Investigation of springback behaviour of sheet steels with finite element simulations in bending type of forming conditions

Formed sheet metals are being used in a wide range of applications. These applications of forming sheet metals are very common in especially in automotive, aircraft, defence industries as well as home appliances like white goods and kitchen tools. Forming sheet metals with bending opertations is the basic and the most common type of forming application. Bending operations are also differing due to some parameters like the shape of the sheet speciemen, material properties and production quantity etc. Clean and renewable energy generation and efficient usage of energy is one of the most important topic for industrial activities in today's world. Automotive and transportation industry is one of the biggest and most important industry and is in direct interactions with a wide range of other fundamental industrial activities. Increasing the efficiency of fuel usage and decreasing the CO2 emissions are possible with making less heavy cars. This is possible with thining the materials used in a car without making no compromises on safety issues. In this scope, AHSS, advanced high strength steels are being started to use widely in automotive industry. DP, dual phase steels are most used type of AHSS in applications. Dual phase steels contains two phases in the same microstructure, 10-30 % of martensite in a ferrite matrix, as it is said in its name. Martensite phase is the stiff phase giving and making the structure is high strength, besides ferrite phase is ductile and making the structure formable. Both phases are in same structure is making the structure more feautured. Springback phenomena is depending on so many parameters at the same time making it a complex forming problem. It is depending on dimensional and mechanical material properties and forming operation variables etc. Preventing springback 100% is not possible but it is possible to predict and prevent it to a acceptable point. Mechanical properties of DP600 steel are determined with uniaxial tension tests. ASTM A 370 – 14 and ISO 10275 standarts are used in tests and data collecting. Total 12 number of specimen are divided in 3 groups due to their rolling direction in order to determine anisotropic properties from the tests. 4 specimen are tested in 00 of rolling direction, the other 4 specimen are tested in 450 of rolling direction and the last 4 specimen tested in 900 of rolling direction. Tests are resulted with very same behaviours for both elastic and plastic regions. That means no anisotropic behaviour obtained for DP600 sheet steel. In addition, plastic strain ratio and degree of planar anisotropy coefficients are calculated with respect to BS EN ISO 10113-2014 standart and checked if there is a significant anisotropic behaviour. These values are also showed that deformation of DP600 material is close to ideal isotropic behaviour. Therefore isotropic elasticity, vonMises yield criterion and isotropic hardening rules are used in finite element analysis in ANSYS. Deformation behaviour is mapped with true stress-strain data which is determined from tensile tests. Finite element method analyses and softwares usage is very wide in today's world in order to determine how the design will react within some applied loading conditions. Finite element simulations allows us to get very important estimations and observations about the critical points of your design or system. This estimations are important to get before prototyping or production stages in order to save money and time. Metal forming or sheet metal forming simulations are possible with some softwares. In a general prospect, there are two basic iterative approach behind the physical formulations. These are called explicit and implicit methods. Explicit methods are calculating next steps without balancing equations about the condition of the system at that moment. Implicit formulation is making calculation with respect to now and the next step. Nonlinear equations for every node with respect to their degrees of freedom are being solved in implicit method. These nonlinear equations are being translated to linear equaitons using Newton-Raphson method. Convergence is needed in order to continue to next steps. Convergence is a balancing issue in the case of applied loading conditions. The previous step solution results are being used as a trial result parameters for the each new step. When the convergence is reached, new results are determined with using new boundry condition of this step. Implicit method allows us to get very correct results but convergence is a challenging isuue to get. Therefore, convergence is hard to have if you have a complex problem with conditions like complex geometry, large deformations, very high deformation speeds etc. The main difference between explicit and implicit method is, explicit method is iterating the solution system independent from previous and next steps. There is not a convergence issue in explicit method but the results are less trustable. Explicit method is more appropiate for analyzing cases which large amount of deformation is occuring in a very small amount of time steps like crash, explosion conditions etc. In this study, implicit method is used to simulate bending operation of sheet DP600 steel. Comparisons are made with an experimental and an explicit method study. It is shown that implicit results are more colloborating with experimental results. In this study, springback behaviour of DP600 sheet steel investigated within many different bending conditions. First of all, these behaviour are determined with respect to ratio of male die radius to sheet thickness (Rer/t) in order to get a conditional and extensive idea about this behaviour. First 2 mm's of male die radius are used with 0.5, 1, 1.5, 2 mm's of sheet thicknesses, after then 4 mm's of male die radius are used with 1, 2, 3, 4 mm's of sheet thicknesses to get the answer of the question; when we increase the both components of Rer/t with same ratio, if we will have the same results. The answer is resulted with yes majorly for all 4 bending angles (U, 600, 900, 1200) even if there are small amout of differences. All these analyzes are conducted within 4 different bending angles; U, 600, 900, 1200, gives us 32 total number of different analysis cases with 2 mm's and 4 mm' s of male die closing radius and the sheet thicknesses. In addition, male die closing radius or bending radius on the other hand, is investigated alone when the all other parameters are constant with extra 3 more cases for each bending angle of U, 600, 900, 1200 gives us 12 more analyses. In conclusion, the effects of bending angle, sheet metal thickness and male die closing radius on springback is investigated. Analytical comparisons are also made but it seemed that analytical formulation which is used in this study working more accurately with bigger bending radiuses. Increase in sheet metal thicknesses are resulted with decreases in springback results. When all the other parameters are constant, increase in male die radiuses increased springback results. Increase in bending angle resulted with a decrease in springback angle with respect to decrease in deformation. When the male die radius and sheet thickness ratio components are increased with a same ratio, results yielded to very close values. In addition, some additional analyses are also simulated before conclusive analyses which are mentioned in above paragraph. The effect of female die pocket lenght (w), the effect of friction coefficient used in analyses are investigated and also mesh sensitivity analyses are conducted in order to get a better understanding of the problem and so have a more efficient results with a better analysis methodology in conclusive analyses. Increase in the lenght of the female die pocket is resulted with increase in springback values. Contact friction coefficient changes is not effecting springback and the other investigated parameters in such an analysis. According to mesh sensitivity analyses, 6, 8 or 10 elements along the sheet thickness will be enough in such a bending analysis. Elimination of effects of springback behaviour is an important study area. In this scope, 3 different model of stamping down analyses are created. First of all an appropiate w, female die mouth width are calculated for all 600, 900, 1200 of forming angles after some trial analyses in order to find the situation that male die end radius, workpiece and female die inside area align oneself with. After then, stroke of male die is increased with a value of 3%, 6% and 9% of workpiece thickness which is 1 mm used for all models of stamping down analyses. As a result, springback is being decreased as stamping down amount is increased. Also, attention should be paid about the deviation of final desired forming angle value is getting more different as we increase the amount of stamping down. As a result, springback prediction on forming application is important in order to save money and time. In this study, springback behaviour of DP600 sheet metal which has wide range of applications in industrial applications, especially in automotive industry, is investigated. Results are determined with respect to changes in bending angle, sheet thickness and male die radius or bending radius on the other hand, in conclusive analyses. Analytical comparisons are also made. Additional analyses are also made about the effect of lenght of female die pocket, friction coefficient and mesh sensitivity issue. The main aim is to have a wide sense of springback behavior of DP600.