Experimental indication of interaction between viscoelastic and mechano-sorptive creep

Summary This paper describes the study of possible interactions between the viscoelastic and mechano-sorptive creep inPinus sylvestris. When two periods of constant-moisture creep were separated by a period of mechano-sorptive creep, the expected creep rate during the second viscoelastic period was decreased. A similar effect was observed during creep recovery. These results were taken to indicate an interaction between the two types of creep, although not necessarily a common mechanism. During recovery with uniform moisture cycling, the effects of one extra low and one extra high humidity were observed. The low humidity showed no significant effect, but the high humidity significantly increased the recovery. This was attributed to a parallel effect to that of creep: namely, the creep acceleration that occurs at a higher moisture level than has previously been reached during the test.     

Fibre morphological effects on mechano-sorptive creep

The increased creep rate of paper under load during moisture cycling conditions as compared to that at high constant humidity is a problem in the use of packaging materials. In order to investigate the influence of morphological factors of the fibres on the occurrence and magnitude of this phenomenon, i.e. the occurrence of mechano-sorptive creep, studies on wood fibres isolated from different parts of spruce wood were performed. Thus, creep properties were studied on earlywood and latewood fibres from both juvenile wood and mature wood. In general, latewood fibres showed a higher degree of mechano-sorptive creep than earlywood fibres, and mature wood showed a higher degree of mechano-sorptive creep than juvenile fibres. The difference in mechano-sorptive creep rate between different fibres was shown to be correlated to the differences in fibril angle. The smaller the fibril angle the higher was the mechano-sorptive creep ratio. It was suggested that at fibril angles approaching 45° wood fibres do not exhibit mechano-sorptive creep.     

Mechano-sorptive creep in pulp fibres and paper

The factors affecting mechano-sorptive creep of wood and paper have been investigated for a long time. It has also been argued that single wood fibres do not exhibit mechano-sorptive creep and that the reasons for the accelerated creep under moisture cycling conditions instead are related to the bonds between the fibres. In order to examine the relevance of this argument, measurements on single pulp fibres of different composition were performed in tension, and the mechano-sorptive creep was compared to that of papers made from the same source of pulp fibres. All fibres tested were found to exhibit an increased creep rate during moisture cycling as compared to constant humidity conditions. Thus, pulp fibres show mechano-sorptive creep and in this sense behave similar to solid wood or paper products made thereof. A linear relation between the creep strain rate during cyclic humidity and the creep strain rate at a constant humidity was also noted for both fibres and paper. This relation was not affected by changes in hemicelluloses content or composition, neither for fibres nor for papers made of these fibres. However, in all cases, papers showed a much higher mechano-sorptive creep than the corresponding fibres they were made of.     

Modelling of bending creep of low- and high-temperature-dried spruce timber

In the current project, a finite element model is developed to analyse the long-term behaviour of timber beams. The time-dependent response of wood subjected to bending and moisture changes is investigated in terms of strains and stresses. A rheological model is implemented to capture the effects of creep, mechano-sorption and hygroexpansion. The model is validated against test results from Bengtsson and Kliger (Holzforschung 57:95–100, 2003). The results of the analysis showed that the mechano-sorptive creep of low- (LT) and high-temperature-dried (HT) timber beams can be sufficiently modelled with a spring and a single Kelvin body. The different mechano-sorptive behaviour of LT- and HT-dried specimens is considered with different mechano-sorptive and shrinkage–swelling parameters. The presented model could be used to derive general mechano-sorptive parameters: (1) for better prediction of creep over the service life and (2) to provide a basis of time-dependent probabilistic calculations for structural-sized timber in serviceability limit state.     

Transient moisture effects on wood creep

To gain insight into the physical nature of the coupling between mechanical stress and humidity variations, the behaviour of thin wood strips was studied using specially developed apparatus for creep/recovery and relaxation/blotting-out tests in a controlled humidity environment. The load time and the rate of viscoelastic creep were found to have little influence on mechano-sorptive creep. Moreover, creep trajectory curves for specimens with continuous and interrupted humidity cycles indicated divergence from simple creep-limit behaviour. The effect of transient moisture was also modelled numerically at the molecular level using an idealized cellulose-based composite. Preliminary results suggest that: (i) during free shrinkage, the cellulose chains in elementary fibrils may bend perpendicular to the planes of the hydrogen bonded sheets which form the crystalline lattice; (ii) transient hydrogen bonding between the crystalline cellulose and amorphous polymer owing to the introduction or removal of water may accelerate shear slip between the two phases in the presence of an external load. Received 6 July 2000 The financial support of the Swiss Federal Office for Education and Science is gratefully acknowledged.     

