Geometrically Nonlinear Analysis
Geometrically nonlinear analysis (parameter NLGEOM) can be used in
topology optimization (both, controller and sensitivity-based), shape, and bead
(both, controller and sensitivity-based) optimization as well as in sizing
optimization. An excessive deformation of soft elements can occur during topology
optimization. This does not occur for a linear analysis. However, in the case of a
geometrically nonlinear analysis this leads to an adverse effect on the convergence,
which eventually leads to the termination of the analysis. This must be considered
when applying topology optimization using hyperelastic material.
Optimizing Using Abaqus/Explicit
The use of Abaqus/Explicit is permitted for shape controller optimization of quasi-static problems.
The ODB result interface must be activated which is also default,
see Files and Formats. The result
of one explicit analysis step is divided into 20 increments, which are interpreted
as 20 single substeps in Tosca Structure. A step in the finite element input file must be divided into several steps if
the results of more than 20 substeps should be included in the optimization.
Topology, bead, and shape sensitivity optimization in combination with Abaqus/Explicit is not supported.
Allowed Analysis Types for Sensitivity-Based Optimizations
In Abaqus responses from the following analysis types are allowed:
*STEP
*STATIC
...
*END STEP
and
*STEP, PERTURBATION
*STATIC
...
*END STEP
and
*STEP
*FREQUENCY
...
*END STEP
and
just for Topology Optimization with Tosca sensitivities
*STEP
*Heat Transfer, steady state
...
*END STEP
and
just for Sizing Optimization with Tosca sensitivities
*STEP
*STEADY STATE DYNAMIC
...
*END STEP
and
Optimization with AMS Eigensolver
The AMS solver from Abaqus can be used through the keyword *FREQUENCY, EIGENSOLVER=AMS. It
is an advanced feature and should be only used when necessary and by advanced
users.
Unlike the Lanczos solver, the AMS solver requires the specification of a frequency
range (at least an upper limit of the range). This range is enough to run the
frequency analysis. When using an AMS step in the optimization, the number of Eigenmodes still needs to be defined in the input
file.
While using the AMS solver during Tosca Structure optimizations, certain aspects need to be considered:
- When specifying a lower limit of the frequency range (for example,
10, 1, 100 specifying 10 Eigenmodes, 1Hz as lower and
100 Hz as upper limit) and there exist additional modes below 1Hz: These
additional modes are not calculated.
- If one wants to obtain the first Eigenmode within the specified range, one
needs to define a frequency design response with
LC_SET=MODAL,xxx,1. It may not be the first eigenmode
of the model, but the first within the range.
- It is strongly recommended to ensure that enough Eigenmodes exist within the
specified frequency range, especially when many frequency design responses
are defined. For example, if 10 Eigenmodes are requested and Abaqus only finds nine, the last design response for the 10th Eigenmode does not
obtain a value. In many cases, this can lead to an error in the optimization workflow,
especially when modetracking is used..
- To maintain a high-performance workflow, ensure that the requested frequency
range is not too large compared to the number of needed Eigenmodes. For
example, defining a frequency range that may contain 1000 modes, but only
the first 10 are required. Abaqus then calculates all 1000 modes, while providing output only for the first
10. This decreases performance.