| Due to long-term environmental corrosion and fatigue load,many steel structure bridges have performance degradation and other problems,which can not meet the design and service requirements,so they need to be strengthened urgently.Prestressed CFRP material reinforcement technology has been proved to significantly improve the bearing capacity and fatigue performance of steel structures,but this technology needs mechanical assistance,which greatly limits its application in practical engineering.The shape memory alloy（SMA）anchored at both ends can produce sufficient prestress after thermal excitation,which can be used to inhibit the development of cracks in steel structures.Combining SMA and CFRP to form SMA-CFRP composite reinforcement technology can give full play to the advantages of the two materials to better improve the fatigue performance of steel structures.However,the relevant research has just started and is mainly aimed at steel plate reinforcement,while the fatigue reinforcement of defective steel beams has not been reported.In this thesis,the fatigue properties of defective steel beams strengthened with CFRP,SMA and SMA-CFRP are studied under four different stress levels.The fatigue reinforcement effects of each method are analyzed from the perspectives of failure mode of strengthened beams,crack propagation process of steel beams and interface debonding process.The specific research contents and conclusions are as follows:（1）The failure modes of three reinforcement methods under different stress amplitude fatigue loads are compared and analyzed.For SMA strengthened specimens,the web crack of the steel beam increases with the increase of fatigue times,but the crack propagation of the web is the largest at the stress level of 0.1Pu-0.5Pu,which is 71 mm,and the SMA patch does not peel off;For CFRP strengthened specimens,with the increase of fatigue times,the defects crack first.When the specimens are fatigue damaged,the CFRP cloth peels and breaks;For SMA-CFRP composite strengthened specimens,the defects still crack first.Under four different stress amplitudes,the web cracks of steel beams expand by 53 mm,63mm,88 mm and 88 mm respectively,and the CFRP cloth is not broken,only peeling off occurs.（2）The effects of three different reinforcement methods on the cracking life,interface peeling life and ultimate fatigue life of steel beams are compared and analyzed.The results show that the three reinforcement methods can improve the fatigue life and fatigue performance of steel beams with defects.Compared with CFRP reinforcement and SMA reinforcement,composite reinforcement can significantly improve the fatigue times of Web cracks and bond interface cracks of steel beams,and slow down the growth rate of steel beam cracks and interface cracks.For example,under the stress level of 0.1Pu-0.8Pu,the cracking life of steel beams strengthened with CFRP is 463 times and that of steel beams strengthened with SMA is 107 times.The fatigue life of the specimens strengthened with SMA-1240 and CFRP is greater than that of the specimens strengthened with SMA-1240 alone.In addition,the lower limit of fatigue load remains unchanged.With the continuous reduction of the upper limit,the better the effect of SMA-CFRP composite reinforcement on improving the fatigue life of steel beams with defects.Under the action of 0.1Pu-0.5Pu,the fatigue life has reached 2.88 times the sum of the life of SMA or CFRP alone.（3）The suppression effects of three different reinforcement methods on crack propagation,interface peeling and stiffness degradation of steel beams are compared and analyzed.The results show that the crack growth rate of SMA-CFRP composite reinforced specimens under four different stress amplitudes is about 50% of that of CFRP reinforced specimens,and only 25% of that of SMA reinforced specimens,which also proves that the effect of composite reinforcement on inhibiting crack growth of steel beams is the best.In the process of fatigue loading,compared with the other two reinforcement methods,SMA-CFRP composite reinforcement specimens can effectively reduce the deflection growth rate.When the reinforced specimen is fatigue damaged,the residual stiffness of SMA reinforced specimen and CFRP reinforced specimen is about 40%,while the residual stiffness of SMA-CFRP composite reinforced specimen is about 30%.This shows that SMA-CFRP composite reinforcement technology has the best effect on slowing down the stiffness degradation of defective beams,and can make full use of the residual strength of steel beams and increase the safety reserve of reinforced beams. |
