Mechanism Of Setting On Drying Strain And Cracking Influence Of Pinus Sylvestris Var.Mongolica Litv.Boxed-Heart Square Timber

Pinus sylvestris var.mongolica Litv,as one of the main natural forest trees in northeast of China,has good mechanical properties and visual properties,and its small-diameter boxedheart square timber is suitable for column of Japanese wood structure building,which has broad application prospect in the field of wood structure building.However,due to the anisotropy of drying shrinkage,even if the drying is slow and the moisture content gradient is very small,it still produces great stress due to the difference of the radial drying shrinkage,which leads to the hard cracking of the drying surface and seriously reduces the use value.Therefore,it is of great significance to study the drying cracking control technology and mechanism of the boxed-heart square timber.Based on the section 120mm x 120mm Mongolian pine boxed-heart timber as the research object,in this paper,the free shrinkage characteristics and accurate detection method of mechanical adsorption creep,on the basis of the effects of softening and setting process on actual shrinkage strain,elastic strain,viscoelastic creep strain,mechanical adsorption creep and cracking were studied,and the suitable setting process was determined.The variation of tensile strength of transverse grain with temperature and moisture content and the effect of setting treatment on tensile strength of transverse grain were studied.By analyzing the effect of stress steering on cracking suppression and the effect of stress steering on strain,tensile strength of transverse grain and cracking,the mechanism of cracking suppression by high temperature setting treatment was discussed.The grey GM-BP neural network model was used to predict and simulate the drying stress development trend.The specific research content and important conclusions are summarized as follows:(1)Free drying shrinkage characteristics and mechanical adsorption creep strain detection method of Mongolian pine boxed-heart timbe and the effect of temperature and location on free drying shrinkage properties.Based on the moisture content and size of the thin test strip when it slowly and freely shrinks to the equilibrium moisture content corresponding to each drying condition,the free drying shrinkage characteristics of each layer on the thickness of the test material of Mongolian pine boxed-heart timbe at different temperatures were analyzed.The mechanical adsorption creep strain was calculated based on the free drying shrinkage coefficient and the measured free drying shrinkage size after the plastic deformation was eliminated by steam respectively.By comparing the two results,the accurate detection method of mechanical adsorption creep strain was determined.The results showed that there was an obvious linear function between the free drying shrinkage rate and moisture content of each layer of Mongolian pine boxed-heart timbe,and the coefficient of determination R2 was above 95%.The free drying shrinkage coefficient of the near chord layer is greater than that of the near radial layer,the sapwood is greater than that of the heartwood,and the other layers outside the core layer increase first and then decrease with the increase of temperature(between 60?and 120?),while the core layer decreases continuously with the increase of temperature.In addition,the free drying shrinkage size based on the free drying shrinkage coefficient can realize the accurate detection of mechanical adsorption creep strain.(2)Effect of high temperature setting process on strain change and cracking of test material under high moisture content.Saturated wet air at 95? was used to soften the test material for different time,and then setted for different time with high temperature and low humidity.The effects of softening time and setting time on the actual shrinkage strain,elastic strain,viscoelastic creep strain,mechanical adsorption creep strain and cracking of the drying process after setting treatment and subsequent drying process were studied.The suitable softening and setting process was determined.The results show that under the condition of high moistur content,the softening time is prolonged,and the rheology and plasticity of square wood are enhanced,and the enhanced part extends from surface layer to core layer.It is beneficial to the generation and recovery of viscoelastic creep strain in the process of setting,and increases the mechanical adsorption creep(surface stretching,internal compression)of each layer of square wood in the process of setting.The internal compression creep weakens the inhibition of surface drying shrinkage,reduces the elastic strain level,and makes the overall shrinkage of square wood more uniform.The mechanical adsorption creep of each layer is still large after drying.The tensile mechanical adsorption creep of the surface layer and the compressive mechanical adsorption creep of the core layer are increased with the extension of the setting time.The difference of actual drying shrinkage strain of each layer decreases.The elastic strain develops and turns in advance in the process of setting,and increases the elastic strain level in the later drying period.The viscoelastic creep strain is easy to produce and recover.Under the condition of high moisture content,the optimal process of high temperature setting treatment is:The saturated wet air at 95? was softened for 8h,the dry bulb temperature was 120?,the wet bulb temperature was 90? and the wet air was setted for 12h.