Research On Damage Mechanism And Fracture Behavior Of 304 Stainless Steel Bar With Low-Stress Controllable Rotary Bending Cropping Process

The metal bar cropping is the starting point and the most important process in industrial production.It is widely used in the processing and manufacturing of various mechanical parts such as gears,bearings,shaft sleeves,chains,screws,etc.Its cropping efficiency and cross-section quality directly affect the processing quality,production efficiency and cost of subsequent parts.Aiming at the problems of low cutting efficiency,waste of materials and low quality of cross-section in traditional cropping methods,this paper put forward a new low-stress controlled rotary bending cropping method by using the notch effect of artificially prefabricated annular V-notch,the stress concentration effect of micro-crack initiation and the control technology of fatigue crack propagation,and made an in-depth study on the damage mechanism and fracture behavior in the cropping process by using theoretical analysis,numerical simulation and experimental methods.According to the ductile fracture law of 304 stainless steel bar,the macroscopic constitutive model of ductile damage of 304 stainless steel bar was established.The effects of annular notch angle,notch depth and notch root radius on the deformation behavior and damage evolution of 304 stainless steel bar were analyzed by numerical simulation and tensile test,and the deformation law and damage evolution mechanism of 304 stainless steel bar were revealed.The macroscopic constitutive model of ductile damage and the single factor experiment method were used to numerically simulate the influence law of notch root damage field with different cropping process parameters,and the ideal cropping process parameters were obtained,such as loading speed of 5mm/s,linear increasing load,clamping position L1=5mm,L2=10mm.The macroscopic constitutive model of ductile damage established lays the theoretical foundation for the relationship between damage state parameters and stress state function for the coupled meso-damage model.The artificially prefabricated annular notch on the surface of 304 stainless steel bar is an efficient cutting aid method due to notch effect and stress concentration effect.Through the secondary development platform of finite element software,the coupled constitutive model of ductile damage and microscopic damage is established.This model parameters were identified by theoretical analysis,microstructure analysis and reverse solution method.The influence of tensile specimens with different stress states on the damage evolution of 304 stainless steel bar was analyzed,and the micro-damage evolution mechanism of 304 stainless steel bar was revealed.By using the coupled constitutive model and single factor experiment method,the influence law of different notch geometric parameters on the crack initiation damage field at the notch root was analyzed,and the ideal cropping process parameters with the notch angle symmetry 30,notch root radius 0.2mm and notch depth 0.8mm were obtained.According to the characteristics of the low-stress cropping process,the working principle of the new low-stress controllable rotary cropping method is described in detail.A new type of low stress controllable rotary bending cropping system is constructed,and it is debugged and tested.According to the cross-section characteristics obtained by the new low-stress controllable rotary cropping method,a cross-section evaluation index suitable for the cropping method is proposed,which are cropping time and section flatness.The cropping test results show that the cropping method is more reasonable.According to the fatigue crack propagation law of 304 stainless steel bar,the fatigue crack propagation model of 304 stainless steel bar with annular notch was established by using the theory of linear elastic fracture mechanics,and the load control strategy was designed.The cropping test of 304 stainless steel bar with diameter D=10mm was carried out.The test results show that the ideal cross-section can be obtained when the cropping test is carried out with a linear incremental load control strategy with a loading speed of lmm/s.In order to further verify the rationality of the established theory,when the notch opening angles is 30°,the notch root radius is 0.2mm and the bar clamping positions is L1=5mm and L2=10mm,the proportion of fatigue crack propagation area of the cross-section is the largest,and the ideal cross-section and cropping time can be obtained.