Combining Material Behaviors

Some general rules apply for combining material behaviors in Abaqus.

Abaqus provides a broad range of possible material behaviors. A material is defined by choosing the appropriate behaviors for the purpose of an analysis. This section describes the general rules for combining material behaviors. Specific information for each material behavior is also summarized at the end of each material behavior description section in this chapter.

Some of the material behaviors in Abaqus are completely unrestricted: they can be used alone or together with other behaviors. For example, thermal properties such as conductivity can be used in any material definition. They are used in an analysis if the material is associated with elements that can solve heat transfer problems and if the analysis procedure allows for the thermal equilibrium equation to be solved.

Some material behaviors in Abaqus require the presence of other material behaviors, and some exclude the use of other material behaviors. For example, metal plasticity requires the definition of elastic material behavior or an equation of state and excludes all other rate-independent plasticity behaviors.

This page discusses:

Complete Material Definitions

Abaqus requires that the material be sufficiently defined to provide suitable properties for those elements with which the material is associated and for all of the analysis procedures through which the model will be run. Thus, a material associated with displacement or structural elements must include either a “Complete mechanical” category behavior or an “Elasticity” category behavior, as discussed below. In Abaqus/Explicit density (Density) is required for all materials except hydrostatic fluids.

It is not possible to modify or add to material definitions once an analysis is started. However, material definitions can be modified in an import analysis. For example, a static analysis can be run in Abaqus/Standard using a material definition that does not include a density specification. Density can be added to the material definition when the analysis is imported into Abaqus/Explicit.

All aspects of a material's behavior need not be fully defined; any behavior that is omitted is assumed not to exist in that part of the model. For example, if elastic material behavior is defined for a metal but metal plasticity is not defined, the material is assumed not to have a yield stress. You must ensure that the material is adequately defined for the purpose of the analysis. The material can include behaviors that are not relevant for the analysis, as described in About the Material Library. Thus, you can include general material behavior libraries, without having to delete those behaviors that are not needed for a particular application. This generality offers great flexibility in material modeling.

In Abaqus/Standard any material behaviors defined using a distribution (Distribution Definition) can be combined with almost all material behaviors in a manner identical to how they are combined when no distributions are used. For example, if the linear elastic material behavior is defined using a distribution, it can be combined with metal plasticity or any other material behavior that can normally be combined with linear elastic behavior. In addition, more than one material behavior defined with a distribution (linear elastic behavior and thermal expansion, for example) can be included in the same material definition. The only exception is that a material defined with concrete damaged plasticity (Concrete Damaged Plasticity) cannot have any material behaviors defined with a distribution.

Material Behavior Combination Tables

The material behavior combination tables that follow explain which behaviors must be used together. The tables also show the material behaviors that cannot be combined. Behaviors designated with an (S) are available only in Abaqus/Standard; behaviors designated with an (E) are available only in Abaqus/Explicit.

The behaviors are assigned to categories because exclusions are best described in terms of those categories. Some of the categories require explanation:

  • “Complete mechanical behaviors” are those behaviors in Abaqus that, individually, completely define a material's mechanical (stress-strain) behavior. A behavior in this category, therefore, excludes any other such behavior and also excludes any behavior that defines part of a material's mechanical behavior: those behaviors that belong to the elasticity and plasticity categories.

  • “Elasticity, fabric, and equation of state behaviors” contains all of the basic elasticity behaviors in Abaqus. If a behavior from the “Complete mechanical behaviors” category is not used and mechanical behavior is required, a behavior must be selected from this category. This selection then excludes any other elasticity behavior.

  • “Enhancements for elasticity behaviors” contains behaviors that extend the modeling provided by the elasticity behaviors in Abaqus.

  • “Rate-independent plasticity behaviors” contains all of the basic plasticity behaviors in Abaqus except deformation plasticity, which is in the “Complete mechanical behaviors” category because it completely defines the material's mechanical behavior.

  • “Rate-dependent plasticity behaviors” contains behaviors that extend the modeling provided by the rate-independent plasticity behaviors and by the linear elastic material behavior.

If elastic-plastic behavior must be modeled, you should select an appropriate plasticity behavior from one of the plasticity behaviors categories and an elasticity behavior from one of the elasticity behaviors categories.

General Behaviors:

These behaviors are unrestricted.

