Simulation Advisor

Pro/ENGINEER®

Wildfire™ 2.0

Simulation Advisor

Help Topic Collection

Parametric Technology Corporation

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vii

Table of Contents Simulation Advisor.......................................................................................... 1

What Mechanica Does................................................................................... 1

General Workflow ........................................................................................ 1

Understanding the General Workflow.............................................................. 3

Using Single Parts and Assemblies ................................................................. 3

Dealing With Interfaces Between Parts in an Assembly...................................... 4

Welded Connections ..................................................................................... 4

Making Sure That Units Are Consistent ........................................................... 5

Defining Suitable Geometry........................................................................... 5

Creating Well-Defined Geometry for Mechanica................................................ 6

Safeguarding Against Unintentional Relationships............................................. 7

Refining Your Geometry................................................................................ 7

Techniques for Fully Specifying Your Geometry ................................................ 8

Preparing Your Model for Quicker Results ........................................................ 8

Using Family Tables and Layers ..................................................................... 9

Modeling Thin Features with Shells and Beams ...............................................10

Using Geometric Simplifications ....................................................................11

Setting Up the Simulation Model ...................................................................11

Assigning Material Properties........................................................................12

Specifying Other Physical Properties ..............................................................13

Using Physical Properties .............................................................................13

Applying Loads and Other Boundary Conditions...............................................15

Applying Loads ...........................................................................................15

Applying Constraints ...................................................................................16

Taking Advantage of Planar Symmetry ..........................................................17

Controlling Solution Quality ..........................................................................18

Simulations You Can Perform .......................................................................19

Defining the Correct Type of Analysis ............................................................20

Static Analysis............................................................................................21

Table of Contents

viii

Static Analysis Types...................................................................................22

Modal Analysis............................................................................................22

Mass of a Supported Part .............................................................................23

Constraints and Modal Analysis.....................................................................24

Dynamic Analysis........................................................................................24

Guidelines for Using Dynamic Analysis ...........................................................25

Steps for Dynamic Analyses .........................................................................26

Thermal Analysis ........................................................................................26

Fatigue Analysis..........................................................................................27

Running the Solution and Viewing the Results.................................................27

Allocating Computer Resources.....................................................................28

Reviewing Error Estimates and Results Quality................................................28

Assessing Results Quality.............................................................................29

Displaying and Interpreting Results ...............................................................30

Improving the Design ..................................................................................30

Determining Sensitivity to Design Changes.....................................................31

Optimizing Your Design ...............................................................................32

Reducing Optimization Resource Use .............................................................33

Why an Optimization Might Not Run ..............................................................33

1

Simulation Advisor

What Mechanica Does

Mechanica is a CAE (Computer Aided Engineering) product that allows you to

simulate the physical behavior of a part or assembly, to understand and improve the

mechanical performance of a design.

As a Mechanica user, you work directly on Pro/ENGINEER geometry within the

Pro/ENGINEER user interface. You can directly calculate stresses, deflections,

temperatures, frequencies, and other factors, to simulate the mechanical behavior of

your design as if it were in a test lab, or being used in the real world.

By combining the simulation functionality of Mechanica with the parametric solid

modeling capability of Pro/ENGINEER, you can create parametric "what-if?" studies

to investigate design changes, or to automatically optimize the design to meet

specific performance criteria and goals.

Mechanica is an integrated module of Pro/ENGINEER. However, you can also use it

independently with its own geometry engine or with other CAD systems.

Within Simulation Advisor and in the Mechanica online help, these terms are used in

the following ways:

• Mechanica refers to Structure, and to anything that is common to both Structure

and Thermal.

• Thermal refers to anything that is relevant to Thermal only.

General Workflow

To perform a successful Mechanica simulation, you need to carefully consider various

key decisions and steps you will take. Understanding these issues and making the

right choices can save significant time and effort later on.

As you read through this topic, take a look at the workflow illustration to understand

the order and flow of building your model and analyzing what you have built.

