Generally, structural loads can be divided into two categories: mechanical loads and non-mechanical loads. Non-mechanical loads relevant to this procedure include thermal, swelling, initial stresses/strains, and other initial conditions. In the base analysis, a combination of mechanical and non-mechanical loads can be freely applied without any restrictions; however, for the linear perturbation analysis, only mechanical loads (and thermal loads in the case of linear perturbation static analysis and linear perturbation eigenvalue buckling analysis) are allowed.
It is good practice to delete all loads from the base analysis in the first phase of
linear perturbation. By default, the program deletes all loads from the restart step,
except for displacement boundary conditions, inertia loads, and non-mechanical loads
(see the LoadControl argument on the
PERTURB command). Non-mechanical loads (including thermal loads)
must remain unmodified so that a detailed nonlinear snapshot for various solution
matrices and element history variables can be regenerated in the first phase of the
linear perturbation procedure.
You can apply new mechanical loads in the second phase of the linear perturbation process. However, new non-mechanical loads are not allowed except in a few cases. See Generating and Controlling Non-mechanical Loads for more information.
Note: When performing a linear perturbation full harmonic analysis, the following loads can act as harmonically varying loads:
Nonzero displacements specified in the base analysis (D command).
MPC bonded contact with initial penetration, gap, or offset. The effect of initial penetration, gap, or offset can be excluded by setting KEYOPT(9) = 1 on the contact elements in the base analysis.
Pretension loads (SLOAD command) specified in the base analysis.
External constraint equations (CE command) with nonzero constant terms specified in the base analysis.
In a linear perturbation modal analysis, the total loads obtained from the second phase of the analysis may be used in a downstream analysis following the linear perturbation modal analysis. If a downstream analysis is not needed, then this load generation step can be ignored.
The total perturbed loads are calculated as follows:
where:
| {Fend} = total loads at the end of the load step of the current restart point (load applications are read from the .LDHI file). By default, all loads of {Fend} are deleted except for displacement boundary conditions, inertia loads, and non-mechanical loads. |
| {Fadd} = additional (new) loads prescribed by the user in the second phase of the linear perturbation analysis (after the first SOLVE,ELFORM command is invoked). This additional loading is optional. |
In the first phase of a linear perturbation analysis, the ANTYPE,,RESTART command resumes the Jobname.rdb database and read in the Jobname.ldhi file to establish the {Fend} load.
New load application (adding to {Fadd}) or load removal (changing {Fend}) can occur only in the second phase of the linear perturbation analysis (after the first SOLVE,ELFORM command), allowing flexibility in controlling the final {Fperturbed} that will be used.
{Fperturbed} is used differently in each linear perturbation analysis type:
In a linear perturbation static analysis, {Fperturbed} is used to compute the static analysis solution.
In a linear perturbation modal analysis, {Fperturbed} is calculated and stored in the Jobname.FULL and Jobname.mode files for a subsequent mode-superposition, PSD, or other type of modal-based linear dynamic analysis.
In a linear perturbation eigenvalue buckling analysis, {Fperturbed} is used to calculate the linearly perturbed displacements; these displacements are used for generation of the linearly perturbed stress stiffening matrix and thus the load factor for eigenvalue buckling analysis. Note that this load can be totally independent of the load used in the base analysis.
In a linear perturbation harmonic analysis, {Fperturbed} is used in the frequency steps for the harmonic solution. {Fperturbed} can be frequency dependent and can use complex input.
Non-mechanical loads (including thermal loads) must remain unmodified in the first phase of the linear perturbation analysis so that a detailed nonlinear snapshot for various solution matrices and element history variables can be regenerated; thus, total load contributions to {Fperturbed} include non-mechanical loads in the first phase of a linear perturbation analysis.
In the second phase of a linear perturbation analysis, you cannot change, add, or remove non-mechanical loads with these exceptions: thermal loads can be defined in the second phase of a linear perturbation static analysis or a linear perturbation eigenvalue buckling analysis by specifying a new temperature. In a linear perturbation static or buckling analysis, assuming that the base analysis is nonlinear, the reference temperature is the temperature from which the linear perturbation analysis is restarted (and not the reference temperature (TREF) from the base analysis). If the base analysis is linear, then the reference temperature used in the linear perturbation is the TREF temperature from the base analysis. See Loads, Initial Conditions, and Other Limitations in Linear Perturbation in the Element Reference for more information.
In the stress expansion pass of a linear perturbation modal or buckling analysis, thermal effects contribute no strain in the total strain calculation; that is, the thermal strain is zero in this expansion pass.