Static And Fatigue Properties For Nano-Modified CFRP-Steel Bonded Interface

In the field of civil engineering,carbon fiber reinforced polymer(CFRP)has been widely used in the repair of existing structures and the reinforcement of new structures due to its excellent properties,such as lightweight,high strength,durability,fatigue resistance,and corrosion resistance.However,the application of CFRP in steel structures is still relatively scarce compared to concrete structures.Previous studies have shown that when the epoxy adhesive is used to bond CFRP and steel structures,the bonded interface between them is the weak part.In this paper,the nanofiber film was used to modify and toughen the bonded interface,and the bonding performance of the interface under static and fatigue loading was also investigated.This study mainly includes the following contents:(1)The type II(shear type)fracture behavior of nano-modified CFRP-steel interface under static loading was studied.The nanofiber film was used to modify and toughen the interface.Firstly,an aluminum-aluminum single-lap shear test was carried out to select the appropriate curing temperature,and the areal density and the number of layers of nanofiber film.Then,the tensile shear tests of CFRP-steel double lap joints with different nanofiber film densities and bond lengths were carried out,combined with 3D digital image correlation(3DDIC)technology.The failure mode,load-displacement relationship,strain distribution of the CFRP,and interfacial bond stress distribution were obtained.The effective bond length of the nano-modified interface was determined,and the excellent performance of the nanofiber film in improving bond strength and toughness was proved.(2)The type I(opening type)fracture behavior of nano-modified CFRPsteel interface under static loading was studied.The effects of nanofiber film areal desity and service environment temperature(25℃,50℃,and 75 ℃)on the failure mode,load-displacement relationship,R curve,and fracture toughness were studied through double cantilever beam(DCB)joints.The interface crack propagation was monitored using 2D-DIC technology.Moreover,the applicability of three fracture toughness calculation methods,namely modified beam theory,flexibility method,and modified flexibility method,were compared.(3)The fatigue tests of CFRP plate-steel plate double lap joints under different load levels were carried out.The failure mode,load-displacement relationship,overall stiffness degradation,and interface damage process were obtained,and the fatigue damage mechanism of the bonded interface was clarified.In addition,the applicability of the mean and the traditional engineering design S-N curves based on the mean or local bond stress range was compared,and the fatigue limit of the nano-modified CFRP-steel bonded interface was determined.(4)The P-S-N curves for the nano-modified CFRP-steel bonded interface were established to characterize the inherent scattering characteristics of fatigue life based on four statistical models,including ISO 12107,ASTM E739-91,twoparameter Weibull,and Gumbel models.Subsequently,the performance of the P-S-N curves was comprehensively evaluated by direct graphic observation and quantitative evaluation,and a fatigue reliability design method method for CFRP-steel bonded joints was proposed.