Review of Available Options and Possibilities

Within this section useful options and possibilities are reviewed.

SIMULIA Tosca Structure Options

In SIMULIA Tosca Structure all the following described options are set in OPT_PARAM block in the *.par file.

Option: Description:
DENSITY_MOVE This option decides maximum possible change of the pseudo density values within each optimization iteration. Smaller values of this parameter help to overcome convergence issues. But at the same time, the convergence rate will be slower when compared to the default settings. The default value is DENSITY_MOVE=0.25. In cases where it is observed that either the objective function or some of the constraints are oscillating it is recommended to set DENSITY_MOVE=0.1.
DENSITY_UPDATE This option affects the behavior of MMA (Method of moving asymptotes) with respect to setting of the asymptotes. The two available schemes of density update are CONSERVATIVE and NORMAL. Conservative update method is considered to be more stable. The default is DENSITY_UPDATE=NORMAL. In cases where it is observed that either the objective function or some of the constraints are oscillating it is recommended to set DENSITY_UPDATE=CONSERVATIVE.
DENSITY_INITIAL Material properties of design elements are scaled (solid and void properties are interpolated) based on the pseudo density values. Different material interpolation schemes like SIMP and RAMP are available in SIMULIA Tosca Structure. The initial pseudo density values can be set by the DENSITY_INITIAL option. If one of the constraint is based on volume design response, then the initial density is set equal to the value of the volume constraint by default. In all other cases the initial density value is set equal to 0.5. Note, to compare the DRESP value of SIMULIA Tosca Structure with the solver results set DENSITY_INITIAL = 1 and use the DRESP value from design cycle 0.
DENSITY_LOWER In topology optimization the voids are not created by deleting the finite elements from the mesh but are simulated scaling the stiffness of design elements to a very low value. Zero elemental stiffness would lead to the loss of positive definiteness of the global structural stiffness matrix. The corresponding system of equations would not be solved. Hence a minimum value of pseudo density is used within topology optimization. The default value is DENSITY_LOWER=0.001 but can be modified by the user.
SOFT_DELETE_METHOD In a topology optimization process, the structure consists of hard (filled) and soft (void) elements. The soft elements have a negligible influence on the stiffness of the resulting structure but they are still relevant for the solution of system of equilibrium equations. The soft delete option activates a procedure which checks for elements that could be potentially distorted during optimization procedure based on the user defined criteria and deletes them from finite element mesh. The deleted elements are reactivated if needed, in case that the force flow changes during the optimization. Note, this procedure is useful to stabilize the optimization in the context of non-linear structural analysis and helps to overcome solver instabilities. By default it is not activated.

Solver Specific Settings and Comments: SIMULIA Abaqus

In following SIMULIA Abaqus related remarks are summarized which should be taken into account optimizing structures with non-linear behavior.

Topic: Remark:
Time increment By default SIMULIA Abaqus uses the automatic time incrementation based on the initial, minimum and maximum time increment. In case of non-linear analysis it is recommended to use small initial time increment in order to run the analysis in a stable manner. The following is the syntax for the SIMULIA Abaqus input file: 
*STATIC
0.11, 1.0, 1e-07, 1.0
More details can be found in the SIMULIA Abaqus documentation.
Element type Based on the utilized integration schemes the element types are split into fully integrated elements like C3D8 or C3D4 and reduced integrated elements like C3D8R. In case of reduced integration elements in the model, it is recommended to activate the enhanced hourglass technique. The following is the syntax for the SIMULIA Abaqus input file: 
*SECTION CONTROLS, NAME=< SEC NAME>, HOURGLASS = ENHANCED
Solver instabilities If the optimization fails due to SIMULIA Abaqus convergence issues, further details can be investigated in the *.msg or *.dat file generated by SIMULIA Abaqus. The following are the expected warnings or errors indicating the local buckling of the low density elements:
  • ELEMENTS ARE DISTORTING EXCESSIVELY
  • DISPLACEMENT CORRECTION TOO LARGE COMPARED TO DISPLACEMENT INCREMENT
  • THE STRAIN HAS EXCEEDED FIFTY TIMES THE STRAIN TO CAUSE FIRST YIELD AT 'n' POINTS