三种典型竹质工程材料纵向弹性模量评价

以重组竹、竹束单板层积材（BLVL）、竹集成材为代表的竹质工程材料受到越来越多的关注和应用。文中采用超声波法、自由横向振动法和力学法分别对上述3种典型竹质工程材料的纵向动态弹性模量（MOE）进行评价比较,结果表明,横向自由振动法能较为快速、准确和无损地评价出竹质工程材料的MOE,MOE的变异系数与其自身铺装结构有关;重组竹、BLVL和竹集成材的一阶共振频率分别为455.73、380.41和487.62 Hz;超声波在竹集成材的纵向上传播速度最快,重组竹其次,BLVL最慢;三点弯曲力学测试发现3种竹质工程材料的断裂模式不同,全顺向的重组竹为竹纤维拉断和界面剪切破坏,纵横组坯的BLVL为横向竹束拉断以及竹/木复合界面分层,而更多体现实竹性能的竹集成材为底层竹材维管束拉断和拔出破坏,其断裂载荷为重组竹> BLVL >竹集成材,而断裂位移为竹集成材> BLVL >重组竹。     

竹质双拼梁构件研发及其在装配式建筑中的应用

以轻质高强的竹质双拼梁结构构件为研究主线,以整张化的竹束单板增强人工林木材为突破点,开发新型竹束单板层积复合材料（BLVL）及双拼梁构件并将其应用于装配式建筑。文中从BLVL材料的层积结构优化与开发、物理力学性能表征到竹质双拼梁构件的结构设计与承载力计算、加工组装工艺和不同类型双拼梁的抗弯特性研究等方面,系统阐述了竹质双拼梁在材料和结构上的特点及优势;在此基础上,以模块化连接组装技术制造的竹质生态停车棚为典型案例,论述了竹质双拼梁构件在装配式建筑中的多场景应用情况;最后,针对研发与推广过程中亟待解决的关键技术,指出竹质双拼梁构件及其在装配式建筑中应用的重点研究方向。     

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。     

Influence of stress level on compression deformation of wood in 170°C transient steam conditions

Abstract

Compression creep experiments of hybrid poplar (Populus deltoides×Populus trichocarpa) were performed in a pressurized vessel equipped with a heated hydraulic press. The viscoelastic response at various stress levels (2–7 MPa), a temperature of 170°C and transient steam conditions was studied. Moisture content and oven-dry density of compressed specimens were determined. While some recovery of compression strain occurred, compression resulted in permanent deformation and increased wood density. The influence of stress level on the amount of set recovery of compressive deformation was evaluated after 24 h water soaking. Applied stress level had a significant effect on the compression deformation. The initial strain, as well as creep strain, varied depending on the applied stress level. The highest oven-dry density was obtained at a stress level of 6.9 MPa. Lower stress levels resulted in lower moisture content after the compression process, while the equilibrium moisture content of compressed specimens was not significantly affected by stress level. Set recovery increased from 20% to 65% with increased stress level from 1.7 MPa to 4.1 MPa, then decreased to 53% for specimens compressed at 6.9 MPa. Moisture content after the compression process significantly affected the set recovery.     

Creep in chipboard

Summary Previous linear and curvilinear regression models for predicting the creep deflection of timber and timber products have failed to provide an adequately good fit. However, this paper shows that the 4-element (and to a lesser degree the 3-element) rheological model provides an extremely good fit to chipboard creep data.A set of experiments has been carried out on the creep behaviour of five commercially-available types of chipboard under 3-point sustained loading at constant temperature and humidity. This range of board types encompassed three types of glue — UF, MF/UF and Pf- and was loaded at two stress levels-30% and 60% of the short term ultimate stress. The lifetime of these specimens ranged from 25 days to over 31/2 years until either failure occurred or the load was removed.Creep curves based on 3- and 4-element rheological models have been fitted to the data from each specimen using an iterative least squares computer program which we developed. The validity of the two models is discussed, together with studies on the comparative behaviour of different board types and the use of the models as predictive tools.     

