Composite materials have become popular in the application of a variety of structures. The need for innovative designs has posed a great challenge. This tutorial discusses the optimization-driven design approach of an open-hole tension specimen using OptiStruct.

The design takes a three-phased approach:

The process expands upon three important and advanced optimization techniques; free-size optimization, size optimization and ply stacking sequence optimization. By stringing these three techniques together, OptiStruct offers a unique and comprehensive process for the design and optimization of composite laminates. The process is automated and integrated in HyperWorks by generating the input data for a subsequent phase automatically from the previous design phase.

Together with these steps, an initial and final analysis will be utilized to determine the baseline and final characteristics of the designed part.

Problem Definition

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The composite design optimization methodology presented within this tutorial was developed to solve very complex composite design optimization problems. The methodology breaks down the complex composite design optimization problem, which is not solvable by itself, into several simpler composite design optimization problems, which are solvable by themselves. The cumulative solution to each of the simpler composite design optimization problems provides a solution to the complex design optimization problem. This process of breaking down complex problems into several simpler problems is consistent with the engineering method.

Model overview

Initial Model Setup and Baseline Analysis

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The following set of steps completes the analysis setup of the initial model and provides a baseline analysis for comparison with the final optimized structure.

Step 1: Load the OptiStruct user profile and open the model into HyperMesh

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

Step 2: Create a new material for carbon epoxy

Step 3: Create a new element set containing the initial shape of each ply

Step 4: Create the basic plies for the analysis

Step 5: Create the laminate for the analysis

Step 6: Create a property and assign it to all elements in the model

Step 7: Review the model

This will thicken all shells in the model to their total thickness, displaying them as 3-dimensional representations of their thicknesses.

This separates the view into individual plies.

This represents each of the plies in the model according to the color of its ply as shown in the Model browser. If all of the plies in the model are the same color, change the ply colors in the Model browser so that each is different to help differentiate the plies in the graphics area.

Step 8: Create control cards to set the request and output parameters

This requests that NO homogeneous stress be output. This control card output must be explicitly added as a request since homogeneous stress is output by default. In turning it off, the values will not be calculated by the solver for output.

This turns off HTML output for all analyses and optimizations which have this keyword combination.

This request that OptiStruct’s .h3d output file be output at the first (F) and last (L) iteration. For analysis, this does not matter, but for optimization it applies when you get to the optimization runs.

Step 9: Run the baseline analysis

Step 10: Post-process the baseline analysis in HyperView

Step 11: Return to HyperMesh and deactivate the composite visualization enhancements

 

Phase 1 – Concept Design Synthesis (free-size optimization)

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In free-size optimization, the thickness of each designable element is defined as a design variable. Applying this concept to the design of composites implies that the design variables are the thickness of each ‘Super-ply’ (total designable thickness of a ply orientation) per element.

The following optimization setup is defined in the concept design phase to identify the stiffest design for the given fraction of the material. To obtain more meaningful results, manufacturing constraints are incorporated and carried through all design phases automatically.

Step 1: Create the design variables for free-size optimization

Figure 1: Field entries for the free-size panel

The design variable free-size is created for the free-size optimization. The manufacturing constraints on ply percentage and ply balance will be defined next.

Two PLYPCT continuation lines are added to the DSIZE data entry.

These values constrain the zero- and ninety-degree plies to between twenty and seventy percent of the total thickness of the laminate for any element in the design space.

Figure 2: DSIZE data entry fields for the PLYPCT cards

A BALANCE continuation line is added to the DSIZE data entry.

Figure 3: DSIZE data entry fields for the BALANCE card

A PLYDRP continuation line is added to the DSIZE data entry.

Figure 4: DSIZE data entry fields for the PLYDRP card using the PLYSLP method

Step 2: Create the responses

Step 3: Define constraints for optimization

Step 4: Define the objective function

Figure 5: Optimization Objective Panel (Compliance)

Step 5: Add an output request to output the next optimization step

The output control on composite strain and stress results are defined here. OUTPUT,FSTOSZ (free size to size) is used to output a ply-based input deck for size optimization.

Figure 6: Requesting the free-size to size (FSTOSZ) optimization output file for Phase 2.

With this keyword, OptiStruct automatically generates a sizing model after free-size optimization.

