27.1.1 Prescribed conditions: overview

The following types of external conditions can be prescribed in an ABAQUS model:

**Initial conditions**: Nonzero initial conditions can be defined for many variables, as described in “Initial conditions,” Section 27.2.1.

**Boundary conditions**: Boundary conditions are used to prescribe values of basic solution variables: displacements and rotations in stress/displacement analysis, temperature in heat transfer or coupled thermal-stress analysis, electrical potential in coupled thermal-electrical analysis, pore pressure in soils analysis, acoustic pressure in acoustic analysis, etc. Boundary conditions can be defined as described in “Boundary conditions,” Section 27.3.1.

**Loads**: Many types of loading are available, depending on the analysis procedure. “Applying loads: overview,” Section 27.4.1, gives an overview of loading in ABAQUS. Load types specific to one analysis procedure are described in the appropriate procedure section in Part III, “Analysis Procedures, Solution, and Control.” General loads, which can be applied in multiple analysis types, are described in:

**Prescribed assembly loads**: Pre-tension sections can be defined in ABAQUS/Standard to prescribe assembly loads in bolts or any other type of fastener. Pre-tension sections are described in “Prescribed assembly loads,” Section 27.5.1.

**Connector loads and motions**: Connector elements can be used to define complex mechanical connections between parts, including actuation with prescribed loads or motions. Connector elements are described in “Connectors: overview,” Section 25.1.1.

**Predefined fields**: Predefined fields are time-dependent, non-solution-dependent fields that exist over the spatial domain of the model. Temperature is the most commonly defined field. Predefined fields are described in “Predefined fields,” Section 27.6.1.

Amplitude variations

Complex time- or frequency-dependent boundary conditions, loads, and predefined fields can be specified by referring to an amplitude curve in the prescribed condition definition. Amplitude curves are explained in “Amplitude curves,” Section 27.1.2.

In ABAQUS/Standard if no amplitude is referenced from the boundary condition, loading, or predefined field definition, the total magnitude can be applied instantaneously at the start of the step and remain constant throughout the step (a “step” variation) or it can vary linearly over the step from the value at the end of the previous step (or from zero at the start of the analysis) to the magnitude given (a “ramp” variation). You choose the type of variation when you define the step; the default variation depends on the procedure chosen, as shown in “Procedures: overview,” Section 6.1.1.

In ABAQUS/Explicit if no amplitude is referenced from the boundary condition or loading definition, the total value will be applied instantaneously at the start of the step and will remain constant throughout the step (a “step” variation), although ABAQUS/Explicit does not admit jumps in displacement (see “Boundary conditions,” Section 27.3.1). If no amplitude is referenced from a predefined field definition, the total magnitude will vary linearly over the step from the value at the end of the previous step (or from zero at the start of the analysis) to the magnitude given (a “ramp” variation).

When boundary conditions are removed (see “Boundary conditions,” Section 27.3.1), the boundary condition (displacement or rotation constraint in stress/displacement analysis) is converted to an applied conjugate flux (force or moment in stress/displacement analysis) at the beginning of the step. This flux magnitude is set to zero with a “step” or “ramp” variation depending on the procedure chosen, as discussed in “Procedures: overview,” Section 6.1.1. Similarly, when loads and predefined fields are removed, the load is set to zero and the predefined field is set to its initial value.

In ABAQUS/Standard the variation of many prescribed conditions can be defined in user subroutines. In this case the magnitude of the variable can vary in any way with position and time. The magnitude variation for prescribing and removing conditions must be specified in the subroutine (see “User subroutines and utilities,” Section 13.2”).

Applying boundary conditions and loads in a local coordinate system

You can define a local coordinate system at a node as described in “Transformed coordinate systems,” Section 2.1.5. Then, all input data for concentrated force and moment loading and for displacement and rotation boundary conditions are given in the local system.

Loads and predefined fields available for various procedures

**Table 27.1.1–1** Available loads and predefined fields.

Loads and predefined fields | Procedures |
---|---|

Added mass (concentrated and distributed) | ABAQUS/Aqua eigenfrequency extraction analysis (“Natural frequency extraction,” Section 6.3.5) |

Base motion | Procedures based on eigenmodes: |

“Transient modal dynamic analysis,” Section 6.3.7 | |

“Mode-based steady-state dynamic analysis,” Section 6.3.8 | |

“Response spectrum analysis,” Section 6.3.10 | |

“Random response analysis,” Section 6.3.11 | |

Boundary condition with a nonzero prescribed boundary | All procedures except those based on eigenmodes |

Connector motion Connector load | All relevant procedures except modal extraction, buckling, those based on eigenmodes, and direct steady-state dynamics |

Cross-correlation property | “Random response analysis,” Section 6.3.11 |

Current density (concentrated and distributed) | “Coupled thermal-electrical analysis,” Section 6.6.2 |

Electric charge (concentrated and distributed) | “Piezoelectric analysis,” Section 6.6.3 |

Equivalent pressure stress | “Mass diffusion analysis,” Section 6.8.1 |

Film coefficient and associated sink temperature | All procedures involving temperature degrees of freedom |

Fluid flux | Analysis involving hydrostatic fluid elements |

Fluid mass flow rate | Analysis involving convective heat transfer elements |

Flux (concentrated and distributed) | All procedures involving temperature degrees of freedom “Mass diffusion analysis,” Section 6.8.1 |

Force and moment (concentrated and distributed) | All procedures with displacement degrees of freedom except response spectrum |

Incident wave loading | Direct-integration dynamic analysis (“Implicit dynamic analysis using direct integration,” Section 6.3.2) involving solid and/or fluid elements undergoing shock loading |

Predefined field variable | All procedures except those based on eigenmodes |

Seepage coefficient and associated sink pore pressure Distributed seepage flow | “Coupled pore fluid diffusion and stress analysis,” Section 6.7.1 |

Substructure load | All procedures involving the use of substructures |

Temperature as a predefined field | All procedures except adiabatic analysis, mode-based procedures, and procedures involving temperature degrees of freedom |

With the exception of concentrated added mass and distributed added mass, no loads can be applied in eigenfrequency extraction analysis.