Fatigue Behavior And Design Method Of Corroded Steel Plate Strengthened With CFRP Plates

The appearance of rust pit on the surface of steel structure will lead to the decrease of plasticity and fracture toughness of steel and the local stress concentration of steel,which will accelerate the initiation and propagation of fatigue crack and have a great influence on the fatigue performance of steel structure.The strengthening system with CFRP bonded to steel substrate can effectively recover the local stiffness loss due to corrosion,reduce the stress concentration in the rust pit,and delay the crack initiation and propagation,so as to improve the fatigue life of the corroded steel structure.In this paper,experimental study,theoretical analysis and numerical simulation were adopted together,the surface characteristics of corroded steel structure,the bond properties between CFRP plate and corroded steel plate,the fatigue properties,the stress concentration and crack propagation,the fatigue life prediction and design method of the corroded steel plate strengthened with CFRP plates were systematically investigated.The specific research contents and results are as follows:(1)The artificial accelerated corrosion test of 7 batches of Q235 B hot rolled H-beam was carried out.The surface morphology and apparent contact angle of H-beam flange were measured by means of three-dimensional non-contact profilometer and video optical contact angle measuring instrument.The fractal dimension of the surface morphology and the identification,extraction and evaluation method of rust pit of corroded steel plate were proposed,and the corresponding calculation programs were compiled.The variation law of the primary surface characteristic index,which included surface roughness and fractal dimension,pit size and shape parameter,surface free energy of the corroded steel structure with corrosion durations were analyzed.(2)The tensile tests of 34 double lap joint specimens were carried out,and the effects of corrosion durations,adhesive thickness and bonded length of CFRP plates on the failure modes,the load displacement curves,the strain and interfacial shear stress distribution,the ultimate strength and the effective bond length were investigated together with effect of corrosion duration on the surface characteristic.According to the stress equilibrium condition of the micro element of the CFRP plate,the computing method of the interfacial bond stress and relative slip between CFRP plate and corroded steel plate was proposed,and the bond-slip curves of the interface between CFRP plate and corroded steel plate with different adhesive thickness and corrosion duration were obtained.The interface bond-slip model between CFRP plate and corroded steel plate,within which the influence of rusty surface were taken into consideration,were proposed.The stress analysis of the interface between CFRP plate and corroded steel plate was carried out,the distribution equations of shear stress and normal peel stress of the interface between CFRP plate and corroded steel plate are established.The models for calculating ultimate bearing capacity and effective bond length of the interface between CFRP plate and corroded steel plate were also proposed.(3)The fatigue loading tests of 16 corroded steel plates strengthened with CFRP plates were carried out,the effects of corrosion degree,CFRP plate stiffness,pre-stressing degree,end anchorage and fatigue stress amplitude on the fatigue performance indexes,including fatigue failure mode,fracture characteristics,fatigue life and crack propagation of the corroded steel plate strengthened by CFRP plate were investigated.Results showed that the fatigue life of corroded steel plate can be significantly prolonged by strengthening with CFRP plates.As for the corroded steel plate with the corrosion ratio of 9.38%,13.39%,16.61% and 21.24%,the fatigue life of the specimens strengthened by double-sided sticking CFRP plates under the condition of end anchorage is 18.4 times,9.3 times,8.9 times and 11.4 times higher than that of the specimens without reinforcement,respectively.Methods of reinforcement have a significant influence on the fatigue performance of the corroded steel plates strengthened with CFRP plates.The end anchorage can effectively delay the failure of bonding interface.The crack propagation rate and the fatigue life can be significantly reduced by increasing the rigidity of CFRP plates and(or)adopting prestressed reinforcement.(4)The finite element model of "double layer variable thickness shell-spring-equal thickness shell" was established,and the influence of corrosion duration,adhesive thickness,CFRP stiffness and pre-stressing degree on the stress concentration coefficient of the corroded steel plate strengthened with CFRP plates were investigated.A two-stage crack growth analysis model of corroded steel plate strengthened with CFRP was proposed.The finite element model of "three-dimensional solid-spring-shell" was established,combined with the virtual crack closure method,the stress intensity factors at the crack tip were calculated,and the effects of corrosion duration,adhesive thickness,CFRP stiffness and pre-stressing degree on the crack propagation were analyzed.(5)The calculation equations of surface stress amplitude of corroded steel plate with and without reinforcement were established,and the calculation models of residual crack initiation life and crack propagation life of corroded steel plate strengthened with CFRP were proposed.The rationality of the model was verified by the comparison of the calculation results and the fatigue test results.Finally the design method of improving fatigue performance of corroded steel plate strengthened with CFRP plates was proposed.