Analysis And Application Of The Instability Of Thin-Walled Materials In Plastic Forming Process

Nowadays,under the trend of energysaving,the thin-walled structure material has become one of the important development directions of the metal material research.The parts made from thin-walled materials have been widely used inkey industrial fields,such as civil,military and medical.In order to avoid catastrophic failure,thin-walled material instability analysis has become a top priority.In this paper,according to the classical plastic theory along with the experiments and finite element simulations,forming performance,especially the instability law and instability mechanismof metal thin-walled tube and sheet metal have been analyzed and applications of the instabilityhave been discussed.And the processing technology and test method to optimize blanking and hole expansion process were put forward according to the special characteristics of high strength steel sheet.The main contents and innovations are as follows:The research method of inner and outer wall displacement and tensile fracture angle of thin-walled tube under uniaxial tensile instability was established.The influences of the original diameter and wall thickness on the inner and outer wallsoffsets and displacementswere analyzed.The stress-strain analysis was used to explain the effect of the breaking angle of the thin-walled tube.The reason of the fracture angle of the thin-walled tube is explained by the stresses analysis.The instability of 5A03 aluminum alloy thin-walled tube under uniaxial compressive load wasinvestigated.It is found that the uniaxial buckling failure of aluminum alloy thin-walled tube is a kind of non-axisymmetric buckling collapse with certain regularity.The influences of the ratio of height to diameter and the frictions of the upper and lower end faces on the crush mode were analyzed through experiments and finite element simulation.A finite element simulation method of sheet uniaxial tension is presented to show the instability and fracture process.Based on the experiments and classical plastic mechanics theory,the crossing necking phenomenon,the three dimensional stresses distribution and the rotation of the maximum principal stress direction in the process of tensile instability are analyzed.It is proved that the initial crack appears in the center of the necking zone.The reason of the difference between the theoretical value of the fracture angle 54.7 ° and the actual larger angle was explained from the second principal stress.The tensile instability deformation law of advanced high strength steel(AHSS)sheet was applied to investigate the forming quality of blanking process and hole flanging process.Since the strain value of cross section is positively correlated with the forming quality,the effects of punch shape,hump height and blank holder force on blanking quality were studied.By adjusting the punch diameter of 10 mm to 75 mm,the stress-strain state of the material is changed during the flanging process and the standard hole-turning experimental scheme was optimized.Two forming limit prediction methods of AHSS sheet in hole forming process were shown,which are critical thinning and critical strain.The applicable ranges of the two methods were compared.In addition,a simple finite element simulation method was developed to predict the cross-sectional strain values of non-perfect edge quality in hole forming process.