Step 6: Submit the free-size optimization job

The name and location of the oht_opti_ph1.fem file displays in the input file field.

Figure 7: OptiStruct Panel

If the job was completed successfully, new results files can be seen in the directory from which oht_opti_ph1.fem was saved.

Figure 8: The HyperWorks Solver View window

A few default files which will be written to your directory are:

Step 7: View the element thickness results

This launches the HyperView client from HyperMesh Desktop and opens the session file oht_opti_ph1.mvw which contains three pages with the results from two H3D files (click Close in the Message Log window):

Page 2 – optimization results in oht_opti_ph1_des.h3d

Page 3 – analysis results of subcase 1 in oht_opti_ph1_s1.h3d

Note: If opening these files from standalone HyperMesh, the page numbers will be decremented.

Figure 9: Contour panel plot options (free-size optimization results)

Figure 10: Selecting the final iteration

The element thickness results from the free-size optimization are shown in the image below. The regions indicated in red or in colors tending towards red (from the legend) can be interpreted as thicker regions, while those in blue or tending towards blue are thinner regions. The contour plot indicated above is the total thickness distribution that includes contributions from each ply orientation, i.e. a thickness contribution from the 0s, +/-45s and the 90s. It also indicates the shape and layout of plies per orientation as can be seen in the ply thickness plot.

Figure 11: Element thicknesses contour plot after free-size optimization

Step 8: View the ply thickness results

Figure 12: Ply Thicknesses contour plot

The thickness distribution of 0 degree super ply is generated. It represents the ply shapes and patch locations of the 0 degree ply bundles.

Figure 13: Ply Thickness Contour plot of the 0 degree super-ply

Figure 14: Ply Thickness Contour plot of the -45/+45 degree super-plies

Figure 15: Ply Thickness Contour plot of the 90 degree super-ply

Step 9: View the ply bundles in the optimization result fem

The optimized ‘Super-ply’ thickness is subsequently represented as ‘Ply Bundles’. Four ply bundles per fiber orientation (Super ply) are output by default, based on an intelligent algorithm in OptiStruct. These ply bundles represent the shape and location of the plies per fiber orientation through element sets. In this case, a total of 16 ply bundles are created after free size optimization converges: plies 1 through 4 represent the ply bundles for 0 degree super-ply; plies 5 through 8 represent ply bundles for 90 degree super-ply; plies 9 through 12 represent ply bundle +45 degree super-ply; and applies 13 through 16 represent ply bundles for -45 degree super-ply.

Figure 16: Model browser view showing ply 11300 selected (Laminate 1, Ply 1, Shape 3)

Figure 17: Graphics Area view of ply 11300 (Laminate 1, Ply 1, Shape 3)

The shapes of the plies as indicated through the element set can be used as-is in design Phase 2, or modified easily by updating the element sets in HyperMesh or using ply smoothing to improve the manufacturability. Ply smoothing operations are shown in the next section.

Step 10: Use OSSmooth to perform ply smoothing operations on the model

Ply smoothing is an automated method to further reduce the ply shapes into more manufactural ply shapes. Although ply smoothing significantly improves the ply shapes for manufacturability, often it is still necessary to manually edit the ply shapes after this step.

Figure 18: Graphics Area view of ply 11300 after smoothing operations in OSSmooth (Laminate 1, Ply 1, Shape 3)

Phase 2 – Design Fine Tuning (size optimization)

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In the second design phase, a size optimization is performed to fine tune the thicknesses of the optimized ply bundles from Phase 1. To ensure that the optimization design meets the design requirements, additional performance criteria on may be incorporated into the problem formulation. These new criteria will be fiber strain, matrix strain, and mass.

The following is the modified optimization setup:

Manufacturing constraints previously applied are preserved and transferred to the DCOMP card.

Step 1: Save the model as oht_opti_ph2.hm

Step 2: Review and edit the size design variables

Step 3: Edit the manufacturable thickness and initial value of each ply

Figure 19: A view of the Model browser showing all plies selected

This changes the values for all plies simultaneously.

