Study On Drying Stress And Strain In White Birch Discs During Conventional Drying

With the development of modern ecological civilization,the pursuit of green living concept and the improvement of people’s living standards,the contradiction between increasing demand and growing shortage for wood resources is deepening year by year.The poor wood logs,such as bending wood and small diameter wood,are sawed into wood discs in suitable thickness,processed into kitchen board,watch dial,standing solid wood floor and other handicrafts after drying,which not only can increase the added value,but the high utilization rate is of great significance to ease the contradiction.The key to the improvement of yield rate is the high quality drying.However,due to the existence of shrinkage anisotropy,the drying stress will emerge even if there is no moisture content gradient below the fiber saturation point.Therefore the drying crack is easily produced in wood during and after drying,which affect the application and value seriously.Wood drying stress and strain is the foremost cause of drying defects,regardless of the impact of growth stress,it is mainly due to the nonsynchronous shrinkage caused by the uneven distribution of moisture content,as well as the different shrinkage coefficient between tangential and radial direction.Consequently,it is obviously necessary to analyze the drying stress and strain during drying and explore the appropriate means to alleviate drying stress,thereby reducing the risk of drying defects.For the above causes,the online detection method for moisture content and the image analysis calculation method for drying strain were discussed during wood discs conventional drying.The effect of shrinkage anisotropy alone and its interaction with moisture content gradient on actual shrinkage strain,elastic strain,viscoelastic strain and mechano-sorptive creep were investigated.According to the drying strain characteristics at different drying conditions and moisture content,the variation of drying stress was analyzed and the mechanism of generation,development and reversal for drying stress was clarified.The influence of saturated wet air and vapor pretreatment on wood drying characteristics and drying strains were studied.On the basis of above,the artificial neural network model was used to the simulation and prediction of drying strain.The specific research contents and main conclusions are summarized as follows:(1)To improve the online detection precision of moisture content during drying,the effect of ambient temperature on measurement accuracy of resistance strain type weighing device was analyzed,and the binary regression equation for calculating exact weight was proposed by using ambient temperature and electrical measuring weight.The wood moisture content meter was experimentally calibrated,the influence of probe depth,probe spacing,probe position and temperature compensation on the detection precision was explored under the fiber saturation point.The modified formula of the moisture content measurement with the higher detection precision was obtained.In addition,the image analysis calculation method for drying strain measurement was discussed.These established a foundation for the accurate detection of moisture content and drying strain during drying process.(2)The actual shrinkage strain,elastic strain,viscoelastic creep strain,and mechano-sorptive creep characteristics of white birch discs caused by shrinkage anisotropy alone was explored during slowing conventional drying.The effect of moisture content,variation of wood property and grain direction on each drying strain were discussed.As findings showed,under the individual effect of shrinkage anisotropy,the elastic strain and viscoelastic creep strain were positively correlated with moisture content,but the mechano-sorptive creep increased with decreasing moisture content and the opposite mechano-sorptive creep was presented in tangential and radial direction.In addition,the material didn’t easily recover from viscoelastic creep strain and a large compressive mechano-sorptive creep may occur before viscoelastic creep strain recover.(3)Two kinds of drying schedules were employed to investigate the effects of shrinkage anisotropy and its interaction with moisture content gradient on actual shrinkage strain,elastic strain,viscoelastic creep strain and mechano-sorptive creep in tangential direction.As results,under the interaction of shrinkage anisotropy and moisture content gradient,the stress reversal in sapwood was advanced due to the generation of tensile plastic deformation;and the viscoelastic creep recovered easier and produced reverse creep strain,even reducing or partially restoring the mechano-sorptive creep.(4)According to the charateristics of drying strain at different drying conditions and target moisture content(26%,18%and 10%),the representative microelement was conducted to stress analysis.The generation,development and reversal mechanism of drying stress were discussed,thereby clarified the control mechanism of cracking.Results revealed that,with the effect of shrinkage anisotropy alone,the stress reversal(from tensile to compressive stress)occurred in the tangential direction as the moisture content dropped from 26%to 18%.With interaction between shrinkage anisotropy and the moisture content gradient:For the moisture content in heartwood being higher than in sapwood,the stress reversal has been finished in sapwood when moisture content was higher than 26%;before stress reversal,the vectors of the shrinkage anisotropy stress and the moisture content gradient stress were pointing in the same direction,and after stress reversal the two pointing in opposite direction.For the moisture content in heartwood being lower than in sapwood,the vectors of the shrinkage anisotropy stress and the moisture gradient are pointing in opposite directions.The shrinkage anisotropy stress was eased and neutralized by the non-synchronous shrinkage stress caused by moisture content gradient,thus the actual drying stress depends on the magnitudes of’ the shrinkage anisotropy stress and the moisture content gradient stress.(5)The saturated wet air and vapor pretreatment was applied in wood discs before conventional drying.The effects of two different pretreatment processes on moisture content,drying rate,shrinkage radio,drying crack,actual shrinkage strain,elastic strain,viscoelastic creep strain and mechano-sorptive creep were discussed.In summary,the pre-steaming could reduce the initial moisture content and the variation of moisture content distribution from the pith to bark direction in wood discs,and also caused increase in heartwood but reduction in mix-wood and sapwood for restrained shrinkage strain.In addition,the tensile elastic strain and viscoelastic creep strain for pre-steaming group at the initial stage below the fiber saturation point were reduced,and caused a generation for compressive viscoelastic creep strain at the later stage of drying and slightly eased mechano-sorptive creep.(6)The elastic strain and mechano-sorptive creep were simulated and predicted by artificial neural network.Through the training,validation and test process of the network,a reasonable prediction model was obtained,which provided a feasible basis for the application of artificial neural network in the investigation of drying stress and strain.Results revealed that the predicted values were all in good agreement with the experimental ones for the two established prediction models.For the prediction model of elastic strain and mechano-sorptive creep,the best validation performance of the mean square error was 1.21×10-6and 1.26×10-6,respectively,and the coefficient of determination for the test group was 0.956 and 0.939,respectively.