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.     

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.     

Mechano-sorptive experiments perpendicular to grain under tensile and compressive loads

The present paper is part of a study concerning the long-term deformations and stresses in wood perpendicular to grain when subjected to variations of humidity. Experiments under sustained loading both in tension and compression are described. The effect of different humidity cycles, slow and fast cycles in high and low humidity ranges, on the deformations is studied. It can be observed from the test results, that the mechano-sorptive strains are highly significant, about five times the elastic deformation, although the load level is low and only a single humidity cycle is induced. The deformations are slightly higher in compression than in tension. The moisture range, where the humidity cycle occurred, did not have any major influence. The deformation was mostly dependant on the magnitude of the moisture change. The speed of the humidity cycle did not show any effect on the induced deformations over most of the humidity range, at the high humidity end some rate effects could be noticed. Received 24 November 1997     

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.     

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.     

Use of Leicester's “rheological model for mechano-sorptive deflections of beams”

Summary The model for mechano-sorptive deformation proposed by Leicester is used to predict stress relaxation of wood drying under constraint from apparent creep of drying wood under constant load. The predictions are compared with measurements on four pairs of beams, one beam in each pair being tested under constant load, the other at constant deflection. Agreement was sufficiently close to demonstrate the value of the model.     

Longitudinal moisture-shrinkage coefficients of softwood at the mechano-sorptive creep limit

Summary An adaptor for the conversion of a high-accuracy tensile creep machine to compression loading is described. It was found that a stable mechano-sorptive creep limit could be obtained by a suitable load reduction after moisture cycling; after which further creep and creep recovery just balanced each other. In this stable state the value of the longitudinal moisture-swelling coefficient depended on the strain; being less with tensile strain and more with compressive strain then in the unloaded condition. These differences in the swelling coefficient could explain the apparent recovery during subsequent sorptions in mechanosorptive creep in bending. Such a hypothesis was strongly supported by numerical comparisons of strains in bending, tension and compression.Part of the equipment used in this project was purchased with a grant from the U.K. Science and Engineering Research Council     

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。     

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.     

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     

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     

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     

Non-linear creep caused by slip plane formation

Summary The minute compression failures of the cell walls known as slip planes are quantified with respect to their occurrence in the compression side of beams subjected to mechano-sorptive creep. It is shown that slip planes occur at values of stress/strain lower than previously assumed. The slip plane intensity proves to be linearly related to creep and non-linearly related to stress. A permanent increase, S, of longitudinal shrinkage /swelling results from slip plane formation, and this parameter therefore serves as a substitute for slip plane intensity, which is not easily measured. Slip plane intensity is considered a measure of accumulated damage, and the substitute parameter S is used to quantify the accumulated damage of structural timber beams. The load level experienced by structural timber beams subjected to mechano-sorptive creep is shown to be reflected in the accumulated damage as assessed by S.The project is a part of a USDA funded project Initiation and Propagation of Failure in Wood. Support is also provided by the Danish Technical Research Council. The N.C. Brown Centre for Ultrastructure Studies, CESF, SUNY, Syracuse provided research facilities. The support is gratefully acknowledged     

Mechano-sorptive effects on timber creep

Summary Fick's law of predicting the moisture content of beams, combined with a simple mechano-sorptive model is applied to the analysis of creep resulting from moisture variations due to changing environmental conditions. The natural climatic conditions are modelled with the daily and annual cyclic variations represented by sine waves. As the moisture change responding to varying climatic conditions is always greater near the boundaries of a timber section, the creep rate close to the surface layer is higher than that in the middle of the cross-section. Therefore with time, an increased portion of the load will be carried by the inner part of the material.