Mechano-sorptive Strain Research Articles

The mechanosorptive effect in Pinus radiata D. Don.

Abstract Measurements are reported of the mechanosorptive strain in Pinus radiata specimens stressed either in torsion or in bending. It is demonstrated that, to secure valid data, correction must be made for the moisture-induced distortion at zero load. A series of measurements can be made on a single specimen if two successive mechano-sorptive loading cycles are used and the sense of the stress is reversed for the second cycle. At the end of this procedure the specimen has reverted to its original dimensions. The mechanosorptive strain is shown to vary linearly with the applied stress; the ratio of the mechanosorptive strain to the initial elastic strain is therefore an appropriate way of quantifying the mechanosorptive effect. Analysis of torsion and bending data reveals that there is a strong correlation between the magnitude of the mechanosorptive strain and the shear stress component of the applied stress along the cellulose microfibril direction. It is suggested that the mechanosorptive effect arises from the effect of stress on the distribution of hydrogen bonds in hemicelluloses. A detailed model must await more information about the molecular structure of hemicelluloses in the cell walls.

Mechanical response of wood perpendicular to grain when subjected to changes of humidity

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.

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.

Deformation properties of Finnish spruce and pine wood in tangential and radial directions in association to high temperature drying¶Part IV. Modelling

) and Scots pine (Pinus sylvestris) wood under conditions relevant in the high temperature drying process. The programme included tension experiments on perpendicular specimens at temperatures 95 °C–125 °C. Based on the experiments, a constitutive model to describe the behaviour was developed for use in numerical simulation of drying stresses and is reported in this paper. The model takes into account all the needed strain terms: hygroexpansion (shrinkage, swelling), hygrothermal, elastic, viscoelastic and mechano-sorptive strains. Elastic and viscoelastic strains are modelled jointly as generalised Kelvin material, using the time–temperature–moisture-content superposition principle. Mechano-sorptive creep is modelled with a series of modified `mechano-sorptive Kelvin elements', in which a part of the strain is irrecoverable unless the direction of the acting stress changes.

DETERMINATION OF CROSS-GRAIN PROPERTIES OF CLEARWOOD SAMPLES UNDER KILN-DRYING CONDITIONS AT TEMPERATURE UP TO 140° C

ABSTRACT Small specimens of Pinus radiata have been tested to determine the creep strain that occurs during the kiln drying of boards. The samples have been tested over a range of temperatures from 20°C to 140°C. The samples, measuring 150 × 50 × 5 mm, were conditioned at various relative humidities in a pilot-plant kiln, in which the experiments at constant moisture content (MC) in the range of 5-20% MC were undertaken to eliminate mechano-sorptive strains. To determine the creep strain, the samples were brought to their equilibrium moisture content (EMC), then mechanically loaded under tension in the direction perpendicular to the grain. The strain was measured using small linear position sensors (LPS) which detect any elongation or shrinkage in the sample. The instantaneous compliance was measured within 60 sec of the application of the load (stress). The subsequent creep was monitored by the continued logging of strain data from the LPS units. The results of these experiments are consistent with previous studies of Wu and Milota (1995) on Douglas-fir ( Pseudotsuga menziesii ). An increase in temperature or moisture content causes a rise in the creep straw while the sample is under tension. Values for the instantaneous compliance range from 1.7 × 10−3 to 1.28 × 10−7 MPa−1 at temperatures between 20°C and 140°C and moisture content in the range of 5-20%. The rates of change of the creep strains are in the Order of magnitude 10−7to10−8s−1 for these temperatures and moisture contents. The experimental data have been fitted to the constitutive equations of Wu and Miloia (1996) for Douglas-fir to give material parameters for the instantaneous and Creep strain components for Pinus radiata.

Acoustic Emission Generated upon Mechano-Sorptive Creep of Wood Bent Across to the Grain under Asymmetrical Moistening

Summary Acoustic emission (AE) generated in pine wood samples upon bending across the grain under simultaneous moistening of the tension zone, was measured. The aim of the study was to determine the effect of external tension load on AE generated during asymmetrical moistening of dry wood samples in water. The cumulative counts of AE signals recorded during simultaneous moistening of the tension zone of sample were proportional to the intensity of the mechano-sorptive strain. AE starts when the mechano-sorptive strain in the tension zone (εT) reaches a value close to 0.8%.

