perturb

Mapdl.perturb(type_='', matkey='', contkey='', loadcontrol='', **kwargs)

Sets linear perturbation analysis options.

APDL Command: PERTURB

Parameters:

type_

Type of linear perturbation analysis to be performed:

STATIC - Perform a linear perturbation static analysis.

MODAL - Perform a linear perturbation modal analysis.

BUCKLE - Perform a linear perturbation eigenvalue buckling analysis.

HARMONIC - Perform a linear perturbation full harmonic analysis.

SUBSTR - Perform a linear perturbation substructure generation pass.

OFF - Do not perform a linear perturbation analysis (default).

matkey

Key for specifying how the linear perturbation analysis uses material properties, valid for all structural elements except contact elements. For more information, see Linear Perturbation Analysis in the Mechanical APDL Theory Reference.

AUTO - The program selects the material properties for the linear perturbation

analysis automatically (default). The materials are handled in the following way:

For pure linear elastic materials used in the base analysis, the same properties are used in the linear perturbation analysis. - For hyperelastic materials used in the base analysis, the material properties

are assumed to be linear elastic in the linear perturbation analysis. The material property data (or material Jacobian) is obtained based on the tangent of the hyperelastic material’s constitutive law at the point where restart occurs.

For any nonlinear materials other than hyperelastic materials used in the base analysis, the material properties are assumed to be linear elastic in the linear perturbation analysis. The material data is the same as the linear portion of the nonlinear materials (that is, the parts defined by MP commands). - For COMBIN39, the stiffness is that of the first segment of the force-

deflection curve.

TANGENT - Use the tangent (material Jacobian) on the material constitutive curve as the

material property. The material property remains linear in the linear perturbation analysis and is obtained at the point of the base analysis where restart occurs. The materials are handled in the following way:

For pure linear elastic materials used in the base analysis, the same properties are used in the linear perturbation analysis. Because the material constitutive curve is linear, the tangent is the same as the base analysis. - For hyperelastic materials used in the base analysis, the program uses the same

tangent as that used for MatKey = AUTO, and the results are therefore identical.

For any nonlinear materials other than hyperelastic materials used in the base analysis, the material properties are obtained via the material tangent on the material constitutive curve at the restart point of the base analysis. - The materials and properties typically differ from Matkey = AUTO, but it is

possible the results could be identical or very similar if a.) the material is elasto-plastic rate-independent and is unloading (or has neutral loading) at the restart point, or b.) the material is rate-dependent, depending on the material properties and loading conditions.

For COMBIN39, the stiffness is equal to the tangent of the current segment of the force-deflection curve. - In a modal restart solution that follows a linear perturbation modal analysis,

the TANGENT option is overridden by the AUTO option and linear material properties are used for stress calculations in the modal restart. See the discussion in the Notes for more information.

contkey

Key that controls contact status for the linear perturbation analysis. This key controls all contact elements (TARGE169, TARGE170, and CONTA171 through CONTA178) globally for all contact pairs. Alternatively, contact status can be controlled locally per contact pair by using the CNKMOD command. Note that the contact status from the base analysis solution is always adjusted by the local contact controls specified by CNKMOD first and then modified by the global sticking or bonded control (ContKey = STICKING or BONDED). The tables in the Notes section show how the contact status is adjusted by CNKMOD and/or the ContKey setting.

CURRENT - Use the current contact status from the restart

snapshot (default). If the previous run is nonlinear, then the nonlinear contact status at the point of restart is frozen and used throughout the linear perturbation analysis.

STICKING - For frictional contact pairs (MU > 0), use

sticking contact (e.g., MU*KN for tangential contact stiffness) everywhere the contact state is closed (i.e., status is sticking or sliding). This option only applies to contact pairs that are in contact and have a frictional coefficient MU greater than zero. Contact pairs without friction (MU = 0) and in a sliding state remain free to slide in the linear perturbation analysis.

BONDED - Any contact pairs that are in the closed

(sticking or sliding) state are moved to bonded (for example, KN for both normal and tangential contact stiffness). Contact pairs that have a status of far-field or near-field remain open.

loadcontrol

Key that controls how the load vector of {Fperturbed} is calculated. This control is provided for convenience of load generation for linear perturbation analysis. In general, a new set of loads is required for a linear perturbation analysis. This key controls all mechanical loads; it does not affect non-mechanical loads. Non-mechanical loads (including thermal loads) are always kept (i.e., not deleted).

ALLKEEP - Keep all the boundary conditions (loads and

constraints) from the end of the load step of the current restart point. This option is convenient for further load application and is useful for a linear perturbation analysis restarted from a previous linear analysis. For this option, {Fend} is the total load vector at the end of the load step at the restart point.

INERKEEP - Delete all loads and constraints from the

restart step, except for displacement constraints and inertia loads (default). All displacement constraints and inertia loads are kept for convenience when performing the linear perturbation analysis. Note that nonzero and tabular displacement constraints can be considered as external loads; however, they are not deleted when using this option.

PARKEEP - Delete all loads and constraints from the

restart step, except for displacement constraints. All displacement constraints are kept for convenience when performing the linear perturbation analysis. Note that nonzero and tabular displacement constraints can be considered as external loads; however, they are not deleted when using this option.

DZEROKEEP - Behaves the same as the PARKEEP option, except

that all nonzero displacement constraints are set to zero upon the onset of linear perturbation.

NOKEEP - Delete all the loads and constraints, including

all displacement constraints. For this option, {Fend} is zero unless non-mechanical loads (e.g., thermal loads) are present.

Notes

This command controls options relating to linear perturbation analyses. It must be issued in the first phase of a linear perturbation analysis.

This command is also valid in PREP7.