Study On Joints Bonding Performance Of Structural Finger-Jointed Lumber Of Spotted Gum(Eucalyptus Maculate H.f.)

Spotted gum(Eucalyptus maculate H.f.)has excellent mechanical properties,but its gluing is difficult,and the difficulty in gluing has led to its failure to be widely used in the field of wood structures.Focusing on the technical requirements of structural engineering wood,this topic studies the gluing properties of finger-joint joints of spotted gum lamber from the aspects of surface wettability,gluing interface performance,and finger-joint joint force behavior.The main conclusions are as follows:1)Spotted gum lamber,compared with spruce and sylvestris pine,its physical and mechanical properties have obvious advantages.The physical and mechanical properties of spotted gum lamber,such as hardness,density,flexural elastic modulus and flexural strength,are significantly better than those of spruce and sycamore pine.The air-drying shrinkage rate of spotted gum lamber is higher,and its dimensional stability is not as good as that of spruce and sycamore pine.There were differences in the wettability of different sections of spotted gum lamber,and the wettability of the cross section was slightly better than that of the chord section and the radial section.After treatment with three different surfactants,the wood surface wettability of spotted gum lamber was improved,and 10 wt%NaOH solution had the most obvious improvement,followed by absolute ethanol solution,and 10 wt%ammonia solution had the smallest improvement.High-temperature surface treatment is not conducive to improving the surface wettability of spotted gum lamber.2)The single-shear specimen of spotted gum lamber is inclined under compression,resulting in its ultimate shear strength much greater than that of the double-shear specimen.The double-shear specimen can effectively avoid the inclination of the specimen,and is suitable for the shear performance test of the gluing interface of spotted gum lamber.Adhesives have a great influence on the shear strength of the wood bonding interface of spotted gum lamber.Among the five selected adhesives,polyurethane adhesives are the best,followed by modified polyvinyl acetate,and unmodified polyvinyl acetate is better than modified isocyanate adhesives,epoxy adhesives performed the worst.When the modified polyvinyl acetate glue was used to glue the wood of spotted gum lamber,the interfacial shear strength increased by 47%after treatment with absolute ethanol solution;the interfacial shear strength increased by 0.8%after treatment with 10 wt%NaOH solution.3)The glued shear strength and average wood rupture rate of spotted gum lamber increased gradually with the increase of the inclination angle of the oblique section.The correlation fitting analysis was carried out on the interfacial shear strength and average wood failure rate of spotted gum lamber with the inclination angle of the oblique section(15°～90°),respectively,and the fitting expressions were obtained as y=0.0004x~2-0.01x+0.7682,y=-0.0003x~2+0.1322x-0.074;the fitting coefficients R~2 are 0.9723,0.9968,respectively.4)Finger tenon structure and finger joint end pressure are the main factors affecting the mechanical properties of structural finger joint materials.In a certain range,the increase of finger tenon length,the decrease of finger tenon slope and the increase of finger joint end pressure can improve the bonding strength of finger-jointed joints.The finger-tenon top spring-back and relaxation test showed that the finger joints used in the structural fingered-jointed lamber of spotted gum should adopt a fitting degree of 0.1 mm and an end pressure slightly greater than 9.23 MPa.5)There are differences in the gluing performance of structural finger-jointed joints of spotted gum among the five selected adhesives,among which the tensile properties of the modified polyvinyl acetate adhesive are better than other adhesives,and the bending performance of unmodified polyvinyl acetate adhesive is better than other adhesives.After high-temperature surface treatment and absolute ethanol solution treatment,the bonding tensile strength and bending strength of the joints of finger joint of spotted gum were increased by 5.7%and 3.8%respectively,and 0.6%and 0.8%respectively.6)The type of tensile load-longitudinal strain curves of two structural finger-jointed timber joints of spotted gums.The A-type curve has a plateau period,and the B-type curve increases approximately linearly.The strain at the finger joint node of the specimen changes abruptly when the finger joint material fails.The longitudinal strain-time curve obtained by the image acquisition system is similar the static strain curve.It is found from the mechanical failure process of the structural finger-joint joints of spotted gum that both types of node damage started from the end of the finger-joint node,and then extended to the whole finger-joint node.The overall strain of the specimen expanded and gradually weakened from the finger-joint node to the surrounding area.