THE MECHANO-SORPT1VE BEHAVIOR OF POPLAR DURING HIGH-TEMPERATURE DRYING

In this study the mechano-sorptive creep perpendicular to the grain was demonstrated by an experimental technique based on the analysis of strain of 30mm thick fast-growing poplar lumber. Four levels of drying temperatures (85 °C, 105 °C, 115 °C and 125 °C) were used to dry the wood. The overall board shrinkage and the immediately released and set strains were measured as a function of lime. The total strain that occurs in lumber drying is the sum of four components: elastic strain, shrinkage strain, visco-elastic creep strain and mechano-sorptive creep strain. The elastic and visco-elastic components of creep are small and produced immediately after the creation of stress. Mechano-sorptive creep is the major strain that causes caseharderning during drying.

The drying stress and check development on high-temperature kiln seasoning of sapwoodPinus radiata boards

In the first part of this paper, a model of moisture movement and strain development for a sapwood board being dried under high-temperature conditions was developed. In this second part of the set, the stress development inside the board is examined. It is postulated that the stresses of the board must satisfy both the force equilibrium and geometric compatibility condition over the cross-section. Thus, as the evaporative front recedes into the board, significant tensile stresses will be successively induced at these locations, which are subject to the relaxation of the mechano-sorptive and creep strains. During the first drying period, it is found that the mechano-sorptive strain will have a principal influence on the magnitude of stress, whereas the creep strain can cause the large tensile stress falling significantly just after it has reached the peak value. After that, the relief effects of mechano-sorptive and creep strains tend to become much weaker. Because of the enhanced creep and softened material behaviour, wood subject to high temperatures may not necessarily lead to excessive degrade. It is demonstrated that the present model is capable of describing all the important drying phenomena observed in practice, including case-hardening (plastic set) and stress reversal. By accounting for the relief effect of mechano-sorptive and creep strains, a smaller board total shrinkage is obtained than that of a pure elasto-plastic model ignoring these stress-induced strains.

The simulation of stresses and strains in the drying of Pinus radiata sapwood: the effects of board geometry

One- and two-dimensional structural analyses have been combined with a drying model to predict the stress levels in sapwood boards of softwood timber as a function of time and position within the boards during the high-temperature drying of this timber. As examples, stress levels in boards with a thickness of 50 mm and widths of 50 mm, 100 mm and 200 mm, respectively, have been estimated during a typical high-temperature drying schedule for Pinus radiata timber, considering only shrinkage and instantaneous elastic strains. The two-dimensional analysis suggests that the stress patterns only become significantly two dimensional within 25 mm of the edges of the timber. For the purposes of predicting the maximum stress levels in boards of timber, the one-dimensional analysis yields predictions of stress levels which are not significantly different from those of the more complex two-dimensional model for non-square boards. The one-dimensional approach is therefore useful for predicting crack formation inside timber boards. The two-dimensional approach has applications when it is necessary to predict collapse in a board or when the grain angle varies significantly across the board. The over-prediction of the stress levels inside the timber compared with typical measured rupture stresses (up to 4 MPa predicted, 3 MPa measured) is consistent with the neglect of the visco-elastic and mechano-sorptive elements of strain in the model, both of which would be expected to decrease the predicted stress levels inside the timber. The inclusion of visco-elastic and mechano-sorptive strains would decrease the predicted stresses for both one- and two-dimensional models by the same proportion, so the comparison between the two models would not be greatly affected.

EFFECT OF CREEP AND MECHANO-SORPTIVE EFFECT ON STRESS DEVELOPMENT DURING DRYING

ABSTRACT Constitutive equations to quantify wood deformation under combined mechanical loading and moisture content change (1] were coupled with the moisture distribution developed during drying to predict stress and strain in 50 by 190-mm Douglas-fir heartwood lumber. Two combinations of temperature and relative humidity were used to dry the wood. The overall board shrinkage and the immediately released and set strains were measured as a function of time. Those strains were compared with analytic results, which showed good agreement. The roles that four strain components played in the development of stress-both at board surface and center were compared for different drying conditions. The significance of creep and mechano-sorptive strain in relieving the stress was demonstrated by varying the model parameters.