• Before You Begin — It is important to understand what Mechanica can do

to simulate real mechanical behavior, including the different types of analyses

that can be performed. You must also decide whether the part you are

interested in simulating or improving can be modeled in isolation, or whether

you need to take into account other parts in the assembly.

• Develop the Model — Mechanica works best with well-constructed

Pro/ENGINEER geometry, but even then you can make the simulation more

efficient by substituting a simpler representation. In any case, always make

sure that the geometry you use is as clean as possible, and all the parts are

defined using consistent units of measurement (Mechanica has a units

manager that will help you).

Building a Mechanica model involves defining loads, constraints, enforced

displacements, temperatures, and other environmental factors, which the part

Simulation Advisor - Help Topic Collection

2

or product has been designed to withstand. You can apply and distribute

these boundary conditions in different ways. Thus, you must be careful to use

a method that most closely reflects the real situation you are attempting to

simulate—the results depend on the method you choose. You also need to use

consistent units of measurement for boundary conditions in order to produce

meaningful results.

Another important aspect of simulation model building is choosing the right

material properties, especially if there are multiple materials in the model or if

the properties change with temperature. Mechanica provides tools to help you

manage such situations.

• Analyze the Results — Once the simulation model has been created,

Mechanica uses error checking tools and solution setup parameters to help

ensure that your problem solves successfully. Solution time depends

necessarily on computer memory—always make sure that you allocate an

appropriate memory amount. Mechanica also requires temporary disk space

to solve larger problems, and you can specify where that disk space is

located. Once your solution is complete, there are diagnostic files to help you

understand the quality of the solution so you can view and use the results of

your analysis with confidence.

• Define Model Changes — After studying the results of an analysis, you

may want to improve the design by taking advantage of Pro/ENGINEER's

parametric geometry. You can create design parameters and, using a

sensitivity design study, understand how changing one or more parameters

can affect the analysis results. You can even create named results measures

that track quantities of interest at defined locations in your model.

• Optimize the Model — If you want to find the best design to meet certain

design criteria (such as a maximum stress), while also satisfying a specific

design goal (such as a minimum weight), you can perform an optimization

design study.

Simulation Advisor

3

Understanding the General Workflow

Using Single Parts and Assemblies

Your model can consist of single Pro/ENGINEER parts or assemblies of parts.

Modeling a single part is typically simpler and faster than modeling an assembly.

Modeling assemblies requires additional consideration of how the parts interact or

interface with each other. Each case has certain limitations and advantages to

consider.

When modeling a single part, you must understand the component's boundary

conditions (for example, the loads and constraints associated with it). Understanding

how the particular component interacts with the entire assembly is not always

straightforward, and may require you to model the assembly as a whole.

You must consider many factors when modeling an assembly.

• Most importantly, you need to determine how the components should interact

with each other.

• You must decide if the components for the simulation will be modeled using

solids, shells, or a combination of both.

• If the components are "welded" or rigidly fixed together, you should model the

components using rigid connections.

• If you are interested in stresses due to contact, however, you should model the

assembly using contact regions.


4

When you model an assembly, you can assign different material properties to

different components, whereas you can assign only one material to a single part.

Dealing With Interfaces Between Parts in an Assembly

When you are working with solid parts in a simulation assembly, there are two

typical ways to handle how the part surfaces interact. Mechanica allows surfaces to

be merged (glued together) or to come in contact. When two parts have a shared

(mated) surface, the surfaces behave as though they are the same single merged

surface. If you want to model the contact between the parts, you can create contact

regions and then perform a contact analysis.

If you are creating a simulation model using midsurfaces or shells, there are several

options for handling the component interactions:

• In cases where the edges or surfaces of midsurface shell components meet and

can be considered glued, Mechanica automatically links the geometry to ensure

that the relative degrees of freedom and stiffness behave accordingly.

• Similarly, you can define rigid connections that will effectively replace a

component whose rigidity is far greater than the others, and whose only

contribution is to maintain set displacements between attached components.

Components so connected are free to move in any manner, but will maintain

their relative positions.

