Study On Surface Damage Tolerance Of CFRP/Steel Laminated Composite Structure Based On Boundary Effect Mode

Lots of studies have shown that carbon fiber reinforced polymer(CFRP)can improve the bearing capacity,fatigue life and corrosion resistance of steel structures due to its advantages of high strength,high elastic modulus,strong corrosion resistance and good fatigue performance,which has been widely used in the reinforcement of steel structures.However,brittle carbon fiber is prone to surface damage such as scratches in the actual operation process of CFRP/Steel composite structures.Consequently,it is necessary to study the damage tolerance of CFRP/Steel composite structure in order to ensure the safe operation of damaged composite structures.An analytical mode was established based on the Boundary Effect Theory Model(BEM)and the surface damage tolerance of CFRP/Steel laminated composite structure was studied by the method of extreme load in order to solve this problem.The main contents of this paper are as follows:(1)An analytical model for the ultimate bearing capacity of CFRP/Steel laminated composite structure with surface damage was established based on BEM and macroscopic mechanical analysis considering the surface crack damage of CFRP/Steel laminated composite structure and the structural characteristics of brittle carbon fiber.And the analytical model can predict the surface damage tolerance of microcracks in CFRP/Steel laminated composite structure;(2)The structural characteristic parameter C_ch has an influence on the fracture of the crack tip of the composite structure in the analytical model.Therefore,the fracture characteristics of 2D-CFRP/Steel composite structure with surface scratch damage under three-point bending limit load and the damage area of the crack tip were observed by mean of extreme load method with metallographic microscope.The tensile strength of CFRP in CFRP/Steel composite structure was obtained by taking the structural characteristic parameter C_ch into the analytical model and was compared with that of 2D CFRP by direct tensile test,the deviation of them was 1.39%.It has been indicated the applicability of the analytical model to 2D CFRP/Steel composite structure.(3)The fracture characteristics and structural characteristic parametersC_ch of composite structure are different due to the different structure of carbon fiber.2D carbon fiber bundles are laminated weaved structure from the perspective of fiber structure.However,unidirectional carbon fiber bundles are in the same direction and there is no laminated weaved structure.The tensile strength of unidirectional carbon fiber is higher than that of 2D carbon fiber from the perspective of tensile strength.Therefore,it is necessary to further study the size of the surface damage zone of unidirectional CFRP/Steel composite structure,the determination method of structural characteristic parameter C_ch and the feasibility of the analytical model for unidirectional CFRP/Steel composite structure.The surface damage tolerance of unidirectional CFRP/Steel composite structure was analyzed by using the same test as 2D-CFRP/Steel composite structure.The results have been showed that the size of damage zone varies with the carbon fiber structure.The deviation between the tensile strength obtained by the analytical model and the tensile strength obtained by the direct tensile test was 6.02%,it has been showed that the analytical model is also feasible for unidirectional CFRP/Steel composite structure.(4)"Fracture load=tensile strength×equivalent area"is considered to the linear relation of the analytical model.And the"equivalent area"is related to the laminated structure and geometric parameters of CFRP/Steel composite structure.The influence of structural characteristic parameters C_ch on the fracture properties of 2D and unidirectional CFRP/Steel composite structures was analyzed.The results have been showed that an accurate fracture performance estimates can be obtained within the range of approximate structural characteristic parameters.Therefore,the ultimate load and fracture strength of 2D and unidirectional CFRP/Steel composite structures with different surface microcracks are predicted by tensile strength and the surface damage tolerance of CFRP/Steel composite structures is realized.