Mechano-sorptive Creep Research Articles

Microprestress Theory for the Prediction of Mechanosorptive Creep in Wood

Microprestress Theory for the Prediction of Mechanosorptive Creep in Wood

Study on interaction of Viscoelastic and Mechano-sorptive creep and effect of Fibre Reinforcement on creep of Timber beams

Timber is a sustainable material widely used in structural elements. Its viscoelastic nature, in which the stress-strain relation varies with time, poses significant challenge to the durability of timber structures. Creep of timber under constant loading depends on various factors like stress level, moisture content, load-to-grain angle, temperature, relative humidity etc. Among these, relative humidity and moisture content in timber are the most crucial. Drastically varying moisture content causes mechano-sorptive creep. However, the interaction and contribution of various factors causing creep is still a matter of research. Various codes provide generalized creep coefficient for long term deflection and load reduction factors. A detailed rheological model was proposed by Toratti which addresses mechano-sorptive creep explicitly, incorporating both recoverable and irrecoverable part in creep. This paper aims to review the study on interaction between viscoelastic creep and Mechano-sorptive creep in various environmental conditions. Effect of reinforcing timber beams, on the long term deflection due to creep is also made a subject of study to identify a possible solution to the accelerated deflection by Mechano-sorptive creep, which will be unavoidable in tropical areas with extreme climactic changes. A comparative study between the various standard codes with Indian Standard Codes for timber with respect to creep, is also made, to identify the gaps in IS codes.

Study on free shrinkage and rheological characteristics of Eucalyptus urophylla ×E. grandis during conventional drying

ABSTRACT The collapse-prone nature of eucalyptus wood has implications for the measurement of drying rheological characteristics, particularly free shrinkage. The study outlined the calculation method for drying strain and the determination of drying stress direction. It also analyzed the free shrinkage properties of the wood, as well as the variations in elastic strain, viscoelastic strain, and mechano-sorptive creep strain throughout the drying process. The findings indicate the following: 1, there were no significant differences in free tangential shrinkage between the layers of the specimens. 2, the free tangential shrinkage of the layers was greater than that of the specimens. 3, the use of fresh thin wood that has not been subjected to drying stresses was recommended for accurate measurement of free shrinkage strains, as steaming does not fully reverse the plasticizing deformation caused by drying. 4, the collapse of eucalyptus wood in the early stages of drying hindered the reversal of tensile strain in elastic, viscoelastic, and mechano-sorptive behaviors.

A cyclic (intermittent) drying schedule for Macassar ebony—the prevention of collapse deformation and the utilization of mechano-sorptive creep gradient

Ebony wood is often defective, showing severe cracks and deformations during manufacturing processes, and requires ample times for drying. The continuous drying has been proved to result in enormous collapses during the entire drying stage. In order to control the extent of collapses and further prevent these defects, a low temperature pre-freezing procedure was projected and followed by a cyclic drying program. This cyclic (intermittent) drying schedule was proposed based upon a so-called “mechano-sorptive creep gradient” (MSCG) strategy and elaborated by a “corresponding temperature differentia” (CTD) concept. The MSCG and CTD variables provided the theoretical explanation and the operational instruction to the cyclic drying test, respectively. The overall evolutions of collapse and transversal shrinkage of the ebony lumber boards were quantitatively investigated during the entire period of a cyclic drying process. Furthermore, the relationships between collapse and moisture content as well as cross section orientation were determined quantitatively. Under this specified pre-freezing plus cyclic drying method, the averaged collapse deformations were significantly decreased, with the maximum collapse variables accounting for 20 ∼ 30% of those under continuous cases, fully achieving the collapse controlling objective.