When you model sheet metal or welded assemblies, midsurfaces are typically used

for speed. Mechanica enables you to create welds between the different parts. The

welds are simulation features that enable you to complete the connections between

parts without changing the existing design geometry.

Advanced techniques for connecting assembly components may include using beams

or springs.

• Use beams to connect components to transfer loads and displacements. You

model beams in Pro/ENGINEER on datum curves.

• Use springs to model a flexible connection between parts or between a part and

ground. When defining a spring, you capture the lateral and torsional stiffnesses

in all directions.

Welded Connections

Welded connections enable you to connect shelled assemblies or combinations of

solids.

When using welded connections, be aware that stress in the weld may appear higher

than it actually is, due to singularities—areas of theoretically infinite stress—in the

vicinity of the weld. Be sure to use welded connections when you are predicting the

failure of the components in the structure, not in the weld itself. If you are interested

in the stress in the weld itself, model this area using solid elements to accurately

model the weld geometry.

Simulation Advisor

5

Welded connections you can use in Mechanica include:

• End welds — enable you to extend a surface to meet another part's midplane.

For example, an assembly of two parts that form a "T" produces a gap when

creating the midsurface model. The weld assumes the same material property

and thickness for the extension.

• Perimeter welds — allow you to create a weld between the midsurfaces of two

mated plates—for example, "doubler" plates. The weld consists of surfaces that

connect the two components perpendicular to the first surface selected. You

enter the material thickness for a perimeter weld—the material property value,

however, is taken from the first doubler surface. If you are interested in

modeling intermittent or skip welds, you can do this by using a combination of

end or perimeter welds and surface regions to break the surface into sections for

the weld.

• Spot welds — enable you to connect components simulating a spot weld, rivet,

or simple bolt by entering a value for the material property and weld/bolt

diameter.

Making Sure That Units Are Consistent

Mechanica stores all simulation data and performs analyses using a principal system

of units. Before creating your simulation model, you should define the system of

units you want to use. You can select predefined systems of units, such as foot-

pound-second (FPS) and meter-kilogram-second (MKS), or you can create custom

units.

The Principal System of Units you define can be easily converted in Pro/Engineer

from one system to another. When the software converts systems of units, it also

converts the Mechanica modeling entities, such as loads and material properties.

When working in assembly mode, Mechanica requires the units for all the parts to be

the same as those in the assembly. Therefore, you need to make sure that the parts

and assembly use consistent systems of units. However, if you try to enter

Mechanica with inconsistent units, you can convert units on-the-fly.

When you transfer models from integrated to independent mode, all data values

remain in the principal system of units. Once you transfer the model, it loses any

associated system of units. In independent mode, you do not specify units—the

software interprets unit values from those defined for material properties.

Defining Suitable Geometry

When building a part for use with Mechanica, the geometry you create should be

simple and flexible, so it can be easily optimized later on. You will also want to be

aware of any features that you will later want to modify or suppress.

If your part has more than one feature, develop the large features first, and build all

features as simply as possible. Add the smaller details as separate features. This

approach lets you change one portion of your model without affecting another.


6

As you build your geometry, be sure to

• make sure it is fully specified

• consider ways in which the geometry can be simplified and streamlined

• make use of the features available to help you model complex geometry, such as

shell and beam elements

Creating Well-Defined Geometry for Mechanica

When creating geometry for use with Mechanica, always consider how you want your

part's features to move, and try to avoid relationships that either cause unexpected

movement, or prevent necessary movement. As you build your part, keep in mind

the following points:

• Each feature depends on one or more of the earlier features (its parents) and is

considered a child of those features. In any parent–child relationship, if you move

the parent, the child moves with it. You can modify parent–child relationships,

the Edit>References>Reroute Feat or Replace Ref commands. You can also select

the feature in the Model Tree, right-mouse click, and select the Redefine

command.

• If you define relations between your part's dimensions and assign a design

parameter to the independent dimension, the dependent dimension changes

according to the relation you established.

