OptiStruct Optimization v2017.2 · EXERCISE 4G: FREE-SIZE OPTIMIZATION OF A I-BEAM WEB Steps 1. Import the model in HyperMesh Desktop with OptiStruct user profile selected 2. Review

OPTISTRUCT OPTIMIZATION V2019

CONCEPT DESIGN – FREE-SIZE한국알테어이광원

© 2018 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved.

OptiStruct Optimization, v2017.2.3

2

• Topology – Given a design envelope, topology optimization finds the optimum material placement within that space according to the constraints and objective

• Free Size – Given a shell structure, free size optimization finds the optimum thickness on an element-by-element basis that meets the constraints and objective

• Topography – Given a shell structure, topography optimization creates a bead pattern from the elements that meets the constraints and objective

CONCEPT LEVEL OPTIMIZATION TECHNIQUES

Topology

Free Size

Topography

December 19, 2019 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved.©

FREE-SIZE OPTIMIZATION & DESIGN INTERPRETATION


FREE-SIZE OPTIMIZATION

Topology optimization

• Design variable = Density

• Design space = Total – Base Thickness

• Poor bending representation of

semi-dense elements

• Truss-like design concepts, no

shear panels

Free-size Optimization

• Design variables = Thickness of each element

• Design space = max. element thickness

• Accurate bending representation

• Shear panels possible if they represent the best

concept

• Expandable to composites


topology

→ truss concept

FREE-SIZE OPTIMIZATION

Free-size optimization can propose designs which are quite different from topology-based

optimization with a similar or improved performance.

• Concept-level designs shown below on the left

• Finished structures to the right (refined by size optimization with stress and buckling constraints)

1,00

1,50

2,00

2,50

3,00

3,50

4,00

4,50

5,00

5,50

1 1,5 2 2,5 3 3,5 4 4,5 5

Maximum dispacement

Op

tim

um

mass

Truss Concept

Plate Concept

free-size

→ plate concept


OPTISTRUCT INPUT

DSIZE card – Design variable definition for free-size optimization

• Can be used for shell (PSHELL) or composite (PCOMP, PCOMPG, or

STACK) structures

• Design space parameters

• Minimum thickness

• Maximum thickness

• Specialized design parameters

• Minimum member size

• Stress and fatigue constraints bounds (see chapter Topology Optimization)

• Manufacturing constraints

• Pattern grouping

• Pattern repetition

• Zone Based

HyperMesh Free-Size Entity Editor


OPTISTRUCT INPUT – DESIGN SPACE – MIN AND MAX THICKNESS

For shell (PSHELL) structures, the design

space is the difference between the minimum

(allowed) thickness and the maximum

(allowed) thickness.

Free-size optimization allows thickness to vary

freely between Tmin and Tmax for each element;

this is in contrast to topology optimization

which targets a discrete thickness of either

Tbase or T.

Default values

• With ‘Minimum Thickness’ blank, OS takes 0.0 (resp. legacy field value T0 from PSHELL card)

• With ‘Maximum Thickness’ blank, OS takes T

from PSHELL card

TminTmax


OPTISTRUCT INPUT – MANUFACTURING CONSTRAINTS

Maximum thickness gradient (TG, TGX, TGY, TGZ)*

Minimum member size control (MINDIM, see chapter Topology Optimization)

Pattern grouping

• 1,2,3-plane symmetry (TYP = 1,2,3)

• Uniform pattern grouping (TYP = 9)*

• Cyclic (TYP = 10)

• Cyclic with symmetry (TYP = 11)

• Linear and planar pattern grouping (TYP = 20,21)*

Pattern repetition, i.e. the repetition of a thickness pattern from one design space to the other

Zone based, where each zone (defined by element sets) will have same thickness

* Used for example for tailor rolled blanks, not supported by HyperMesh 2017.2


A concern in topology optimization is that the design concepts

developed are very often not manufacturable.

OptiStruct offers a number of different methods to account for

manufacturability when performing topology (and free-size)

optimization

Why are the manufacturing constraints so important?

• Make it much easier to interpret optimization results

• Use of standard profiles/manufacturing tools/processes

• Optimized structures are of no value if nobody can manufacture them

MANUFACTURABILITY – MANUFACTURING CONSTRAINTS

?

?

?


Input mindim – approximate

minimum diameter d in two

dimensions

For 2D and 3D design spaces

Controls the size of small

structural features

Easier interpretation of the

resulting layout

Controls “checkerboarding”

Higher computation cost

The smallest mindim available

in a run is dependent on

average mesh size

• Min member > 3x average mesh size

• Min member < 12x average mesh size

MINIMUM MEMBER SIZE CONTROLw/o mindim

d = 60

d = 90


Difficult to interpret resp. to

manufacture due to micro

structures

Results are mesh depended

“Checkerboarding”

periodic pattern of high and

low values of densities

arranged in a fashion of

checkerboards

WITHOUT MINIMUM MEMBER SIZE CONTROL


Input MAXDIM – definition of

maximum allowable

structural member size

For 2D and 3D design spaces

Eliminates material

concentrations

Resolves transitional structural

features

The smallest MAXDIM

available

in a run is dependent on

average mesh size

• Max member

> 2 x min member size

> 6 x average mesh size

MAXIMUM MEMBER SIZE CONTROL w/o max dim

w/ max dim


DESIGN INTERPRETATION OF FREE-SIZE OPTIMIZATION RESULTS

In HyperMesh run tcl file .HM.comp.tcl or .HM.ent.tcl in

HyperMesh to organize elements, which formed a topology or free-size

design space, into components resp. sets, based on their optimized

densities/thicknesses.