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.     

基于纳米压痕技术的木材胶合界面力学行为

【目的】研究木材胶合界面的静态和动态力学行为,探讨树脂渗透对木材管胞壁层力学性能的影响,为木质复合材料制造工艺优化和增强改性提供理论依据。【方法】采用纳米压痕静态和动态力学测试技术(Nano-DMA),对针叶材火炬松与酚醛树脂(PF)、脲醛树脂(UF)胶黏剂所形成胶合界面区域各相材料的静态弹性模量、硬度、蠕变性能以及储能模量和损耗模量等力学行为进行分析。【结果】静态力学行为方面,在界面区域,PF和UF渗透进入管胞壁层后,木材管胞壁的弹性模量( E r)和硬度( H )提高;经PF渗透后,木材管胞壁的 E r和 H 分别增加7%和26%;Burgers蠕变力学模型可有效描述胶合界面区域管胞壁的纳米压痕蠕变特性,经树脂渗透后,木材管胞壁的瞬时弹性模量增加,黏弹性模量和黏性系数减小;在保载初期,PF界面区域木材管胞壁的蠕变柔量约下降60%,UF界面区域木材管胞壁的蠕变柔量约下降58%。动态力学行为方面,随着加载频率增加,界面材料的储能模量( E ′ r)逐渐增大,而损耗模量( E ″ r)和损耗因子(tan δ)呈减小趋势;当加载频率为10 Hz时,PF和UF树脂渗透使得管胞壁层的储能模量分别增加16%和29%。【结论】胶合界面区域胶黏剂进入管胞壁层,对木材管胞的静态力学性能具有增强作用,同时胶黏剂可提高管胞壁的短期抗蠕变能力;木材管胞壁具有较高的储能模量和损耗模量,而树脂的储能模量和损耗模量较低,经树脂渗透后,木材管胞壁的储能模量增加,但损耗模量和损耗因子呈下降趋势,可能对界面传递和分散应力产生不利影响。     

Modelling the creep behaviour of chipboard: The rheological approach

Summary The process of applying the rheological approach to describe creep deformation has been investigated using the 5-parameter model developed by Pierce and Dinwoodie (1977).A method utilising exponential curve fitting techniques was examined by attempting to produce fits to the first 6 months' data from long-term experiments (12 years). The procedure included a means of assessing the variability occurring in the derived model parameters which showed that acceptable fits were obtainable for only 3 out of the 20 data sets available. Further analysis of one of these data sets (that for cement-bonded particleboard, subjected to a stress level of 30% of the short-term ultimate in an environment of 25 °C and 90% relative humidity) revealed that the model attributed most of the creep deformation to the viscous component; resulting in an over-estimate of the deformation at 12 years when the 6-month fit was projected.An alternative approach to deriving the value of the model parameters is proposed, based on the theory underlying the model. The applicability of this approach was investigated using the above mentioned data set. It was found that the model can be constrained to give a more realistic ratio between the time-dependent components of deformation (ie. viscoelastic and viscous creep) for the material considered, thereby improving the accuracy of the projected creep deformation over the 12-year period.     

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.     

国内竹材人造板构成及生产和市场现状

由于竹材具有密度适中、纹理美观、强度高、硬度大和耐磨性好等优点,因此竹材人造板被广泛的应用于交通、建筑、造船、以及家具和装饰材料领域,在国内的一些领域已逐步的取代了木材、钢铁和橡胶等。本文简要介绍了竹材人造板的构成单元:竹片、竹篾和竹刨花,同时指出竹材人造板设计应遵循的构成原则和方法,即对称原则、表面构成方式、以及等应力原则。依据构成单元的形成和处理方法,作者把国内竹材人造板划分为13种类型,同时详细介绍了竹材人造板的制造工艺及其产品的利用,如竹材胶合板、竹材地板及竹木复合产品。文章最后介绍了各种竹材人造板的生产、市场现状及销售状况。图7参21。     

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.     

