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Sealing Successful FEA

Tue, 01/21/2014 - 3:06pm
Trelleborg

Rod contact pressure of Zurcon L-Cup at different operating pressures.FEA has become commonplace in all industries, from aerospace to life sciences, but software, like any tool, is only as good as the people using it.

Finite Element Analysis (FEA) is a method, typically performed by software, used to create models of materials or designs that can then be subjected to simulated stresses. Engineers can use the model to predict material and product performance under a range of operating conditions, providing real cost benefits over manufacturing and testing prototypes and providing fascinating insights into the behavior of seals.

However, the model is run by complex algorithms in a hypothetical and idealized world - real-world testing, expertise and human input is required to bring it to life and significantly enhance the reliability of the results. Perfect results are not always generated.

“For a model to be successful, numerous factors have to be understood and formulated. A model is only as good as the conditions set, which relies on the expertise of engineers to understand a system as much as the programming and logic of the software,” says Matthias Keck, Manager Development Portfolio R&D Europe for Trelleborg Sealing Solutions.

Setting the Stage

Trelleborg Sealing Solutions has developed its FEA software to provide reliable seal-related modeling on a per application basis. This involves a clear, accurate and comprehensive plan of boundary conditions to be created, taking into account hardware interaction with the product or whether the seal needs to be stretched or contorted for installation. In addition, a library of accurately modeled materials, both metallic and non-metallic, is needed to provide results as close to real-world performance as possible.

Trelleborg Sealing Solutions performs the majority of its analysis on non-metallic materials, which require non-linear modeling. This relies heavily on the quality of the material properties used in the model. Therefore, all materials need to be extensively tested in the real world to determine stress-strain curves, Poisson ratios and all other physical material properties.

The accurate modeling of the seal before parameters like stresses are applied is paramount to a successful analysis. “The data used to plan the analysis benefits from decades of experience – drawing from diverse knowledge sources such as sales engineers, polymer testing laboratories, real-world experimentation and applications” explains Matthias.

A Step Further

Once a detailed model of the seal has been developed, a comprehensive understanding of the forces and stresses that the product or seal will be subjected to is required. A range of operational conditions can be calculated, such as flat or varying temperature and pressure conditions or dynamic movement of the hardware surrounding the seal. Accounting for the myriad of possibilities can be complex and must be done on a case-by-case basis, requiring human analysis and experienced operators.

The FEA software needs to be robust and must be capable of contact, large strain and multi-physics analysis for static and dynamic non-linear problems. Flexibility is also crucial to allow for adaption or a new direction in analysis.

The Human Touch

The software on its own is relatively useless. “A team of analysts from a range of specialties is the best compliment to well designed software. No FEA analysis is routine and a team of experienced people provide valuable input and ideas to circumvent potential pitfalls. The best analyses are created when the FEA analysts work in close cooperation with design and product engineers,” continues Matthias.

FEA result of Zurcon Scraper DA24 in a 3-D sectional view.Whether a 2-D or 3-D model is chosen depends on the application. 2-D models are for axi-symmetrical, plane strain and plane stress analysis, which help to demonstrate how a product will install, grow from thermal expansion and react to forces or pressures applied.

3-D models offer a more complete idealization of a product, such as a seal that operates under complex and frequently varying stresses, allowing for non-uniform profiles or voids. For example, for a gasket with holes stamped into it for bolts to pass through, 2-D analysis would not capture both cross sections with and without the holes. Therefore, 3-D analysis is needed to visualize more complex profiles, increasing the reliability of the results obtained and for simulating eccentric positioning or loads for axi-symmetrical geometries.

Mullins Effect

The Mullins effect was named after the scientist who headed up the team that intensively researched the phenomena in the mid-20th century. It describes the softening of a filled rubber or pure gum following extension beyond a previous maximum. Repeatedly deforming the rubber in this way causes the stress-strain curve of the material to progressively approach an equilibrium point, with the effect being more profound in elastomers with a higher proportion of reinforcing fillers. If the strain exerted on the rubber is less than the previous extension, regular elastic behaviors are observed.

The Mullins effect has been widely studied in academic literature, and though the resulting physical changes are well understood, a consistent determination of cause has not been reached. It has been blamed on everything from bond ruptures and chain retraction, to network rearrangement and filler-cluster rupture.

What Can FEA Do?

The pressure distribution pattern is an important factor during development of dynamic seals.FEA is extremely useful in ensuring maximum seal life in an application. It is possible to predict seal deformation under load, the sealing contact forces or the contact pressure distribution, the latter is especially useful for optimizing dynamic seals - or to “reverse-engineer” existing problems to discover the root causes.

Assembly force requirements to install the seal can be calculated and determine if specific tooling is needed. Various temperature conditions can be simulated to see the effects on the sealing system. Thermo-viscoelasticity simulation can be studied for each material to analyze effects of creep, viscoelasticity, hyperelasticity and Mullin’s Effect over a period of time to ascertain sealing load long after installation. Frictional Analysis uncovers breakout torque and running torque of rotary seals, or actuation force of linear seals.

Any seal manufacturer you work with should have FEA analysis as part of their services offering. It shortens the design optimization process, while improving design and seal performance through integrated software simulation. Reliable seal analysis capabilities reduce development costs and lead times and lead to a lower cost, better performing product for the end-user.

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