Comparative investigation on viscoelastic and mechanosorptive creep behavior of two tropical hardwoods and one temperate softwood

Comparative investigation on viscoelastic and mechanosorptive creep behavior of two tropical hardwoods and one temperate softwood

Magnetic resonance imaging of water vapor absorption in simulated corrugated board

Water vapor absorption into a multi-layer simulated corrugated board structure was investigated using magnetic resonance imaging (MRI). Humidity and other ambient conditions considerably affect the strength and durability of corrugated board, and through creep, collapse of the corrugated board structures at unexpectedly low load levels. Currently, water vapor permeation and absorption are investigated with methods that do not elucidate the behavior of individual sheet layers in corrugated board. With MRI, signal intensity changes caused by water absorption can be spatially located and identified, providing a promising tool for the assessment of internal changes in a board, and its interaction with humidity. In this work, simulated corrugated board assemblies, consisting of alternating straight layers of linerboard and fluting material, were constructed, subjected to one-sided high humidity environment and periodically MR imaged to obtain temporal information on water accumulation at high relative humidity. It was observed that, in general, the fluting layers would exhibit higher signal intensities sooner, and by extension, higher water accumulation, when compared to the linerboards. The presented study provides insight into the behavior of paperboards in a double-wall corrugated board structure while under constant high humidity. In the future, actual corrugated board samples could be investigated in periodically fluctuating relative humidity and under constant stress to obtain information on the mechanosorptive creep.

Čimbenici koji utječu na proces savijanja cjelovitog drva

The effect of temperature and moisture on the behaviour of solid wood is a well-known fact that directly affects wood creep as well. Wood creep includes three types of behaviour, such as viscoelastic creep, mechano-sorptive creep, and pseudo-creep and recovery. All these types can occur simultaneously, and it is sometimes complicated for researchers to isolate or distinct one from another. This paper presents a review of literature on wood rheology and creep properties, as well as factors that influence them, mainly time, temperature, and moisture content. The study of the viscoelasticity and wood creep is very important for gaining knowledge to be applied in solid wood bending.

Accelerated relaxation behavior during water desorption in the mechano-sorptive creep of wood: modeling and analysis based on the free volume concept and Kohlausch–Williams–Watts function

Abstract The accelerated creep behavior during water desorption of wood was modeled on the Kohlausch–Williams–Watts (KWW) function and free volume concept. Assuming that the water desorption rate dmc/dln t $\text{dmc/dln\,t}$ is involved in the creation of a new free volume, the net creep compliance during desorption J du(t) was obtained as ln J du ( t ) = β c ln t − β c ( ln τ c 0 + k f dmc / d ln t ) + ln J c ∞ $\mathrm{ln} {J}_{\text{du}}\left(t\right)={\beta }_{\text{c}} \mathrm{ln} t-{\beta }_{\text{c}}\left(\mathrm{ln} {\tau }_{\text{c}0}+{k}_{\text{f}}\,\text{dmc}/\text{d}\mathrm{ln} t\right)+\mathrm{ln} {J}_{\text{c}\infty }$ where β c, τ c0 and J c∞ are the stretching parameter, characteristic time of the relaxation, and long-time asymptote compliance at a constant moisture content (mc) equal to the initial mc of the accelerated creep, respectively, and k f is the coefficient of the water desorption rate. These parameters were determined by simulation using experimental data and literature results and were found to be reasonable. The simulation results agreed well with the experimental ones. This verifies the validity of the model and its derived equations.

An improved model for the time-dependent material response of wood under mechanical loading and varying humidity conditions

Timber is one of the most widely applied engineering materials due to its favorable physical and mechanical characteristics. Nevertheless, over the service life, the combination of mechanical load and varying environmental conditions can have a critical influence on the mechanical response of timber structures, considering especially the mechano-sorptive characteristics. In this work, the performance analysis of the existing time- and moisture-dependent models identifies the need of a more comprehensive 3D constitutive rheological model, which can take into account orthotropic, elastic-plastic, viscoelastic and mechano-sorptive aspects, including the complex mechano-sorptive recovery phases that may occur over the lifetime. Therefore, a comprehensive algorithm of such a model is developed here. An improvement is made for the mechano-sorptive component by introducing an ordinary and a transcending mechano-sorptive part, which are governed by the absolute change of moisture content and the change exceeding the historical highest moisture content at each loading phase, respectively. Simulations are run under cases with mechanical loading of tension and compression perpendicular and parallel to the direction of the fibers. Both medium-term (up to 2500 h) as well as short-term (up to 350 h) mechanical behavior of the material under constant temperature is analyzed and good agreements are achieved with literature results. Comparison with existing models indicates that the proposed model gives an improved prediction of the mechano-sorptive creep and recovery, which has a non-linear dependence on the moisture history. Furthermore, sensitivity analyses are performed and the applicability of the model to different species under different combinations of load and moisture variation is studied here.

