| Strengthening steel structures with carbon fiber reinforced polymer（CFRP）can significantly extend the fatigue life of steel bridges caused by local damage.In practical engineering,mixed-mode fatigue fracture problems occur in most steel structures under mixed-mode loading.However,there are few studies on the crack growth mechanism of steel plates with mixed-mode cracks in the field of CFRP reinforcement.The primary focus of this study is to simulate the crack propagation of mix-mode cracks,select the reasonable crack growth criterion and prediction formula of crack growth rate through the fracture analysis program,and verify the consistency of the numerical simulation and the test results in terms of crack growth mechanism and fatigue life based on the fatigue test of damaged steel plate reinforced with CFRP plate and the FEM software ABAQUS.The main research work and analysis results are as follows:（1）According to the fatigue test result and the existing research,whether mix-mode cracks（Ⅰ/Ⅱ）under mixed load or an inclined crack under uniaxial tensile fatigue load,the crack growth process of mix-mode cracks（Ⅰ/Ⅱ）is simplified into three stages:Mix-mode crack condition,Transition condition and Mode Ⅰ crack condition based on experimental study on fatigue properties of CFRP-repaired steel plates with a mixed-mode crack.As a result,the ratio of stress intensity factor（KⅡ/KⅠ）in Mix-mode crack condition determines the crack growth direction of Transition condition,while Mode Ⅰ crack condition basically propagates along a straight line;as for the mix-mode cracks（Ⅰ/Ⅱ）,the proportion of the life of Transition condition in the total fatigue life is basically unchanged,and the life law of Transition condition significantly affects the total fatigue life.（2）The FEM model was established based on experimental study on fatigue properties of CFRP-repaired steel plates with an inclined crack under uniaxial tensile fatigue load.The theoretical formula is deduced to obtain the variation law of stress intensity factor in Mix-mode crack condition and the crack growth path of Transition condition.As a result,whether strengthened or not,the crack growth depends on the initial loading angleβ.Compared with the mix-mode cracks（Ⅰ/Ⅱ）and inclined crack,the variation of equivalent stress intensity factor rangeΔKeff0 with the initial loading angle in Mix-mode crack condition shows opposite,which affects the fatigue crack growth rate in Transition condition,resulting in the difference in the variation of total fatigue life.（3）Taking the CFRP-repaired steel plate with an inclined crack under uniaxial tensile fatigue load as the benchmark analysis model,the parametric research is carried out for the initial damage,single or double-sided reinforcement and prestressed level of prestressed CFRP plate respectively.The crack growth behavior,variation trend of stress intensity factor and fatigue life were investigated under different initial loading angles.As a result,the greater the initial damage,the higher the fatigue life improvement coefficient after CFRP reinforcement.Reinforcement of steel plates repaired with single-sided with one side is less efficient in extending fatigue life.If the prestressed level of the prestressed CFRP plate is so high that the stress intensity factor Kmin≤0 always occurs in crack growth process,there is significant improvement in fatigue life due to crack closure effect.When the initial loading angle is in the range of 0°-30°,the fatigue life improvement coefficient of CFRP-reinforced specimens is relatively high.The results provide a research approach for the fatigue crack growth behavior and residual fatigue life evaluation of CFRP strengthened steel plates with mix-mode crack,and provide a theoretical reference for the selection of a safe and applicable CFRP strengthening scheme. |
