Lattice Optimization in OptiStruct is a two-phase process to create blended Solid and Lattice structures from concept to the detailed final design. The first phase performs a concept-level topology optimization to optimally partition solid, void, and intermediate space and create the lattice elements. The second phase optimizes the size of each lattice element to determine the final optimized structure. The completed model can then be manually reinterpreted to fit a number of different manufacturing operations, including, but not limited to, 3D printing.

In this tutorial, you will learn to:

Problem Statement

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Phase I: Concept Level Design

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In the model, the non-design regions surround the load-bearing points and without the design space, the non-design area is discontinuous. This allows the optimization to determine the most efficient structure to transition the load and stress throughout the structure.

The Model browser shows that there is already an optimization set up.

Lattice optimization differs from topology as a concept-level optimization in that elements with intermediate densities will be mathematically interpreted as solid, lattice, or void, depending on your settings in the setup phase. You will add continuation cards and parameters in a text editor to change this from standard topology to lattice optimization.

Step 1: Import the model controlarm.fem into HyperMesh Desktop

User Profiles can also be accessed from the Preferences menu on the toolbar.

Step 2: Add the LATTICE continuation card to the DTPL card

+       LATTICE 1       0.1     0.7     200.

Each field begins with the field’s data and is padded with spaces to a total of 8 characters long.

Step 3: Add the optimization parameters DOPTPRM to control the optimization

DOPTPRM,POROSITY,MED

DOPTPRM,LATLB,CHECK

DOPTPRM indicates that optimization parameters to control the optimization are being added.

DOPTPRM, POROSITY determines the penalty factor applied to all designable elements throughout the optimization. In regular topology optimization, a penalty is applied to avoid medium dense elements by significantly reducing their stiffness. In Lattice optimization, medium dense elements can be accepted as they can be manufactured as a lattice structure. You can apply a smaller penalty or nothing, which controls the number of medium dense elements and consequently lattice type elements. This parameter has three options:

LOW: Sets the penalty factor to 1.8. This setting accurately represents the structural relationship between intermediate dense solid elements and lattice type elements, which result in a model that has few lattice elements, as medium dense elements are widely removed, due to the penalty.

MED: Sets the penalty factor to 1.25. This penalizes intermediate dense elements to some extent and results in a moderate presence of lattice zones.

HIGH: Sets the penalty factor to 1, negating any penalty to the Young’s Modulus and density relationship. This usually leads to a high number of lattice elements, but also produces a mathematical over-estimation of stiffness for structures known to be porous, which leads to a larger discrepancy between the structural performance of Phases I and II.

In some cases a certain value for LB can lead to important parts of the structure being interpreted as void. For instance, for a high number, such as 0.5 or where important parts of the structure have low densities. In those cases, the removal of the void elements can lead to a non-functioning structure. DOPTPRM, LATLB aims at preventing such scenarios. It will force OptiStruct to check the structure for large differences in compliance between the final iteration of Phase I and the one where the elements with density below LB are removed. If the structure is found to be too ‘soft’, the lower bound for lattice zones can be adjusted downward during the optimization to create a more continuous or stiff structure that more closely represents the original mesh. This parameter has the following options:

CHECK: Requests that OptiStruct check the lattice optimized representation of the model for differences in compliance.

AUTO: Allows OptiStruct to automatically adjust the lower bound of the lattice-optimized model downward to increase the model continuity.

NONE: No check for performance loss is made.

Step 4: Run the optimization

OptiStruct provides a summary of the lattice optimization and its effects on the model, at the end of the output file.

Since DOPTPRM, LATLB, CHECK was applied, OptiStruct checked the drop-in compliance, due to the removal of elements with density below LB. The drop is relatively small and this LB can be considered a good choice.

Phase II: Fine-Tuning Design

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Step 1: Open the model controlarm_lattice.fem in a text editor

At the end of Phase I, OptiStruct automatically created a new file, controlarm_lattice.fem, from your original optimization model. In this model, the original designable elements with a density below LB have been removed, those above UP remain unchanged and those in-between are replaced by bars (CBAR) representing the lattice structure.

The second phase of this optimization will optimize the radius of each joint in the lattice structure to determine where there is a need for material. The necessary design variables (DESVAR) and design variable property relationships (DVPREL1), along with the stress constraints are automatically created. In many cases, the model can be run as is, but it is advised to always review the optimization setup.

Step 2: Review the model to verify the changes to the original model

Note: The volume fraction response in the original model has been replaced by total volume in the size optimization phase, which will now be minimized and a new response has been added to constrain the von Mises stress in the lattice region to 200. This was created from the value in the last field of the LATTICE continuation card in the first phase.

Step 3: Run the second optimization phase

Verify that the optimization is fully converged and that the constraint is satisfied.

Note: The difference in lattice penalty (lower than 1.8, set by the optimization parameters) causes the compliance of the final Phase I model to differ from the initial Phase II model. This compliance difference is also affected by solid elements retained in the Phase II model, which recover their full density/stiffness. For this reason, post-processing a lattice optimization requires that you analyze changes to the model compliance between the Phase I final optimization compliance and the Phase II initial compliance calculation and again at the end of Phase II.

Since the optimization removes CBAR elements of small radius after the last optimization, the compliance for the last optimized run should be confirmed against the controlarm_lattice_optimized.fem file, an analysis of the optimized structure provided by OptiStruct.

Final compliance from the optimization run

Final compliance from the analysis model

Step 4: Post-process the results from the Phase II optimization

An overall view of the optimized solid/lattice model

Detail of the front of the optimized lattice/solid model showing the variation in CBAR radius

Iteration 0 (Initial Sizing Design)

Last Iteration (Final Design)

See Also:

OptiStruct Tutorials