Defining the gasket behavior directly using a gasket behavior model

To define the gasket behavior by a gasket behavior model, you must assign a gasket section definition to a region of your model and assign the name of a gasket behavior definition to the gasket section definition. The gasket behavior for this region is defined entirely by the properties specified by the gasket behavior definition referring to the same name.

GASKET SECTION, ELSET=name, BEHAVIOR=name
GASKET BEHAVIOR, NAME=name

Property module: 
Material editor: Name: name, enter data for any materials found under 
OtherGasket
Create Section: select Other as the section Category and Gasket as the section Type: Material: name

The thickness-direction, transverse shear, and membrane behaviors are defined to be uncoupled. Each behavior is specified independently.

You must specify the thickness-direction behavior. You can specify multiple thickness-direction behaviors to define the loading and unloading characteristics. You can obtain an average contact pressure output when the thickness-direction behavior is defined as force or force per unit length versus closure.

The transverse shear and membrane behaviors are optional for gasket elements that have all displacement degrees of freedom active at their nodes. You can define one or both of these behaviors.

When thermal and rate-dependent effects are important, you can define thermal expansion and creep behavior for gaskets; user subroutines UEXPAN and CREEP can be used to define these behaviors.

You cannot specify density for gasket elements since they have no mass matrix.

GASKET BEHAVIOR, NAME=name
GASKET THICKNESS BEHAVIOR
GASKET ELASTICITY
GASKET CONTACT AREA
EXPANSION
CREEP
DEPVAR
USER OUTPUT VARIABLES

Property module: material editor: 
OtherGasketGasket Thickness Behavior
OtherGasketGasket Transverse Shear Elasticity and/or Gasket 
Membrane Elasticity
MechanicalExpansion
MechanicalPlasticityCreep
GeneralDepvar
GeneralUser Output Variables

To define the thickness-direction behavior of gaskets, Abaqus/Standard offers a nonlinear elastic model with damage and a nonlinear elastic-plastic model with the possibility of considering creep effects. Thermal effects in the thickness direction can also be accounted for.

Abaqus/Standard measures the thickness-direction deformation as the closure between the bottom and top faces of the gasket element; therefore, the thickness-direction behavior must always be defined in terms of closure. The closure is the sum of the elastic closure, plastic closure, creep closure, thermal closure, plus any initial gap in the thickness direction. As explained below, the behavior can be defined as pressure versus closure, force versus closure, or force per unit length versus closure. In all cases the thickness-direction behavior can be defined as a function of temperature and/or field variables.

GASKET THICKNESS BEHAVIOR, DEPENDENCIES

Property module: material editor: OtherGasketGasket Thickness Behavior

You can define the elastic transverse shear stiffness of the gasket. Abaqus/Standard measures the relative displacement between the bottom and top of the gasket element along the local 2- or 3-directions to define the transverse shear in the gasket. Therefore, you should always define the elastic transverse stiffness as stress (or force, or force per unit length) per unit displacement. You can specify the stiffness as a function of temperature and field variables. The same stiffness is used for the shear in the 1–2 plane and the shear in the 1–3 plane. For each set of temperature and/or field variables, the first slope of the initial loading curve for the gasket's thickness-direction behavior will be used to compute the transverse shear stiffness if the transverse shear behavior is not defined explicitly.

GASKET ELASTICITY, COMPONENT=TRANSVERSE SHEAR, DEPENDENCIES

Property module: material editor: OtherGasketGasket Transverse Shear Elasticity

You can define the linear elastic behavior of the gasket by giving Young's modulus and Poisson's ratio. These data can be provided as a function of temperature and/or field variables. If you do not specify the linear elastic behavior of the gasket, the gasket has no membrane stiffness. In this case you must ensure that the nodes of the elements are restrained adequately in the directions orthogonal to the thickness direction of the gasket.

GASKET ELASTICITY, COMPONENT=MEMBRANE, DEPENDENCIES

Property module: material editor: OtherGasketGasket Membrane Elasticity

You can define isotropic thermal expansion to specify the same coefficient of thermal expansion for the membrane and thickness-direction behaviors.

Alternatively, you can define orthotropic thermal expansion to specify three different coefficients of thermal expansion. The first coefficient will apply to the thermal expansion of the gasket in the thickness direction; the other two coefficients will apply to the expansion of the gasket in the local 2- and 3-directions, respectively.

The membrane thermal strains, ${\epsilon }^{th}$, are obtained as explained in Thermal expansion. Abaqus/Standard computes the thermal closure for the thickness direction as

so that the “mechanical” closure is obtained as

You can specify the initial gap and initial void as part of the gasket section definition; the initial thickness is obtained directly from the nodal coordinates of the gasket elements, or you can specify it as part of the gasket section definition (see Defining the gasket element's initial geometry).

