21.1.3 Choosing the appropriate element for an analysis type

**Products: **ABAQUS/Standard ABAQUS/Explicit ABAQUS/CAE

The ABAQUS element library contains the following:

stress/displacement elements, including contact elements, connector elements such as springs, and special-purpose elements such as surface elements;

pore pressure elements;

coupled temperature-displacement elements;

heat transfer or mass diffusion elements;

forced convection heat transfer elements;

coupled thermal-electrical elements;

piezoelectric elements;

acoustic elements;

hydrostatic fluid elements; and

user-defined elements.

Within ABAQUS/Standard or ABAQUS/Explicit, a model can contain elements that are not appropriate for the particular analysis type chosen; such elements will be ignored. However, an ABAQUS/Standard model cannot contain elements that are not available in ABAQUS/Standard; likewise, an ABAQUS/Explicit model cannot contain elements that are not available in ABAQUS/Explicit.

Stress/displacement elements

Stress/displacement elements are used in the modeling of linear or complex nonlinear mechanical analyses that possibly involve contact, plasticity, and/or large deformations. Stress/displacement elements can also be used for thermal-stress analysis, where the temperature history can be obtained from a heat transfer analysis carried out with diffusive elements.

Stress/displacement elements can be used in the following analysis types:

static and quasi-static analysis (“Static stress analysis procedures: overview,” Section 6.2.1);

implicit transient dynamic, explicit transient dynamic, modal dynamic, and steady-state dynamic analysis (“Dynamic analysis procedures: overview,” Section 6.3.1);

“Acoustic, shock, and coupled acoustic-structural analysis,” Section 6.9.1; and

Stress/displacement elements have only displacement degrees of freedom. See “Conventions,” Section 1.2.2, for a discussion of the degrees of freedom in ABAQUS.

Pore pressure elements

Pore pressure elements are provided in ABAQUS/Standard for modeling fully or partially saturated fluid flow through a deforming porous medium. The names of all pore pressure elements include the letter P (pore pressure). These elements cannot be used with hydrostatic fluid elements.

Pore pressure elements can be used in the following analysis types:

soils analysis (“Coupled pore fluid diffusion and stress analysis,” Section 6.7.1); and

geostatic analysis (“Geostatic stress state,” Section 6.7.2).

Pore pressure elements have both displacement and pore pressure degrees of freedom. In second-order elements the pore pressure degrees of freedom are active only at the corner nodes. See “Conventions,” Section 1.2.2, for a discussion of the degrees of freedom in ABAQUS.

These elements use either linear- or second-order (quadratic) interpolation for the geometry and displacements in two or three directions. The pore pressure is interpolated linearly from the corner nodes. Curved element edges should be avoided; exact linear spatial pore pressure variations cannot be obtained with curved edges.

For output purposes the pore pressure at the midside nodes of second-order elements is determined by linear interpolation from the corner nodes.

Coupled temperature-displacement elements

Coupled temperature-displacement elements are used in problems for which the stress analysis depends on the temperature solution and the thermal analysis depends on the displacement solution. An example is the heating of a deforming body whose properties are temperature dependent by plastic dissipation or friction. The names of all coupled temperature-displacement elements include the letter T.

Coupled temperature-displacement elements are for use in fully coupled temperature-displacement analysis (“Fully coupled thermal-stress analysis,” Section 6.5.4).

Coupled temperature-displacement elements have both displacement and temperature degrees of freedom. In second-order elements the temperature degrees of freedom are active at the corner nodes. In modified triangle and tetrahedron elements the temperature degrees of freedom are active at every node. See “Conventions,” Section 1.2.2, for a discussion of the degrees of freedom in ABAQUS.

Coupled temperature-displacement elements use either linear or parabolic interpolation for the geometry and displacements. The temperature is always interpolated linearly. In second-order elements curved edges should be avoided; exact linear spatial temperature variations for these elements cannot be obtained with curved edges.

For output purposes the temperature at the midside nodes of second-order elements is determined by linear interpolation from the corner nodes.

Diffusive (heat transfer) elements

Diffusive elements are provided in ABAQUS/Standard for use in heat transfer analysis (“Uncoupled heat transfer analysis,” Section 6.5.2), where they allow for heat storage (specific heat and latent heat effects) and heat conduction. They provide temperature output that can be used directly as input to the equivalent stress elements. The names of all diffusive heat transfer elements begin with the letter D.

The diffusive elements can be used in mass diffusion analysis (“Mass diffusion analysis,” Section 6.8.1) as well as in heat transfer analysis.

When used for heat transfer analysis, the diffusive elements have only temperature degrees of freedom. When they are used in a mass diffusion analysis, they have normalized concentration, instead of temperature, degrees of freedom. See “Conventions,” Section 1.2.2, for a discussion of the degrees of freedom in ABAQUS.

The diffusive elements use either first-order (linear) interpolation or second-order (quadratic) interpolation in one, two, or three dimensions.

