Computer aided engineering tool for seals.

Seals play a very important role in automotive engineering. The performance and reliability of many major components such as engines, transmissions, brakes, shocks and air-conditioners depend to a great extent on the quality of the seals used in these components. The seals used in windows, doors, radio housing, etc., in the passenger compartment influence the noise, vibration and harshness characteristics of the vehicle.

With the increasing consumer demands for quality, comfort, safety, durability and environmental protection the specifications for seals/gaskets are becoming more and more stringent. The additives and modifications of fuel, oils and fluids, higher fuel economy and tougher emission standards for reducing environmental impact subject seals to an aggressive thermal and chemical environment (ref. 1).

Many new elastomeric materials developed in recent years, meet the challenge. The economic thermoplastic  manufacturing processes of these new elastomeric alloys offer greater design flexibility. The challenge now is to select the proper material, process and design.

The rubber-like elastomeric materials used for seal/gasket applications undergo large deformation and in most cases have nonlinear stress-strain relations. The strength and stiffness of these materials also depend on the strain rate, temperature, stress and exposure time.

Material model

The small-strain theory used to describe deformations in metals and other engineering materials is not adequate for seal materials which experience large strain even at relatively small loads. The constitutive relation can not be characterized in terms of uniaxial tension test data. Special forms of material constitutive models (Mooney-Rivlin, Frazer-Nash, Ogden) are available to characterize rubber-like incompressible (Poisson's ratio = 0.5) and nearly incompressible seal materials.

The time dependent behavior of rubber/elastomers can be characterized in general as viscoelastic. The viscoelastic behavior can be explained by the example given by Wineman (ref. 2).

If an elastomer part is subjected to instantaneous strain (at time t = O) and held constant for a long time (as in the case of many press-fit bushings), the corresponding stress in the stress-time graph decreases with time from its instantaneous value, until it reaches some non-zero value. This phenomenon is called stress relaxation.

If, instead of a step-strain the elastomer part is subjected to suddenly applied stress, and the stress is held constant for a length of time (as in the case of many seals under constant fluid pressure), the corresponding strain, in the strain-time graph continues to increase with time, until it reaches some limiting value. This phenomenon is called creep. The creep process has three distinct stages, namely primary (transient state), secondary (steady-state) and tertiary (accelerated state).

The failure associated with prolonged steady-state and accelerated state of creep is termed as creep-rupture and the failure in the transient state is termed as stress-rupture.

In reality, the elastomer seals are subjected to continuously changing stress conditions. Corresponding creep strain rates also change continuously. The total creep strain at any instant of time is a function of stress state, total strain and time. Two of the most commonly used methods of accumulating creep strains are the strain-hardening and the time-hardening (age-hardening) laws. The strain-hardening law assumes that the creep rate depends on instantaneous stress and accumulated creep strain, whereas the time-hardening law assumes that creep rate depends on instantaneous stress and length of exposure at that stress level. The two methods of accumulating strains are graphically shown in figure 1.

Under prolonged temperature, stress and strain conditions, elastomers also lose their stiffness, hardness and toughness (tear resistance). The change in stiffness and hardness (which directly relates to friction) significantly influences the natural frequency of the component.

It is very critical to understand and account for the material characteristics in developing sealing applications. A highly squeezed inexpensive seal may hold fluids very well initially, but the higher initial stress results in higher creep rate and hence, lower seal life. The increased squeeze also results in increased friction. The leaks, squeaks, noise and vibrations in an automobile can be related directly to poor seal engineering.

The nonlinear viscoelastic and incompressible behavior of rubber/elastomers makes it very difficult to use most of the general purpose finite element analysis (FEA) codes for design of seals and gaskets. In addition to material behavior, the boundary conditions and loads in sealing applications, are also complex. The boundary conditions are typically in the form of rigid walls with sliding surfaces and gap elements. The pressure loads may vary with elastomer modules and changing geometry.

Obviously, a higher degree of FEA expertise is required to model/analyze rubber/elastomer seals. The cost of engineering seals using general purpose FEA is prohibitively high.

