8.6.2.2. Advanced Analysis Options You Can Set on the Solution Controls Dialog Box
The following sections provide more detail about some of the advanced analysis options that you can
set on the Solution Controls dialog box.
8.6.2.2.1. Equation Solver
Automatic solution control activates the sparse direct solver (EQSLV,SPARSE) for most cases. Other options
include the PCG and ICCG solvers. For applications using solid elements, the PCG solver may be faster,
especially for 3-D modeling.
If using the PCG solver, you may be able to reduce memory usage via the MSAVE command. The MSAVE
command triggers an element-by-element approach for the parts of the model that use SOLID185,
SOLID186,SOLID187 SOLID272,SOLID273, and/or SOLID285 elements with linear material properties.
(MSAVE does not support the layered option of the SOLID185 and SOLID186 elements.) To use MSAVE,
you must be performing a static or a modal analysis with PCG Lanczos enabled. When using SOLID185,
SOLID186, and/or SOLID187, only small strain (NLGEOM,OFF) analyses are allowed. Other parts of the
model that do not meet the above criteria are solved using global assembly for the stiffness matrix.
MSAVE,ON can result in a memory savings of up to 70 percent for the part of the model that meets
the criteria, although the solution time may increase depending on the capabilities of your computer
and the element options selected.
The sparse direct solver, in sharp contrast to the iterative solvers included in the program, is a robust
solver. Although the PCG solver can solve indefinite matrix equations, when the PCG solver encounters
an ill-conditioned matrix, the solver iterates to the specified number of iterations and stop if it fails to
converge. When this happens, it triggers bisection. Aft er completing the bisection, the solver continues
the solution if the resulting matrix is well-conditioned. Eventually, the entire nonlinear load step can
be solved.
Use the following guidelines for selecting either the sparse or the PCG solver for nonlinear structural
analysis:
- If it is a beam/shell or beam/shell and solid structure, choose the sparse direct solver.
- If it is a 3-D solid structure and the number of DOF is relatively large (that is, 200,000 or more DOF),
choose the PCG solver. - If the problem is ill-conditioned (triggered by poor element shapes), or has a big difference in mater-
ial properties in different regions of the model, or has insufficient displacement boundary constraints,
choose the sparse direct solver.
8.6.2.3. Advanced Load Step Options You Can Set on the Solution Controls Dialog Box
The following sections provide more detail about some of the advanced load step options that you can
set on the Solution Controls dialog box.
8.6.2.3.1. Automatic Time Stepping
Automatic solution control turns automatic time stepping on (AUTOTS,ON). An internal auto-time step
scheme ensures that the time step variation is neither too aggressive (resulting in many bisection/cut-
backs) nor too conservative (time step size is too small). At the end of a time step, the size of the next
time step is predicted based on four factors:
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