Experimental investigation on the rheological behavior of timber in longitudinal and transverse compression

Understanding the rheological behavior of timber is vital for evaluating the long-term performance of timber structures. In this study, short-term experiments were first conducted to obtain the short-term mechanical properties of timber including yield strength and elastic modulus in both longitudinal and transverse compression. Subsequently, creep tests were carried out to capture the rheological behavior of timber over 360 days. Different loading types of compression and stress levels were chosen in the creep tests. Environmental temperature and relative humidity, moisture content, environmental strain and creep strain were monitored. The coefficients of environmental deformation and irrecoverable mechano-sorptive creep were obtained in longitudinal and transverse directions. Various creep models of timber were calibrated against the experimental data. Weighted nonlinear regression was adopted to better simulate the secondary creep behavior. The creep coefficients over 50 years were estimated using the calibrated models, which were compared with values from existing national standards and research. The experimental results demonstrate that the limit of linear creep in longitudinal compression was bigger than that in transverse compression. Also, the transverse timber creep was much bigger than the longitudinal creep.

Creep behaviour of Japanese cypress timber under various hygrothermal conditions

ABSTRACT This study presents the creep behaviour of inner- and surface-layers of Japanese cypress boxed-heart timber under constant and cyclic moisture sorption using cantilever creep test at a temperature of 20, 65, and 80°C. The objective was to measure the viscoelastic and mechanosorptive creep parameters of the timber at each temperature. A load representing 20% of the ultimate rupture load under equilibrium moisture content (EMC) of around 12% at each temperature was applied at the free-end of the cantilever specimen. The specimen was exposed to constant moisture conditions and then subjected to cyclic moisture sorption, consist of adsorption and desorption. The surface strains of the specimen were then recorded, and models were used to measure the creep parameters. The results show that the creep behaviour of the specimen depends on the temperature, and its creep compliances tend to increase as the temperature increased. The results confirmed that the viscoelastic and mechanosorptive creep dependence on time and cumulative moisture content change (CMCC), respectively. Fitting the experimental data with the models used in this study shows good agreement, and the creep parameters at each temperature were successfully measured. These findings indicate the potential to model stress occurrence in the timber during the drying.

Orthotropic hygro-mechanical behavior of Chinese fir during cyclical relative humidity variation

The orthotropic free swelling–shrinkage and mechano-sorptive creep (MSC) during cyclical relative humidity (RH) variation (0–80% RH) and time-dependent viscoelastic creep (VEC) in a steady state (0% RH) were examined in Chinese fir (Cunninghamia lanceolata). During RH changing, the occurrences of local maximum or minimum strains of the free swelling–shrinkage and MSC lagged behind the corresponding maximum or minimum RH. The lagged time decreased with increasing cyclical times. Furthermore, the lagged time exhibited an orthotropic behavior: the longer lagged time was found in longitudinal specimen compared to transverse specimens. MSC exhibited a more pronounced anisotropy than VEC. According to the three tests, the performance of free swelling–shrinkage and the mechano-sorptive effect on the orthotropic MSC behavior was addressed. The free swelling–shrinkage mainly dominated the creep strain during cyclic RH variation, especially for longitudinal specimen. The mechano-sorptive effect on the MSC behavior exerted more influence on tangential specimen compared to radial and longitudinal specimens. The mechano-sorptive limit was observed for all specimens. Longitudinal specimen required more cyclic times to approach the mechano-sorptive limit than transverse specimens.

Molecular origin of mechano-sorptive creep in cellulosic fibres

Mechano-sorptive creep, i.e. the increased rate of creep that occurs during changing relative humidity, when loading paper or wood, is a phenomenon still not fully understood. This phenomenon was here investigated by examining the changes occurring at the molecular level utilising FTIR spectroscopy. By subjecting the paper to deuterated water, the changes involving both the crystalline hydroxyls as well as those in accessible regions could be examined. During loading, the cellulose molecular chains are stretched taking the load. In addition, during mechano-sorptive creep a further exchange from OH to OD groups occurred. This was interpreted as caused by slippage between cellulose fibrils allowing previously non-accessible hydroxyls to become available for deuterium exchange. Thus, the loosening of the structure, during the changing moisture conditions, is interpreted as what has led to the increased creep and the possibility for new areas of cellulose fibril/fibril aggregates to be exposed to the deuterium exchange.

