Creep in chipboard

Summary Samples of five chipboards, one waferboard, one plywood, one fibre building board and Scots pine timber were loaded in four point bending at a stress equivalent to 60 per cent of the short term failing stress under five combinations of temperature and relative humidity for a period of six months, or until prior failure.Irrespective of whether creep behaviour was assessed in terms of total deflection, viscous component of deflection, relative creep, creep modulus, or deflection at, or time to failure, significant differences were obtained with variations both in temperature and relative humidity. The effect of temperature, at fixed humidity, was slightly greater between 20°C and 30°C than between 10°C and 20°C, while the effect of humidity was appreciably greater between 65% and 90% rh than between 30% and 65% rh. Fitting of hyperbolic curves to the data permitted the prediction of creep behaviour with a good degree of fit at any combination of temperature and humidity within the experimental range.Differences in response to environmental conditions existed among some of the boards. Thus the ranking order of the materials was slightly different under variable humidity than it was under variable temperature. It was possible to describe these differences in terms of the variation that occurred among the materials in the relative proportion of the elastic, viscoelastic and viscous components of deflection.     

Creep in chipboard

Summary Deflection under 4-point bending of a range of board types was measured for up to six months. Results were obtained under steady-state conditions of stress level (30%, 45%, 60%, 67.5% and 75% of the short term breaking stress), relative humidity (30%, 65% and 90% rh) and temperature (10°C, 20°C and 30°C). These results are presented in terms of relative creep and creep modulus. The relative creep for all board types increased with increasing stress level, increasing relative humidity and increasing temperature. An analysis of variance investigating variations between materials showed significant differences in relative creep. When the relative creep of all materials was compared over all conditions and all stress levels, plywood and waferboard had consistently low relative creep values. High alkaline cured PF chipboard and non-British Standard UF chipboard had consistently high relative creep values. The creep modulus of MUF bonded chipboard decreased with increasing log₁₀ (time) under all conditions. Creep modulus also decreased with increasing stress level.The statistical analysis in this paper was performed by Anne Hasted and Laura New of the Statistical Services Centre, Reading University     

Fatigue and creep in chipboard

Summary Matched samples of chipboard were loaded in four-point bending under either a 7 hours loaded/17 hours unloaded cycle, a 17 hours loaded/7 hours unloaded cycle, or constant load in order to asses the effect of cyclic loading on creep deformation. Tests were carried out at three humidity levels (30, 45 and 90% RH) and at three stress levels (30, 45, and 60% of the short term strength). The creep rate of samples under constant load was greater than under cyclic load, with the 7 hours loaded/17 hours unloaded samples giving the lowest creep rate. Analysis of the data on an accumulated time-under-load basis showed that the law of superposition did not apply to the 7 hours loaded/17 hours unloaded samples at the 30% and 45% stress levels, with these samples giving lower deflection than for the other loading conditions. The 17 hours loaded/7 hours unloaded samples also did not appear to obey the law of superposition, although the difference between them and the constantly loaded samples was slight. Considerable scatter in data accumulated at the 60% stress level did not allow any differences in the superposition analysis of cyclic and constant loaded samples to become apparent. Increasing the stress level applied to samples produced an almost linear increase in relative creep for all loading regimes. The 7 hours loaded/17 hours unloaded samples gave consistently lower relative creep values at all levels of stressing, but particularly at the 30 and 45% stress levels. Increasing the relative humidity from 30% to 65% RH had an almost negligible effect on relative creep of all samples, but increasing the humidity to 90% RH caused a marked increase in relative creep.     

Creep in chipboard

Summary Samples of five chipboards, one waferboard, one plywood, one fibre building board and redwood timber (Pinus sylvestris) were loaded in 4-point bending under four stress levels: 30, 45, 67.5 and 75% of the short term ultimate, thereby extending earlier work at 60% stress level. Five environmental conditions were used at each stress level: 10°C 62% relative humidity (rh); 20°C 30% rh; 20 °C 65% rh; 20°C 90% rh and 30°C 68% rh. Deflections were recorded at least on a daily basis for a period of 6 months, unless failure of the sample occurred earlier. Values for the elastic, viscoelastic and viscous components of creep deflection were calculated for two time periods using the 4-element rheological model developed in an earlier paper: comparisons were made on the basis of the percentage contribution of each component. The relative proportions of the elastic, viscoelastic and viscous components making up total deflection were found to vary considerably with time, temperature, relative humidity, stress level and material. The relationships were complex with strong interactions occurring among environmental condition, stress level and material. Thus, for example, the effect of high temperature, or high relative humidity, in determining the magnitude of the viscous component was much greater at higher levels of stressing: waferboard appeared to be less sensitive to high humidity and solid timber more sensitive to high temperature than the other materials. T-tests further confirmed the complexity of the relationships. It is concluded that much of the variability in creep deflection recorded in Part 8 of this series is primarily due to changes in the percentage contribution of the viscous component.     