• The part-building techniques you employ, such as sweeps, blends, mirroring, and

patterning, may cause conflict in later modeling tasks if they restrict the

movement of your part in any way.

• When using family tables to create alternative representations of the geometry,

be aware that simulation entities defined in an assembly are stored in the .asm

file, and do not transfer to the part.

• If you change the principal system of units, you can keep your model the same

size or scale it.

• If you use a Pro/NOTEBOOK layout when building your part or assembly,

Pro/ENGINEER defines relations between the aspects of the part or assembly you

declare to the layout and the associated aspect of the layout.

Because Pro/ENGINEER treats any part dimension you declare to a layout as

dependent, you cannot select the dimension as a Mechanica design parameter

without first undeclaring it.

As a general guideline, try to safeguard against unintentional relationships that may

constrain your model in unexpected ways.

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Safeguarding Against Unintentional Relationships

To avoid setting up unintentional relationships while building your part, perform the

following checks before using Mechanica:

• Use the Info>Parent/Child command to review parent–child relationships and

reassign dimensions when necessary.

• Select the feature in the Model Tree, right-mouse click, and select Redefine or

Edit References to cycle through each of the dimension references. When

necessary, redefine the dimensioning scheme or redesign the feature.

• Test your design by animating or reviewing the shape changes using the DSN

CONTROLS menu. If you see any problems or Pro/ENGINEER fails to regenerate

the part, redesign the part in a way that prevents conflicts.

• Add cosmetic features, rounds, and chamfers later in the model's history, and

avoid using these features as references for other features.

• Try to make external rounds suppressible, and leave internal rounds.

• Reference datum entities whenever possible.

Refining Your Geometry

In Mechanica Structure and Thermal, element generation is the first step in the

analysis of a part or assembly. Mechanica automatically creates elements as part of

the analysis. In most cases, Mechanica will automatically mesh a properly built

model without any user intervention. In certain situations, including models with

geometric inconsistencies or small features, you can take several steps to ensure

that the Pro/ENGINEER geometry will mesh:

• Make sure your geometry is fully specified, as this will help meshing succeed.

• Modify the model to clean up any questionable geometry. In cases where small

surfaces or edges with rounds create problems and cannot be redefined, use

Edit>Replace, Edit>Patch, or Edit>Thicken. Adding sections to blends and sweeps

and adding points in variable radius rounds may be helpful.

• If auto-meshing fails within an assembly, try to assess the individual parts in

Mechanica. This can help you determine why Mechanica cannot create elements—

whether an individual part is creating the problem or if the assembly constraints

are overconstraining the automesher. If any part cannot mesh, use the

suggested techniques to fully specify the geometry. Otherwise, merging touching

parts that are of the same material and modifying assembly constraints may

help.

• Suppressing all unnecessary features reduces the total number of elements

generated and may also simplify the geometry so it can mesh successfully. It is

especially important to suppress features that can create small or difficult to

mesh surfaces. Features likely to create these surfaces include rounds, unaligned

features, and variable section blends and sweeps. Model rounds as separate

features. If a round is part of the feature sketch instead of a separate feature,


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select the feature in the Model Tree, right-mouse click, and select Redefine to

change the sketch to a straight-edged part.

Techniques for Fully Specifying Your Geometry

Use the following techniques to ensure that the geometry is fully specified:

• Check geometry — When you select the Info>Geometry Checks command, the

software highlights any geometry with unclear Pro/ENGINEER design intent. All

geometry checks must be cleaned up for successful mesh generation. If

Geometry Check is inactive, the design intent of all features is clear.

• Check the surface slope — When you select the Analysis>Surface Analysis

command, the Surface Analysis dialog box opens. Select Slope from the Type

drop-down list, and select a surface and a reference plane. The software colors

the selected surface according to the slope. This allows easy determination of

whether or not edges are sharp and also ensures that geometry changes are

smooth in blends and sweeps.

