Study On Drying Stress Model And Continuous Measurement Of Drying Strain Of Pinus Massoniana Board

Wood drying stress and strain is the theoretical basis of making the optimum wood drying schedule. The drying stress model can describe the cause of generation of wood drying stress and it's developing regularity by mathematical method, and it is the theoretical basis which can accomplish the control technology of wood drying; therefore, it is theoretically important and practically significant to study wood drying stress model.The object of this paper was 30mm thick Pinus massoniana boards. The actual shrinkage during the process of drying was divided into four components, including free shrinkage strain, instantaneous strain, viscoelastic strain and mechano-sorptive strain. The mathematical model of drying stress of Pinus massoniana boards was established by diverging wood drying time, the difference between the free shrinkage of one layer in the board and the actual shrinkage of the board could be described using the model, which was the cause that drying stress generates in that layer. The model could also explain the changing regularity of stress, the cause of generation of residual stress during drying and the relievable effect of viscoelastic strain and mechano-sorptive strain on drying stress. The experiment of model verification indicated that the drying stress model presented in this paper could describe the changing regularity of drying stress and strain of wood during drying, but the drying stress model couldn't calculate and explain the drying stress during process of conditioning.In this paper the traditionally analytical method of drying stress was broken through, the study on drying stress and strain of Pinus massoniana board by mathematical method showed that viscoelastic strain and mechano-sorptive strain released the drying stress to a significant extent Because the moisture content of inner layers was orderly below FSP during drying, free shrinkage strain occured in the board, tensile stress took place orderly from outer layer to core layer. The instantaneous strain was the smallest during drying, viscoelastic strain and mechano-sorptive strain were larger. The ratio of viscoelastic strain to instantaneous strain of all layers was 2.49; but that of mechano-sorptive strain to instantaneous strain was in the large range of fluctuantion which was between 8.73 and 45.34; Because of its superimposition, the mechano-sorptive strain was larger than viscoelastic strain before stress reversal, but after that, mechano-sorptive strain might be smaller than viscoelastic strain.In this paper a wood shrinkage sensor of resistance strain was designed and made. The sensor realized the continuous measurement of wood shrinkage and viscoelastic strain and it provided a new measuring method for the study on the mechanical property and drying stress and strain of wood.A new method of continuous measurement for drying stress was also presented in this paperand the critical drying stress of the surface layer of Pinus massoniana board was measured by that method. Pinus massoniana board of 30mm thick was dried by regarding the critical drying stress as controEing parameter. The results showed that the drying period was 7.5h faster than that of conventional drying, the drying efficiency was improved by 13.89%. The surface check was not found out during drying, and the drying quality could come up to National second degree drying quality standard.The physical and mechanical properties were obtained in this paper and the theoretical basis of profound research for Pinus massoniana board was established. The research indicated that temperature had no effect on the ratio of total shrinkage and the coefficient for free shrinkage, the coefficient for free shrinkage was irrelevant to moisture content below FSP. The shrinkage coefficient for Pinus massoniana board in tangential direction was not affected by the change of load, but it changed with the change of moisture content and temperature; the tangential mechano-sorptive coefficient was not affected by the change of load or moisture content, but it increased with the increase of temperature.