Fatigue and creep of chipboard

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.     

Bending creep behavior of hot-pressed wood under cyclic moisture change conditions

This study examined the bending creep behavior of hot-pressed wood during cyclic moisture changes. Sugi (Cryptomerica japonica D. Don) specimens were pressed in the radial direction under six combinations of nominal compressive strain (33% and 50%) and press temperatures (140°C, 170°C, 200°C). Creep tests were conducted at 20°C with three cyclic relative humidity changes between 65% and 95% under 25% of short-breaking stress. The effect of moisture content (MC) change on elastic compliance and mechanosorptive (MS) compliance was investigated. The relation between MS compliance and thickness swelling was studied. The results indicated that total compliance increased over the history of cyclic moisture changes; and its behavior was closely related to the changes in MC and thickness swelling. The total compliance increased during adsorption and decreased during desorption. Elastic compliance increased linearly with MC and was dependent on press temperature and compression. With increasing MC change, MS compliance increased during adsorption and decreased during desorption. The first adsorption led to greater MS compliance than did the subsequent adsorption with the same amount of MC change. In general, the elastic parameterK _E and the MS parameterK _Mincreased with compression and decreased as the press temperature increased. The MS parameterK _M was apparently greater than the elastic parameterK _E. The MS parameterK _M increased with swelling coefficient K_SW of the hot-pressed specimen during adsorption and decreased with an increasing shrinkage coefficientK _SH during desorption.     

Bending creep behavior of wood under cyclic moisture changes

This study examined the bending creep behavior in the longitudinal direction of six species under cyclic moisture content (MC) changes. For each species, tests were made at 20°C with five cyclic relative humidity changes between 65% and 95%, beginning from moisture adsorption. A load corresponding to 25% of short-term breaking load of the species was applied to the radial section of each specimen with four-point bending. The effect of MC change on instantaneous compliance was also investigated under the same condition. The quantitative relation between mechano-sorptive (MS) compliance and MC change was examined, and the material parameter KM for the relation in specific sorption was determined. Results indicated that the total compliance in the six species with different behavior increases with sorption time. As an integral part of total compliance, instantaneous compliance changes linearly with MC and influences to a greater or lesser extent the total compliance behavior. In general, with increasing MC change, the MS compliance linearly increases during the first adsorption and all desorption and decreases slightly during subsequent adsorption. The material parameterK _M varies markedly not only with species but also with specific sorption. The first adsorption causes the largest deformation, followed by desorption.This paper was presented at the 48th annual meeting of the Japan Wood Research Society in Shizuoka, Japan. April 1998     

Cantilever experimental setup for rheological parameter identification in relation to wood drying

Wood exhibits a pronounced time dependent deformation behavior which is usually split into ‘viscoelastic’ creep at constant moisture content (MC) and ‘mechano-sorptive’ creep in varying MC conditions. Experimental determination of model rheological parameters on a material level remains a serious challenge, and diversity of experimental methods makes published results difficult to compare. In this study, a cantilever experimental setup is proposed for creep tests because of its close analogy with the mechanical behavior of wood during drying. Creep measurements were conducted at different load levels (LL) under controlled temperature and humidity conditions. Radial specimens of white spruce wood [Picea glauca (Moench.) Voss.] with dimensions of 110 mm in length (R), 25 mm in width (T), and 7 mm in thickness (L) were used. The influence of LL and MC on creep behavior of wood was exhibited. In constant MC conditions, no significant difference was observed between creep of tensile and compressive faces of wood cantilever. For load not greater than 50% of the ultimate load, the material exhibited a linear viscoelastic creep behavior at the three equilibrium moisture contents considered in the study. The mechano-sorptive creep after the first sorption phase was several times greater than creep at constant moisture conditions. Experimental data were fitted with numerical simulation of the global rheological model developed by authors for rheological parameter identification.     

Investigation of changes in compressed moso bamboo (<Emphasis Type="Italic">Phyllostachys pubescens</Emphasis>) after hot-press molding

In this study, molding moso bamboo strips to a curved shape using hot-press molding operation was explored. Bamboo strips with different thickness and moisture content (MC) were subjected to press molding under 120–210 °C for different time. Changes in the chemical components of bamboo were analyzed by Fourier-transform infrared spectroscopy (FTIR). Effect of MC on thermal mechanical behavior of bamboo was investigated using dynamic mechanical analysis (DMA). Results showed that the influencing degree of four variables on compression and recovery ratios decreased as: temperature?>?time?>?thickness?>?MC. Compression ratio increased and recovery ratio decreased dramatically when pressing temperature exceeded 180 °C. FTIR analysis indicated that polysaccharide (especially hemicelluloses) underwent a progressive thermal degradation during compression at 180 and 210 °C for 40 min, whereas relative content of lignin increased. DMA results showed that bamboo samples with a higher MC had a lower storage modulus value, confirmed water had a plasticizing effect. The loss factor of bamboo with higher MC (12 and 16%) exhibited two major transitions centred around 100 °C (α₁) and 50 °C (α₂), respectively. The temperature of these α transitions kept almost unchanged as moisture level increased from 12 to 16%. These findings provide fundamental information for the future preparation of curved bamboo as profiled components in engineered products.     

