Experimental Investigation And Characterization Of Fracture Behavior During AHSS Sheet Metal Forming

Advanced high strength steel is more and more widely used in automotive industry.Stamping process is a very important forming method and has a lot of advantages.However, in order to meet automotive lightweight requirements, the thickness of sheetsshould be reduced and strength should be increased. As strength increases, fractureduring AHSS sheet metal forming becomes more and more serious. Therefore it isnecessary to investigate the fracture phenomenon.The research work was funded by National Natural Science Foundation of China(No.51105246). Uniaxial tension, piercing, hole expansion and nakazima tests wereconducted on DP590, DP780, DP980, MS1180and MS1300steel. The fracture modesof AHSS were analyzed and the characterization of fracture behavior under differentmaterials and different stress states was compared. The conclusions are as follows:(1) A dimple measurement method based on DigitalMicrograph software wasdeveloped. This method was used to investigate the fracture morphologies of DP590,DP780, DP980, MS1180and MS1300steel during uniaxial tension, piercing, holeexpansion and nakazima tests.(2) The fracture mode of tensile fracture for DP590and DP780isdimple-dominant ductile fracture, while dimple fracture and cleavage fracture coexistin tensile fracture for DP980, MS1180and MS1300. With increase of tensile strength,elongation and plastic strain ratio decrease, and the average dimple diameter reduces.The quantitative correlation between the average dimple diameter and tensile strengthisd10502.321.21. However the relation between dimple density and tensile strengthis not monotonic. The fracture morphologies of different tensile directions are similar.(3) The fracture morphologies under three stamping processes were studied. Forshearing-induced fracture during piercing process, the quantitative relation between thepercentage of burnish zone and tensile strength isf239.90.36. The fracture zone ofhigher grade DP steel is smoother. As for MS1180and MS1300, the fracture zones consist of very small dimples. The dimple size of M1300(~1.2μm) is smaller than thatof M1180(~1.6μm). According to the results of hole-expansion tests, thehole-expansion ratio decreases as strength increases. The fracture mode of DP590,DP780, DP980, MS1180and MS1300steel is dimple-dominant ductile fracture. Theaverage dimple diameter decreases with increase of tensile strength. However therelation between dimple density and tensile strength is not monotonic. DuringNakazima tests, the fracture mode of DP780and DP980steel is dimple-dominantductile fracture. The dimple of DP980is smaller and shallower than that of DP780.(4) The fracture types and stress states under different forming processes weresummarized. The fracture type of uniaxial tension is middle-through fracture and thestress state is tensile stresses from three directions. The fracture type of piercing isthickness-through fracture and the stress state is shear stress. The fracture type ofhole-expansion is edge fracture and the stress state is circular tensile stress. The fracturetype of Nakazima test is middle fracture and the stress state is tensile stresses from twodirections.(5) The fracture morphologies of DP780under different stress states werecompared. The fracture morphology under tensile stresses from three directions isuniform equiaxial dimples. The fracture surface under shear stress is rough fracturezone with none dimples. The fracture surface under circular tensile stress is dominatedby a large number of shallow dimples. The fracture surface under equal tensile stressesfrom two directions is dominated by equiaxial dimples, and the dimple depth isshallower than that under tensile stresses from three directions but deeper than thatunder uniaxial tensile stress. The fracture morphology under unequal tensile stressesfrom two directions is inclined dimples.