2005 ABAQUS Users’ Conference 1

Modeling Thermoplastic Vulcanizate Seals with ABAQUS

Scott D. Schrader

Applications Engineer, Santoprene® Specialty Products

ExxonMobil Chemical

Abstract: The use of Abaqus for analysis of seals made from thermoplastic vulcanizate (TPV) materials is explored. Of particular interest are the material models that are available to model nonlinear, time dependent behavior. This includes currently available material models in Abaqus, such as hyperelastic and viscoelastic. The typical TPV material behavior is explained and then the corresponding test methods that are required to capture this behavior are reviewed. This is followed by a discussion of the procedures used to develop the TPV material models. A summary of the material model validation process and results is provided. Finally, some practical applications of the models used for TPVs are reported along with the experimental results comparing the FEA predictions to testing performed on actual seal profiles. Keywords: Compression Set, Constitutive Model, Cyclic Softening, Damage, Elastomer, Experimental Verification, Hyperelasticity, Hysteresis, Load Loss, Rubber Seal, Thermoplastic, and Viscoelasticity.

1. Introduction

The use of thermoplastic vulcanizates (TPVs) in various automotive sealing applications has grown significantly as automotive manufacturers and part suppliers have discovered an alternative to thermoset rubber seals. TPV materials offer the performance of an elastomer while providing the benefits of a thermoplastic, including weight reduction, recycling, colorability, and ease of processing (Rader, 1988).

As manufacturers begin to use TPVs, they need to consider necessary design changes in order to take advantage of TPV performance and benefits. One area of interest is the ability to model and predict TPV sealing performance using finite element analysis (FEA). In this paper, the use of hyperelastic and viscoelastic material models in Abaqus to predict sealing performance is evaluated.

Two sealing performance indicators used by most sealing system manufacturers are compression load deflection test and load loss set test. The compression load deflection test determines the seal stiffness as it is squeezed between two more rigid bodies. This has implications related to closing effort seen in doors, hoods, and trunks. The load loss set test determines how much the sealing

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force dissipates over time and how much the seal returns to its original shape. The seal integrity depends on maintaining the sealing force and filling the original gap.

The use of FEA to compare several seal designs is critical in order to reduce time to market. The ability of FEA to predict seal performance is dependent on the seal geometry, material model, and boundary conditions. Some examples will be shown where FEA was used to simulate the compression load deflection test and load loss set test.

First, a brief background will be given on thermoplastic vulcanizates. This will be followed by an overview of the material testing and results. Then an overview of the hyperelastic and viscoelastic material model development process is given. Finally, some practical examples comparing FEA simulations to experimental results will be given.

2. Thermoplastic Vulcanizates

A thermoplastic elastomer (TPE) is a material that behaves like rubber, but can be processed like a plastic. Thermoplastic vulcanizates (TPVs) are a family of TPEs that contain fully cured, or vulcanized, rubber particles. A TPV can be described as a composite material with a thermoplastic matrix containing rubber particles. See Figure 1 for an illustration of the morphology.

Figure 2 shows a scanning electron micrograph of the rubber particle distribution (Boyce, 2001). The use of vulcanized rubber particles provides improved compression set and better flex fatigue. It also allows better physical property retention over a broader temperature range. The TPV materials used in this work are polypropylene and ethylene-propylene-diene-monomer (PP and EPDM) blends prepared using a dynamic vulcanization process (Coran, 1996).

3. Material Testing

Material testing was performed to characterize the TPV behavior and to determine the appropriate hyperelastic and viscoelastic material parameters. Material testing was performed using elastomer testing procedures that have been developed by Axel Products, Inc. The testing included simple tension, planar tension, biaxial tension, and simple compression using a slow cyclic loading procedure (Miller, 2001) to determine hyperelastic material parameters. Volumetric compression and short term stress relaxation testing were also performed (Miller, 2000) to determine material compressibility and viscoelastic parameters. All test specimens were die cut from the same extruded sheet sample which was approximately 2 mm thick. The nominal strain rate was 0.01 sec-1 for all tests.

Several observations can be made from results of the slow cyclic loading tests. Figures 3 and 4 show the results for TPV 67A. The material exhibits hysteresis. The first cycle loading to a specified strain level is unique. Subsequent loading cycles to the same specified strain levels are more repeatable and eventually stabilize around five cycles. There is some degree of cyclic softening or Mullins effect. There is permanent set in the material. As strain levels are increased, so does the level of permanent set. It should be noted that all of these characteristics have also

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been observed in thermoset EPDM rubber compounds commonly used in automotive sealing applications.

