Failure Mechanical Analyses Of Cracked Steel Structures Strengthened With FRP

The tensile strength of fiber reinforced composites (FRP) is2-10times of that of the steel.FRP strengthening technology applied to the steel structures has achieved great concern for itsadvantages of free from the secondary stress and defects, good durability, easy constructionand low maintenance costs. But within the body of cracked steel structure strengthened withFRP, the bonding layer is the weakest part other than the initial defect of the steel structure.Damage of the bonding layer will result the debonding of FRP and ultimately caused thefailure of the entire strengthened structure. Therefore, this research focused on the twoweakest parts of FRP strengthened cracked steel structures:1) the bonding layer between FRPand steel structures,2) the initial defect local detail. The bonding layer between CFRP plateand steel plate specimen, the three-dimensional surface crack of the carbon fiber laminates(CFL) strengthened cracked steel plate specimen, are separately taken as the objects for theinvestigation on the mechanical property of the strengthened body. Compositive researchmethod of theoretical derivation, numerical analysis and experiment was employed to conductthe mechanical analysis of the failure mechanism of the CFRP strengthened cracked steelstructures. The main research content and conclusions were summarized as bellow:1) Establishement of the theoretical derivation of stress calculation formula of bondinglayer between CFRP and steel plate.The bi-shear model of steel plate strengthened with CFRP was constructed. Thecalculation formulas of shear stress, normal stress and effective bonding length of the bondinglayer were established. The shear stress distribution in the boding layer was also discussed.The relation of shear stress and failure mechanism of bonding interface was analysized.Theoretical analysis results showed that the size of steel plate, shear modulus and thickness ofbonding layer, elastic modulus of FRP and steel plate, thickness of FRP were the main factorsinfluence the bonding strength and strengthening effect.2) Finite element analysis on the stress distribution within the bonding layer.Bi-shear finite element model (FEM) was set up with ANSYS v10.0for the mechanicalanalysis of steel plate strengthened with CFRP. The FEM was used to simulate test processand stress state within the bi-shear specimen of steel plate strengthened with CFRP. Variation and distribution of interface shear stress in bonding layer between CFRP plate and steel platewere essentially studied. The interface shear stress result obtained from both the FEM and theabove theoretical formula were analyzed and compared. It was showed that the results of thetheoretical formula and FEM were basically consistent. This proved the effectiveness of thetheoretical method proposed in this paper for the calculation of bonding shear stress.3) Experimental study on the bi-shear specimen of CFRP strengthened steel plate.The CFRP and steel plate bonding specimen charaterized with Single Lap Jointrecommended in ASTM D3166and ISO9644was made to conduct bi-shear tests on thebonding layer between CFRP plate and steel plate. Interface shear stress between CFRP plateand steel plate was determined through the test. The stress distribution and the stress transfercharacteristics of the bonding layer were analyzed, and then, the interface shearing failureprocess and damage characteristics were discussed. The research showed: the interface shearstress obtained from the test, theoretical formula and FEM agreed well. This is a further proofto the effectiveness of the theoretical formula proposed in this paper.4) Numerical analysis on the stress intensity factor of three-dimensional surface crackin steel plate strengthened with CFL under tension load.First, the FEM of three-dimensional semi-elliptical surface crack was established toanalyze the stress intensity factor. The crack front meshing method for fracture mechanicsanalysis was explored, the model and calculation parameters were optimized. The calculationaccuracy of the FEM method was checked with the reference of Newman-Raju formula. Then,the FEM of steel plate with three-dimensional surface crack and strengthened with CFL undertension load was established to investigate the influences of crack parameters, strengtheningmethod and CFL parameters on the stress intensity factor of surface crack. A semi-empiricalcalculation formula of stress intensity factor of strengthened specimen was proposed. Thefracture mechanism of cracked steel structures strengthened with CFL under tension loadswas discussed.5) Numerical analysis on the stress intensity factor of three-dimensional surface crackin steel plate strengthened with CFL under bending load.The FEM of steel plate with three-dimensional surface crack and strengthened with CFLunder bending load was established to investigate the influences of crack parameters, strengthening method and CFL parameters on the stress intensity factor of surface crack. Asemi-empirical calculation formula of stress intensity factor of strengthened specimen underbending load was proposed. The fracture mechanism of cracked steel structures strengthenedwith CFL under bending loads was discussed. The research showed that the long and shallowcrack were the most dangerous defects in steel structures. The employment of strengtheningmethod with FRP sheet to the cracked steel structures was effective and proved to be anadvanced technology.