Probabilistic evaluation of the final moisture content of kiln-dried lumber using the bootstrap method

This paper presents a probabilistic method of evaluating the final moisture content (MC) of lumber obtained at the end of the kiln-drying process. The final MC data of three different drying tests conducted in past studies were analyzed using the bootstrap method. Target MC was tentatively set below 20 % in the analysis. Two characteristic parameters representing the final MC were estimated with bootstrap confidence intervals. These parameters were the standard deviation (SD) and the percentage of the population that met the MC requirement of less than 20 % (P ₂₀). The histograms of the final MC and the subsequent goodness-of-fit tests revealed that the final MC data of two drying tests did not follow any classical probability distributions, including Normal, Log-Normal, Weibull, and Gamma distributions, thus indicating the need for nonparametric statistics. The uncertainty of the final MC could be evaluated with the estimated SD and P ₂₀. After deriving the relationships between P ₂₀ and the corresponding probability that P ₂₀ is not achieved, we demonstrated how such relationships could provide a kiln operator with information to facilitate better decision-making in optimizing a drying schedule.     

Predicting oven-dry density of Sugi (Cryptomeria japonica) using near infrared (NIR) spectroscopy and its effect on performance of wood moisture meter

We propose a non-destructive method to predict the oven-dry density of Sugi (Cryptomeria japonica D. Don) using near infrared (NIR) spectroscopy so as to calibrate a commercial moisture meter. A prediction model for oven-dry density was developed using NIR spectra obtained from Sugi samples with a known density. The density of air-dried Sugi boards was predicted with the developed model. Then, the moisture content (MC) of the boards was measured by a hand-held capacitance-type and an in-line microwave moisture meters. For each board, the moisture meters were calibrated by the predicted density. The predicted density was correlated with the measured one with an R ² of 0.81 and a standard error of prediction (SEP) of 15.3 kg/m³ within the measured density of 279.2–436.4 kg/m³, indicating that the developed model was applicable for predicting oven-dry density of Sugi. The MC readings of both moisture meters showed a good correlation with the oven-dry MC that ranged from 12.1 to 28.9 %. For both moisture meters, the density calibration with the NIR-predicted density gave a higher R ² and a lower SEP than with the conventional calibration with the mean density. These results demonstrate that the present density calibration using NIR spectroscopy could improve the performance of the moisture meters for the air-dried Sugi boards with varying densities.     

Creep-rupture life of Douglas-fir under cyclic environmental conditions

Summary Small beams of Douglas-fir were subjected to constant loads and the time to failure was investigated while the specimens were subjected to cyclicly varying conditions of relative humidity and temperature. At a given load level, periodic variations in environment led to significant reductions in the average time to failure. The extent ofthis reduction was primarily related to the range in integral moisture content attained by the specimens.

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Zusammenfassung Douglasien-Stäbe wurden einer konstanten Dauer-Belastung unterworfen und die Zeit bis zum Biegebruch gemessen. Während der Versuchsdauer wurden relative Luftfeuchtigkeit und Temperature zyklisch verändert. In einem bestimmten Belastungsbereich führen derartige periodische Änderungen der Umgebungsbedingungen zu einer signifikanten Verkürzung der Zeitdauer bis zum Bruch. Das Ausmaß der Verkürzung hängt in erster Linie mit der Größe der sich in der Probe einstellenden Gesamtholzfeuchtigkeit zusammen.

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Paper presented at Session 3-Anatomy and Fundamental Properties-of the 21st Annual Meeting of the Forest Products Research Society, July 3, 1967, in Vancouver, British Columbia, Canada.     

