| Carbon fiber reinforced composites have been widely used in aerospace,marine,construction and other fields due to their excellent performance.However,composite structures are inevitably subject to a variety of defects introduced during manufacturing and service.For these large integrated components;however,it is infeasible to achieve structural replacement due to high cost and long period of service.Therefore,composite repair technologies have received significant attention,and bonding repair technology were widely used in various fields duo to it’s great advantages.The main objective of this study was to improve the strength of the joints by various methods,and to enhance the reliability of repaired structures.The following studies are mainly conducted:T300 carbon-fiber/epoxy-prepreg with the fiber density of 1.77 g/cm3 and the thickness of each layer of 0.15 mm was used to fabricate the carbon fiber composite laminate by autoclave curing,and was cut to produce the composite specimens according to the test standard.Three types of joints,including step lap joint,scarf adhesive joint,and mortise and tenon joints,were fabricated with the same bonding area.The average failure loads and strength of various specimens were obtained through the tensile and bending tests,and the failure morphology of various specimens was analyzed as well.The experimental results showed that the mechanical properties of different types of adhesive joints were quite different,and the scarf adhesive joint obtained the best mechanical properties.Four grades of sandpaper with grit sizes up to 60,220,400,800,and polish were used to prepare the bonding surface,and two typical adhesive joints(single lap joint and scarf adhesive joint)were tested to investigate the effects of the grit size and sanding direction of the sandpaper(parallel to the fiber direction,perpendicular to the fiber direction,and random direction)on the tensile properties.Moreover,the surface parameters,including the surface profile,surface roughness,surface contact angle,and surface energy of the specimens,were measured to study the influence of grit sizes of sandpaper on the surface characteristic.Experimental results showed that the highest bonding strength could be achieved when the surface was sanded in the random direction,and the shear strength was 10%higher than that of unidirectional grinding(parallel to the fiber direction and perpendicular to the fiber direction).For the single lap joints,optimum shear strength was obtained when samples were abraded using sandpaper with grit size of 220,and the shear strength increased by 17.6 and 22.3%,respectively,compared to the grit sizes of 60 and polish processing.For the scarf adhesive joints,the maximum tensile strength was achieved when samples were abraded with 400 grit size,and the tensile strength increased by 11.0 and 8.9%,respectively,compared to the grit sizes of 60 and polish processing.Four ultrasonic application time of 5,20,40,60 min,four ultrasonic power of 200,300,400,500 W,and two types of adhesive(rigid adhesive DP270 and tough adhesive Araldite 2015)were used to study the influence of ultrasonic parameters and adhesive types on the toughness of joints by experimental.The experimental results showed that water immersion ultrasonic could enhance the bonding performance significantly,and the effect of ultrasonic time and power on the bonding performance of the joint were influenced by the adhesive properties.The mode I fracture energy(GIC)were found to be improved by 88.6%and 46.8%for the joints bonded with rigid adhesive DP270 and tough adhesive Araldite 2015,respectively,under the optimum ultrasonic conditions compared to reference groups. |
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