Research On Forming Limits Of Sheet Metal Under Complex Loading Conditions

Sheet metal stamping is a kind of plastic forming process,widely used in the production of parts in automotive,aerospace,instrumentation and other industries.Forming limit is an important index in the evaluation of forming quality and technological design.Forming limit diagram,as the most important reference in determining the forming limit,is of great significance to the industrial production.Traditional forming limit diagram is obtained based on the hypothesis of linear or nearly linear strain path,not considering the effect of bending effect.However,in actual stamping forming of sheet metal,nonlinear strain path is very common during the forming process,also accompanied with strong bending deformation.Under such complicated loading conditions,the change of strain path and the participation of bending deformation have great influence on the forming limit of sheet metal.Therefore,forming limit under the condition of the nonlinear strain path and bending deformation is researched,of great significance for the improvement of evaluation in the process of production and process of design rationality.This article mainly focus on the forming limit of sheet metal under complex loading conditions.Experimental research of forming limit under the condition of nonlinear strain path.Experimental research of forming limit under non-linear pre-strained condition.Numerical simulation and experimental research of forming limit under the condition of continuous strain path and Core mechanism of increased forming limit under complex loads.The main contents of this paper are as following:1)With combination of test and theoretical analytical,choosing the low carbon steel as the research object.Taking the process of sheet metal flowing through draw-bead as the research breakthrough point,studying the nonlinear strain effects on forming limit,by adopting the draw-bead test machine.Designing and manufacturing a series of draw-bead inserts with different rounded corners and heights independently,two kinds of test methods are designed to explore the formability of sheet metal through draw-bead inserts.Analytical model is constructed to explain the experimental results in detail.2)Adopting the method of measuring forming limit curve in the existing international standard in laboratory.Low carbon steel widely used in automobile covering parts is chosen as the research object.Designing and manufacturing a set of 50 mm punch and a set of 20 mm punch test mould independently.The first step of strain loading is the process of sheet metal flowing through draw-bead inserts with different geometric parameter.Then samples with different width were obtained by linear cutting,and punch stretching experiment was carried out.At the same time,the ARAMIS system,the online-grid strain measurement system,was employed,recording and analysing the strain state of samples from start until failure.Then obtaining limit strains under the limit strain state,and drawing the forming limit curves.Through experiment analysis of the punch diameter with 100 mm punch(standard),50 mm and 20 mm,the difference of forming limit curves under different punches were analysed.The bending effect on the mechanism of deformation of sheet metal forming limit was discussed in detail.3)The forming limit under continuous strain path was analyzed by means of numerical simulation and experiment,and the forming limit diagram was modified.By choosing the U-shaped parts as the analysis model,the process of sheet metal flowing through draw-bead inserts is as the breakthrough point of the continuous strain path.Low carbon steel commonly used in automobile covering parts is the material.Analysing the difference of forming limits under different strain paths between equivalent draw-bead and real draw-bead by using various numerical simulation softwares.Then choosing a typical car fender as example,by analysis of the two methods of the equivalent draw-bead and draw-bead,different continuous deformation path of sheet metal forming were understood.Then the actual fender was stamped and measurement data were analyzed.Then,based on the modification of the forming limit diagram in this case,the local grid strain gradient criterion based on the deformation region was proposed.Choosing the low carbon steel and aluminum alloy as material for the test.Repeated loading-unloading-reloading of uniaxial tensile tests were done to study the forming limits under continuous strain path,suggesting that different forms of continuous strain path has different effects on the sheet metal formability.4)Choosing the existing typical forming process with enhanced forming limits as the main research content.Incremental forming,thinning drawing and forming process under combined tension and bending were selected as specific study case,analyzing the forming mechanism of increased forming limit in detail.Then trying to put forward the affine deformation mechanism and make a more clear definition on the formability of sheet metal.