| Chinese fir(Cunninghamia lanceolata)is the structural timber tree species,which plantation area ranks first in China.Chinese fir clones have been widely cultivated and planted for their advantages of high genetic gain,low cost and excellent parental traits.The evaluation of mechanical properties and exploration of microstructure factors affecting its mechanical properties of new clones wood are very important for clone tree breeding and efficient wood processing and utilization.Therefore,six newly bred clones of Chinese fir(Kaihua 3,Kaihua13,Daba 8,F24x na 1-1,Yang 020 and Yang 061)were selected as experimental materials in this paper.Using Universal Capability Testing Machine,the bending strength,bending modulus of elasticity,compressive strength parallel to grain and hardness of 6 clones wood were measured with reference to relevant national standards.The calibration and prediction models of wood mechanical properties of Chinese fir clones were established by analyzing the value of wood mechanics and near infrared spectroscopy using near infrared spectroscopy technology.By means of optical microscopy,X-ray diffraction and Fourier microscopic infrared imaging system,the characteristics of microstructure factors affecting wood mechanical properties were analyzed and measured,such as the microscopic morphology of wood cells,ultrastructure of cell wall,which including the angle of microfibrils,crystallinity and relative content of lignin.The main conclusions of this paper are as follows:1.Variation of mechanical properties of Chinese fir clones(1)The bending strength grades of six clones wood were all at low lever,from high to low of 20-year-old clones,they were F24x na 1-1(57.60 Mpa),Daba 8(55.36 MPa)and Kaihua 3(51.83 MPa).The bending strength of 10-year-old clones Yang020 and Yang061 was lower than20-year-old clones,it is 51.36 Mpa and 42.56 MPa,respectively.And the bending strength values gradually increased from pith to bark in the radial direction.(2)The bending modulus of elasticity grades of six clones wood were all at low lever,from high to low of 20-year-old clones,they were F24 Na 1-1(11.08 GPa),Kaihua 13(10.95GPa),Daba 8(10.85 GPa)and Kaihua 3(10.04 GPa),The bending modulus of elasticity of10-year-old clones Yang020 and Yang061 was lower than 20-year-old clones,it is 10.18 GPa and 8.98 GPa,respectively.And the bending modulus of elasticity gradually increased from pith to bark in the radial direction.(3)The compressive strength parallel to grain grades of the six clones wood were all at low lever.From high to low of 20-year-old clones,the clone timbers were Kaihua 13(33.52MPa),Daba 8(32.55 MPa),F24 na 1-1(32.71 MPa)and Kaihua 3(30.98 MPa).The compressive strength parallel of 10-year-old clones Yang020 and Yang061 was lower than20-year-old clones,it is 30.27 Mpa and 27.20 MPa.And the compressive strength parallel to grain gradually increased from pith to bark in the radial direction.(4)The hardness grades of six clones wood are very soft,from high to low of 20-year-old clones,they were F24x Na1-1(1715 N),Kaihua 13(1682 N),Kaihua 3(1606 N)and Daba 8(1575 N),and 10-year-old clones Yang020 and Yang061 were slightly lower,1486 N and 1340N,respectively.The hardness values of each surface were different greatly,ranked by cross hardness>tangential hardness>radial hardness in all the clones.(5)All six clones were low-strength wood,and Kaihua 13,F24xna 1-1 and Yang 020 were Chinese fir clones with excellent mechanical properties in this study.2.Prediction of Mechanical Properties of Chinese fir Clones using Near Infrared(1)The correlation coefficients(Rc)of correction models for bending strength were above0.91 in all clones,and the correlation coefficients(Rp)of prediction models were between 0.84and 0.87.Those of F24na 1-1 and Kaihua 3 models had the best prediction effect with Rp of0.87.The prediction effect of PLS model based on the information of six clones was better,and Rc was 0.89.(2)The PLS model of bending modulus of elasticity of all clones had Rc between 0.86 and0.93 and Rp between 0.84 and 0.88.And the prediction effect of Daba 8 was the best,Rc was0.88.The Rc of prediction models using synthesizing information of six clones was 0.88.(3)The PLS models of compressive strength parallel to grain of all clones had good prediction effects with Rc above 0.92 and Rp between 0.87 and 0.90.But the accuracy of the model established by synthesizing information of six clones was reduced(Rc=0.86).(4)The PLS model based on the information of wood hardness of all six clones had a good prediction effect.The correlation coefficient of the corrected model was 0.95 and that of the predicted model was 0.88.3.Microstructure Factors Affecting Wood Mechanic Properties(1)The average wall-lumen ratio of six clones were 0.167-0.212,which was positively correlated with the density.There was a positive linear correlation between the density and compressive strength parallel to grain,bending strength and bending modulus of elasticity,with the correlation coefficients of 0.84,0.81 and 0.69.(2)The wood microfibril angle of all clones was from 10.47~oto 13.18 ~o,and that of Kaihua13 was the smallest and Daba 8 was the largest.The microfibril angle decreased gradually from pith to bark in the radial direction.This was negatively correlated with bending strength,bending modulus of elasticity and compressive strength parallel to grain,as they were all increased from pith to bark.That is to say,the angle of microfibril decreases and the mechanical properties of wood increase.(3)The wood crystallinity of all clones ranged from 35.88%to 44.63%,and that of Yang061 was the lowest and Kaihua 13wasthe largest.And in the radial direction,it gradually increased from pith to bark.This trend was the same as that of bending strength,bending modulus of elasticity and compressive strength parallel to grain,which showed a positive correlation between crystallinity and these properties.That is to say,the mechanical properties of wood increase with the increase of crystallinity.(4)Among the six clones,the hardness value of F24x na1-1 with the highest relative content of lignin was the highest,and that of Yang061 with the lowest lignin relative content was the lowest.Generally speaking,the lignin content was positively proportional to wood hardness,that is,the higher the lignin content was,the higher the wood hardness was.As a heterogeneous material composed of a variety of substances,wood has overlapping influence factors such as microstructure,microfibril angle,crystallinity and lignin content,which reflect the mechanical properties of wood. |
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