Abstract
Steel structures are widely used in civil engineering,including steel bridges,steel structure plants and marine platforms,etc.A large number of steel structures are under cyclic loads,for instance,wind,traffic and waves.However,these cyclic loads may induce crack propagation in steel structures,which influence their fatigue performance.Thus,methods should be taken to repair these structures and prolong their fatigue life.Traditional repairing methods,such as welding and stop holes may induce secondary cracks.By contrast,repairing with carbon fiber reinforced polymer(CFRP)may avoid introducing further damage to the structure.
Scholars have studied the fatigue performance of central cracked steel plates repaired with CFRP,using doublesided repairing method.Liu et al.[1]suggested the experimental fatigue life of steel specimens can be increased to 2.2～7.9 times.Emdad et al.[2]considered the ef fect of prestress in CFRP,experimental results showed that fatigue life extension ratio was 4.0～35.2,generally higher than that of specimens repaired with nonprestress CFRP,which was 2.9～4.7.
Nevertheless,prestressing CFRP requires adequate operation space,which may limit its application when repairing structures in tiny spaces.When heated in specified temperature range,shape memory alloy(SMA)has a unique property that it would return to its shape before heated.Thus,by attaching prestrained SMA to structural members,prestress can be set when SMA is heated.
SMA can be easily used to prestress structural members,while CFRP has light weight,high tensile strength and high elastic modulus,fitting for structural repairing.Therefore,the author took full advantage of both materials by combining them and then studied the fatigue performance of central cracked steel plates(Figure 1)strengthened by SMA/CFRP composites(Figure 2),using finite element method(FEM).
Figure 1 Unstrengthened central cracked steel plate(unit:mm)
Figure 2 Strengthened central cracked steel plate(unit:mm)
In this research,the author conducted finite element analysis of several specimens strengthened by double-sided composites under tensile fatigue loading.The finite element analysis was carried out by ABAQUS and a typical model mainly comprised four kinds of materials,including steel,adhesive,CFRP and SMA(Figure 3).Three parameters were considered:(1)elastic modulus of CFRP,including normal modulus(230 GPa)and high modulus(640 GPa);(2)whether using SMA or not;(3)if using SMA,considering different prestress levels of SMA.The prestress in SMA was adjusted by cooling method.
Figure 3 Finite element model
The finite element analysis results showed that using SMA/CFRP composites could reduce the stress intensity factor(SIF)in the surrounding area of the crack tip and thus prolong the fatigue life of central cracked steel plate under tensile fatigue loading.The fatigue life of each specimen was calculated based on the modified Paris model[3].As for the specimens strengthened only by normal and high modulus CFRP,the fatigue life extension ratios were 1.5 and 2.1,respectively.When using SMA,these indexes increased to 2.9 and 3.8 respectively,and further increased as the prestress level rose(Figure 4).
Figure 4 Fatigue life extension ratio
The following conclusions can be drawn:(1)Comparing with CFRP repairing,SMA/CFRP composites repairing can prolong more fatigue life of central cracked steel plates under tensile fatigue loading.(2)Higher prestress level in SMA can lead to longer fatigue life of central cracked steel plates strengthened by SMA/CFRP composites.