Friction Measurement And Calculation On Contact Surface Of Metal Plastic Forming

The friction between tool and workpiece is a complex physical phenomenon during metal deforming,which is also a critical issue that directly affect the feasibility of forming process and the quality of products.And as one of the key input boundary conditions in finite element simulations,it plays an important role in controlling the accuracy of necessary output results predicted.In order to meet the needs of practical computing,many researchers have done a lot of research from various angles.Some proposed friction models and some invented friction measurement methods.However,it is mainly confined to the improvement and determination of the friction coefficient,as a result,the distribution of shear stress in plastic forming has not been revealed.Based on the rigid plastic finite element theory,a software system for calculating and analyzing the friction characteristics is developed by using FORTRAN language.This system can provide a function that input nodes’ velocity of contact surface and then the node force will be calculated.According to the relationship between surface force and equivalent nodal force in the finite element theory,the friction shear stress on contact surface will be got by inverse operation of the nodal forces that have been known.The displacement of the contact surface node is carried out by upsetting the cylindrical specimen with a concentric circular grid etched on the surface,and then measuring the diameter of the mesh.In the experiment,this paper takes Q235 as the experimental object.There are seven kinds of specimens with different height to diameter ratio and two different contact surface roughness,all of which take 1mm as incremental loading.The diameter of the concentric circles is recorded at each pressure 1mm,which is converted to the relative nodal displacement velocity and then as the boundary conditions of numerical input to the system.At last,the distribution of the friction stress on the contact surface and the influence of the different height diameter ratio and roughness on the friction force distribution of the contact surface are summarized.The results show that the shear stress increases linearly from the center to the edge of the cylinder,and then decreases after reaching a certain value.And with the increase of the pressure,the position of the highest point shifts to the center.Under the same amount of reduction,the position of the maximum shear stress of the specimen with high ratio of height to diameter is higher than that of the specimen with small diameter ratio.With the increase of the height diameter ratio,the shear stress decreases.For specimens with a height to diameter ratio of 1~1.4,the increase of the surface roughness of the specimen does not have much influence on the friction distribution of the shear stress on the whole face.It only causes the increase of the friction shear stress and the increase of the shear stress decrease area near the edge of the specimen.