Diffusive elements are available in the following element families:

“Shell elements: overview,” Section 23.6.1 (these elements cannot be used in a mass diffusion analysis); and

Forced convection heat transfer elements

Forced convection heat transfer elements are provided in ABAQUS/Standard to allow for heat storage (specific heat) and heat conduction, as well as the convection of heat by a fluid flowing through the mesh (forced convection). All forced convection heat transfer elements provide temperature output, which can be used directly as input to the equivalent stress elements. The names of all forced convection heat transfer elements begin with the letters DCC.

The forced convection heat transfer elements can be used in heat transfer analyses (“Uncoupled heat transfer analysis,” Section 6.5.2), including cavity radiation modeling (“Cavity radiation,” Section 32.1.1). The forced convection heat transfer elements can be used together with the diffusive elements.

The forced convection heat transfer elements have temperature degrees of freedom. See “Conventions,” Section 1.2.2, for a discussion of the degrees of freedom in ABAQUS.

The forced convection heat transfer elements use only first-order (linear) interpolation in one, two, or three dimensions.

Coupled thermal-electrical elements

Coupled thermal-electrical elements are provided in ABAQUS/Standard for use in modeling heating that arises when an electrical current flows through a conductor (Joule heating).

The Joule heating effect requires full coupling of the thermal and electrical problems (see “Coupled thermal-electrical analysis,” Section 6.6.2). The coupling arises from two sources: temperature-dependent electrical conductivity and the heat generated in the thermal problem by electric conduction.

These elements can also be used to perform uncoupled electric conduction analysis in all or part of the model. In such analysis only the electric potential degree of freedom is activated, and all heat transfer effects are ignored. This capability is available by omitting the thermal conductivity from the material definition.

The coupled thermal-electrical elements can also be used in heat transfer analysis (“Uncoupled heat transfer analysis,” Section 6.5.2), in which case all electric conduction effects are ignored. This feature is quite useful if a coupled thermal-electrical analysis is followed by a pure heat conduction analysis (such as a welding simulation followed by cool down).

The elements cannot be used in any of the stress/displacement analysis procedures.

Coupled thermal-electrical elements have both temperature and electrical potential degrees of freedom. See “Conventions,” Section 1.2.2, for a discussion of the degrees of freedom in ABAQUS.

Coupled thermal-electrical elements are provided with first- or second-order interpolation of the temperature and electrical potential.

Piezoelectric elements

Piezoelectric elements are provided in ABAQUS/Standard for problems in which a coupling between the stress and electrical potential (the piezoelectric effect) must be modeled.

Piezoelectric elements are for use in piezoelectric analysis (“Piezoelectric analysis,” Section 6.6.3).

The piezoelectric elements have both displacement and electric potential degrees of freedom. See “Conventions,” Section 1.2.2, for a discussion of the degrees of freedom in ABAQUS. The piezoelectric effect is discussed further in “Piezoelectric analysis,” Section 6.6.3.

Piezoelectric elements are available with first- or second-order interpolation of displacement and electrical potential.

Acoustic elements

Acoustic elements are used for modeling an acoustic medium undergoing small pressure changes. The solution in the acoustic medium is defined by a single pressure variable. Impedance boundary conditions representing absorbing surfaces or radiation to an infinite exterior are available on the surfaces of these acoustic elements.

Acoustic infinite elements, which improve the accuracy of analyses involving exterior domains, and acoustic-structural interface elements, which couple an acoustic medium to a structural model, are also provided.

Acoustic elements are for use in acoustic and coupled acoustic-structural analysis (“Acoustic, shock, and coupled acoustic-structural analysis,” Section 6.9.1).

Acoustic elements have acoustic pressure as a degree of freedom. Coupled acoustic-structural elements also have displacement degrees of freedom. See “Conventions,” Section 1.2.2, for a discussion of the degrees of freedom in ABAQUS.

Acoustic elements are available in the following element families:

The acoustic elements can be used alone but are often used with a structural model in a coupled analysis. “Acoustic interface elements,” Section 26.14.1, describes interface elements that allow this acoustic pressure field to be coupled to the displacements of the surface of the structure. Acoustic elements can also interact with solid elements through the use of surface-based tie constraints; see “Acoustic, shock, and coupled acoustic-structural analysis,” Section 6.9.1.

Using the same mesh with different analysis or element types

You may want to use the same mesh with different analysis or element types. This may occur, for example, if both stress and heat transfer analyses are intended for a particular geometry or if the effect of using either reduced- or full-integration elements is being investigated. Care should be taken when doing this since unexpected error messages may result for one of the two element types if the mesh is distorted. For example, a stress analysis with C3D10 elements may run successfully, but a heat transfer analysis using the same mesh with DC3D10 elements may terminate during the **datacheck** portion of the analysis with an error message stating that the elements are excessively distorted or have negative volumes. This apparent inconsistency is caused by the different integration locations for the different element types. Such problems can be avoided by ensuring that the mesh is not distorted excessively.