Effect Of Damp-hot Set On The Conventional Drying Performance Ofpinus Sylvestris Var.Mongolica Square Timber

In order to solve the problem of cracking of large square timbers of Pinus sylvestris var,especially the square timbers with pith core in the conventional drying process,the 120 mm cross-section small-diameter square timbers with pith core were taken as the research object,and pretreated by different damp-hot processes,i.e.,softened by saturated wet air at different time and temperature,and drying set by different temperature,humidity and time(the surface layer was plasticized and set under the action of softening state and stress),and then dried regularly.The conventional drying rate,cracking and drying strain of square timber pretreated by different damp-hot processes were measured.The mechanism of the influence of pretreatment on the conventional drying performance of small-diameter square timber of Pinus sylvestris var.mongolica was analyzed,and the appropriate damp-hot pretreatment process was discussed;On this basis,the artificial neural network model is constructed to simulate and predict the conventional drying quality of Pinus sylvestris var.mongolica square timber after the damp-hot pretreatment by adjusting the process parameters.It is not only of great significance for realize the high-efficient and high-quality drying of large cross-section Pinus sylvestris Carr,but also provide basic data for the research of mechanism of drying set treatment to inhibit dry surface cracking,and provide reference in the development of rapid and high quality drying technology of large square timbers of other tree species.It can be used for reference in the development of rapid and high-quality drying technology of large section square timber of other tree species.The main research contents and results are as follows:(1)The effect of damp-hot pretreatment on the drying speed and surface crack of Pinus sylvestris Carr.Under the limited experimental conditions of this study,the surface crack was reduced and the drying rate was increased by increase the time and temperature of softening treatment and increase the temperature,reduce the time of drying set treatment.The best technological conditions obtained in this study are:softening treatment of saturated wet air at 95℃ for 24 hours,followed by variable treatment of wet air at 120℃ and 30%humidity for 18 hours.Compared with untreated test materials,which are dried according to the same standard directly,the surface crack is greatly reduced and the drying time is shortened by about 50%with conventional drying after pretreatment according to the process conditions.(2)The change of drying strain distribution of specimen was measured before and after the different damp-hot pretreatment processes and after the follow-up troditional drying,And explored the cracking mechanism based on the degree of surface cracking.The effect of wet heat pretreatment on the dry strain of the specimen surface was obvious.In the process of pretreatment,the creep of the specimen surface under the action of tensile stress causes the stress relaxation,the actual dry shrinkage strain decreases and the shrinkage degree of the core and sapwood is uniform.The specific performance is the decrease of tensile elastic strain reduces the influence of internal tensile stress on the surface layer;the decrease of viscoelastic creep strain makes the stress distribution in the wood uniform and the cracking decreases;the increase of mechanical adsorption creep improves the dimensional stability of the surface layer,inhibits the surface cracking,but intensifies the internal cracking.The inhibition of surface crack could be promoted by increased of softening treatment temperature and time,and the increase of setting treatment temperature,and the generation of surface crack and internal crack could be aggravated by increased of setting treatment time.(3)The conventional drying process of pithy core of Pinus sylvestris var.mongolica was used as the research object and Python was used to develop it in a PyCharm integrated development environment.Based on a BP(back propagation)neural network algorithm,the model input was steaming treatment time,steaming temperature,setting time,setting temperature,initial moisture content of the wood,and location of the wood core and sapwood.The wood drying rate and longitudinal fissure degree were output to predict the conventional drying rate and surface cracking degree of wood.The correlation coefficient of the wood drying rate prediction was 0.96,and that the longitudinal fissure degree prediction was 0.99.The experimental results and predicted values were nearly identical,which proved that the BP neural network model can predict the effect of damp-hot pretreatment on conventional drying of square-edged timber with heartwood.