| The viscoelastic behavior of wood,which mainly depends on the anisotropic structure,the moisture of wood,the temperature-humidity environment and the treatment history of wood,play an important role in the utilization and manufacture processes.In this study,the orthotropic viscoelastic properties of specimens with different moisture contents(MCs)ranging from 0.6% to 300% during the cooling,heating and cyclical temperature variations were systematically investigated,and the response mechanism of orthotropic viscoelastic behavior of wood to moisture and temperature was revealed.In this study,the viscoelastic properties of the heartwood of Chinese fir(Cunninghamia lanceolata)were determined in tensile mode using Dynamic Mechanical Analyzer(DMA 2980).The temperature spectrum of storage modulus(E′),loss modulus(E′′)and loss factor(tanδ)were measured for the longitudinal(L),radial(R),and tangential(T)specimens.At constant MC conditions,the effects of moisture(bound water and free water)content and its state on the orthotropic viscoelastic behavior was discussed.During the water-loss process,the changes in orthotropic viscoelasticity of wood with different MCs were investigated,and the interaction between adsorbed water molecules and wood in non-equilibrium state was also revealed.The effect of quenching treatment(temperature automatically decreased from room temperature to-120°C over 11 min)on the viscoelastic behavior of wood with different MCs was analyzed in the temperature range between-120°C and 280°C.Furthermore,the effect of thermal history on the orthotropic viscoelasticity of dry wood specimens were revealed by measuring the temperature spectrum of viscoelastic behavior.The main conclusions of this study were summarized as follows:1.The viscoelastic behavior of dry wood specimens showed evident grain orientation: the values of E′for the L specimens were higher than that of the transverse specimens,and the E′of the R specimens were higher than that of the T specimens.Within the temperature ranging from-120°C to 280°C,the loss peak temperature of relaxations for the L specimens were lower than that of the transverse specimens.The temperature effect exhibited a more pronounced influence in the T direction,followed by the R and L direction.When temperature increased from 25°C to 280°C,the E′of dry wood specimens in the L,R and T direction decreased by 54.69%,69.87%and 77.22%;and the values of tanδin the L,R and T direction increased by 255.30%,517.88%and 695.34%.2.At constant MC conditions,the higher adsorbed water content,the higher the plasticization effect,which aggravated the change rate of E′(ΔE′)of specimens.The values of ΔE′ was more affected by per unit of MC that that by per unit temperature.At each MC level,the ΔE′ for the L specimens were lower than that of the R and T specimens.During the water-loss process,the reformed hydrogen bonds effect was increased with increasing amount of moisture,and exhibited more pronounced influence on the enhancement of stiffness for the transverse specimens than that of the L specimens.Compared with the L specimens,the transverse specimens are more susceptible to temperature and moisture.3.At constant MC conditions,the loss peak temperature of γ-relaxation shifted to relative lower temperature range as a function of the plasticization effect of adsorbed water.At each MC level,the loss peak temperature of γ-relaxation for the L specimens were lower than that of the transverse specimens,and the difference was reduced with increasing amount of moisture.One secondary mechanical relaxation was also observed at around-110°C for specimens with the formation of ice.During the water-loss process,the moisture desorption promoted the reduction of instability within wood cell walls,and the damping of specimens with different MCs trended to accord.4.The viscoelastic behavior of specimens ranged from-120°C to 25°C were not influenced by the quenching treatment,while the viscoelasticity of specimens ranged from 25°C to 90°C were changed by the quenching treatment: the relaxation induced by the segmental motion of low-molecular-weight hemicellulose was observed in dry wood specimens;the relaxations in the shape of "double peak" were found in the transverse specimens with adsorbed water,and that was more obvious at rather low frequencies,which may be attributable to the superposition of the transitions of lignin and hemicellulose.5.The mechanical relaxation behavior of dry wood specimens in low temperature was changed by the thermal history: another δ-relaxation was observed in the temperature range between-129°C and-110°C,and its peak temperature and intensity showed obvious frequency-dependence.During cyclical temperature variation,the relaxation assigned to the movement of low-molecular-weight hemicellulose was only observed in the transverse specimens during the heating process,and its temperature showed no frequency dependence,but its intensity decreased with increasing frequency.6.The E′ values of specimens in the three main directions increased with increasing frequency,which was independent of its MC and the measured temperature range.On the contrary,the mechanical relaxation behavior of specimens was dependent of its anisotropic structure,MC and the measured temperature range. |
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