竹材刨花板蠕变性能的研究

结构用刨花板由木质刨花和胶粘剂热压而成,属高分子材料,具有粘弹性。在负荷作用下产生弹性变形,在长期负荷下发生蠕变。试验表明,竹材制成的酚醛胶刨花板,在高湿环境下,比竹材脲醛胶刨花板有较好的抗蠕变能力;如在竹材刨花板上贴上一层竹席,则抗蠕变性能进一步提高。竹材脲醛胶刨花板比木材脲醛胶刨花板有较好的抗蠕变性能。在低湿度环境下,所有板材的蠕变性能差别较小。     

Effect of two relative humidity environments on the performance properties of MDF, OSB and chipboard.

The fatigue and creep performance of MDF, OSB and chipboard have been examined in two environments, namely 65%RH (standard environment) and 85%RH (high humidity). Parallel fatigue and creep tests have been performed in four-point bending on the three wood-based panel products in the two environments. The constant 65%RH environment is service class 1 and the constant 85% RH environment is indicative of a service class 2 environment as detailed in Eurocode 5. The non-interruptive technique of stress-strain hysteresis loop capture has been utilised to follow property changes of the fatigue samples during cyclic loading at a stress ratio of R equal to 0.1. Loop parameters such as loop area, dynamic modulus, and fatigue modulus have been used to characterise the response of these materials to fatigue loads in the two environments. Creep microstrains for the creep samples were recorded in parallel with the fatigue parameters. Fatigue and creep results at 85%RH were more variable than those reported at 65%RH for MDF, OSB and chipboard. In general, at R=0.1 and 85%RH, fatigue and creep microstrains were higher, dynamic stiffnesses were lower and hysteresis loop areas were higher than corresponding properties measured at 65%RH. MDF and chipboard were less moisture tolerant than OSB, this is reflected in the large changes in fatigue and creep parameters.     

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.     

The variable parameter rheological model of wood

How to establish the rheological model to simulate creep behavior of wood and wood-based composites under change-load has not been solved in research of wood rheology. This paper presents a new model—variable parameter rheological model. The bending creep behavior of small clear poplar specimens under different constant load levels were examined. The load levels within 50% of rupture load of the specimens, and the experimental creep behavior were simulated by the variable parameter Maxwell model. The results show that using only one model of variable parameters may simulate the creep behavior of wood under different constant load levels very well. Applying the generalized Boltzmanns superposition principle, the variable parameter rheological model can be used to simulate the creep behavior of wood under change-loads conveniently and accurately.     

Long-term deformation of MDF panels under alternating humidity conditions

Summary Creep tests were carried out on 19 mm thick commercial MDF panels, raw and overlaid with melamin resin-impregnated 80 g/m² paper.The tests were carried out under cyclic environmental conditions with relative humidity levels of 30%–90%, a steady temperature of 20 °C, 3 different stress levels, 20%, 30% and 40%, and using mid-span loading.The relative creep of the MDF panels was considerably higher than that in particleboards with similar characteristics. Creep behaviour of the raw panels seems to be strongly influenced by melamin coating of the boards' surfaces but not by the edge coating, except when both were combined.This study debates the convenience of comparing the rheological behaviour of MDF and particleboards by means of the relative creep.     

Mechano-sorptive creep in wood fibres

In order to investigate the way in which fibre properties affect the mechano-sorptive creep phenomenon in paper, single wood fibres were exposed to tensile stresses at a constant humidity of 80% relative humidity (RH) and in a cyclic humidity environment varying between 80 and 30% RH. Contrary to earlier claims, it was demonstrated that single wood fibres exposed to a cyclic RH show a considerably higher creep than that corresponding to the highest RH experienced in the cycle, i.e., a mechano-sorptive behaviour. The creep strain rate at cyclic humidity was shown to be a function of the creep rate at constant climate, and to be an apparent linear function of the applied stress.     

