| Carbon fiber reinforced polymer have been widely applied to the reinforcement of steel structures because of their excellent mechanical and material properties,such as high specific strength,large specific modulus and corrosion resistance.The bonding layer is the weakest part in the CFRP-steel composite structure.The strength of the bonding layer must be improved to increase the overall strength of the CFRP-steel composite structure.The strength of the bonding layer depends on the bonding interface and the strength of the resin material.The strengths of the bonding interface and the bonding layer were increased by using grooved steel surface and other high-performance materials.Based on the significant effect of kevlar fibers(aramid fiber)on bridging toughening of resin adhesive layer,the fracture properties of Mode II of the CFRP-steel composite structure reinforced by short aramid fiber were tested.In order to improve the shear strength of the bonding interface,ended notch flexible(ENF)test was adopted.Bridging toughening mechanism based on aramid fiber,the various ENF specimens with three typies of adhesive layer structures,i.e,the smooth steel surface,grooved steel surface and grooved steel surface reinforced by short aramid fiber were prepared by using a homogeneously distributed short aramid fiber film.The mechanism of bridging toughening of short aramid fiber was studied through load displacement curve,specimen profile and fracture surface characteristics of ENF test.Then,the length and density of short aramid fiber with ENF and short aramid fiber were optimized and analyzed.The interlaminar shear fracture properties of three typies of CFRP-steel composite structures have been studied,including smooth steel surface,grooved steel surface and grooved steel surface reinforced by short aramid fiber.Although the grooved steel surface can significantly increase the mechanical"interlocking"between the adhesive layer and steel interface,the epoxy(including the hardener)is more difficult to penetrate into the rough surface of the grooved area during the preparation of the specimen.The extreme load of grooved specimen compared with that of plane specimen is decreased by 16%,and the fracture energy is decreased by 30.2%.However,the short aramid fibers were effectively mixed into the epoxy to increase the the adhesive strength.The interfacial shear strengths,i.e,the interfaces of epoxy-CFRP and epoxy-steel,and the shear strength of epoxy,were enhanced by short aramid fiber.Although the average thickness of specimens with grooved steel surface reinforced by short aramid fiber is increased by 0.1mm,the extreme load and the fracture energy can be increased by 15.7%and 6.8%than the plane specimen respectively.The effects of length and density of short aramid fiber film on interlaminar shear strength of CFRP-steel composite structure were studied.When the density is 30g/m~2,the number of longer fibers embedded in the grooved steel surface is decreased,and the loading effect of the aramid fibers is reduced.When the length of aramid fiber is 6mm,the wettability of fiber film with higher density is decreased resultlng to the more micro defects and the number of aramid fiber embedded in grooved steel is significantly reduced.When the length and density of aramid short fiber are 6mm and 30g/m~2 respectively,the optimized interlaminar shear strength of CFRP-steel composite structure was achieved. |
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