21.2.1 Density
Products: Abaqus/Standard Abaqus/Explicit Abaqus/CFD Abaqus/CAE
References
Overview
A material's mass density:
must be defined in Abaqus/Standard for eigenfrequency and transient dynamic analysis, transient heat transfer analysis, adiabatic stress analysis, and acoustic analysis;
must be defined in Abaqus/Standard for gravity, centrifugal, and rotary acceleration loading;
must be defined in Abaqus/Explicit for all materials except hydrostatic fluids;
must be defined in Abaqus/CFD for all fluids;
can be specified as a function of temperature and predefined variables;
can be distributed from nonstructural features (such as paint on sheet metal panels in a car) to the underlying elements using a nonstructural mass definition; and
can be defined with a distribution for solid continuum elements in Abaqus/Standard.
Defining density
Density can be defined as a function of temperature and field variables. Based on user-defined data Abaqus internally estimates the material density as follows:
For acoustic, heat transfer, and coupled thermal-electrical elements in Abaqus/Standard and acoustic elements in Abaqus/Explicit, the density is continually updated to the value corresponding to the current temperature and field variables.
For coupled temperature-displacement elements in Abaqus/Standard, the density is continually updated to the value corresponding to the current temperature and field variables for heat transfer computations only. Structural body force computations ensure mass conservation during the analysis by assuming the density to be a function of the initial temperature and field variables and changes in volume only.
For all other elements in Abaqus/Standard and Abaqus/Explicit, the density is taken to be a function of the initial temperature and field variables and changes in volume only. It is not updated if temperatures and field variables change during the analysis.
For Abaqus/CFD the density is considered constant for incompressible flows.
In an Abaqus/Standard analysis a spatially varying mass density can be defined for homogeneous solid continuum elements by using a distribution (“Distribution definition,” Section 2.8.1). The distribution must include a default value for the density. If a distribution is used, no dependencies on temperature and/or field variables for the density can be defined.
| Abaqus/CAE Usage: | Property module: material editor: General |
You can toggle on Use temperature-dependent data to define the density as a function of temperature and/or select the Number of field variables to define the density as a function of field variables. |
Units
Since Abaqus has no built-in dimensions, you must ensure that the density is given in consistent units. The use of consistent units, and density in particular, is discussed in “Conventions,” Section 1.2.2. If American or English units are used, you must be particularly careful that the density used is in units of 
, where mass is defined in units of 
.
Elements
The density behavior described in this section is used to specify mass density for all elements, except rigid elements. Mass density for rigid elements is specified as part of the rigid body definition (see “Rigid elements,” Section 30.3.1).
In Abaqus/Explicit a nonzero mass density must be defined for all elements that are not part of a rigid body.
In Abaqus/Standard density must be defined for heat transfer elements and acoustic elements; mass density can be defined for stress/displacement elements, coupled temperature-displacement elements, and elements including pore pressure. For elements that include pore pressure as a degree of freedom, the density of the dry material should be given for the porous medium in a coupled pore fluid flow/stress analysis.
If you have a complex density for an acoustic medium, you should enter its real part here and convert the imaginary part into a volumetric drag, as discussed in “Acoustic medium,” Section 26.3.1.
The mass contribution from features that have negligible structural stiffness can be added to the model by smearing the mass over an element set that is typically adjacent to the nonstructural feature. The nonstructural mass can be specified in the form of a total mass value, a mass per unit volume, a mass per unit area, or a mass per unit length (see “Nonstructural mass definition,” Section 2.7.1). A nonstructural mass definition contributes additional mass to the specified element set and does not alter the underlying material density.