Step 4: Update the laminate calculation type

The laminate option defines the laminate behavior. In this case SMEAR theory is used to define the laminate behavior; that is the A-matrix is calculated exactly since it is stacking sequence independent, the D-matrix is calculated as AT2/12, and finally the B-matrix is set to zero. Adding the Symmetric option to the SMEAR theory just assures a symmetric laminate will be output by adding/removing 2 plies at a time vs. 1 ply at a time.

Step 5: Delete the existing responses, constraints, and objective

When responses are deleted in HyperMesh, all constraints and objectives which depend on those responses are deleted.

Step 6: Create the new responses for size optimization

The responses of volume, natural frequency, and composite strain are created for size optimization.

Figure 20: The fiber mechanical strain response

Figure 21: The matrix mechanical strain response

Step 7: Create optimization constraints

The responses of fiber and matrix strain are defined as the optimization constraints.

Step 8: Create objective function for the optimization

Step 9: Edit the composite size design variable to update the manufacturing constraints

This DESVAR has been carried over from the previous optimization as a size optimization with manufacturing constraints preserved from the original optimization.

Step 10: Define the output request for shuffling deck

The output control on composite strain and stress results defined in the previous phase are carried over automatically. OUTPUT,SZTOSH (sizing to shuffling) writes a ply stacking optimization input deck.

Step 11: Submit the size optimization job

If the job was completed successfully, new results files can be seen in the same directory where oht_opti_ph2.fem was saved.

A few default files are:

Step 12: View the thickness results in HyperView

Figure 22: Element thickness contour plot (final iteration) after phase-2 size optimization

This brings us to the third and final phase of the design process in which you try to identify a proposal for the optimal stacking sequence of the plies.

Phase 3 – Ply Stacking Sequence Optimization

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This algorithm is aimed at providing a global view of what the optimal stacking sequence could be. An input deck for the ply stacking sequence optimization, oht_opti_ph2_shuffling.*.fem, was generated from a previous design stage. Each ply bundle is divided into multiple PLYs whose thickness is equal to the manufacturable thickness (0.01 in this case), and the STACK card is updated accordingly. In this design phase, composite plies are shuffled to determine the optimal stacking sequence.

It is important that design performances are preserved. Hence, the optimization problem is retained as previously formulated in the size optimization phase. Two manufacturing constraints are applied:

Step 1: Load the OptiStruct user profile and import the model into a new session

Follow Step 1 from the free-size phase to load the oht_opti_ph2_shuffling.*.fem file into a new HyperMesh session.

Step 2: Save the model as oht_opti_ph3.hm

Step 3: Update the design variable to input the constraints for the shuffling optimization

This sets the outermost layer of the shuffling sequence to maintain one repetition of a set of -45, 0, 45, 90 degree plies. This ply set is applied to both faces of this optimization, since the laminate is symmetric.

Step 4: Submit the shuffling job

The following result files are generated:

Step 5: Post-process the results

It is automatically loaded in your default Internet browser window.

The plies are color coded based on their fiber orientations.

Figure 23: Shuffling Optimization History

The above image shows the history of the shuffling optimization. The columns represent the global trend of the ply stacking sequence at a particular iteration, with the last column being the final solution.

The weight of the part has not been changed during the shuffling design phase, rather the plies were reordered to obtain the maximum performance.

This light weight design therefore meets all of the performance requirements, is feasible and manufacturable.

Post-Optimization Final Analysis

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The following set of steps provides a final optimized structure for comparison with the initial model.

Step 1: Load the OptiStruct user profile and open the model into HyperMesh

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

Step 2: Import the final optimization properties over the existing model

Step 3: Alter the control cards

Step 4: Delete the optimization entities

Step 5: Review the model

This will thicken all shells in the model to their total thickness, displaying them as 3-dimensional representations of their thicknesses.

This will separate the view into individual plies.

This represents each of the plies in the model according to the color of its ply as shown in the Model browser. If all of the plies in the model are the same color, change the ply colors in the Model browser so that each is different to help differentiate the plies in the graphics area.

Figure 24: Visualization of model thickness in HyperMesh with half of the elements in the model masked

Step 6: Run the final analysis

Step 7: Post-process the final analysis in HyperView

This corresponds to the fiber strain in the model.

The final design weighs ~0.33 lbs.

Figure 25: A plot of maximum Normal Y strain across all plies, showing that no element exceeds 0.007 me

See Also:

OptiStruct Tutorials