Sasaki Masahiko Toda Okumu Gordon Wanyama Kei Sawata 《Journal of Wood Science》2012,58(5):408-416
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.
P. W. Bonfield C. L. Hacker M. P. Ansell J. M. Dinwoodie 《Wood Science and Technology》1994,28(6):423-435
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.
Acoustic emission analysis of industrial plywood materials exposed to destructive tensile load 总被引：1，自引：0，他引：1
Franziska Ritschel Yang Zhou Andreas J. Brunner Thomas Fillbrandt Peter Niemz 《Wood Science and Technology》2014,48(3):611-631
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.
Takahisa Nakai Hiroyuki Yamamoto Tetsuya Nakao Masatoshi Hamatake 《Wood Science and Technology》2006,40(8):683-695
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
11.
The results from a duration-of-load and creep testing program on a thick strand-based composite product, using wood from forests attacked by the mountain pine beetle, are presented. The constant loading of experimental beams lasted for 1?year. The long-term deflection was monitored and recorded at a pre-set frequency. Time-to-failure data were also obtained for all the broken specimens. Ramp load test was carried out at different rates of loading to investigate its influence on short-term strength. Finally, the fatigue test was conducted with a triangular cyclic load history, and the strain history was obtained to elucidate the low-cycle fatigue behavior of the composite. 相似文献
12.
13.
介绍轻型木结构楼、屋面板冲击荷载性能试验的条件、装置和方法,根据规范要求研制加载装置,并进行木结构用意杨结构胶合板的冲击荷载性能试验研究,旨在为国产速生林用于轻型木结构的研究提供一些试验数据。试验结果表明：与国外结构胶合板具有相同厚度的轻型木结构用意杨屋面板和楼面板,其冲击荷载受弯性能均优于我国木结构工程施工质量验收规范标准要求。 相似文献
14.
The performance of plywood-sheathed shear walls is determined at the plywood-to-timber joints. In joints with dowel-type fasteners, such as nails and screws, the fastener is fractured under reversed cyclic loading (e.g., seismic force), reducing the ductility of the joint. The fracture is caused by low-cycle fatigue due to the reversed cyclic bending of the fastener. Therefore, evaluating the fatigue life is important for estimating the ultimate displacement. The main objective of this study is to estimate the ultimate displacement of the joints and to enable load–displacement calculation of single shear joints under reversed cyclic displacement when bending fatigue failure of the fastener occurs. Single shear tests were conducted under different loading protocols, and the damage performances of the fasteners were determined by subjecting them to reversed cyclic bending tests. Based on the results, the failure lifetimes of joints with dowel-type fasteners were estimated. In addition, the fracture mechanism of these dowel-type fasteners was elucidated. CN50-type nails and wood screws with dimensions of 4.1?×?38 and 4.5?×?50 mm were used as fasteners. The single shear tests showed that the smaller the displacements per cycle, the lower are the ultimate displacement and ductilities of the joints. Moreover, load–displacement relationship up to fastener failure can be approximately estimated by combining the yield model and failure lifetime. 相似文献
15.
A. Miksic M. Myntti J. Koivisto L. Salminen M. Alava 《Wood Science and Technology》2013,47(6):1117-1133
The mechanics of fresh wood with and without a fatigue pre-treatment that mimics a mechanical pulping process was experimentally studied. The mechanical properties of Norway spruce samples under compression are considered with the macroscopic stress–strain data and from local strain properties via digital image correlation technique. The results highlight the effects of the orientation of the wood annual rings compared to the loading direction and of the pre-fatigue. The wood presents a low yield point when the annual rings are tilted compared to the load axis, but the Young’s modulus and yield stress are higher when the annual rings are either parallel or perpendicular to the load direction. In the last case, buckling of softest layers occurs. The fatigue treatment makes the wood less stiff as deduced from the decreases of Young’s modulus and yield stress, whatever the orientation of annual rings. Secondly, it creates a thin and localized softened layer. 相似文献
16.
对塑木地板进行抗弯性能测试,分别选取25％和75％两种应力水平进行疲劳／蠕变试验。研究了塑木地板在疲劳／蠕变复合作用下的断裂机理。结果表明：随着应力水平的增加,蠕变应变也随之增加,75％应力水平时产生的应变为25％应力水平时的3倍;其疲劳／蠕变曲线与纯蠕变曲线十分相似;在25％的应力水平作用后,材料的剩余抗弯强度为原来的94％～97％。 相似文献
17.
广东高州丛生竹林下棘托竹荪栽培试验 总被引：1，自引：0，他引：1
引进 3 个棘托竹荪 Dictyophora echinovolvata 菌株在广东省高州市丛生竹林下进行不同菌株及
不同竹林郁闭度栽培对比试验，结果是：D89 号菌株在郁闭度 0.9 的条件下取得最高产量，其带托与去托
竹荪产量分别为 21 608.5 和 11 057.3 g，折合每平方种植面积的产量为 5 402.1 和 2 764.3 g，达到了较高
的产量水平。各个菌株在 3 种竹林郁闭度下栽培的平均产量从高到低排列的顺序为 D89、D- 古优 1 号、
D1 号，三者的去托竹荪平均产量数据非常相近。说明 3 个菌株均很适合在高州的竹林下栽培，并且均可
获得较高的产量。不同菌株对竹林郁闭度的适应能力表现出一定的差异，D- 古优 1 号较其他两个菌株适
应性强，3 个菌株均较适合在竹林郁闭度为 0.7 和 0.9 的条件下栽培。 相似文献
18.
太浦河大桥静载试验研究 总被引：1，自引：0，他引：1
吴江市苏同黎太浦河大桥主桥上部结构为三跨预应力混凝土变截面连续箱梁。本文详细介绍太浦河大桥静载试验设计方案,包括：测试截面的选择、加载车型、重量以及加载工况、荷载效率系数的确定等。通过静载试验来确定主梁控制截面在试验荷载下的应变（应力）,主梁控制截面在试验荷载作用下的最大挠度,主梁在偏载作用下的影响系数以及梁体裂缝的开展情况等数据。由试验得出本桥在试验荷载作用下各测试截面的应力、挠度校验系数均小于1.0,表明该桥的承载能力满足正常使用的要求;各测试截面的相对残余应变和相对残余变位均小于20%,说明结构处于弹性工作状态。在该桥静载试验过程中完全按照既定的试验方案进行,通过静载试验得出该桥质量良好,安全可靠。 相似文献
19.
20.
Formaldehyde emission from wood products: relationship between the values by the Chamber method and those by the Desiccator test 总被引：1，自引：1，他引：1
The formaldehyde off-gassing properties of various wood products were evaluated under environment conditions that simulate
conditions of mobile home. The formaldehyde concentrations observed in the chamber were correlated to the 24-h desiccator
formaldehyde potentials. It was found that 24-h desiccator values did correlate to formaldehyde levels in the chamber at a
given temperature, relative humidity, air change rate, and sample loading. The 24-h desiccator method correlated to each other
for a given wood product. Formaldehyde levels of wood products tested in combination reflected the levels of the highest single
emitter. Formaldehyde concentrations from combinations of particleboards and plywood panels may be predicted from their single
chamber K factor using the modified Hoetjer equation. Double loading of a single wood product did not double the chamber concentration
at the low air change rate and sample loading. Finally, the formaldehyde concentrations observed in the chamber under conditions
that simulated mobile loadings of wood product, air change rate, temperature, and humidity relate to real wood formaldehyde
levels. 相似文献