Machine Design

A better way to Simulate Gaskets

The gasket feature includes a range of 2D and 3D gasket or interface elements. The 3D, eight-node version, for example, lets users apply different temperatures at each node and assign several thermalloading schemes.

The graph shows experimental pressure versus displacement (relative displacement of top and bottom gasket surfaces) for a graphite composite. Part of gasket complexity is that their material behaves nonlinearly. The sample was loaded (blue curves) and unloaded (brown curves) five times along the loading path and then unloaded at the end of the test to determine the materials unloading stiffness.

Ansys 6.1 has calculated deflections for a gasket and the surrounding cylinder head. The lower view shows deflections on the gasket surface.

Gaskets seem fairly simple. Seen one, you've seen them all. But they are not. And until now, gasket design has been a trial-and-error effort. A recent software release from Ansys devotes special features to gasket design and simulation.

Gaskets are usually thin with layers of several materials. Before applying a gasket, its material should have a known behavior in the thickness direction (from top to bottom) to ensure that joints remain sealed under complex loading cycles. "This behavior has been difficult to characterize with standard material models," says Guoyu Lin, a development engineer with Ansys, Cannonsburg, Pa. "It's also impractical to directly model gaskets using conventional solid elements because it requires such a large number due to the elements' aspect ratios."

When gaskets are thin, say 0.003 in., aspect ratios (heightto-width ratios) for the elements that mesh them can reach 25:1, much higher than the recommended 3:1. With a courser mesh or thinner gasket, the ratio heads up. So a standard element typically will not give good answers.

The simulation software overcomes these hurdles by including several elements and material options to simulate gaskets. New elements let users directly measure relative deformation of the gasket's top and bottom surfaces. Users also can define pressureclosure characteristics to model gaskets' throughthickness behavior. At present, the simulations ignore membrane and transverse shear.

"Interface elements, based on the relative deformation of the top and bottom surfaces, quantify gasket pressure. Therefore, graphs of experimentally measured pressure versus closure can be used to characterize gasket materials," says Lin. Several 3D and 2D interface elements are available, and temperatures can be included as element-body loads at the nodes. To calculate the life of a gasket, users can take advantage of fatigue tools in the simulation software, or conventional fatigue analysis (an S-N curve) methods. Effects of aging are not taken into account. However, temperature cycling can be accounted for by defining a set of temperature-dependent material data.

The software also includes new material models with options for defining complex loading and unloading curves. For example, a gasket-table option lets users simulate gasket joints with interface elements that are only one element thick.

The gasket material, usually under compression, behaves nonlinearly. But the material shows a complicated unloading behavior as well.

A table option lets users input an experimentally measured pressure-closure curve for a material model, and several unloading curves. The curves are often not the same. When no unloading curves are defined, the material behavior follows the compression curve.

Users need not account for the surface conditions of the materials on either side of the gasket. And sealing compounds can be considered several ways. For example, sealant can be treated as another layer of material. Or, model the combination of sealant and gasket as one layer and define material properties that reflect the combination.

"The software calculates results for gasket pressure, closures, and thermal closures," says Liu. He adds that experienced Ansys users should be able to learn the gasket feature within a few days.

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