Development And Performance Research Of SMA-CFRP Composite Prestressed Patc

Fatigue failure is a common disease of steel components,among which 80% to 90% of the failure of steel structures is related to fatigue.In view of the problem of fatigue cracking,there are many reinforcement methods,such as crack stop hole,welding,additional steel plate,etc.These methods still have some limitations,such as crack stop hole will weaken the cross-section,welding will affect the fracture toughness of steel,and additional steel plate will increase self weight of components.To address these issues,SMA and CFRP composite prestressed patch(hereinafter referred to as SMA-CFRP composite pre-stressed patch)can effectively reinforce damaged components without damaging the original structure and significantly increasing self weight.SMA can provide prestress,CFRP can provide large stiffness,and the combination of the two can significantly improve the reinforcement effect.In response to some issues in the SMA-CFRP composite prestressed patch,such as insufficient research on SMA recovery stress,which results in significant differences in recovery forces obtained by different researchers,this article conducted in-depth research on the recovery force of SMA and identified key factors affecting recovery stress.At the same time,in response to the problem that the expensive price of NiTi based SMA is not conducive to large-scale engineering utilization,lower priced iron based shape memory alloy(Fe-SMA)have been introduced for the production and research of SMA-CFRP composite prestressed patch.The research contents and conclusions of this article are as follows:(1).For commonly used NiTi based SMA,NiTi and NiTiNb,heat treatment at different temperatures is carried out.Mechanical and phase transformation behaviors are analyzed by universal testing machine and DSC(differential scanning calorimetry),and then recovery stress is tested.The results show that the yield strength,superelasticity of SMA decrease with the increase of heat treatment temperature while the shape memory effect of SMA increase with the increase of heat treatment temperature.During the heating process of recovery stress test,the yield strength will play as the upper limit of the recovery stress.During the cooling process,the positive phase transition temperature will determine the loss of stress.By selecting the appropriate heat treatment temperature,the optimal recovery effect can be achieved.(2).Apply different prestrains to NiTi and NiTiNb heat treated at 800℃,analyze the effect of prestrain on phase transformation behavior,and conduct recovery stress experiments.The results show that prestrain affects the recovery stress by affecting the number of stress-induced martensite.Too small prestrain will lead to insufficient potential in the recovery process,and too large prestrain will damage the structure of stress-induced martensite and hinder the recovery process.Therefore,to obtain the maximum recovery stress,it is necessary to choose an appropriate prestrain.In this article,NiTi achieved room temperature recovery stress above 300 MPa through appropriate heat treatment and prestrain,which exceeded the maximum stress recorded.NiTiNb can achieve room temperature recovery stress above 560 MPa,which is close to the reported maximum value.(3).The bonding behaviors of between three types of SMA(NiTi,NiTiNb,Fe-SMA)and adhesive layer were studied.The results indicate that the pull-out process of SMA includes three stages:(1)elastic deformation of the adhesive layer,(2)the end of the SMA and adhesive layers first debonding,ultimately leading to complete debonding,(3)sliding of SMA.Sandblasting treatment can significantly improve the debonding strength,which can increase by over 100 MPa.(4).A study was conducted on the reinforcement performance of SMA-CFRP composite pre-stressed patches made of three types of SMA.Fatigue testing was conducted on the cracked steel plates reinforced with three types of patches,and the failure modes and reasons for performance differences of three patches were analyzed.The results show that SMA-CFRP composite prestressed patch can significantly improve the fatigue life of cracked steel plate,NiTi and NiTiNb composite patches can increase the fatigue life of cracked steel plate by more than 4 times,and the use of Fe-SMA composite patch can increase the fatigue life by more than 7 times,mainly due to the high elastic modulus and good fatigue performance of Fe-SMA.