• Highlight small edges — When you select the Analysis>Model Analysis

command, the Model Analysis dialog box opens. Select Short Edge from the Type

drop-down list, and select a part. The software highlights small edges on your

model. Small edges will typically not cause AutoGEM failures, but they will cause

a higher number of elements to be created.

• Check for interference in assembly models — When you select the

Analysis>Model Analysis command, the Model Analysis dialog box opens. Select

Global Interference from the Type drop-down list, and specify the other options

on the dialog box. The software highlights any areas of interference for

assemblies. Always avoid interference in assemblies.

• Modifying feature dimensions — aligns surfaces and edges that are already

nearly aligned.

Preparing Your Model for Quicker Results

Many features and components that are necessary for the manufacture and

aesthetics of a model are not needed for simulation. Examples of these features are

external rounds, chamfers, and cosmetic holes. Pro/ENGINEER offers a number of

techniques that allow you to remove and manage these features and components:

• Use feature suppression to temporarily remove features or components not

pertinent to the analysis.

• Use layers to organize which features will be suppressed

• Use simplified representations to increase efficiency by allowing you to exclude

components or features from a model.

• Maintain additional simplified models for analysis by using family tables or layers

or simplified representations.

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• Use deletion of features or parts only if your simulation model is based on an

offline copy of your enterprise's development model.

Idealizations offer another method of simplification by allowing you to use more

efficient element types and/or symmetry. Pro/ENGINEER and Mechanica provide

several methods of managing idealized geometry:

• Use shells for thin structures such as sheet metal parts. Shells are based on thin

solids or free surfaces.

• Use beams for structures composed of long narrow members (trusses or lattices).

Beams are created from edges, datum curves or points.

• Use mass elements to represent mass as a rigid body. Mass elements are created

from datum points.

• Use springs to connect to ground or to another part. Springs are based on datum

points.

• Investigate a 2D section of your geometry using plane strain, plane stress, and

axisymmetric model types. These 2D sections require free planar surfaces or

surfaces on solid geometry.

• Investigate a 3D section of your geometry where planar or cyclic symmetry

exists.

If there is ever any doubt whether a simplification or idealization is valid,

compare the results to the full 3D model, to other element types, or to other

model types.

Using Family Tables and Layers

To maintain a simplified model for analysis, it will probably be necessary for you to

maintain two or more versions—one for design and documentation, and one or more

for Mechanica. This can be easily achieved by using family tables or layers.

• Family Tables — Use family tables to quickly set a model for analysis by

retrieving an instance of the model that has the appropriate features suppressed

or resumed. You can maintain several models for different analysis purposes or

to segregate analysis-specific portions of the design model.

• Layers — Use layers to identify features you want to suppress or resume. By

giving layers easily recognized names, such as "Rounds" or "Analysis_1," you can

select the features to suppress or resume by selecting Show>Layer Tree on the

Model Tree.


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Modeling Thin Features with Shells and Beams

Shell and beam elements are extremely useful in engineering simulation. They let

you model complex three-dimensional geometry with less complex elements than

those used for solid modeling. You can apply beam and shell elements to greatly

reduce model size, disk usage, RAM usage during solution, and results storage

space, with no degradation in accuracy or solution quality.

Mechanica can use all types of Pro/ENGINEER geometry. This includes complex 3D

solids, meshed as solid elements, as well as free surfaces, datum curves, datum

points, and quilts.

It is important to remember that all element types can exist together comfortably in

one simulation model, at both the part and assembly level.

• Meshing Geometry with Elements — You mesh surfaces and quilts with shell

elements when a shell property is assigned to them. Shell pairs can have the

same effect on solid geometry, but they inherit their properties from the

geometry. Like shell properties, you assign laminate layup properties to a solid

surface, surface, or quilt.

You mesh curves and edges with beam elements when a beam section is

assigned. Typically, you use datum curves as beams between solids or shells.

Note: You can also create models made of a combination of shells and solids,

called mixed-mesh models.

The geometry of 2D model types consists of surfaces, curves, and points only.

This reduces model complexity to the extent that you can effectively ignore the

geometric aspects in this type of model.