• By default there are 10 components/sets named e.g. “THICK 0.500- 1.050”,

“THICK 1.050- 1.600” and so on, containing all elements with a thickness

between (Tmin) and (Tmin + 10% x (Tmax-Tmin)) an so on.

• This helps you visualize results by turning components on and off resp. by

masking the according entity sets.

• PARAM, TCLTINIT, value (default=0.0) defines the lower threshold value

used for the tcl files.

• PARAM, TCLTSTEP, value (default=0.1) defines the step or interval value

used for the tcl files.

• Hint: These both cards are currently not supported in HyperMesh 2017.2,

please use bulk unsupported control cards.



For a free-size optimization OptiStruct automatically creates a .fsthick file.

• This file contains the element definitions for those elements that were part of the design space.

• The optimized thickness of these elements are provided as nodal thickness values (Ti). These

thicknesses overwrite the thickness specified on the PSHELL entry.

• Importing this file in HyperMesh, pay attention that import option ‘FE overwrite’ is activated to overwrite

the existing elements with new ones having grid point thicknesses defined.

• For a shaped-based representation for 2D shell elements activate the ‘2D Detailed Element

Representation’ on the Visualization Toolbar in HyperMesh. This can be combined with color ‘By

Thickness’ to color the elements according to their thickness values.



In most cases, variable thickness of a shell structure is achieved through step-wise change of

thickness.

Free-size results provide a different concept about how the zones of different thicknesses

should be designed. Detailed size optimization can then be performed to fine tune the final

design.

With the output request OUTPUT,FSTOSZ a sizing model is automatically generated. This is

applicable to both composite and non-composite (PSHELL) optimization.

• The name of this file _sizing.#.fem with # number of the last iteration.

• Please see online help for details and options.

• Main usage is the optimization of composite structures, please see chapter Optimization of Composite

Structures in the user’s guide of the manual.


Subcase independent responses

• Mass (MASS)

• Fraction of mass (MASSFRAC)

• only used in topology optimization

• fraction of the initial design space

• takes into account the non-design space

• Volume (VOLUME)

• Fraction of design volume (VOLFRAC)

• only used in topology optimization

• fraction of the initial design space

• only considers the design space volume

• Center of Gravity (COG)

• Moment of Inertia (INERTIA)

• Bead discreteness fraction (BEADFRAC)

• only used in topography optimization

OPTIMIZATION RESPONSES

1.0

0.25

initial

design spaceoptimized

design space

0.5

VOLFRAC = 0.25 / 1.0 = 0.25

MASSFRAC = (0.25+0.5) / (1.0+0.5) = 0.5

0.5

Example


Static subcase dependent responses

• Compliance of a static subcase (COMP)

• The compliance is the strain energy of the structure

• For a structure with applied forces (f) subcase the compliance (C) can be considered a reciprocal measure for the

stiffness (K): C = ½ u f = ½ f² / K with ½ f² = const.

For maximal stiffness (K) the compliance (C) has to be minimized!

• For a structure with applied displacements (u) subcase the compliance (C) can be considered a measure for the

stiffness (K): C = ½ u f = ½ u K u = ½ u² K with ½ u² = const.

For maximal stiffness (K) the compliance (C) has to be maximized!

• The compliance can be defined for the whole structure, for individual properties and materials, or for groups of

properties (components) and materials.

• Static compliance weighted across all subcases (WCOMP)

• The weighted compliance is a method used to consider multiple subcases (loadsteps, load cases) in a classical

topology optimization. The response is the weighted sum of the compliance of each individual subcase (loadstep, load

case).

OPTIMIZATION RESPONSES

f, u

𝐶 =1

𝐾


EXERCISE 4G: FREE-SIZE OPTIMIZATION OF A I-BEAM WEB

File Name and Location

…\STUDENT-EXERCISE\4g_I-Beam\I-Beam.fem

Exercise Goal

The purpose of this exercise is to set up a free-size

optimization and post-process the results.