The mechano-sorptive creep behaviour of basalt FRP reinforced timber elements in a variable climate

The use of Fibre Reinforced Polymer (FRP) reinforcement has been shown to improve the short-term flexural behaviour of timber elements. This is particularly important when reinforced elements are subjected to a variable climate condition, which is known to accelerate long-term or creep behaviour. In this paper, both unreinforced and Basalt FRP reinforced beams are subjected to creep tests at a common maximum compressive stress of 8 MPa over a 75-week period. Results demonstrated a significant reduction in total creep deflection due to the FRP reinforcement. Using matched groups, experimentally measured total strain behaviour is decomposed into the elastic, viscoelastic, mechano-sorptive and swelling/shrinkage strain components. Analysis has shown that the mechano-sorptive component is similar in unreinforced and reinforced beams. The reduction in creep behaviour of the reinforced members was primarily due to the restrained swelling/shrinkage response of the reinforced beams and was independent of the mechano-sorptive effect. This finding demonstrates the positive influence of FRP reinforcement on the long-term behaviour of timber elements and indicates a potential to describe the long-term deflection performance of FRP reinforced elements from short-term swelling/shrinkage tests.

Creep behavior of laminated veneer lumber from poplar under cyclic humidity changes

Many academic studies over the years have confirmed that mechano-sorptive (MS) creep is an inherent characteristic of wood. Unlike solid wood, bond lines are introduced into laminated veneer lumber (LVL), creating a laminated structure with different hygroscopicity. What are the effects of these differences on the MS creep of LVL? In this study, three groups of well-matched LVL samples were subjected to four-point bending loading within different relative humidity cycles. For each group, the applied load ranged from 15% to 35% of the short-term fracture load. The results showed that after the first hygroscopic process, LVL showed irreversible expansion (0.11 mm) and a relatively slow moisture adsorption rate. These made it difficult for LVL to show partial creep recovery during the first adsorption process no matter how low the load level was, while solid wood showed partial creep recovery when the load level was ≤ 25%. The following creep behavior of LVL was similar to that of solid wood: partial creep recovery started from the second adsorption stage when a moderate load level was applied.

Orthotropic mechano-sorptive creep behavior of Chinese fir during moisture desorption process determined in tensile mode

Time-dependent creep behavior is one of the most important material characteristics in wood products exposed to various environmental conditions. The creep behaviors, including the general viscoelastic creep (VEC) at constant moisture content and the mechano-sorptive creep (MSC) during moisture content variations, are, however, not sufficiently described. Given the anisotropy of wood, comprehensive data on the creep behavior will require knowledge of its moisture-dependent properties in relation to the three principal axes of anisotropy. The present study examined the tensile orthotropic creep behaviors of Chinese fir (Cunninghamia lanceolata): VEC at constant moisture content and MSC during desorption process, at 0, 20, 40, and 60% relative humidity (30 °C). The creep anisotropy of MSC was more pronounced than that of VEC. The free shrinkage caused the specimens to deform in the opposite direction to load in MSC tests. Based on the databases of free shrinkage, VEC, and MSC, it is obvious that the mechanical stress has a positive effect on MSC, which is presumed to be a result of the double effect composed of the rearrangement of hydrogen bonds and unstable state. The unstable state could be evaluated quantitatively by the mechano-sorptive strain. The mechano-sorptive strain of radial and tangential specimens is affected by the relative humidity to a higher degree than that of longitudinal specimens. Further, the unstable state exerted more influence on tangential specimens and less on longitudinal specimens.

