Many of the tasks that you need to perform in a nonlinear transient analysis are the same as (or similar to) those that you perform in nonlinear static analyses (described in Performing a Nonlinear Static Analysis) and linear full transient dynamic analyses (described in Structural Static Analysis). However, this section describes some additional considerations for performing a nonlinear transient analysis.
You cannot use the Solution Controls dialog box (Performing a Nonlinear Static Analysis) to set solution controls for a thermal analysis. Instead, use the standard set of solution commands and the standard corresponding menu paths.
This step is the same as for a nonlinear static analysis. If your analysis includes time-integration effects, however, include a value for mass density (MP,DENS). You can also define material-dependent structural damping (MP,BETD, MP,ALPD, TB,SDAMP,,,,BETD, or TB,SDAMP,,,,ALPD).
Specify transient analysis type and define analysis options as you would for a nonlinear static analysis:
Apply loads and specify load step options in the same manner as you would for a linear full transient dynamic analysis. A transient load history usually requires multiple load steps, with the first load step typically used to establish initial conditions (see the Basic Analysis Guide). The general, nonlinear, birth and death, and output control options available for a nonlinear static analysis are also available for a nonlinear transient analysis.
In a nonlinear transient analysis, time must be greater than zero. See Transient Dynamic Analysis for procedures for defining nonzero initial conditions.
For a nonlinear transient analysis, you must specify whether you want stepped or ramped loads (KBC). See the Basic Analysis Guide for further discussion about ramped vs. stepped loads.
You can also specify dynamics options: alpha (ALPHAD) and beta (BETAD) damping, time-integration effects (TIMINT), and transient integration parameters (TINTP):
Damping
Rayleigh damping constants are defined using the constant mass (ALPHAD) and stiffness (BETAD) matrix multipliers. In a nonlinear analysis, the stiffness may change drastically; therefore, use BETAD with care. For more information, see See Damping.
Time-Integration Effects (TIMINT)
Time-integration effects are ON by default in a transient analysis. For creep, viscoelasticity, viscoplasticity, or swelling, you should turn the time-integration effects off (that is, use a static analysis). These time-dependent effects are usually not included in dynamic analyses because the transient dynamic time-step sizes are often too short for any significant amount of long-term deformation to occur.
Except in kinematic (rigid-body motion) analyses, you rarely need to adjust the transient integration parameters (TINTP), which provide numerical damping to the Newmark and HHT methods. (See your Mechanical APDL Theory Reference for more information about these parameters.)
Automatic solution control sets the defaults to a new time-integration scheme for use by first order transient equations. This is typically used for unsteady-state thermal problems where θ = 1.0 (backward Euler scheme). It is unconditionally stable and more robust for highly nonlinear thermal problems such as phase changes. The oscillation limit tolerance defaults to 0.0, so that the response first order eigenvalues can be used to more precisely determine a new time-step value.
Write load data for each load step (LSWRITE) to a load step file.
Save a backup copy of the database to a named file (SAVE).
Start solution calculations (LSSOLVE). Other methods for multiple load steps are described in Getting Started in the Basic Analysis Guide.
After solving all load steps, exit the SOLUTION processor (FINISH).
As in a nonlinear static analysis, you can use POST1 to postprocess results at a specific moment in time. Procedures are much the same as described previously for nonlinear static analyses. Again, you should verify that your solution has converged before you attempt to postprocess the results.
Time-history postprocessing using POST26 is essentially the same for nonlinear as for linear transient analyses. See the postprocessing procedures outlined in Transient Dynamic Analysis.
More information about postprocessing is available in the Basic Analysis Guide.
! Build the Model:
/PREP7
--- ! Similar to a linear full transient model, with
--- ! these possible additions: nonlinear material
--- ! properties, nonlinear elements
---
FINISH
!
! Apply Loads and Obtain the Solution:
/SOLU
ANTYPE,TRANS
! TRNOPT,FULL by default
--- ! Establish initial conditions as in linear full
--- ! transient analysis
LSWRITE ! Initial-condition load step
NLGEOM,ON ! Nonlinear geometric effects (large deformations)
! NROPT=AUTO by default: Program chooses appropriate Newton-Raphson and
! Adaptive Descent options, depending on
! nonlinearities encountered
! Loads:
F,...
D,...
! Load Step Options:
TIME,... ! TIME at end of load step
DELTIM,... ! Time-step controls (starting, min, max)
AUTOTS,ON ! Automatic time-stepping, including bisection
! KBC=0 by default (ramped loading)
! Dynamic Options:
ALPHAD,... ! Mass damping
TIMINT,ON ! TIMINT,ON by default, unless you turned it OFF for
! initial-condition load step
! Nonlinear Options:
CNVTOL,... ! Convergence criteria
NCNV,,,... ! Nonconvergence termination controls
PRED,ON ! Predictor ON
OUTRES,ALL,ALL ! Results for every substep written to database
LSWRITE ! First "real" transient load step
--- ! Additional load steps, as needed
---
LSSOLVE,1,3 ! Initiate multiple l.s. solution
SAVE
FINISH
!
! Review the Results:
/POST26 ! Time-History Postprocessor
SOLU,2,CNVG ! Check convergence
SOLU,3,FOCV
PRVAR,2,3
NSOL,... ! Store results (displacements, stresses, etc.) as
! variables
PLVAR,... ! Graph results vs. TIME to evaluate general quality
! of analysis, determine critical time step, etc.
FINISH
!
/POST1 ! General Postprocessor
SET,... ! Read results from desired time step
PLDISP,... ! Postprocess as desired
PLNSOL,...
NSORT,...
PRNSOL,...
FINISH