| This study characterized the basic characteristics of Taxodium hybrid‘zhongshanshan’wood at the macroscopic and microscopic levels to reveal the mechanism and behavior of its hygrothermal deformation under the effect of moisture and heat.This study also conducted predictive analysis and simulation of hygrothermal deformation of Taxodium hybrid‘zhongshanshan’wood and its rotary-cut veneers.The results presented here are of profound theoretical significance for understanding the characteristics of Taxodium hybrid‘zhongshanshan’wood.In addition,this research increases the number of species that have been examined by wood science research,providing a scientific basis for the utilization and development of Taxodium hybrid‘zhongshanshan’wood and relevant guidelines for its processing and utilization.This study characterized the macroscopic and microscopic characteristics and the mechanism of hygrothermal deformation of Taxodium hybrid‘zhongshanshan’wood through high-resolution3D laser scanning,observations of randomly selected quadrilateral areas of wood slices,dynamic thermomechanical analysis,collaborative testing of thermal conductivity and the thermal expansion coefficient,and numerical simulation.The results are presented below.（1）Taxodium hybrid‘zhongshanshan’wood is a heartwood species with clear annual ring demarcations and small latewood bands.Change from earlywood to latewood is gradual,and its wood rays are slender and densely distributed.Its resin channels are rare,and its knots are rich.Its surface is smooth,with a"V"pattern on the tangential section.Its structure is fine,and its texture is straight,as it is considered a light,softwood.From the pith to the outside,the moisture content of the greenwood decreases and then increases,the air-drying equilibrium moisture content increases linearly,and the basic density increases parabolically.The number of years separating juvenile wood and mature wood is 12 years.（2）The tracheid wall of earlywood is thin,and the tracheid wall in the radial direction is larger than the tracheid in the tangential direction.The tracheid wall of latewood is thick,and the tracheid wall in the radial direction is smaller than the tracheid wall in the tangential direction.In the same annual ring,the tracheid width in the tangential direction does not change and the tracheid width in the radial direction decreases from earlywood to latewood.The tracheid has uniseriate or biseriate-bordered pit pairs in radial cell walls.The tangential double cell wall thickness of latewood tracheid is smaller than that of the tracheid cell lumen,and the ratio ranges from 0.21 to 0.35.It has diffuse and diffuse-in-aggregate axial parenchyma,with nodular thickening at end walls.The wood ray is composed of ray parenchyma,the end wall has no nodular thickening,and the single pit pairs are visible.The cupressiod cross-field pits can be observed,and the number of cross-field pits is 2 or 3（occasionally 1 or 4）.The wood ray at the tangential section has a large volume,forming more interlaced space with tracheids.（3）The fiber saturation point of the Taxodium hybrid‘zhongshanshan’wood was determined according to arbitrary quadrilateral areas of its wood slices at the microscopic level,which was between 33.33 and 38.33%.Under 103±2°C,when the moisture content of Taxodium hybrid‘zhongshanshan’wood was above its fiber saturation point,the tracheids showed weak elastic deformation with"shrink-recovery"fluctuations.When its moisture content was below its fiber saturation point,the tracheid shrinkage of latewood was greater than that of earlywood,and the tracheids had greater shrinkage in the tangential direction than in the radial direction,stemming from the restraint of ray parenchyma cells and the resultant plastic deformation.Consistent with the macroscopic shrinkage behavior of wood,the shrinkage rate of wood increased from the pith to the outside,and the difference between tangential and radial shrinkage decreased.High-resolution three-dimensional laser scanning provided a straightforward,fast,and accurate measurement of deformation,cracks on the loose sides,and microscopic shrinkage behaviors of wood veneers.After Taxodium hybrid‘zhongshanshan’wood veneers were air-dried,the volume shrinkage in the transverse direction was significantly larger than that in the parallel direction,and its warpage degree ranged from 21.18 to 40.88%.The axis of the veneer was skewed by 7°to 12°along the direction parallel to the fiber.（4）When Taxodium hybrid‘zhongshanshan’wood of initial moisture content between 6%and 18%experienced a temperature change from-50 to 150°C,its dynamic loss modulus showed a loss peak around 72°C.This mechanical relaxation process reflected the glass transition of hemicellulose.At room temperature,the storage modulus and loss modulus of heartwood were larger than the storage modulus and loss modulus of sapwood.The radial storage modulus was larger than the tangential storage modulus,and the loss modulus in radial and tangential directions was similar.The glass transition temperature of the radial hemicellulose was slightly lower than that of the tangential hemicellulose.At room temperature,the storage modulus and glass transition temperature of hemicellulose decreased as the wood moisture content increased.The loss modulus was not correlated with moisture content.（5）During the process of isothermal humidification of Taxodium hybrid‘zhongshanshan’wood,the wood moisture content increased linearly as environmental relative humidity increased.The growth in moisture content at high temperature（90°C）was more rapid than at low temperature（30°C）,while the moisture stain at high temperature（90°C）was less than that at low temperature（30°C）.The high to low moisture expansion coefficients of Taxodium hybrid‘zhongshanshan’wood of different directions were the following:tangential(1400×10-5),radial(698×10-5),and longitudinal(96×10-5).The thermal conductivity of Taxodium hybrid ‘zhongshanshan’wood increased linearly with heat transfer temperature and wood moisture content.When the moisture content was 12%and the heat transfer temperature was 30°C,the average thermal conductivity was 0.1257 W/（m·K）.The thermal expansion coefficient of Taxodium hybrid‘zhongshanshan’wood and its longitudinal moisture expansion coefficient were of the different order of magnitude,indicating that the hydrothermal effects were dominated by the moisture effect.During the steady-state heat transfer process,the thermal expansion coefficients of Taxodium hybrid‘zhongshanshan’wood oscillated between negative and positive values,and the wood showed alternating shrinkage and expansion.（6）Taxodium hybrid‘zhongshanshan’wood is a natural composite material.Based on the lamination theory of the composite,it is considered a multi-layer composite;that is,lamination is present in three directions:longitudinal,radial,and tangential.Considering that hygrothermal deformation represents a stress-strain relationship of a single-layer board,a hygrothermal deformation model of Taxodium hybrid‘zhongshanshan’wood was established.The established hygrothermal deformation model was numerically simulated without considering the viscoelastic effect of wood.The simulated value of deformation during the shrinking process at 30°C was lower than the measured value,while its trend was consistent with the measured value.During the isothermal humidification process,the simulated value of deformation was extremely small,suggesting that the thermal effect of the hydrothermal effects was not apparent.The simulation showed that the hygrothermal deformation of Taxodium hybrid‘zhongshanshan’veneer arose from the moisture content gradient between the surface layer and the core layer of the veneer.The difference between the simulated value and the measured value was associated with the deviation in thickness of the veneers and the cracks on the loose side of the veneers. |
