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Prediction And Numerical Simulation Of Ductile Fracture And Forming Limit In Microscaled Sheet Metal Forming Process

Posted on:2022-04-12Degree:MasterType:Thesis
Country:ChinaCandidate:Z F XiaoFull Text:PDF
GTID:2481306311960659Subject:Mechanical engineering
Abstract/Summary:PDF Full Text Request
As the demand for product portability increases,the demand for micro parts is growing.Microforming is a method of manufacturing micro parts directly from plastic forming of sheet metal material with the advantages of low cost and high efficiency compared to micromachining techniques.In the metal forming process,the ductile fracture behaviour of the metal is the main factor affecting the forming performance of the sheet.In order to improve the forming quality and efficiency of microformed parts and to achieve a rational forming design,it is necessary to understand the forming mechanism and to build an accurate simulation model to predict the forming process.Because of the presence of dimensional effects in the microforming process,macroscopic fracture models and numerical simulation methods cannot be directly applied to the prediction of material deformation and fracture behaviour at microscale.The lack of maturity in the prediction of forming fracture of sheet materials at microscale has restricted the development of microforming technology,so there is a need to develop models for numerical simulation applied to microforming.Metal toughness fracture is the result of void accumulation,and the evolution of void includes three parts:nucleation,growth and aggregation.The most widely used hole fracture model is the GTN(Gurson-Tvergaard-Needleman)model,which is highly accurate in predicting the fracture of high.stress triaxiality sheet materials at the macroscopic scale,but cannot directly predict the ductile fracture behaviour of microformed parts due to the existence of size effects and the inability to reflect the effect of stress state on material fracture in the model.This paper combines the Thomason model with the Lemaitre mechanical damage model to develop a shear-corrected GTN model and the corresponding numerical algorithms and damage parameter determination methods,thus enabling the new model to predict ductile fracture in complex stress states at the micro mesoscopic scale.Firstly,this study investigated the mechanism of ductile fracture and developed a shear-modified GTN model.In the ductile fracture process of metallic materials,there are two fine-scale damage mechanisms that depend on the stress state:One is the cavity damage fracture mainly caused by tensile fracture and the other is the slip damage fracture mainly caused by shear fracture,which is dominated by shear fracture.In this paper,by considering the effect of size effects on the evolution of void and the effect of stress state on the fracture behaviour of the material,an improved toughness fracture model is developed based on the GTN model by adding a size factor and using the Lode parameter to characterise the stress state,and is embedded into ABAQUS using the VUMAT subroutine for numerical calculations.Then,this study investigates the forming of progressive sheet at the micro mesoscopic scale,designs a forming tool for the manufacture of micro gears and investigates the effect of size effects on the forming effect of Magnesium-Lithium alloys.Numerical simulations were carried out with a shear modified GTN model to investigate the effect of size effect and void evolution on the material during forming at the micro mesoscopic scale,and the effect of size effect on forming defects such as rollover and burr during forming of formed parts was observed using microscopy.Finally,the forming limits of TA2 materials at microscale were tested using the Holmberg method using DIC(Digital Image Correlation)equipment.In this paper,the influence of size effect and grain growth twinning on the forming limit of materials is studied.At the same time,the influence of stress state,cavity evolution,size effect and deformation twinning on the forming limit of materials is analyzed by numerical simulation.In addition,in order to verify the accuracy of the shear modified GTN model in predicting the forming limit,a 0.1mm titanium sheet stamping and forming experiment was carried out and the model was found to be able to accurately predict the forming limit at material fracture.
Keywords/Search Tags:Microforming, Forming limits, Size effects, Ductile fracture, Shear-modified GTN model
PDF Full Text Request
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