J. Cent. South Univ. Technol. (2008) 15(s1): 545-549
DOI: 10.1007/s11771-008-418-8
Advances and expectations of study on wood rheology
MA Yuan-rong(马远荣)1, 2, LUO Ying-she(罗迎社)1, LI Xian-jun(李贤军)1, 3
(1. Institute of Rheological Mechanics and Material Engineering, Central South University of Forestry and Technology, Changsha 410004, China;
2. College of Civil Engineering and Mechanics, Central South University of Forestry and Technology,
Changsha 410004, China;
3. College of Material Science and Engineering, Central South University of Forestry and Technology,
Changsha 410004, China)
Abstract: By studying and summarizing the characteristics of wood rheology, the mathematic models of creep and mechano-sorptive creep of wood were analyzed. Rheology behaviors in process, especially drying stress and deformation set were discussed. Application of wood rheology in woodcraft process was elaborated and the research prospects and orientation were forecasted.
Key words: wood rheology; creep, mechano-sorptive creep; drying stress; deformation set
1 Introduction
As a kind of excellent regenerative nature engineering material, wood has fine strength-weight ratio, low heat-conductibility and beautiful appearance, which are flattered by many people. But because of the specialties of its inner structure, its rheology behavior is more complicated and conspicuous than other engineering materials such as steel and concrete, which makes the quality of wood products decrease and substandard products increase. Wood elements deform badly or collapse earlier under long-term loads, which bring about large loss of economy. Studying the principles of wood rheology will be of great importance to the technology of shaping woods and fixation of set, improving the quality of the wood products and design of engineering elements.
2 Mathematical models of woods rheology
2.1 Creep
As a kind of visco-elasticity polymer materials, when the rheology properties of wood in the range of visco-elasticity are studied, combination of Hookes and dampers is usually adopted[1]. The mechanical models of wood creep are shown in Fig.1 Burger body is a simple model describing the creep behavior of woods and the relating equation is:
(1)
From Eqn.(1), three parts of woods’ creep, that is, elastic, visco-elasticity and creep can be determined. But it should be noted that Burger body is only applied to the initial and the second stages of creep, and it cannot be applied to the end and breakage stage.
Furthermore, when the above models are adopted, the solution of so many creep characteristic constants will be obtained difficultly. DIAO and LIANG[2] measured the creep characteristic constants of pinus masoniana and eucalyptus camaldulensis by means of electricity medium. LU only measured creep constants of some main tree species in their short-term bending creep. But there are not uniform experimental standard and evaluating methods for the solution of rheological coefficients of different tree species under different conditions. SHAO put forward a concept of variable coefficients rheological model[3]. He used two-unit Maxwell body of variable coefficients to draw up the creep of polar accurately, by which the rheological coefficients could be solved easily.
2.2 Mechano-sorptive creep
The above creep model considers the deformation under constant temperature and moisture. In fact, woods are usually used in a dynamic environment of temperature and moisture. Mechano-sorptive creep is the deformation of loaded wood under dynamic temperature and moisture.
Fig.1 Mechanical models of wood creep: (a) Maxwell body; (b) Kelvin body; (c) Element model; (d) Burger body
It has been paid close attention to since ARMSTRONG and KINGSTON[4] found it 1950s. SALIN[5] presented its mechanical model by means of numerical analyzing method, which was approved by most scholars. The total stain includes 4 independent parts, elasticity, deformation caused by free moisture, common creep and mechano- sorptive creep, as expressed by Eqn.(2). The relevant mechanical model is shown in Fig.2.
Fig. 2 Physical creep model in series
3 Study of rheology in wood processing
3.1 Drying stress
Drying is the absolutely necessary segment in wood industry. The drying quality and velocity depend mostly on drying technique, while drying stress is the most pivotal parameter and basis when we explore the drying mechanism and establish drying technique.
The total stain includes elasticity, visco-elasticity, mechano-sorptive creep and dry shrinkage during drying at normal temperature. The mathematic representation is shown in Eqn.(3), where dry shrinkage is the inherent property of wood and shall not be studied emphatically.
(3)
where
.
Since MCMILLEN presented slice method in 1963, it has become the classical technique to analyze drying stress. Furthermore, there are other methods such as non-contact[6], ultrasonic, laser and numeric picture technique. Lately, many scholars improved the slice method. The schematic illustration of slice method is shown in Fig.3. So every item in Eqn.(3) can be calculated respectively.