If user subroutine UEXPAN is used to define the thermal expansion of the gasket, the incremental thermal strains must be provided in the subroutine. The thermal closure will be obtained from the thermal strain in the thickness direction, as described above.

EXPANSION, TYPE=ISO
EXPANSION, TYPE=ORTHO

EXPANSION, TYPE=ISO, USER
EXPANSION, TYPE=ORTHO, USER

Property module: material editor: MechanicalExpansion: Use user subroutine UEXPAN (optional)

You can define creep behavior in the thickness direction of the gasket only when the elastic-plastic model (see Defining a nonlinear elastic-plastic model above) is used. The creep closure rate will be obtained as

where ${\dot{\epsilon }}^{cr}$ is obtained as explained in Rate-dependent plasticity: creep and swelling. You can specify the initial gap and initial void as part of the gasket section definition; the initial thickness is obtained directly from the nodal coordinates of the gasket elements, or you can specify it as part of the gasket section definition (see Defining the gasket element's initial geometry).

If user subroutine CREEP is used to define the rate-dependent thickness-direction response of the gasket, the compressive creep strain increment must be provided in the subroutine. The creep closure will be obtained from the creep strain, as described above.

CREEP

CREEP, LAW=USER

Property module: material editor: MechanicalPlasticityCreep: Law: User-defined (optional)

You can define viscoelastic behavior in the thickness direction of the gasket only when the elastic-damage model (see Defining a nonlinear elastic model with damage above) is used. Only frequency domain viscoelastic behavior is supported. This behavior is useful for modeling the steady-state dynamic response of automotive components with gaskets about some pre-loaded base state, such as would be obtained at the end of a nonlinear sealing analysis, to determine the noise-vibration-harshness (NVH) characteristics of the system.

During the nonlinear sealing analysis step the frequency-domain viscoelastic behavior is ignored, and the material response is determined by the long-term elastic properties of the material. It is generally accepted (Zubeck and Marlow, 2002) that the dynamic stiffness and damping characteristics of automotive components such as gaskets and grommets vary with the frequency of excitation as well as the level of preload. These structural properties also depend on the geometry and the level of confinement of the gasket. This capability allows the direct specification of such dynamic properties as quantified by the effective storage and loss moduli in the thickness-direction, as tabular functions of the frequency of excitation and the level of preload. The preload is quantified by the amount of closure in the base state about which the steady-state dynamic response is desired.

In determining the dynamic response of the gasket, the long-term elastic response is assumed to be defined by the nonlinear elastic model with damage. The steady-state dynamic response is assumed to be a perturbation about a base state defined by this elastic damage behavior at a certain value of closure. The viscoelastic response can be specified using two approaches, as discussed below.

When the thickness-direction behavior of the gasket is defined in terms of force or force per unit length versus closure, Abaqus/Standard will provide the thickness-direction force or force per unit length as output variable S11. In this case you can define either a contact width or contact area versus closure curve that will be used to obtain the average “contact” pressure at each integration point as output variable CS11. This average pressure considers the changing contact area that occurs as a result of the deformation of a gasket, as shown in Figure 1. The closure used for input of this curve corresponds to the total mechanical closure, defined as the sum of the elastic, plastic, and creep closures.

When two- and three-dimensional link gasket elements are used, you should specify the contact area versus mechanical closure in tabular form. When axisymmetric link and three-dimensional line elements are used, you should specify the contact width versus mechanical closure in tabular form. A typical curve is shown in Figure 7.

Figure 7. Specification of contact area versus mechanical closure for output of average pressure.

You must specify the area at zero closure, then the area at increasing closures. The area is constant when the mechanical closure is negative and is extrapolated from the slope computed from the last two user-specified data points if the closure reaches values that are greater than the last user-specified closure. Area versus closure curves can be provided as a function of temperature and field variables.

GASKET CONTACT AREA, DEPENDENCIES

Property module: material editor: OtherGasketGasket Thickness Behavior: Units: Force, SuboptionsContact Area

Output variable E is usually used in Abaqus/Standard to output strain. For gasket elements with behavior defined by a gasket behavior model this output variable has thickness-direction and transverse shear components with units of displacement and membrane strains. Output variable NE is used to output an effective strain. The effective strain components are computed as follows:

The output variables THE, PE, or CE can also be used for gasket elements to output generalized thermal strains, plastic strains, or creep strains, respectively.

For all stress/strain output variables the 11-component refers to the through-thickness direction; the 22-, 33- and 23-components refer to two direct and one shear membrane component, respectively; the remaining 12- and 13-components refer to the transverse shear components. For details about these definitions, see About gasket elements.

The output of the elastic strain energy (output variable ALLSE) also contains the energy due to damage or change in elasticity as a function of plasticity. Therefore, this energy is usually not fully recoverable.