Behavior Keyword Requires
Material damping DAMPING Elasticity, fabric, equation of state (except ignition and growth equation of state), hyperelasticity, hyperfoam, low-density foam, or anisotropic hyperelasticity (except when used with beam or shell general sections or substructures)
Density DENSITY Required in Abaqus/Explicit, except for hydrostatic fluid elements
Solution-dependent state variables DEPVAR  
Thermal expansion EXPANSION  

Complete Mechanical Behaviors:

These behaviors are mutually exclusive and exclude all behaviors listed for elasticity, plasticity, and hydrostatic fluid behaviors, including all related enhancements. However, when an element is enriched (Modeling Discontinuities as an Enriched Feature Using the Extended Finite Element Method), the mechanical user material behavior can be used in conjunction with the progressive damage and failure behavior, which is in the enhancement for elasticity or plasticity behaviors category.

Behavior Keyword Requires
Acoustic medium ACOUSTIC MEDIUM Density
Deformation plasticity(S) DEFORMATION PLASTICITY  
Mechanical user material USER MATERIAL, TYPE=MECHANICAL  

Enhancements for Elasticity Behaviors:

Behavior Keyword Requires
Elastic shear behavior for an equation of state(E) ELASTIC, TYPE=SHEAR Equation of state
Strain-based failure measures FAIL STRAIN Elasticity
Stress-based failure measures FAIL STRESS Elasticity
Hysteresis(S) HYSTERESIS Hyperelasticity (excludes all plasticity behaviors and Mullins effect)
Mullins effect MULLINS EFFECT Hyperelasticity (excludes hysteresis), hyperfoam or anisotropic hyperelasticity
Compressive failure theory(S) NO COMPRESSION Elasticity
Tension failure theory(S) NO TENSION Elasticity
Viscoelasticity VISCOELASTIC Elasticity, hyperelasticity, or hyperfoam (excludes all plasticity behaviors and all associated plasticity enhancements); or anisotropic hyperelasticity
Shear viscosity for an equation of state(E) VISCOSITY Equation of state

Rate-Independent Plasticity Behaviors:

These behaviors are mutually exclusive.

Behavior Keyword Requires
Brittle cracking(E) BRITTLE CRACKING Isotropic elasticity and brittle shear
Modified Drucker-Prager/Cap plasticity CAP PLASTICITY Drucker-Prager/Cap plasticity hardening and isotropic elasticity or porous elasticity
Cast iron plasticity CAST IRON PLASTICITY Cast iron compression hardening, cast iron tension hardening, and isotropic elasticity
Cam-clay plasticity CLAY PLASTICITY Isotropic elasticity, orthotropic elasticity, or porous elasticity (in Abaqus/Standard)
Isotropic elasticity or orthotropic elasticity (in Abaqus/Explicit)
Concrete(S) CONCRETE Isotropic elasticity 
Concrete damaged plasticity CONCRETE DAMAGED PLASTICITY Concrete compression hardening, concrete tension stiffening, and isotropic elasticity
Crushable foam plasticity CRUSHABLE FOAM Crushable foam hardening and isotropic elasticity
Drucker-Prager plasticity DRUCKER PRAGER Drucker-Prager hardening and isotropic elasticity or porous elasticity (in Abaqus/Standard)
Drucker-Prager hardening and isotropic elasticity or the combination of an equation of state and isotropic linear elastic shear behavior for an equation of state (in Abaqus/Explicit)
Plastic compaction behavior for an equation of state(E) EOS COMPACTION Linear Us-Up equation of state
Jointed material(S) JOINTED MATERIAL Isotropic elasticity and a local orientation
Mohr-Coulomb plasticity MOHR COULOMB Mohr-Coulomb hardening and isotropic elasticity
Metal plasticity PLASTIC Elasticity or hyperelasticity (in Abaqus/Standard)
Isotropic elasticity, orthotropic elasticity (requires anisotropic yield), hyperelasticity, or the combination of an equation of state and isotropic linear elastic shear behavior for an equation of state (in Abaqus/Explicit)

Rate-Dependent Plasticity Behaviors:

These behaviors are mutually exclusive, except metal creep and time-dependent volumetric swelling.