Compressive fatigue in wood

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     

Behavior of acoustic emission generation during tensile tests perpendicular to the plane of particleboard II: effects of particle sizes and moisture content of boards

Particleboard specimens with various particle sizes were conditioned into two ranges: low and high moisture content. One set was investigated for internal bond (IB) strength and acoustic emission (AE) events during tensile tests perpendicular to the plane and the other for ultrasonic wave transmission characteristics in the thickness direction. The particleboard structural mechanics were changed as a result of the moisture effect. Specimens conditioning to higher moisture content had lower IB strength and lower cumulative acoustic emission event counts (T _AE). The decrease in IB strength indicated that the irreversible thickness swelling was seen when recovery forces of the particles exceed the restraining action of the adhesive. This was attributed to stress release, which resulted in internal failure of the board. The change in the internal structure caused an increased stress level at the initiation of AE generation. No events were recorded before this stress level, obeying theKaiser effect. The decrease inT _AE was not only related to the decrease in IB strength but was also affected by the transformation (attenuation) of the AE signals during IB tests according to the mesh size used.Part of this paper was presented at the 45th and 50th Annual Meetings of the Japan Wood Research Society, Tokyo and Kyoto, 1995 and 2000, respectively     

杉木生材含水率分布及其对气干的影响

以人工林杉木为研究对象,研究生材状态下不同厚度锯材含水率的差异,比较含髓心、钝棱与其他类型等3类锯材的含水率差异,建立生长轮宽度与锯材含水率的关系,分析生材状态含水率对锯材气干周期的影响。研究结果显示,25mm和40 mm的锯材含水率差异极显著;含髓心锯材含水率最低,其次是其他类的,最大是含钝棱锯材;随着生长轮宽度减少,锯材含水率增大;生长轮宽度为4 mm时,是生材含水率分界线,即当生长轮平均宽度小于和不小于4.0 mm时,锯材含水率差异极显著;25 mm和40 mm厚不同含水率锯材各2组气干至含水率25%,低含水率锯材较高含水率的气干周期分别缩短57%和66%。     

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.     

Creep in chipboard

Summary The effect of moisture and level of stressing on the rate of creep and time to failure is studied for UF and MUF bonded particleboard. Response curves were fitted to a 4-element rheological model with a high degree of accuracy for four levels of stressing and for three levels of relative humidity at 20°C. Values of relative creep increased with time and also with increased levels of stress and r.h. The ratio of stress to deflection — presented either as the creep modulus or as isochronous curves — decreased rapidly with time. Particleboard is demonstrated to exhibit non-linear viscoelastic properties, particularly at the higher levels of stress. Deflection was increased and time to failure decreased when r.h. was raised from 65 to 90 per cent, but no significant statistical change was noted between 30 and 65 per cent r.h. Increased levels of stress caused a decrease in both deflection and time to failure. Greater deflection at failure and longer time to failure were recorded for MUF board, but for the UF board the 90 per cent r.h. had a more pronounced effect in increasing deflection and decreasing the time to failure. Predictions are made on the long-term stress loadings for different humidity conditions.     

Preplanting indicators of survival and growth of desiccated Abies procera bareroot planting stock

Plant moisture loss during preplanting handling is considered one of the major elements in transplanting shock. The aim of the present study was to evaluate test parameters that may be suitable for assessment of the physiological condition of noble fir (Abies procera Rehd.) planting stock that has been subjected to desiccation stress. The transplanting-sensitive species was subjected to full exposure (roots and shoots) for 0, 0.5, 1.5, 3.0 and 6?h, or partial exposure (shoots) for 0, 3, 10, and 24?h before planting. The effects of desiccation on moisture content (MC), shoot water potential (ψ), root growth potential (RGP), root electrolyte leakage (REL), survival and height growth were evaluated. Desiccation effects on plant parameters and performance were generally significant after 1.5?h of full or 10?h of partial exposure to treatment. REL explained the largest percentage of performance variation in fully exposed plants, followed by MC and ψ. RGP tended to underestimate performance potential. Factors affecting the reliability of the test parameters, e.g. moisture loss rate, root rewetting and site conditions, are discussed.     

Effect of moisture content on fatigue behavior in the non-linear region for wood

Summary For oven-dried wood applied non-linear deflection, the temperature rising AT was constant after the initially slight temperature rising, and then arose immediately before fatigue failure. Moreover, there was not the rapid reduction of dynamic stress as observed for moist wood. On the other hand, the process to fatigue failure for various moist wood depended upon the moisture content. For heating vs. moisture content, the heating increased with moisture content up to 13 to 20%, while it decreased gradually above 20%. The processes to fatigue failure at more than 13% moisture content were similar to one another. These results were explained on energy balance between the heating due to flexibility of moist molecules in wood substances and the energy consumption for moisture movement and vaporization. The present results supported the mechanism of fatigue behavior in the non-linear region which was proposed in the previous report.     