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.     

The modelling of time-dependant deformation in wood using chemical kinetics

Summary The development of rheological models to predict creep has led to the derivation of quite complex equations that can predict creep reasonably accurately. However, these models are conceptual and are not based on a fundamental understanding of the actual deformation processes occurring within the material. The concept of modelling creep using a chemical kinetic approach is one that attempts to understand creep in wood at a molecular level and, from this, to develop models that more accurately predict creep deflections.This paper presents two models developed from chemical kinetic theory, that describe the time-dependant deformation of wood. The validity of applying these models to experimental data has been assessed by stress relaxation tests on thin samples of Sequoia sempervirens. Two stages of experimentation were carried out. In stage 1, both models were applied to the results of stress relaxation tests on 6 samples. Similar values of activation energy and activation volume were calculated by both models and a single energy barrier was found to dominate the deformation process.In stage 2, the effect of varying the initial applied stress on activation energy and activation volume was assessed by carrying out stress relaxation tests at stress levels of 25%, 30% and 35% of the short-term strength. Values of activation energy and activation were found to increase as the applied stress level decreased.Both models describe the time-dependent behaviour of wood well, however their ability to predict long-term creep deflections may be limited. Future work will develop these models further in order to improve long-term creep prediction and then apply them to the results of both creep and stress relaxation tests at a variety of stress levels and moisture contents in order to test their validity.     

Creep in chipboard

Summary Matched samples to those used in a previously reported investigation (under constant environments employing eight board materials and solid wood) were subjected to five different levels of stress under four different cyclic environments for periods of six months. Relative creep increased slightly and linearly with respect to stress within the range of stress levels adopted, as well as increasing with severity of the environment. All materials showed greater sensitivity to alternating humidity than to alternating temperature. The effects of the three prinicipal variables (stress level, environment, material), and the strong interactions among them, were quantified using analysis of variance and found to be similar to those under constant environments: the effect of environmental condition was found to be far greater on relative creep than that of either stress level or material. Averaging over all conditions and stress levels, relative creep increased progressively among timber, plywood, waferboard, chipboard and fibreboard, though this ranking order varied at different stress levels and environments due to the presence of strong interactions. At a stress level of 30% and a time of 43,200 minutes, the relative creep under alternating humidity of 3090% was on average fourteen times greater than that under a constant humidity of 65% (averaging across the nine materials); however, there was considerable divergence among the materials in this ratio. At the same level of stressing, the mean relative creep for all materials under an alternating temperature of 1030 °C was 1.6 times higher than at a constant temperature of 20°C. The ranking order of materials in their sensitivity to alternating temperature was different to that under alternating humidity.The statistical analyses in this paper were performed under contract to Building Research Establishment by Laura New of the Statistical Services Centre, Reading University     

Time dependence of Poisson’s effect in wood I: the lateral strain behavior

To understand the viscoelasticity of wood three dimensionally, a longitudinal tensile creep test for 12 species was conducted to examine the changes with time in the lateral strain and the viscoelastic, i.e., apparent Poisson’s ratio. The changes in the lateral strain (ɛ _T and ɛ _R) were similar to those in the longitudinal strain (ɛ _L). That is, during creep, the absolute value of lateral strain continued to increase with the gradual reduction in the increase rate; immediately after the removal of the load, it recovered abruptly; then, it recovered slowly and finally reached a certain value. The rate of increase in the longitudinal strain during creep was smaller than that in the absolute value of lateral strains. The apparent Poisson’s ratio became large during creep because the lateral strain increased more than the longitudinal strain. The analysis of lateral strain by decomposition into three components, that is, instantaneous strain, delayed elastic strain, and permanent strain, has revealed that the lateral permanent strain in the transverse direction contributes most to the increase in the apparent Poisson’s ratio during creep.     