The equation established by CHEN[8] is:
(4)
DIAO and HE[9] studied the drying stress of hardwood timber by means of slice technique. In the initial stage of drying, the distribution of drying stress was that the exterior was tensile and the centre was compressive, while in the later period, stress reciprocated. The results accorded with conclusions of TOKUMOTO and other researchers[10]. Temperature had no obvious effect on the average moisture content of extreme strain value, but the higher the temperature, the shorter the time for the strain to reach the extreme value. The drying stress could exist an influence on the shrinkage of woods, in which tensile stress made shrinkage decrease, while compressive stress made shrinkage increase. The higher the temperature was, the bigger the residual deformation of the outside layer would be.
Fig.3 Schematic illustration of slice method
LIU[11] measured the drying stress also by section and established the mathematics models. He found that the elastic modulus of wood was not a constant during drying and increased with the decreasing moisture content, which changed a little above the F.S.P but largely below the F.S.P. The distribution of drying elastic stress and residual stress along the depth were quartic and quadratic polynomial respectively, and the maximum of stress was on the surface and in the middle of the lumber.
HU[12] detected the stress and strain of wood in the process of drying by use of a non-contact detection method. The results show that the main factors influencing wood drying stress are drying-strain velocity, drying temperature, water-content grad and temperature grad. GONG[13] found that vapor treatment enabled the stress transformation point to postpone, and drying stress of wood emancipated a great deal because of high temperature and high humidity treatment.
CHENG[14] studied the shrinkage stresses and visco-elasticity of wood during drying under steam of high temperature and pressure. The shrinkage stress in the radial direction of sample was about twice of that in the tangential direction under high temperature with 0 relative humidity. However, the shrinkage stress along tangential direction was higher than that of radial direction under other relative humidity. The shrinkage stress was effectively restrained and decreased dramatically with the increase of RH during the drying process under superheated steam at 180 ℃. It was found that shrinkage stress would still exist even under superheated steam at 200 ℃.
TAKANORI[15] pointed that creep during drying was higher than that in balance moisture. The temperature and speed of drying influenced creep mostly. SHUICHI[16] studied the effect of moisture grad on drying stress. He presented that drying stress depended on moisture grad in surface chiefly.
3.2 Generation, recovery and fixation of set of processing wood
The set produced by compression or bending will recover when moisture or hydro-thermal was rendered. Some scholars have done a lot of work on stress deliverance under high temperature and high pressure. The results showed that stress delivered with the delay of time until it reached 0 at last. These properties had been used to fix the set produced by compression or bending and made much progress. Thus making the mechanisms of formation and recovery of wood set clear are important to wood processing. Some representative viewpoints are shown in Table 1[17-23].
4 Application of rheology principles in wood process
4.1 Thermal or vapor treatment
When adequate moisture and heat are given to wood, the elastic modular will decrease and wood will soften so that wood is easy to be bent. Based on this principle, steam, high frequency and microwave are used to soften woods.
ZHOU et al[24], dealt with the pretreatment of poplar by using hydrothermal treatment before it was compressed, thereby compared the influence of different treatment conditions and cooling time on the longitudinal compressive properties. The results showed that the temperature of water had obvious effect on the compressive strength. With the increase of temperature, the compressive strength and yield stress all appeared a descending trend, while the yield strain changed a little. The strain at the maximum load and the residual deformation increased. Along with the prolongation of heat-preserving time, the strain at the maximum load increased firstly, reached its maximum value in 1 h, and then decreased until it became mild. In case that water temperature was 100 ℃ and heat-preserving time was 1 h, the sample could be compressed easily, but its strength was minimum and strain was maximum. Along with the prolongation of cooling time, the compressive strength and yield stress both increased slightly.
Based on temperature factor and time factor, XIE and LIU[25] researched heat treatment on mason pine. He found that the teat-treated mason pine timber was darkened with increase of heat treatment temperature and time, and temperature could have more effect on color than time. The wetted swell rate of the heat-treatment samples in low temperature was larger than that in high temperature and the dimensional stability of the samples was also bad. The MOR and MOE of heat-treatment samples decreased with the increase of temperature and time, but the compressive strength along the grain changed a little.
WANG and ZHAO[26] studied the creep of compressed wood of Chinese fir heat treated in air. The heat treated compressed specimens had lower instantaneous
Table 1 Mechanisms of formation and recovery of wood set
compliances and creep compliances under the dry measurement conditions of common temperature, which had higher values during adsorption and desorption. Their creep behaviors during adsorption and desorption didn’t present the typical characteristics of mechano- sorptive creep. Heat-treatment had great effects on its creep behaviors of compressed woods. The higher the temperature, the longer the time, then the higher the instantaneous compliances and creep compliances under certain conditions. The decomposition of the main components of wood cell wall led to the increase of the compliances. The temperature had obvious influence on the value of compliances and set recovery. The higher the temperature was, the lower the recovery would be.
