Research On The Ductile Fracture Criterion And Spinnability Of Shear Spinning For High Strength Steel

Lightweight is one of the main development direction of automobile technology,the use of advanced high strength steel is an important means of lightweight.Shear spinning is semiplastic and local forming technology.The fracture and springback phenomenon is serious during shear spinning of high strength steel.At present,there is a lack of a unified and effective method to predict the forming limit in the field of spinning technology.Therefore,study on fracture criterion and spinnability of shear spinning for high strength steel,and the control of its forming quality,has important theoretical significance and practical value for both the development of high strength steel forming technology and the improvement of spinning technology.Forming mechanism of shear spinning,fracture mechanism,spinnability and forming quality were studied through the aspects of theoretical analysis,finite element simulation and experiments.The main research contents and conclusions in this thesis are as follows:The application range of the commonly used five ductile fracture criterions were analyzed based on the relationship between the stress triaxiality and the equivalent plastic strain of fracture,which determines the criterion to be used in fracture prediction of shear spinning.A new method was proposed to calibrate the ductile fracture criterion considering fracture limit under various deformation conditions based on finite element method and experimental research,and parameters of Oyane criterion and R&T criterion of DP600 were calibrated.Based on the finite element software ABAQUS,the fracture prediction model of shear spinning was established.Oyane criterion was modified considering characteristics of shear spinning and fracture prediction result.Thus a ductile fracture criterion for precisely predicting fracture failure of shear spinning process was obtained.Simulation results show that the fracture location distribution of shear spinning predicted by Oyane criterion and R&T criterion was in good agreement with the experimental rules,which is located at rear of the contact area of the rollers and distributed along the tangential direction.The prediction error of fracture location by revised criterion is 15.9%,which has good accuracy on shear spinning fracture prediction.A method for measuring the spinnability of shear spinning base on ellipsoidal mandrel was presented,which evaluates spinnability by the minimum half cone angle and the maximum thinning rate.Devices for shear spinning was designed and spinninability experiment was carried out.The influences of mechanical properties of materials and forming parameters on shear spinning were analyzed through a series of experiments.Spinnability experiments show that the maximum thinning rate is positively correlated with the elongation of tensile test.The feed rate and radius of roller had little effects on spinnability,while the deviation rate had significant effects on spinnability,a negative deviation in small range can effectively improve the maximum thinning rate.Finally,the shear spinning process of DP600 conical parts was studied,the stress and strain,spinning force and ductile damage distribution were analyzed.Taking the wall thickness deviation,roundness and springback angle as the forming precision indexes of the conical spun parts,the influence law of the forming parameters on the forming quality of the spun parts was obtained.Typical defects and prevention measures were analyzed.The radius and the feed rate of rollers should not be too large or too small during shear spinning of DP600 conical parts,and spun part with good forming precision can be obtained by using zero deviation sate or a small negative deviation rate.Typical defects during forming process include cracking,flange wrinkling and mouth instability,which can be prevented by reducing the feed rate,reducing the gap between mandrel and roller,and keeping a certain margin of the flange.