(3)Effect of high temperature setting process on strain change and cracking of test material under low moisture content.The setting of the actual shrinkage strain,elastic strain,viscoelastic creep strain,mechanical adsorption creep strain and cracking of the specimens after setting and the subsequent drying process were analyzed by using different softening and setting treatments.The results show that the increase of softening temperature increases the plasticity of the surface layer and increases the tensile mechanical adsorption creep strain of the surface layer.The actual dry shrinkage strain decreases and the elastic stress level decreases,which is beneficial to the generation and recovery of viscoelastic creep strain.With the extension of softening time,the increasing plasticity extends to the core layer,but if the softening time is too long,the increasing plasticity is not obvious,and the effect on drying stress is not significant.The surface mechanical adsorption creep increases with the increase of setting temperature.The creep strain of mechanical adsorption is relatively large in advance of elastic stress shift and in late drying period.The lengthening setting time increases the tensile elastic strain of the hardened surface layer and the core layer after drying.The tensile mechanical adsorption creep of the stabilized and dried surface layer and the compressive mechanical adsorption creep of the core layer are both larger.Under the condition of low moisture content,no variable treatment process has been obtained to suppress cracking effectively.The optimal process with cracking inhibition effect is:The saturated wet air at 95?was softened for 8h,the dry bulb temperature was 120?,the wet bulb temperature was 90?and the wet air was setted for 6h.(4)Cracking mechanism was inhibited by high temperature setting treatment.The effect of stress steering on cracking suppression was analyzed comprehensively,and the effect of setting treatment on strain,tensile strength and cracking of transverse grain.The mechanism of cracking inhibition by high temperature setting treatment was discussed.The results show that temperature and moisture content have a significant effect on the tensile strength of transverse grain,and the tensile strength of transverse grain decreases with the increase of temperature and moisture content;The transverse tensile strength of the surface layer of the treated material under air-dry condition is obviously smaller than that of the untreated material,and the difference of the transverse tensile strength of the core layer is small.It can be seen that the tensile strength of the transverse grain on the surface of the specimen decreases due to the high temperature setting treatment,which has a negative effect on the inhibition of surface crack;In the process of high temperature setting,stress steering has no obvious effect on cracking inhibition.In the process of high temperature,the rheological properties of wood are enhanced,and tensile creep and stress relaxation occur on wood surface under tensile stress.At the same time,the internal compression creep occurs under compressive stress.It weakened the inhibition of surface drying shrinkage,weakened the surface tensile stress further,and reduced or even avoided the occurrence of surface crack;The large compressive viscoelastic creep strain and mechanical adsorption creep strain in the early stage of the treatment are beneficial to restrain the internal crack under tensile stress after the stress shift.(5)The GM-BP model is used to predict the development trend of drying strain.The GM(1,1)grey prediction model was used to predict the variation trend of moisture content in the conventional drying process after high temperature setting.The predicted value of moisture content,softening temperature,softening time,setting temperature and setting time were used as input of BP neural network model to predict the development trend of elastic strain of surface layer and core layer in the future.The results show that the GM(1,1)grey prediction model has a high accuracy in predicting the variation trend of moisture content with time in the conventional drying process after high temperature setting.The average absolute error of GMBP model for predicting the development trend of surface elastic strain is 2.19×10-4.The average core absolute error is 1.77×10-4.Compared with the BP neural network model,the surface error is 2.5×10-4.The core layer error value is 2.48×10-4.The results show that the prediction accuracy of GM-BP model is high,and it can simulate and predict the development trend of drying strain in the next adjacent period in the conventional drying process after the high temperature setting.