| Thermal-treated wood has a larger share in the modified-treated wood market,which can improve wood color and dimensional stability,but has a negative impact on the mechanical strength of wood.When thermal-treated wood is used in load-bearing structures,it is necessary to detect its mechanical strength.Therefore,it is of great significance to explore a non-destructive testing method to quickly and accurately detect the mechanical strength of thermal-treated wood.In this study,poplar(Populus tomentosa Carr.)and spruce(Picea asperata Mast.)were used as experimental materials.The wood was thermal treated in different media such as air,nitrogen and steam and at different temperatures and times.The color parameters L*,a*,b*and mechanical strength such as flexural elastic modulus,flexural strength and shear strength were measured.The support vector regression(SVR)machine method was used to optimize the parameters of the"color-mechanical strength"characterization model of thermal-treated wood through cross-validation and grid search algorithms,and the"color-mechanical strength"characterization model of thermal-treated wood with high prediction accuracy and good effect was established.At the same time,the NREL method and infrared spectroscopy(FTIR)analysis were used to explore the response mechanism of the color to the change of the main components of the cell wall.The response mechanism of the color to the microstructure of the cell wall was investigated by scanning electron microscopy(SEM)and X-ray polycrystalline diffractometer.Based on the principal components and the microstructure of the cell wall,the response mechanism of"color-mechanical strength"was studied.The main results are as follows:(1)The correlation analysis showed that the colors L*,a*,b*of poplar and spruce thermal-treated wood had a significant positive correlation with flexural elastic modulus,flexural strength and shear strength.Among them,the correlation between color parameters L*,b*and mechanical strength was better(r was generally higher than 0.5),the correlation between color and mechanical strength of air medium thermal-treated wood was the best(r was 0.908),and the correlation between color and mechanical strength of thermal-treated poplar was better than that of thermal-treated spruce.(2)Through the SVR machine method,with RBF as the kernel function,grid search method and 5-fold cross-validation error,the optimal hyperparameter combination was obtained:the Gamma value was 500,the C value was 10000,and theεvalue was 0.1.A model for characterizing the mechanical strength of thermal-treated materials with high prediction accuracy and good effect was developed.The R~2 values of the SVR model training set of the thermal-treated poplar and spruce were all above 0.9691,and the prediction accuracy was high.Among them,the SVR model of flexural strength has the highest prediction accuracy and the best effect.In the poplar prediction model,the flexural strength prediction model test set of water-vapor therma-treated wood has the best prediction effect,and the R~2 value was 0.9026.In the spruce prediction model,the flexural strength prediction model test set of air-medium thermal-treated wood has the best effect,and the R~2value was 0.8970.The linear fitting of the measured and predicted mechanical strengths of poplar and spruce showed that there was a significant correlation,the fitting effect was good.(3)Thermal treatment causes thermal degradation of the main components of the wood.The relative contents of hemicellulose and lignin in the thermal-treated poplar showed an overall decreasing trend with the decrease of brightness L*,and the relative contents of cellulose showed an overall increasing trend,and the relative contents of brightness L*and the three major constituents were all highly significantly correlated.The relative content of cellulose showed a trend of increasing and then decreasing.The mechanical strength of the wood increased to a certain extent,but when the brightness L*of poplar and spruce significantly to 32.23 and 30.86,the degradation of the principal components was intense,resulting in a significant decrease in mechanical properties.The degradation of the principal components of the cell walls of the thermal-treated spruce was more intense.Infrared spectroscopy shows that deacetylation of hemicellulose,breakage of cellulose molecular chains and breakage of lignin benzene rings occur within the wood during thermal treatment,and that the changes become more pronounced as the luminosity decreases.(4)The thermal degradation of the main components of the cell wall of thermal-treated lumber causes damage to the cell wall structure,with some cells undergoing wrinkling and radial splitting.There is a significant positive correlation between brightness L*and radial cell wall thickness,with the thermal degradation of the main components of the wood increasing as the thermal treatment increases,and the radial cell wall thickness decreasing as the brightness L*decreases,which has a negative impact on the mechanical strength of the treated wood.The radial cell wall thickness decreased from 2.29μm to 1.65μm in treated poplar,from 2.21μm to 1.59μm in earlywood,and from 5.31μm to 2.70μm in latewood.(5)The crystallinity of thermal-treated poplar and spruce showed a trend of first increasing and then decreasing with the decrease of lightness L*.When the lightness L*of poplar decreased to 37.55(YK-6),the crystallinity reached the maximum value of 60.18%,and when the lightness L*of spruce dropped to 34.15(YK-5),the crystallinity reached the maximum value of 51.45%.The crystallinity increases,and the intermolecular force increases,which improves the mechanical strength of wood to a certain extent.When the degree of thermal treatment is deepened,the lightness L*decreased,and the acetic acid generated by the decomposition of hemicellulose would degrade the amorphous area of cellulose,even the microfibrils in the shaped area,which would reduce the crystallinity and reduce the mechanical strength of wood.The width of the crystalline region and crystallinity of the thermal-treated poplar and spruce had the same trend of change.The increase of the crystallinity of thermal-treated poplar was caused by the increase of the crystalline region,and the width of the crystalline region reaches the maximum in the YK-6 group.The width of the crystalline region of heat-treated spruce began to decline in the YS-4 group,and the increase of the crystallinity was the combined effect of the increase of the crystalline region and the decrease of the amorphous region. |
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