A hyperelastic material model can be used to capture a loading segment from the cyclic test sequence. A stabilized set of stress strain data must be selected for use in the hyperelastic fitting routine. The selection should depend on the strain level reached in the sealing application. Stress and strain corrections are applied to the data to shift the strains back to zero and to account for gauge length and area changes caused by permanent set. Figure 5 shows the corrected stress strain data used for TPV 67A for the fifth cycle loading at a specified strain level of ten percent. Figure 6 shows the volumetric compression test results for TPV 67 A. This test data is used to determine the TPV compressibility. Figure 7 shows the results of stress relaxation testing for TPV 67A. This data is used to determine the Prony series coefficients used in the viscoelastic material model.

Table 1 is a listing of the materials and the corresponding material parameters used in the analysis.

4. Experimental Testing

Two lip seals were tested for compression load deflection and load loss set. The initial seal gap was set to 20 mm and then the contacting surface was moved down 10 mm for five cycles. During the 5th cycle loading, the seal was held for 30 minutes at the 10 mm position and the force decay was recorded. At the end of the 30 minute force decay, the lip seal was unloaded. Both lip seals are made using the TPV 67A material. The first lip seal design, Lip A, has a single hinge near the base. The second lip seal design, Lip B, has a double hinge near the base.

Two bulb seals were tested for compression load deflection and load loss set. The initial seal gap was set to 12 mm and then the contacting surface was moved down 5 mm for five cycles. During the 5th cycle loading, the seal was held for 30 minutes at the 5 mm position and the force decay was recorded. At the end of the 30 minute force decay, the bulb seal was unloaded. The first bulb seal, Bulb A, is made using the TPV 67A material. The second bulb seal, Bulb B, is made using the TPV 73A material.

Testing was performed on an Instron 5565A with a 100 N load cell. A lubricant was applied to minimize frictional forces between the seal and deflecting plate. The test temperature was 23° C and the speed was 25 mm/min. The Lip A and Lip B seals were clamped using a fixture at the base. The Bulb A and Bulb B seals were mounted at the base using double sided tape.

5. FEA Models

FEA models were built to simulate the experimental testing described above for each seal. A dual blade cutter was used to cut 2 mm thick sections from the extruded seals. The sections were digitized and the resulting geometry was used to build the finite element mesh.

Figure 8 is a photograph of the lip seal sections for Lip A and Lip B. Figure 9 is a photograph of the bulb seal sections for Bulb A and Bulb B. Figure 10 shows an example of the digitized point

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data and surface geometry used to create the finite element mesh for Bulb A. Similar geometries were created and used to mesh models for Lip A, Lip B, and Bulb B.

All seal models used 2D, plane strain, hybrid (Abaqus CPE4H) elements. The fixtures used to hold and deflect the seals were assumed to be rigid. A friction value of 0.1 was assumed for contact between the seal and upper plate, since a lubricant was used during testing. A friction value of 1.0 was assumed between the holding fixture and Lip A and Lip B. Rough friction with no separation was used for the base of the Bulb A and Bulb B, since a double sided tape was used.

6. Results

Figure 11 shows maximum principal strain results for Lip A, while Figure 12 shows compression load deflection (CLD) and load loss set (LLS) results. The measured CLD and LLS values lie between the predicted CLD and LLS values when a stabilized stress strain model at 5% and 10% are used. Figures 13 and 14 show good agreement for Lip B when a 5% stabilized stress strain model is used.

Figure 15 shows maximum principal strain results for Bulb A, while Figure 16 shows CLD and LLS results. The measured values agree with the predicted values when a 10% stabilized stress strain model is used. Figures 17 and 18 show a similar result for Bulb B when a 5% stabilized stress strain model is used.

7. Conclusions

It appears reasonable compression load deflection and load loss set predictions can be obtained using FEA when the appropriate stabilized stress strain curves are selected. A material model that can capture cyclic loading behavior and permanent set is most likely required to obtain a precise prediction of CLD and LLS values. However, the effect of various design modifications using FEA can still be used to improve sealing performance.

8. References

1. Boyce, M.C., Socrate, S., Yeh, O.C., Kear, K., Shaw, K., Micromechanisms of Deformation and Recovery in Thermoplastic Vulcanizates, Journal of the Mechanics and Physics of Solids, Vol. 49, 2001, pp.1323-1342.

2. Boyce, M.C., Yeh, O.C., Socrate, S., Kear, K., Shaw, K., Micromechanisms of Cyclic Softening in Thermoplastic Vulcanizates, Journal of the Mechanics and Physics of Solids, Vol. 49, 2001, pp. 1343-1360.

3. Boyce, M.C., Kear, K., Socrate, S., Shaw, K., Deformation of Thermoplastic Vulcanizates, Journal of the Mechanics and Physics of Solids, Vol.49, 2001, pp1073-1098.

