| The resources of bamboo are abundant in our country.Lots of researches about bamboo have been conducted on the product development or process modification.But there are few researches about the bonding interface characterization of bamboo composites,and still follows the adhesion theory used in wood.Besides,the structure of bamboo is compact and there is no transverse organization which affects the penetration of the adhesive into bamboo.Therefore,in this paper,the bleached and carbonized bamboo were treated under different ultrasonic processes.The change of surface characteristics of bamboo were analyzed,including the surface roughness and surface wettability.The optimal ultrasonic process was established refer to the shear strength of the bamboo laminated lumber.The scanning electron microscope,fluorescent track technology,digital image correlation method and finite element method were used to analyze the effects of ultrasonic treatment on the interface properties of the bamboo laminated lumber,including microstructure,adhesive penetration,and strain distribution of the bonding interface.The relationship between micro parameter and shear strength was discussed.Research methods and results were as follows:(1)The surface roughness and the surface wettability of bleached and carbonized bamboo before and after different ultrasonic process treatment was calculated.Result showed that ultrasonic treatment increased the surface roughness and the surface free energy and lowered the contact angle of phenol formaldehyde adhesive in bamboo.It indicated that the surface wettability of bamboo was better after ultrasonic treatment.(2)The shear strength of two-ply bamboo laminated lumber after different ultrasonic process treatment was calculated.Result showed that ultrasonic cavitation increased the transverse permeability of bamboo and improved the adhesive penetration,thus increased the shear strength of bamboo laminated lumber.The influence degree of ultrasonic process parameter to shear strength was temperature>power>time.As for the bleached bamboo,the optimal ultrasonic process was as follows: ultrasonic temperature was 60℃,ultrasonic time was 60 minutes and ultrasonic power was 1200 W.As for the carbonized bamboo,the optimal ultrasonic process was as follows: ultrasonic temperature was 60℃,ultrasonic time was 90 minutes and ultrasonic power was 1400 W.(3)The morphology of the adhesive bond-line on the bamboo surface was investigated by a scanning electron microscope and the bond-line thickness was calculated.Result showed that the bond-line thickness of untreated bamboo laminated lumber was relatively small and the bonding interface was flat and smooth.The adhesive was mainly distributed in the broken cell cavities formed during sample preparation,as well as between the cell walls on the surface.After the ultrasonic treatment,the bond-line thickness was increased and the boundary between the bamboo and the adhesive was indistinct.Adhesive not only distributed in the broken cell cavities,but also distributed in the adjacent parenchyma through the bigger cell wall pits.(4)The average depth and effective depth of the adhesive on the bamboo surface were evaluated using fluorescent microscopy characterization.Result showed that the bonding interface in laminated lumber was changed after ultrasonic treatment.The bond-line on treated bamboo surface has more twists and turns and the adhesive was distributed unevenly.The average depth and effective depth of the adhesive on the bamboo surface were increased after ultrasonic treatment.(5)Strain distribution in the bamboo laminated lumber bonding interface had been studied by the method of digital image correlation in the shear strength testing.Result showed that shear strain was started from the bottom of the bonding interface and then extended through bond-line in a vertical direction and through matrix at a horizontal direction.When the load was increased,the longitudinal strain was increased,while the transverse strain extended to the region of 4mm.The maximum strain of bonding interface in bamboo laminated lumber was lower after ultrasonic treatment,which was related to the higher elasticity modulus of the cell wall resulted from the penetration of the adhesive.(6)Stress and strain distribution in the bamboo laminated lumber bonding interface had been investigated by means of finite element method.Result showed that the distribution of stress and shear strain in the bonding interface were basically the same before and after ultrasonic treatment and were symmetrical on both sides.The stress and shear strain on the right hand were higher than those on the left hand,and those in the bottom were higher than those at the top.The curve of stress and strain distribution along the bond-line and normal to the bond-line in the bonding interface were basically similar.The stress was mainly the tensile stress along the X axis,in addition the peel stress along the Y axis.But the peel strain along the Y axis was higher than the tension strain along the X axis.The maximum value of stress and strain were occurred in the contact point between adhesive and matrix.(7)Comparison the surface characteristics and bonding interface properties between bleached and carbonized bamboo laminated lumber.Result showed that the surface roughness of bleached bamboo was higher and the surface wettability was lower than that of carbonized bamboo.While the shear strength of bleached bamboo laminated lumber was higher than that of carbonized bamboo laminated lumber.The surface cells of the carbonized bamboo laminated lumber were crushed under heat and pressure,resulting in higher bond-line thickness and more penetration of the adhesive.Due to the lower elasticity modulus of carbonized bamboo,the stress in the carbonized bamboo laminated lumber bonding interface was more concentrated and the maximum value of strain was higher than those in the bleached bamboo laminated lumber bonding interface.The finite element method confirms these differences. |
PDF Full Text Request