INFLUENCES OF THE FORMING PROCESS

ON CRASH PERFORMANCE

Josh Asokan CReg: 04108

The objective of this seminar is to understand metal

forming simulation parameters (effective plastic strain

distribution, thickness and residual stresses) into the crash

simulation of a structural component.

The process of forming a component changes the

properties of material being used. This is generally

ignored in the design and validation process in automotive

structures even though changes in material strength and

thickness are substantial. Finite element tools are now

able to predict the as formed material properties and use

this in subsequent crash analysis.

Crash analysis

Introduction

A vehicle body structure consists of hundreds of formed components. A detailed analysis of the stamping process can take between 5 to 10 day per part to complete. To reduce the time, it is necessary to identify the key parts in which to include formed properties in the first stage of forming to crash analysis.

Step-1

Parts for which it is important to include formed properties in the crash analysis is identified.

Step-2

• The formed properties are estimated.

Step-3

• The formed properties are predicted using a full forming analysis and are mapped into crash analysis model.

Crash analysis

Numerical and computation tool

The numerical method, which is used for solving, is the Finite Element Method (FEM).

The Finite Element Method requires that a domain is divided into finite number of elements (a mesh). The elements are connected at points called nodes. When the load is applied on the structure, deformation occurs in the element. It invokes nodal displacement.

There are two general schemes for calculating an approximate solution (here, the nodal displacement): explicit and implicit method.

Crash analysis

Test component

Crash analysis

The simulation procedure consist of:

● Design of the hat profile which has to be produced by a stamping process,

● Forming simulations of the component (stamping, coarsening, trimming and two different spring back simulations),

● Transferring the forming parameters into the crash simulation,

● Crash simulation.

Crash analysis

Forming to crash procedureA description of the steps which have been done in

order to map the forming results to the crash simulation. The steps are divided into a design, a forming and a crash process.

•Design

•Forming processStamping process

Coarsening

Trimming of the formed hat profile

Spring back phase

•Crash of the component

Crash analysis

A simple geometry - a top hat profile with a

welded flat lid has been chosen as the suitable

component. The component has to satisfy the following

conditions:

• It has to be able to absorb as much kinetic energy as

possible during the crash test;

• It has to be produced by stamping in one operation.

Design

The first condition is satisfied if the component deforms

in an acceptable deformation mode during the crash

process.

The trigged hat profile has been made formable by

stamping, in one operation.

Crash analysis

Forming process

Forming is divided into two phases: a forming

phase, and a springback phase.• The stamping is a process of the sheet metal forming

which is done by relative motion of a metal sheet

caused by parts of tool (punch and die).•The coarsening is performed after the stamping, and

includes remeshing, i.e., increasing of the element size.•The trimming is a process where the uneven deformed

edges after stamping and excessive material are cut off.•The spring back is a process of relaxation of elastic

residual stresses obtained after the stamping process.

Crash analysis

Stamping process

The forming tool consist of: a die, a punch, two blank holders, and two drawbeads.

Crash analysis

Stamping process

Crash analysis

Crash analysis

Coarsening

Trimming of the formed hat profile

In reality, the trimming is performed after the springback. Usage of the trimming tool causes again additional stresses in material. Then, later, since the trimming tool has been removed, a relaxation of stresses is performed (springback). In order to avoid two springback simulations, the trimming simulation is performed between the stamping and the springback simulations.

Crash analysis

Crash of the componentThree different modeling cases are considered:

1. The geometry of the hat profile has been taken after the forming phase simulation with the later welded lid. The forming parameters (stress tensor, strain and thickness distribution) after springback simulation, performed with only the hat profile (the first case of springback), have been used. This crash simulation has been called TRICK.

2. Both the geometry and the forming parameters after springback simulation, run with the lid welded to the hat profile (the second case of the springback) have been used. This crash simulation has been called FlSB (Forming, lid, SpringBack).

3. Geometry of the designed component with virgin material properties, the thickness of 1 mm and the element mesh size of 5 mm has been used. This crash simulation has been called VIRGIN.

Crash analysis

Crash of the component

The back end of the component is fixed to the unmovable part of the test equipment. The front end (near the triggers) is the part of structure that is exposed to the initial force of the impact body. The impact body is a flat part of test equipment which moves along the y-axis.

The fixed part of component is modeled by constrained nodes on the back edge of the hat profile and the lid. The nodes are constrained in translation and rotation.

Crash analysis

Crash analysis

Crash simulation

The results obtained from the crash simulations are

presented by three diagrams: axial deformation of the

front end, force-displacement response, and energy

absorption diagram.

The axial deformation is presented by a displacement of the front end vs time curve.

The force-displacement response shows how the rigid wall force acts at different global displacement.

The energy absorption is given by a rigid wall work vs displacement of the front end curve.

Crash analysis

Table 4.1:Performed simulations

Crash analysis

Displacement of the front end vs. time

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Force-displacement Response

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Energy absorption

Crash analysis

Crash analysis

Sheet Forming Effect on Crash Example-1

With forming effect

Without forming effect

Crash analysis

Sheet Forming Effect on Crash Example-2

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Sheet Forming Effect on Crash Example-3

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Conclusions

The weakest structures are the structures where the forming parameters are not included.

Thinning of the material causes a weaker response of the structure.

The effective plastic strains always hardens the material response.

The influence of the effective plastic strains is the most eminent.

The residual stresses added to the strains with thinning significantly change the deformation mode.

Crash analysis

References

http://www.ls-dyna.com

http://www.dynamore.de/download/papers/af02_v10_elsaesser_trw.pdf

http://www.metalformingmagazine.com/technical papers/formingparameters/crash analysis

http://www.ulsab.org

Crash analysis

Thank you!