MSME-SEAL

By focusing attention on the materials used for seal applications, general seal geometry, typical boundary conditions and loads encountered in seal problems, it was possible to develop a special purpose analysis and design software (MSME-SEAL) for rubber/elastomer seals.

MSME-SEAL, which is based on a higher order hybrid (stress, displacement) FEA formulation, is very effective in modeling the Mooney-Rivlin, nonlinear viscoelastic material behavior and also the nonlinear traction boundary conditions in seal applications.

The effectiveness and accuracy of the hybrid formulation are shown in figure 2, by comparing MSME-SEAL results with closed-form solutions for an infinitely long cylinder subjected to internal pressure.

In MSME-SEAL the FEA module works as a user transparent solvet in the background. The main objective was to develop a design tool that is as easy to use as a thumb rule or looking up design charts. MSME-SEAL is PC based software with built in graphics. For advanced complex applications the FEA module can be loaded on workstations-Mini/Main computers and the results may be post processed on a PC.

The type of seals that can be engineered using MSME-SEAL software include molded, bonded and extruded forms of seals. MSME-SEAL allows for four different materials in a section. This is useful in modeling dual durometer seals, spring loaded seals and seals with metal (steel, brass, aluminum), plastic (PTEE) inserted or bonded on surface.

The input (pre-processor) and output (post-processor) modules of MSME-SEAL are shown in table 1. The input to the software is limited to few design parameters and materials data. For example, if an o-ring seal has a perfect circular cross section, then the user enters the inner and outer diameters of the ting. The pre-processor automatically generates the mesh for the higher order element. Even for complex geometry the user has to input minimum geometry points.

The results of the analysis are presented in the form of animated deformation, force-deflection plots, stress, strain and displacement plots, stress-time and strain-time plots.

The deformation plot of an example dual durometer EPDM door seal predicted using MSME-SEAL is shown in figure 3. The resultant normal force on the master surface (rigid surface) with respect to seal normal maximum deflection is shown in figure 4. The force-deflection curve in the door seal application is useful to characterize the door closing effort.

The predicted deformation of a standard circular cross-section o-ring seal is shown in figure 5. Figure 6 shows von Mises stress distribution in the seal. MSME-SEAL allows the user to select between strain-hardening and age(time)-hardening creep strain accumulation laws. The solution time for this example was 12 minutes on 486/33 PC.

Summary and conclusions

Seals play a very important role in the performance, reliability and durability of many major components of an automobile. With the increasing consumer demand for quality, economy, comfort, safety and also environmental protection, the seals are being subjected to a very aggressive thermal and chemical environment. New elastomeric alloys are available in the market which meet the challenge. The elastomeric alloys also offer economic thermoplastic moldability, hence greater design flexibility. But, the elastomeric materials like rubber are viscoelastic in nature. This article introduces basic concepts of material modeling for rubber/elastomers. The .article also introduces a special purpose nonlinear viscoelastic, higher order FEA based computer code MSME-SEAL (easy-to-use-yet-versatile) for the analysis and design of seals and gaskets.

References

1. J.R. Dunn, "Elastomeric materials for demanding automotive applications," Automotive Polymers & Design, Vol. 11, No. 3, Feb. 1992.

2. A. Wineman, "Some modeling considerations for rubber-like materials in the development of software for computer-aided design, "SAE paper No. 860812.

3. G.A. Greenbaum, M.F. Rubinstein, "Creep analysis of axisymmetric bodies using finite elements," Nuclear Engineering and Design, vol. 7, 1968, pp. 379-397.

  TABLE 1 -pre- and post-processor modules of
                 MSME-Seal
Pre-processor
  Geometry
  Material properties
  Boundary conditions
  Rigid-walls initial
  and final positions
  Contact surfaces
  Contact friction
  Automatic mesh
  generation
Processor
  Hybrid,
  non-linear,
  hyper-elastic,
  viscoelastic
  formulation
Post processor
  Animated deformation
  Stress/displacement color contours
  Force-deflection/energy
  absorption/
  stress-time/strain-time
  plots