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1.
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. 相似文献
2.
Fibre morphological effects on mechano-sorptive creep 总被引:2,自引:2,他引:0
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. 相似文献
3.
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. 相似文献
4.
J. M. Dinwoodie B. H. Paxton P. W. Bonfield J. S. Mundy 《Wood Science and Technology》1995,29(1):64-76
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. 相似文献
5.
This paper describes the deformations caused by stress and humidity interaction, mechano-sorption, in the cross grain directions of wood and the relaxation or accumulation of internal stresses caused by these deformations. Long-term tests on small clear specimens in cyclic climates with both tensile and compressive loads were carried out. The development of internal stresses in timber was measured indirectly at different times during the adsorption and desorption processes. Released deformations were measured from cross-sections after cutting them to small slices. These deformations were used to estimate the internal stresses caused by the humidity variations. Tests with constant loads and multiple humidity cycles show a mechano-sorptive strain that is ten times higher than the elastic strain. It is shown that existing models for describing mechano-sorption perpendicular to grain are inaccurate when applied to multiple humidity cycles. The present results demonstrate that if the mechano-sorptive behaviour and the moisture gradients in wood can be accurately described, it is possible to predict the stress distribution in a timber cross-section by knowing the climate history. 相似文献
6.
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8.
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. 相似文献
9.
Experimental indication of interaction between viscoelastic and mechano-sorptive creep 总被引:1,自引:1,他引:0
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. 相似文献
10.
P. U. A. Grossman 《Wood Science and Technology》1971,5(3):232-235
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. 相似文献
11.
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. 相似文献
12.
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. 相似文献
13.
14.
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. 相似文献
15.
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 相似文献
16.
J. M. Dinwoodie Jo-Anne Higgins B. H. Paxton D. J. Robson 《Wood Science and Technology》1992,26(6):429-448
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 相似文献
17.
ABSTRACT Acetylated wood is now commercially available and designed to be used in certain outdoor applications as an alternative to preservative-treated wood. Fastener corrosion can be a concern in preservative treated wood when the wood remains wet for long periods. However, little data on the corrosiveness of acetylated wood exists beyond the product literature. Here we examine the corrosiveness of commercially obtained acetylated wood and compare it against unmodified (untreated) southern pine (Pinus spp.). Corrosion rates of plain carbon steel, hot dip galvanized steel, and stainless steel were calculated gravimetrically after a one year exposed in the wood. Four different moisture conditions were examined: 90% relative humidity (RH), 95% RH, 100% RH, and a fully water saturated condition. When compared to literature data on the corrosion of fasteners in preservative treated wood at 100% RH, the acetylated wood had much lower steel corrosion rates than all preservatives examined; the measured corrosion rates for galvanized steel were lower than all preservatives except chromated copper arsenate. These measured corrosion rates across a range of moisture conditions can be used to inform the selection of appropriate corrosion resistant fasteners when building with acetylated wood. 相似文献
18.
J. Pritchard M. P. Ansell R. J. H. Thompson P. W. Bonfield 《Wood Science and Technology》2001,35(5):405-423
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. 相似文献
19.
The effects of temperature on mechano-sorptive (MS) creep of delignified hinoki wood (Chamaecyparis obtusa Endl.) were investigated using longitudinal (L) and radial (R) specimens during adsorption and desorption over the temperature
range of 20°–80°C. The results were compared with those of stepwise delignified specimens tested at a constant temperature
of 20°C. It was found that the effects of temperature on the MS creep of delignified specimens are more remarkable than for
untreated specimens. The tendencies of increasing MS creep with temperature, delignification, and their combination were observed.
The increase in MS creep for L specimens was relatively small and almost equal in both adsorption and desorption processes,
while for R specimens the MS creep was small in desorption, but significantly different in adsorption. In addition, good correlation
was observed between the MS coefficient (K) and instantaneous compliance (J
0). The increase in MS creep occurs as a result of temperature increase or decrease in lignin content, or their interacting
effects. However, in the case of desorption for R specimens, the increase of MS creep was unexpectedly small due to a remarkably
increased J
0.
Part of this report was presented at the 15th Annual Meeting of the Chubu Branch of the Japan Wood Research Society in Fukui,
October 2005 相似文献
20.
M. Z. Fan J. M. Dinwoodie P. W. Bonfield M. C. Breese 《Wood Science and Technology》2002,36(2):125-143
This paper records work carried out to determine the behaviour of cement bonded particleboard (CBPB) under constant relative
humidity (RH) and single change in RH. The movement was analysed with respect to the structure of CBPB and the nature of the
cement paste and wood chips. Exposure of CBPB to a constant RH resulted in considerable increase in mass and decrease in dimensions.
The ratio for change in mass:length:thickness=47:1:5. The change, in both mass and dimensions with time, under a single change
in RH exposure consisted of 2 distinct stages with a point of “inflection” between them: an initial stage in which change
in mass parallels change in dimensions, and a later stage in which this relationship is inverted. Such a relationship indicates
the existence of two different controlling mechanisms, respectively, a general moisture reaction, and a carbonation or condensation
reaction. The relationships between mass and dimensional changes were linear under constant RH and on desorption, but the
nature changed at the “inflection points” under a single change in RH.
Models were developed to predict the changes in both mass and dimensions under both a constant and a single change in RH.
Both mass and dimensional changes with changes in RH were essentially both Fickian and non-Fickian. Fractional changes in
dimensions were very similar for all changes in RH, whilst those for mass were significantly influenced by the level of RH.
Received 21 June 1999 相似文献