4. Coran, A. Y., Patel, R.P., in Thermoplastics Elastomers, 2nd Ed., Holden, G., Legge, N. R., Quirk, R., and Schroeder, H.E., (Eds.), Hanser, New York, 1996, p. 153.

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5. Miller, K., Using Slow Cyclic Loadings to Create Stress Strain Curves for Input into Hyperelastic Curve Fitting Routines, Testing and Analysis (www.axelproducts.com), April 2001.

6. Miller, K., Testing Elastomers for Hyperelastic Material Models in Finite Element Analysis, Testing and Analysis (www.axelproducts.com), July 2000.

7. Qi, H.J., Boyce, M.C., Constitutive Model for Cyclic Softening Stress-Strain Behavior of Thermoplastic Vulcanizates, Research Report, Advanced Elastomer Systems, L.P., October 2003.

8. Rader, C.P., in Handbook of Thermoplastic Elastomers, 2nd Ed., Walker, B.M. and Rader, C.P., (Eds.), Van Nostrand Reinhold, New York, 1988, p. 85.

9. Tables

Table 1. Hyperelastic and viscoelastic material parameters. Reference ID TPV 67A 5 TPV 67A 10 TPV 73A 5 TPV 73A 10

Material

Santoprene® thermoplastic rubber, grade 121-67W175

Santoprene® thermoplastic rubber, grade 121-67W175

Santoprene® thermoplastic rubber, grade 121-73W175

Santoprene® thermoplastic rubber, grade 121-73W175

Test Conditions

5th cycle, 5% strain, 23° C, 0.01 sec-1

5th cycle, 10% strain, 23° C, 0.01 sec-1

5th cycle, 5% strain, 23° C, 0.01 sec-1

5th cycle, 10% strain, 23° C, 0.01 sec-1

Neo Hookean C10 (MPa) 1.6045 1.2456 2.1784 1.6926 D1 6.97E-4 6.97E-4 6.31E-4 6.31E-4 Prony Series G1 9.2058E-2 9.2058E-2 4.56848E-02 4.56848E-02 Tau1 (sec) 4.083 4.083 1.8500 1.8500 G2 0.1716 0.1716 8.01625E-02 8.01625E-02 Tau2 (sec) 71.2 71.2 11.986 11.986 G3 0.1664 0.1664 0.10999 0.10999 Tau3 (sec) 1044.0 1044.0 66.797 66.797 G4 - - 0.11580 0.11580 Tau4 (sec) - - 376.43 376.43 G5 - - 0.12346 0.12346 Tau5 (sec) - - 2783.0 2783.0

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10. Figures

Figure 1. TPV morphology.

Figure 2. Scanning electron micrograph of a TPV.

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Figure 3. Cyclic simple tension and planar tension test data for TPV 67A.

Figure 4. Cyclic equibiaxial tension and simple compression test data for TPV 67A.

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Figure 5. Stabilized, offset, and corrected stress strain data for TPV 67A, 5th cycle, and 10% maximum strain.

Figure 6. Volumetric compression test data for TPV 67A.

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Figure 7. Stress relaxation data from simple tension test, TPV 67A.

Figure 8. Lip A (left) and Lip B (right).

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Figure 9. Bulb A (left) and Bulb B (right).

Figure 10. Bulb A with digitized points and geometry.

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Figure 11. Maximum principle strain for Lip A.

121-67W175Lip A

0

1

2

3

4

0 2 4 6 8 10

Deflection (mm)

Load

(N)

Test ATest BTest CFEA, 5% Max StrainFEA, 10% Max Strain

Figure 12. Load deflection results for Lip A.

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Figure 13. Maximum principle strain for Lip B.

121-67W175Lip B

0

1

2

3

0 2 4 6 8 10

Deflection (mm)

Load

(N)

Test ATest BTest CFEA, 5% Max StrainFEA, 10% Max Strain

Figure 14. Load deflection results for Lip B.

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Figure 15. Maximum principle strain for Bulb A.

121-67W175Bulb A

0

5

10

15

20

25

30

35

40

45

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

Deflection (mm)

Load

(N)

Test 1Test 2Test 3FEA 5%FEA 10%

Figure 16. Load deflection results for Bulb A.

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Figure 17. Maximum principle strain for Bulb B.

121-73W175Bulb 3

0

10

20

30

40

50

60

70

0 0.5 1 1.5 2 2.5 3 3.5

Deflection (mm)

Load

(N/1

00 m

m)

Test 1Test 2Test 3FEA, 5% Max StrainFEA, 10% Max Strain

Figure 18. Load deflection results for Bulb B.