Effects of saturated vapor pre-steaming on drying strain in Asian White Birch: Experimentation and modelling

The effect of low pressure saturated vapor pre-steaming on restrained shrinkage strain, mechano-sorptive creep and the distribution of moisture content was investigated during conventional drying of wood discs. Mechano-sorptive creep was furthermore modelled by artificial neural network theory with five inputs, i.e., pre-steaming and drying temperatures, wood moisture content, relative humidity and distance from the pith. Results revealed that, pre-steaming partly reduced the variation of moisture content distribution along radial direction, increased restrained shrinkage strain in heartwood and decreased in sapwood and slightly decreased the mechano-sorptive creep. The neural network model provided reasonable prediction results, namely, the coefficient of determination for training, validation and test sets greater than 0.95. Keywords: Artificial neural network, Betula platyphylla, mechano-sorptive creep, restrained shrinkage strain, white birch discs

Moisture adsorption in palletised corrugated fibreboard cartons under shipping conditions: A CFD modelling approach

Corrugated fibreboard packages (cartons) must support considerable mechanical loads during long term transport of fresh produce in refrigerated freight containers (RFCs). Fresh produce are transported under high relative humidity to reduce fruit moisture loss and preserve quality. However, these conditions can progressively reduce carton mechanical strength over time as a result of mechano-sorptive creep. Little is known regarding the actual moisture dynamics in stacked cartons in RFCs, which is important for mechanical strength assessments. To this end, a portion of a fully loaded RFC was investigated using a computational fluid dynamics (CFD) model, with respect to moisture transport in the air and the corrugated fibreboard. Simulations included the effects of loading, defrost cycles, fruit respiration and transpiration.Results showed relatively low moisture content gradients in fibreboards through the stacked cartons under optimal shipping conditions. However, the initial activation of the RFC considerably accelerated the development of moisture content gradients in the cartons. Additionally, the most significant factor influencing spatial moisture gradients through the cartons was heat conduction from outside through the container wall.

Orthotropic mechano-sorptive creep behavior of Chinese fir during the moisture adsorption process determined in tensile mode via dynamic mechanical analysis (DMA)

Abstract The orthotropic viscoelastic creep (VEC) at a constant moisture content (MC) and mechano-sorptive creep (MSC) during the adsorption process were examined for Chinese fir (Cunninghamia lanceolata) under tension at 20, 40, 60 and 80% relative humidity (RH) (30°C). Free swelling was performed on matched specimens based on the strain partition assumption to better understand the characteristics of the mechano-sorptive (MS) phenomenon. Expansion, elastic and time-dependent creep behaviors of radial (R) and tangential (T) specimens were affected by the MC to a higher degree than those of the longitudinal (L) specimen. A higher proportion of elastic strain in total strain was found in the L specimen as compared with transverse specimens, regardless of VEC and MSC. The RH level had a greater effect on relaxation behavior in the L specimen for MSC. According to the three tests, expansion mainly dominated the creep strain during adsorption, especially for the L specimen. The MS strain exerted more influence on transverse specimens and had less contribution to the L specimen. Moreover, under all RH isohume (RHI) conditions, the unstable state contributed to MS strain diminishing as MC approached equilibrium moisture content (EMC). A shorter adsorption time to a new equilibrium state was achieved at the expense of intensifying the unstable state of the wood cell wall.

Swelling restraint of thermally modified ash wood perpendicular to the grain

The paper reports on creep of ash wood (Fraxinus excelsior L.) thermally modified at 180 and 200 °C, and subsequently subjected to compression in tangential and radial directions and simultaneously wetted, from the moisture content (MC) of 6% to above the fibre saturation point (FSP). The compressing load made 0.00, 0.25, 0.50 and 0.75 of impact stress at the proportional limit (Rc). The compression stress needed to restrain the swelling of wood, the so-called swelling pressure, was indirectly determined from isochrones of mechano-sorptive creep. The most important finding was that thermal modification reduces the strain of ash wood subjected to compression perpendicular to the grain to a degree proportional to the mass loss. The compression stress needed to restrain the swelling of thermally modified wood is ca. 10 and 20% smaller in the tangential and radial directions, respectively. This effect leads to a reduction in the anisotropy of swelling pressure of thermally modified wood perpendicular to the grain. Moreover, although upon thermal modification the mass loss of wood takes place, at the MC of 6% it shows practically the same modulus of elasticity (MOE) and Rc as the unmodified wood. After wetting to MC higher than the FSP, the thermally modified wood at 200 °C shows significantly higher MOE and Rc than the wood modified at 180 °C and untreated wood. Reduction of wood hygroscopicity, an inevitable effect of thermal modification, also reduces the range of changes in mechanical properties of wood caused by the increase in its MC to the FSP.