Effect of load type on failure mechanisms of spruce in compression parallel to grain

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.     

木材蠕变模拟研究概述

综述树种和材性变异、应力水平、温度、含水率等因素对木材蠕变的影响,介绍分析含水率稳定和变化条件下木材蠕变模拟预测等方面的研究进展情况.我国木材蠕变特别是木材的机械吸附蠕变研究尚缺乏系统性,应综合考虑不同的影响因素对木材蠕变进行充分研究,以建立不同条件下木材蠕变预测的模型,为国内木材蠕变研究及木结构建筑的安全和可靠性设计提供基础.     

Long term bending creep of wood in cyclic relative humidity

Summary Long term creep and recovery test results of wood under a bending load of 10 MPa stress and subjected to relative humidity cycling are presented. In spite of the vast number of humidity cycles during these tests, the results do not give evidence of an existence of a mechano-sorptive creep limit. The results also indicate that the recovery is not complete on unloading when the humidity is cycled. A mechano-sorptive model that fits the test results is proposed. According to the model, the elastic bending deflection in a cyclic load of 10—3 MPa and subjected to natural outdoor relative humidity can be about doubled to account for the creep under ten years loading.     

Shear creep and mechanosorptive behavior of nail-plate-jointed laminated-veneer lumber

The objective of this research was to evaluate the mechanosorptive deflection of shear creep of nail-plate-jointed laminated-veneer lumber. The joint was composed of steel gusset and nails, 40 mm in length and 2.8 mm in diameter (Zn40). The specimens were loaded parallel (PA) and perpendicular (PE) to the grain. Shearing loads applied were 50 and 100kgf, and the load levels were 11% and 22% (PA) and 7% and 14% (PE) of the maximum strength obtained from static testing. The creep test specimens were loaded for 1500h. A few general conclusions could be drawn from this study: The mechanosorptive deflection (_ms) is defined as _ms = _t — (_c + _sh) — _o, where _t, is total deflection, _c is pure creep, _sh is shrinkage-swelling behavior, and _o is the initial deflection. Changes in relative humidity may cause more severe creep deflection than constant humidity, especially during the drying process. The mechanosorptive deflection is greater at the lower load level than that at the higher load level. The mechanosorptive effects seem to be somewhat more resistant in the parallel direction than in the perpendicular direction.     

Creep and stress relaxation behavior for natural cellulose crystal of wood cell wall under uniaxial tensile stress in the fiber direction

We investigated the temporal changes in creep and stress relaxation behavior in both microscopic crystalline cellulose and macroscopic strain of wood specimen using Japanese cypress (Chamaecyparis obtusa Endl.) to understand the viscoelastic properties of wood cell walls. Specimens 600 µm in thickness were observed by the X-ray diffraction and submitted to tensile load. The crystal lattice strain of (004) plane and macroscopic strain of specimen were continuously detected during creep and stress relaxation tests. It was found that the creep compliance based on macroscopic strain showed a gradual increase after instantaneous deformation due to loading and then the parts of creep deformation remained as permanent strain after unloading. On the other hand, crystal lattice strain showed a different behavior for macroscopic strain; it kept a constant value after instantaneous deformation due to loading and then increased gradually after a certain period of time. These differences between macroscopic and microscopic levels were never found in the stress relaxation tests in this study. Relaxation modulus at the macroscopic level only showed a decreasing trend throughout the relaxation process. However crystal lattice strain kept a constant value during the macroscopic relaxation process. In addition, the microfibril angle (MFA) of wood cell wall has a role of mechanical behavior at microscopic level; crystal lattice strains were smaller with increasing MFA at both creep and relaxation processes. Creep compliance and stress relaxation modulus at the macroscopic level decreased and increased with increasing MFA, respectively. Our results on the viscoelastic behavior at microscopic level evidenced its dependency on MFA.     