Creep of the beam in Japanese conventional structures composed of green and kiln-dried timber II: predictive model for relative deflections

We conducted creep tests to evaluate creep behaviors of conventional Japanese framing (jikugumi) structures as reported in a previous article. We measured beam deflections of two structures: one of them was composed of only green timbers (G) and the other with only kiln-dried timbers (D). Besides the two structures, we prepared green and kiln-dried beams to measure moisture content (MC), weight, and dynamic Young’s modulus (E _f) by the longitudinal vibration method. We attempted to predict deflections of beams in the structures by using experimental data for single beam specimens. The proposed simple predictive model was derived from two equations: a relation between MC and equilibrium moisture content calculated with temperature and relative humidity, and a relation between MC change and relative deflection change. Beam deflections were traced for 2.5 years, while the predictions were based on experimental data from loading to the 11th day of the test. It was assumed that sensitivity of deflection change to MC should differ during desorption or adsorption. Although annual cyclic changes were observed in E _f, there was no obvious relationship between E _f and beam deflection. Part of this article was presented at the Annual Meeting of the Architectural Institute of Japan, Kyushu, September 1998     

The critical stress in various stress levels of bending member on fire exposure for mechanical graded lumber

This study focused on the relationship between stress level and the fire resistance of structural lumber. The 210 samples were prepared from 15-year-old trees of Acacia mangium from the Forest Estate Plantation, Indonesia. Specimens were 20 ® × 20 (T) × 500mm (L) and were air-dried. Sixty samples were tested under four-point bending stress to obtain the modulus of elasticity (MOE) and the modulus of rupture (MOR) for classifying the stress grades of the remaining 150 samples. The tests were performed using a four-point load bending position at various stress level while the specimens were exposed to fire along the shear-free region. Time to failure was affected by the stress levels in an exponential trend. Despite changes in stress level, charring rate, and time to failure, the critical stress of a member from the same species was similar. The fire performance under applied load could be predicted by using simplified fire-testing methods. The developed testing apparatus is valid for evaluating the small-scale fire resistant behavior of structural lumber in bending.     

Variation of stress wave velocity with MC and temperature

 The effect of moisture content (MC) and temperature on the stress wave velocity and signal frequency spectrum through sapwood has been investigated. It was discovered that in 2.5 m long green boards only low frequencies were present in the transmitted signal, while for boards less than 500 mm long, the much higher resonance frequency of the transducer was dominant. For green boards between 0.5 and 2.5 m both low and high frequency components were present. The frequency spectrum was monitored for a 540 mm long board over a range of moisture contents and temperatures. When the MC was below 30% the transmitted signal waveform consisted almost entirely of the transducer resonance frequency, while at higher moisture contents, low frequency components predominated. The frequency spectrum of the transmitted signal was little affected by temperature, but it was affected by the type of transducers used. The effect of temperature and moisture content on stress wave velocity was studied and is displayed in the form of a three dimensional graph. Received 3 May 1999     

Shear fracture characterization of green-glued polyurethane wood adhesive bonds at various moisture and gluing conditions

Abstract

The shear fracture properties of green-glued one-component polyurethane (PUR) wood adhesive bonds subjected to kiln drying were investigated. The local shear strength and fracture energy of the wood adhesive bonds were determined from experimentally recorded complete shear stress versus deformation curves of the bond line. A stable test set-up and small specimens that were anti-symmetrically loaded were used in order to get a uniform and pure state of shear stress. Different moisture contents (MCs) and pressing times were investigated. The fracture properties of conventionally dry-glued wood adhesive bonds and of solid wood were used as reference. The results show that the fracture energy of green-glued bonds with PUR adhesive is dependent on the MC of wood and on the pressing time. The same fracture energy and strength can be obtained by green gluing as by dry gluing, but there seems to exist a maximum MC of sapwood, in the range between 78% and 160%, and a minimum pressing time, in the range between 3 h and 48 h, for which it can be achieved. Both dry- and green-glued polyurethane adhesive bonds were more ductile than solid wood.     

山毛榉热处理材的介电式含水率仪读数校正实验

采用介电式含水率仪对山毛榉常规干燥材及热处理材的含水率进行了测量,并与烘干法测量结果进行了对比分析,采用最小二乘法,分别拟合出了180℃、195℃和210℃山毛榉热处理材的校正曲线。180℃、195℃和210℃山毛榉热处理材的介电式含水率仪校正曲线分别为, y＝1.2923 x＋0.2783, y＝1.3920 x＋0.7473, y＝1.6445 x＋1.4892。结果表明,常规干燥材的电测法结果能很好地反映其实际含水率水平,而热处理材的电测法结果则普遍大于烘干法,3种热处理材均呈现出随含水率的增高,差值随之增大的趋势,且随着处理温度的升高,差值愈大。