FEM analysis of advanced high strength steel car body drawing process

S. Al Azraq^a, R. Teti^a, S. Ardolino^b, G. Monacelli^b

^aUniversity of Naples Federico II, Italy

^bElasis SCpA, Pomigliano d’Arco, Italy

INTRODUCTION

• The aim of industrial applications of drawing process simulations is to replace the physical tryout by the computer tryout for time and cost reduction as well as quality improvement in the die design/manufacturing cycle

• In recent years, Advanced High Strength Steels (AHSS) have been intensively applied to automobile components to improve crashworthiness, without increasing the car body weight

• Major negative consequence of these new materials: absence of knowledge about their behaviour during both the drawing process and future physical applications

• Engineers turned to forming simulations in order to closely investigate and try to minimize this problem

MATERIAL REQUIREMENTS FOR SHEET METAL DRAWING

• The steel industry has recently produced a number of advanced high strength steels (AHSS)

• AHSS are highly formable, yet possess an excellent combination of strength, durability, strain rate sensitivity and strain hardening

• These characteristics enable automotive designers to achieve both weight reduction and improved crash safety

MATERIAL REQUIREMENTS FOR SHEET METAL DRAWING

Advantages of AHSS:

• Improved crash safety

• Excellent combination of strength, durability, strain rate sensitivity and strain hardening

• Weight reduction

• High fatigue resistance

MATERIAL REQUIREMENTS FOR SHEET METAL DRAWING

Disadvantages of AHSS:

• Forming difficulties (accentuated deviation shape problems deriving from springback), making difficult the following utilization or assembly

• Lack of knowledge about AHSS behaviour after forming may decrease some of the part characteristics

• Deterioration of press forming accuracy, due to the sheet metal high strength

FEM analysis of sheet metal drawing processes

The FEM codes for sheet metal forming may be classified accordingly to their type of integration: implicit or explicit

• Explicit methods solve for equilibrium at time t by direct time integration. The explicit procedure is conditionally stable since iterative procedures are not implemented to reach equilibrium and also ∆t is limited by natural time.

• Implicit codes solve for equilibrium at every time step (t + ∆t). Depending on the procedure chosen, each iteration requires the formation and solution of the linear system of equations.

FEM analysis of sheet metal drawing processes

Dynamic explicit method:

• Reduces the computational time drastically (adv)

• Small time steps are needed (disadv)

• Equilibrium after each time step cannot be checked (disadv)

• Springback calculations are very time consuming (disadv)

FEM analysis of sheet metal drawing processes

Static implicit method:

• Equilibrium is checked after each time step and thus leads to more accurate results (adv)

• Computation time depends quadratically on the number of d.o.f. (disadv)

• The stiffness matrix is often ill conditioned, which can make the method unstable and deteriorates the performance of  the iterative solvers (disadv)

In sum, explicit codes are favoured for solving large problems although implicit codes yield more accurate results

FEM approaches (one-step vs. incremental)

The FEM numerical computation typology for drawing processes depends on the desired analysis and belongs to one of the following approaches:

• One-Step Simulation

• Provides the preliminary feasibility analysis using simplifying assumptions such as accepting a linear behaviour of deformation, ignoring contact and friction problems, etc.

• Starts from the final deformed metal sheet configuration till the initial plane blank, in very short time, with only one step

• Incremental Simulation

• All the process components must be taken into account (kinematic characteristics, the blankholder, etc)

• Used to simulate the complete drawing process, providing accurate results

• Requires high hardware resources and calculation time

FEM analysis of sheet metal drawing processes

FEM Analysis of sheet metal drawing process adopts one of 3 methods based on the membrane, shell and continuum element:

• Membrane elements:

• Computational efficiency and superior convergence in contact analysis (adv)

• Does not consider bending effect and it is inaccurate in bending dominant problems (disadv)

• Shell elements:

• Can capture the combination of stretching and bending and provides more degrees of freedom to capture accurate stress distribution (adv)

• Notable amount of computational time and computer space for its 3D calculations (disadv)

• Continuum elements:

• Can handle through-thickness compressive straining (adv)

• More elements are needed to describe the shell-type structures, so that a large system of equations must be solved (disadv)

AutoForm Incremental code

AutoForm FEM code

• Is an algorithm for the efficient resolution of typical automotive industry complex drawing problems

• Uses a highly specialized FEM quasi-static implicit code, characterized by an incremental solution procedure

• The stresses in the metal sheet thickness are calculated using convenient elasto-plastic models

• Mesh and remeshing generation are automatic

• The contacts and elements are formulated using:

• Full Shell Elements

• Bending-enhanced membrane elements

AutoForm Incremental code

Procedure in AutoForm:

 

 

Test case with AutoForm Incremental code

An example of a simulation realized in AutoForm for a complex an under floor longitudinal member of Fiat Stilo is presented as test case:

• An incremental simulation was performed

• The process consisted of three operations: forming, trimming and springback

• The simulation was evaluated using the following result parameters: Formability, Thinning and Normal Displacement (Springback)

Test case with AutoForm Incremental code

Formability

Areas undergoing different stresses are coloured differently on the part.

Test case with AutoForm Incremental code

Another AutoForm option to analyse the formability is the Forming Limit Diagram (FLD)

In the current example, all strains are situated below the forming limit curve (FLC). This indicates that the process is quite safe and robust from a formability standpoint.

Test case with AutoForm Incremental code

Thining

The scale displayed in the lower part of the main window presents a range from 30% thinning (-0.3) to 10% thickening (0.1) Areas undergoing different stresses are coloured differently on the part.

springback
Test case with AutoForm Incremental code

The areas more subjected to springback are immediately recognised and the phenomenon can be analysed punctually and quantitatively

Conclusions

• The utilization of advanced simulation technologies (new and more powerful software system) and the AHSS implementation allows for increasing levels of product quality and a more rational employment of the materials

• Drawing process simulations improve and make the die design process much easier, faster and efficient

• It is possible to quickly evaluate numerous design and processing options, in search of the best solution

• The further developments of the research will include the utilization of a neural network approach to the modeling of the behaviour of the AHSS work material