木材静曲蠕变成套测试系统的研发

为解决木材变湿蠕变测试普遍存在的环境参数(温湿度和风速)控制方式和控制精度不足、蠕变变形量测试灵敏度和精度不足、数据智能采集和处理以及系统安全防护不足等问题,在集成现有技术的基础上研发出一套木材蠕变测试系统。该系统包括环境气候箱、蠕变测试机架、木材蠕变变形量及干缩湿胀测试单元、荷载同时装卸单元、数据采集和处理单元、系统保障和防护单元6个部分。该系统可在温度为0~70℃、相对湿度为10%~98%、风速为0.3~5.6 m/s时对木材或木基材料的普通蠕变和机械吸附蠕变进行长时间稳定的测试、记录,蠕变变形量及木材厚度方向的干缩湿胀检测精度为±0.01 mm,数据采样间隔在1 s~24 h范围内可调。与现有木材蠕变测试系统相比,本系统在多参数自动同步获取、数据采样精度、运行稳定性、安全性和环境风速可调性等方面有独特优势。通过前期对木材112 d循环变湿蠕变量、干缩湿胀量及环境温湿度参数的观测和测试结果分析,证实该套检测系统可对木材的蠕变挠度、干缩湿胀量、环境温湿度等参数进行长期连续稳定的检测、记录和显示,所有测试指标均能达到设计预期。同时,由于该系统具有多参数协同检测和精度较高等特性,使得实测木材变湿蠕变比采用喷蒸变湿处理试件所获蠕变测试结果在局部地方存在明显差异,如吸湿过程中的木材静曲挠度并不都是反向减小,而是取决于吸湿速率和外荷载的竞争关系,从而为木材静曲变湿蠕变机理的揭示提供了有力证据。该系统的研制为精确可控变温变湿环境下木材蠕变机理的研究提供了新的平台。     

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.     

Hygroexpansion of wood during moisture adsorption and desorption processes

In order to investigate the shrinking and swelling behavior of wood at a non-equilibrium state, the moisture sorptlon processes of wood under constant and changing conditions were studied. For the static sorption experiment, Chinese fir （Cunninghamia lanceolata） specimens were subjected to the adsorption processes at 25℃, 10 different relative humidity environments and the moisture contents were measured at distinct time intervals of adsorption processes. For the dynamic sorption experiment, the specimens were exposed to periodically and linearly varying relative humidity between 45% and 75% at 25℃. Moisture content as well as radial and tangential dimensional changes in response to the changing relative humidity were measured. The main results from the experiments indicated that： the moisture sorption isotherms of Chinese fir at equilibrium state and different stages of adsorption processes could be characterized by S-shape curves. From the non-equilibrium state to the equilibrium state, the sigmoid moisture sorption isotherms changed from smooth, gradually increasing values to a steep rise at 100% humidity. Furthermore, under dynamic conditions with a constant temperature and a linearly and periodically varying relative humidity, the moisture content as well as radial and tangential dimensional changes of the specimens generally waved but lagged behind the relative humidity change.     

竹材横纹热压流变学性质的研究

以毛竹为对象,研究了在不同应力水平及不同温度、含水率下竹材横纹压缩的蠕变特性。实验结果表明,竹材横纹压缩蠕变组分与含水率、温度、应力水平密切相关;当压应力在屈服极限以下时,竹材横纹压缩蠕变的三种组分总量均与压应力、温度和含水率成正比,但温度和含水率的升高会使弹性组分的比量下降、粘弹性和粘性组分的比量增大。     

温度对重组竹短期受压蠕变性能的影响

通过对重组竹受压试件进行短期蠕变试验,研究温度对重组竹受压试件蠕变特性及蠕变规律的影响。针对不同应力水平下温度对重组竹短期受压蠕变的影响,研究了在同一应力水平7.5%下,重组竹在5种不同温度下的24 h顺纹受压蠕变性能;进一步比较了重组竹在应力水平为7.5%,15%,30%且温度分别为25,50,75℃情况下的24 h顺纹短期受压蠕变特点。最后,采用Burgers模型对上述不同温度变量和不同应力水平变量下重组竹短期受压蠕变曲线进行拟合分析。结果表明:在重组竹顺纹受压蠕变中,温度和材料应力水平越高,瞬时弹性变形越大,重组竹蠕变应变总量越大,重组竹抵抗蠕变性能越弱,且较高温度和较高应力水平的同时作用会对重组竹构件产生不利影响;Burgers模型的拟合决定系数基本均在0.98以上,说明Burgers模型能够较准确地描述温度对重组竹短期受压蠕变曲线特性的影响。根据试验与拟合曲线特性可知,重组竹顺纹受压蠕变中弹性变形占80%以上,说明在不同温度下重组竹顺纹受压蠕变中弹性变形占主要部分,随着温度的升高,弹性变形有所下降,黏性变形逐渐增加。