sheet metal formingcrimping simulation in ansys

8/2/2019 Sheet Metal FormingCrimping Simulation in ANSYS

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Sheet Metal Forming/Crimping Simulation in ANSYSCaner Demirdogen,Ph.D.

DANA Corporation, Kalamazoo, MI

Hakan Oka, MSc.

FIGES, Ltd., Bursa, Turkey

Chad Thomas

Fleetguard, Inc., Cookeville, TN

Tarik Ogut, Ph.D.

FIGES, Ltd., Bursa, Turkey

Abstract

Crimping is a widely used assembly method to attach a metal fitting (crimp-nut) to a sheet metal structure.This method mechanically locks the fitting and the sheet metal together by means of a crimp die. For some

applications, this mechanical joint must be leak proof. For a leak-proof joint, 3-layer-crimp form (fittingflange-sheet metal flange-sheet metal, shown in Figure 11) must be achieved. However, achieving 3- layer

crimp form is not easy. It requires the right combination of the flange lengths, sheet metal thickness andmost importantly crimp-die profile.

Historically, trial and error was the only way to determine these parameters correctly. This used to be atime consuming and expensive process since several prototypes and pilot runs were necessary until the

right combination is found. Fortunately some commercial finite element packages are capable of handling

contacts, plasticity and large strains and displacements efficiently and accurately. Thus, the computersimulation of such a crimping process is possible now. In this paper, the authors modeled the several

combinations of a crimp-nut, a metal shell and a crimp-die in ANSYS 8.0. To obtain the correct stress-

strain curve for the already work hardened low carbon steel metal shell, a simple and practical method wasdeveloped and used.

Introduction

How would you attach a steel fitting to a deep-drawn steel can? Welding and brazing were some of the

methods used in the past for this purpose. However, the joints produced by these methods were nevermechanically consistent and they all required the costly pre and post cleaning operations. Thus, the industry

kept searching new methods and finally a crimping process was developed for this purpose. This process is

a forming process. It rolls the edge of the machined fitting over the shell flange and locks them together. Itrequires a machined fitting with a flange, shell with a flange and a crimp die as shown in Figure 1.

Important parameters for such a crimp joint are shell flange length, fitting flange length and crimp die

profile for a given shell thickness. All these parameters are defined in Figure 1. The right combination of

these parameters is to be found by finite element simulations in ANSYS.

Finite Element modelThe generic finite element model used for this study is shown in Figure 2. Mid-noded axi-symmetric 2D

Plane 183 elements are used to model the shell, fitting and the die. Contact 172, contact 172 elements were

used between the shell and the fitting to model contact. Rigid Contact 169 elements were used to model thecrimp die and fitting flange interaction. The authors chose the rigid contact since the force exerted on the

crimp die was to be extracted from the model and the single anchor point of the rigid contact elements

made this process very simple.


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Figure 1. The components of the crimp joint

Figure 2. The finite element model

Crimp Die

Deep Drawn

Steel Shell

MachinedSteel Fitting(Crimp-nut)

Fitting Flange

ShellFlange

Center-line

Crimp die Profile

Work

hardened

zone

Contact 172

elements

Rigid Contacttarget 169

elements


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The finite element models required the accurate true stress true strain curve for shell and fitting materials.

The fitting is forged steel. However, the only true stress true strain curve available in literature for this

material was for the rolled condition not for the forged condition. Thus, the authors created a finite element

model of the fitting and crimp die only, without the shell, and simulated the crimping process by using theavailable stress-strain curve for the fitting material. Later on, the real crimp die was manufactured and a

fitting was crimped by using this real die. The crimped fitting was sectioned and a digital picture of the

section was recorded. Figure 3 shows the real crimped joint cross-section and the ANSYS predicted cross-

section. They correlate to each other very well. This was the calibration of the finite element model and thematerial properties.

The model shown in Figure 2 also required the stress-strain curve of the shell material. The shell material is

low carbon (1008) steel. However, this material was already work hardened around the flange radius

(shown in Figure 2) during the prior deep drawing and flange forming operations. Thus, the stress-straincurve for 1008 steel for the work hardened condition was required for an accurate model and that,

unfortunately, was not readily available in literature. Thus, the authors developed a simple and practical

method to measure the amount of work hardening in the material first and obtain the stress-strain curve,

later. Due to the confidentiality of this information, the detailed step-by-step approach will not bedescribed.

Determining the stress-strain curve for work hardened low carbon steel

Testing of the material showed that 30% pre-strain was the correct value for the flange area under

consideration. After performing tensile tests on 10 pre-strained specimens, the true stress true strain curvefor the shell material was obtained.

Figure 4 shows the true stress-true strain curve of the work hardened 1008 steel. The scaling on the vertical

axes was intentionally deleted due to the confidentiality of this data. This is the material curve used for allof the ANSYS models presented in this paper.


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Figure 3. The comparison of real cross-section (shown in solid blue) and the ANSYSmodel prediction (shown in wire-frame)

Figure 4. The true stress true strain curve of the shell flange radius material

Simulation of the crimping process for different flange lengths and dieprofile

Figure 5 shows the details of the finite element model used for this study. Six ANSYS models were createdto study the effect of different parameters. The list of the models and their corresponding parameters were

shown in Table 1. Figure 6 through 12, shows the expected crimp joint cross-section, residual Von Mises

stress and the force exerted to the die during the crimping process.

.

Figure 5. The boundary conditions used for the finite element model

STRESS-STRAIN CURVE

23000

28000

33000

38000

43000

48000

53000

58000

0 0.05 0.1 0.15 0.2 0.25

STRAIN

STRESS

/psi

Constrained in all

Directions

Pivot point of the rigid die with given

displacement in Y direction


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Table 1. The list of the ANSYS models

Von Mises stress after spring back Die load during the crimping process

Figure 6. Model 1: asis021_0305

DimensionsModel

No

Model name

Fitting Flange

(inch)

Shell Flange (inch)

1 Asis021_0305 0,305 0,21

2 Asis022_0305 0,305 0,22

3 Asis023_0305 0,305 0,23

4 Asis022_0355 0,355 0,22

5 Asis023_0355 0,355 0,23

6 Asis023_0380 with

different die profile

0,380 0,23


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Kink in the loadcurve means a non-

stable forming

rocess

Kink in the load

curve means a non-stable forming

process


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Figure 11. Model 6: asis023_0380: Von Mises stress after spring back

Figure 12. Model 6: asis023_0380: Die load during the crimping process

Triple layer crimpconfiguration required for

leak proof joint


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Conclusion

In this paper, a method to simulate a crimping process in ANSYS was described. The accuracy of the

method was proven by comparing the calculated model results with the real measurements.

The effects of fitting flange length, shell flange length and crimp die profile were studied in 6 differentmodels to find the right combination of these parameters. The parameters used in Model 6 are the right

parameters for the production since it predicts a 3-layer crimp joint form (important for a leak-tight joint)and a smoothly increasing die load (important for consistent forms).

Models 2 and 3, Figures 7 and 8, predicts a step change in the die load during the crimp-process. This

usually is an indication for a possible buckling state during the crimping process. Since buckling is an

unstable state of the structure, the parameters used for models 2 and 3 should not be selected forproduction.

By using this simulation method, one can also study the effect of tolerances to crimp joint form and die

load.

ANSYS 8.0 is a very capable finite element software package, which can handle contact, plasticity, and

large deflection nonlinearities very accurately as shown in this paper. Thus, forming processes can easilyand accurately be modeled in ANSYS.

sheet metal formingcrimping simulation in ansys

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