Behavior Keyword Requires
Cap creep(S) CAP CREEP Elasticity, modified Drucker-Prager/Cap plasticity, and Drucker-Prager/Cap plasticity hardening
Metal creep(S) CREEP Elasticity (except when used to define rate-dependent gasket behavior; excludes all rate-independent plasticity behaviors except metal plasticity)
Drucker-Prager creep(S) DRUCKER PRAGER CREEP Elasticity, Drucker-Prager plasticity, and Drucker-Prager hardening
Metal plasticity PLASTIC, RATE Elasticity or hyperelasticity (in Abaqus/Standard)
Isotropic elasticity, orthotropic elasticity (requires anisotropic yield), hyperelasticity, or the combination of an equation of state and isotropic linear elastic shear behavior for an equation of state (in Abaqus/Explicit)
Nonlinear viscoelasticity VISCOELASTIC, NONLINEAR Hyperelasticity
Rate-dependent viscoplasticity RATE DEPENDENT Drucker-Prager plasticity, crushable foam plasticity, or metal plasticity
Time-dependent volumetric swelling(S) SWELLING Elasticity (excludes all rate-independent plasticity behaviors except metal plasticity)
Two-layer viscoplasticity(S) VISCOUS Elasticity and metal plasticity

Enhancements for Plasticity Behaviors:

Behavior Keyword Requires
Annealing temperature ANNEAL TEMPERATURE Metal plasticity
Brittle failure(E) BRITTLE FAILURE Brittle cracking and brittle shear
Cyclic hardening CYCLIC HARDENING Metal plasticity with nonlinear isotropic/kinematic hardening
Inelastic heat fraction INELASTIC HEAT FRACTION Metal plasticity and specific heat
Oak Ridge National Laboratory constitutive model(S) ORNL Metal plasticity, cycled yield stress data, and, usually, metal creep
Porous material failure criteria(E) POROUS FAILURE CRITERIA Porous metal plasticity
Porous metal plasticity POROUS METAL PLASTICITY Metal plasticity
Hill anisotropic yield/creep POTENTIAL, TYPE=HILL Metal plasticity, metal creep, Cam-clay plasticity, or two-layer viscoplasticity
Nonquadratic yield(E) POTENTIAL (except TYPE=HILL) Metal plasticity
Shear failure(E) SHEAR FAILURE Metal plasticity
Tension cutoff TENSION CUTOFF Mohr-Coulomb plasticity

Enhancement for Elasticity or Plasticity Behaviors:

Behavior Keyword Requires
Tensile failure(E) TENSILE FAILURE Metal plasticity or equation of state
Damage initiation DAMAGE INITIATION For elasticity behaviors: elasticity based on a traction-separation description for cohesive elements or elasticity model for fiber-reinforced compositesFor plasticity behaviors: elasticity and metal plasticity or Drucker-Prager plasticity
Damage evolution DAMAGE EVOLUTION Damage initiation
Damage stabilization DAMAGE STABILIZATION Damage evolution
Transverse shear TRANSVERSE SHEAR Mechanical behaviors associated with shell or beam elements

Thermal Behaviors:

The following behavior is unrestricted:

Behavior Keyword Requires
Volumetric heat generation HEAT GENERATION  

The following behaviors are unrestricted but exclude thermal user materials:

Behavior Keyword Requires
Thermal conductivity CONDUCTIVITY  
Latent heat LATENT HEAT Density
Specific heat SPECIFIC HEAT Density

Complete Thermal Behavior:

This behavior is unrestricted but excludes the thermal behaviors in the previous table.

Behavior Keyword Requires
Thermal user material USER MATERIAL, TYPE=THERMAL Density

Pore Fluid Flow Behaviors:

These behaviors are unrestricted.

Behavior Keyword Requires
Swelling gel(S) GEL Permeability, porous bulk moduli, and absorption/exsorption behavior
Moisture-driven swelling(S) MOISTURE SWELLING Permeability and absorption/exsorption behavior
Permeability(S) PERMEABILITY  
Porous bulk moduli(S) POROUS BULK MODULI Permeability and either elasticity or porous elasticity
Absorption/exsorption behavior(S) SORPTION Permeability

Mass Diffusion Behaviors:

These behaviors exclude all other behaviors.

Behavior Keyword Requires
Mass diffusivity(S) DIFFUSIVITY Solubility
Solubility(S) SOLUBILITY Mass diffusivity