Model fitting for visco-elastic creep of Pinus radiata during kiln drying

This work examines the applicability of mathematical models for correlating the visco-elastic strains during kiln drying of radiata pine (Pinus radiata D. Don) sapwood at various temperatures and moisture contents. The eventual aim is to use a mathematical model incorporating these strains to optimise the drying schedules and minimise the degradation. Data sets from previous experiments (Keep 1998) obtained at temperatures from 20 to 140 °C for sapwood at 5, 15 and 20% moisture contents (dry basis) were analysed. The data were fitted for various theoretical models, namely the Maxwell, Kelvin and Burgers models, and the Bailey-Norton equation. The parameter values and standard errors for the above models over the range of experimental data have been determined. The results indicate that the Maxwell model did not fit the experimental data well, having only one parameter. In most cases, the Bailey-Norton equation was inadequate, as it is a power-law model with a predicted continuous increase in creep with time and does not predict a plateau in the creep strain, as has been observed for most of Keep's (1998) data. The Kelvin model gave a better fit than the Bailey-Norton equation for most of the data sets. From visual inspection of the plots for the experimental data and the model predictions with time, it was found that both the Kelvin and Burgers models fitted the data satisfactorily. However, the three-parameter Burgers model was not a significant improvement over the two-parameter Kelvin model at the 0.01 level of significance, as shown by an analysis of variance. Received 2 November 1998     

Duration-of-load and creep effects in strand-based wood composite: a creep-rupture model

In order to investigate the creep and duration-of-load (DOL) effects in thick strand-based wood composite products, a creep-rupture model is proposed linking the accumulated damage to creep deformation. Results from long-term constant load tests have been interpreted by means of this creep-rupture model, which is capable of representing the time-dependent deflection and time-to-failure data at different stress levels. The predictions of the model have been verified using results from ramp load test at different rates of loading. The creep-rupture model incorporates the short-term strength of the material, the load history and predicts the deflection history as well as the time-to-failure. As it is a probabilistic model, it allows its incorporation into a time-reliability study of wood composites’ applications.     

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。     

A global rheological model of wood cantilever as applied to wood drying

In the process of wood drying inevitable stresses are induced. This often leads to checking and undesired deformations that may greatly affect the quality of the dried product. The purpose of this study was to propose a new rheological model representation capable to predict the evolution of stresses and deformations in wood cantilever as applied to wood drying. The rheological model considers wood shrinkage, instantaneous stress–strain relationships, time induced creep, and mechano-sorptive creep. The constitutive law is based on an elasto–viscoplastic model that takes into account the moisture content gradient in wood, the effect of external load, and a threshold viscoplastic (permanent) strain which is dependent on stress level and time. The model was implemented into a numerical program that computes stresses and strains of wood cantilever under constant load for various moisture content conditions. The results indicate that linear and nonlinear creep behavior of wood cantilever under various load levels can be simulated using only one Kelvin element model in combination with a threshold-type viscoplastic element. The proposed rheological model was first developed for the identification of model parameters from cantilever creep tests, but it can be easily used to simulate drying stresses of a piece of wood subjected to no external load. It can therefore predict the stress reversal phenomenon, residual stresses and maximum stress through thickness during a typical drying process.     

室外竹质人造板耐潮湿性能研究

室外自然老化法和5种加速耐潮湿试验方法对2种竹质人造板的物理力学性能均有明显降低,试验结果表明:高温干燥和沸煮对板材性能的影响最显著,对于同一种方法处理后的试件,横向上性能降低幅度比纵向上大,静曲强度较弹性模量降低更为显著。15个月的室外自然老化法试验结果接近于6 h水煮-冰冻-干燥法的试验结果,板材性能随时间延长逐渐降低,但降低幅度逐渐变小,还需放置更长时间进行分析比较,以真正反映竹质人造板的耐潮湿性能。