Responses• volume fraction

• yLowerFlange

Objective: min volfrac

Constraints• yLowerFlange > -0.7

Design Variables• Free-size (thickness of shell elements in the design

space, i.e. web) with minimum member size 2.0,

minimum and maximum thickness of 0.5 resp. 6.0



Steps

1. Import the model in HyperMesh Desktop with OptiStruct user profile selected

2. Review the model and check loads, constraints and load step

3. Create the free-size design variable including the ‘Web_Design’ property with minimum and maximum

thickness of 0.5 resp. 6.0 and minimum member size of 6.0

4. Create a volume fraction response

5. Create a static displacement response for y-direction containing all nodes of component

‘LowerFlange’

6. Create a constraint for the static displacement with a lower bound of -0.7

7. Define the objective function to minimize volume fraction

8. Export the model, run the optimization with OptiStruct and review .out file

9. Review results of _hist.mvw file and .mvw file

10. Import .fsthick file in HyperMesh Desktop



Steps with Description

1. Import the model in HyperMesh

Desktop with OptiStruct user profile

selected

2. Review the model and check loads,

constraints and load step

3. Create the free-size design variable

including the ‘Web_Design’ property

with minimum and maximum

thickness of 0.5 resp. 6.0 and

minimum member size of 2.0

4. Create a volume fraction response

5. Create a static displacement

response for y-direction containing

all nodes of component

‘LowerFlange’

3 4 5




6. Create a constraint for the static displacement with a lower bound of -0.7

7. Define the objective function to minimize volume fraction

8. Export the model, run the optimization with OptiStruct and review .out file

6 7 8




9. Review results of _hist.mvw file and

.mvw file

9

9

9




10.Import .fsthick file in HyperMesh Desktop

• Pay attention that import option ‘FE overwrite’ is activated to overwrite the existing elements with new ones having grid point thicknesses defined on CQUAD4 card. These thicknesses overwrite the thickness specified on the PSHELL

entry.

• For a shaped-based representation for 2D shell elements activate the ‘2D Detailed Element Representation’ on the

Visualization Toolbar in HyperMesh. This can be combined with color ‘By Thickness’ to color the elements according to

their thickness values.

10 10

10

10

10


DEMO: FREE-SIZE OPTIMIZATION OF A I-BEAM WEB CONTINUED

File Name and Location

…\INSTRUCTOR-DEMO\4_Free-size\i-beam_freesize_min-vol.fem

Demo Goals

• Change optimization setup to

• Objective minimize compliance

• Constraint volume fraction < 0.2362 (this is the minimized volume

fraction value from previous exercise with different setup)

• Review the results and comment on the design differences

of both setups

• Introduce pattern grouping for new setup to archive

a symmetry design (in split of nonsymmetric loading)

• Review the symmetric design results and comment on the

differences

Responses• compliance

• volume fraction

Objective: min compliance

Constraints• volume fraction < 0.2362

Design Variables• Free-size (thickness of shell elements in the design

space, i.e. web) with minimum member size 2.0,

minimum and maximum thickness of 0.5 resp. 6.0



Comparison of MIN volume fraction (constrained disp.) and MIN compliance (constraint volfrac.)

design

• Volume fraction = 0.363 for both designs (intended)

• Compliance = 5.152E+4 resp. 4.273E+4 (makes sense)

• Maximum thickness in design space = 6.0 for both

• Minimal displacement lower flange = -0.70 resp. -0.92 (makes sense)

• As the load does not act in the middle of the beam, in both designs the load paths are developed mainly to

the left side of the beam (shorter way to support).

• The volume fraction design has additional reinforcements towards the middle of the lower flange, so that its

displacement gets lowered in order to meet the constraint value. This is not needed for the compliance

design as for this case only the overall stiffness is important.



Comparison of non symmetric and symmetric (both MIN compliance) design

• Volume fraction = 0.363 for both designs (intended)

• Compliance = 4.273E+4 resp. 4.553E+4 (makes sense, increase (less stiff) due to additional constraint)

• Maximum thickness in design space = 6.0 resp. 3.8 (makes sense, enforced load path to both left and

right support)

• Minimal displacement lower flange = -0.92 resp. -0.98 (makes sense, increase (less stiff) due to

additional constraint)


SUMMARY FREE-SIZE OPTIMIZATION

In free-size the thickness of each element is the design variable

Topology leads to solid-void designs, free-size to continuous thickness changes

DSIZE card is used to setup a Free-Size design variable

Manufacturing constraints

• Thickness (min, max and max gradient)

• Member size (min member)

• Pattern grouping (symmetry and constant thickness)

• Pattern repetition (master/slave)

• Grouping (zone based free-size)


1. Which of the following statement(s) is not true of free-size

optimization?

a) Each element within the design region can have a unique thickness.

b) Free-size design variables can’t have stress constraints applied as a

parameter.

c) Free-size optimization generates optimization variables per-element per-ply

for composites.

d) Free-size optimization is useful for PSOLID, PSHELL, and PCOMP(G)

elements.

2. Which manufacturing constraint(s) can’t be used in free-size

optimization?

a) Pattern Grouping

b) Pattern Repetition

c) Draw Direction

d) Extrusion

QUESTIONS & ANSWERS

Documents

OptiStruct Optimization v2017.2 · EXERCISE 4G: FREE-SIZE OPTIMIZATION OF A I-BEAM WEB Steps 1. Import the model in HyperMesh Desktop with OptiStruct user profile selected 2. Review