LI and LIU[27], investigated the effect of steaming and heat on fixation of compression set. Dimensional stability could be improved by either steaming or heating woods while woods were in a compressive state. When value of ASE went beyond 50%, the time of high-temperature steam treatment was much shorter than that of heating treatment at the same temperature. Compressive deformation was nearly fixed completely when the temperature was 180 ℃, and heating treatment took 15-20 h but high-temperature steam would take only 8-10 min.
LI et al[28], tested the bent woods at different temperature processing. The result showed that the set recovery of curvature radius decreased as the heat treatment times added and the temperature escalated. LI and TANG[29] studied roughly the high-temperature treatment process of Chinese fir wood. They found that wood after heated treatment had high dimension stability.
4.2 Resin treatment
Water-soluble PF or MF of low molecular weight can fix the set of processing wood, which has been approved conformably. But there exist some divergence points about the mechanism of fixation. Roger M Rowell owed it to stuffing. He thought that there was no cross-linking reaction between resin and ingredients of wood cell. Musafumi Inoue accounted that colophony could cement the compressed wood cells so that they couldn’t rebound effectively, beside stuffing, there would be other reactions between colophony and wood cells
XIE and ZHAO[30] generalized the previous researches on the changes of combination forms between molecules during chemical treatment. He accounted that chemical stress relaxation and chemical creep of wood were related to primary ingredients of wood cell, such as lignin, cellulose, semi-cellulose. Medicament treating made the combination forms between molecules change.
4.3 Cross-linking reaction
Cross-linking reaction between woods and reagent of low molecular weight is an important means to modify woods. Formaldehyde is a staple. INOUE and NORIMOTO[21] used formaldehyde to make fixation set of compressive woods. The set of compressed cryptomeria could be fixed by heated water with formaldehyde and SO2 at 120 ℃, 2 h were needed, while only 20 min were adequate. The study also showed that gas formaldehyde to fixation was better than liquid phase, but gas formaldehyde would make the MOR and MOE bigger.
5 Expectations of study on woods rheology
5.1 Analysis of rheological mechanisms and study on modification of wood composites
For the above summarization，although the set of processing wood has been fixed effectively by the use of various means, there was no definite and conforming viewpoint about the mechanisms of fixation and deformation during processing. Only explain the mechanisms by means of experiment, especially the constitutive relationship under various conditions, can we grasp the complicated rheology behaviors of wood. So the study on modification of wood composites can interpenetrate the crystallography field and make pivotal progress.
5.2 Accelerated characterization of long-term woods rheology
Studying on the equivalency of temperature-time and stress-time of polymer materials has been very sophisticated. The results obtained from a short-term creep test could be used to predict the long-term mechanical properties of polymer. When this principle was used in woods, some progress has been achieved; however the error of prediction was great. Because wood is a kind of porous and anisotropic polymer, its rheological properties are affected by various internal and external factors, such as time, inscape, feature of load, temperature and moisture. Among them, time, temperature moisture and stress level are the most important factors. How to improve the existed principle about equivalency of time-temperature-stress, and consider the reciprocation among them, will contribute greatly to the precision of accelerated characterization of long-term woods rheology.
5.3 Application of genetic algorithm in study of woods rheology
According to the above statement, the factors which influence woods rheological characteristic are so complicated that there are no acknowledged uniform criteria and database to be used so far. The difficulties of measuring and predicting long-term rheological properties accurately are conceivable. Because of the discrepancy of tree species, a large number of research findings can’t be used in Chinese woods directly. While as a kind of searching algorithm of robustness, genetic algorithm can optimize complicated system. Its integral searching strategy and optimal algorithm don’t depend on gradient or other auxiliary information, but it needs objective function of influencing searching orientation and relating sufficiency function. If associated the advanced computer, it would provide a kind of effective and reliable numeric medium.
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(Edited by ZHAO Jun)
Foundation item: Project(30871983) supported by the National Natural Science Foundation of China
Received date: 2008-06-25; Accepted date: 2008-08-05
Corresponding author: LUO Ying-she, Professor, Tel: +86-731-5623376; E-mail: lys0258@sina.com