• Manipulating Geometry Effectively — You must create some geometric

entities in Pro/ENGINEER, and you can create others in Mechanica.

o In Pro/ENGINEER, you create solids, surfaces, and quilts for use within

Mechanica. You can also create geometric simplifications, or substitutions,

which are particularly useful for beam elements replacing solid extrusions

or for surfaces replacing thin plates.

o In Mechanica, you can create datum curves and points, which makes it

easy to create beams on-the-fly and to create usable coordinate systems.

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Using Geometric Simplifications

You can perform geometry simplification substitutions in a number of ways.

• Use deletion of features or parts if your simulation model is based on an offline

copy of your enterprise's development model.

• Use feature or part suppression of simpler geometry to preserve the original

model intact, though in a modified form.

Simplified representations also preserve your original model. Your use of either

method should be based on your specific modeling requirements and comfort level.

Setting Up the Simulation Model

When you are creating a model, you are typically defining a representation of a

portion of your real world situation. Using the tools available in Mechanica, you are

approximating certain aspects of this situation.

The most essential aspects of any engineering simulation are assigning material

properties and simulating the real world mounting, loading, and heating of your

components. The main steps in setting up any simulation model are these:

• Assign material properties. A customizable library of typical engineering materials

is available in Mechanica. You can define materials as either structural or

thermal, with isotropic, orthotropic, or transversely isotropic characteristics. Use

material orientations to specify the alignment of the directionally-dependent

materials.

• Use constraints to simulate the real world mounting or support of your

component. Constraints specify the degrees of freedom available to specific areas

of your geometry. This simulates the mounting of your components.

• Use loads to specify external physical phenomena that affect your model. You can

directly apply forces, pressures, or temperatures to your geometry.

• For thermal effects, define thermal constraints, convection effects, and

temperature distributions. Use Thermal to specify these definitions, as well as

heat loads to define heat or energy fluxes.

• Use idealizations, such as shells, beams, springs and masses, as well as 2D

model types, to minimize computational effort.

A beam model can provide valuable insight into overall design issues, while also

providing accurate loading data for a more detailed solid model—for example,

support connections. Due to the point nature of their connection to your

geometry, which is merely an approximation, use beams only near areas where

you are not interested in specific, evaluated quantities.

Like beams, use springs only near areas where you are not interested in specific,

evaluated quantities. Springs are ideal, however, for determining gross behavior

in your model, when you need to know specific loading input information for a

more detailed solid or shell model.


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• Define contact regions when you anticipate contact between portions or parts of

your model. Mechanica will only look for interpenetration of a model during a

contact analysis, in the regions specified.

• Use the Check Model command to make sure that you have a valid simulation

model. This command checks your model for errors. It will validate the correct

application of properties and boundary conditions, and will perform some

automatic adjustments, such as shell normal alignment.

• Use the Model Tree to list many of the modeling entities associated with your

model and to select these entities easily for various tasks.

Assigning Material Properties

Material properties are required for all simulation models. You use them to define the

physical characteristics of the material to be used within the analyses.

Mechanica supports various types of material properties, such as isotropic,

transversely isotropic and orthotropic materials. The software provides a material

library with a standard set of isotropic material properties. You can also create your

own material sets and maintain your own enhanced material library.

When material properties are assigned to geometric entities like parts, surfaces, and

curves, they are automatically assigned to the resulting elements.

When you create or assign a material property to a model, the specific property

values are determined by the principal system of units (such as IPS and MKS)

previously set for the model. However, you can change units for individual properties

from the Material Properties dialog box. It is important to note that even though you

can define each material property in different sets of units, when you run an

analysis, the units for materials are always interpreted as the principal system of

units set for the model. You can create and display custom units from

Pro/ENGINEER's Units Manager.

When specifying material properties, it is important to understand and identify failure

criteria up front. Some other things to consider include:

• What failure criteria and safety factors will be applied?

• Over what temperature range are the properties valid?

• Are the properties dependent upon processing conditions?

• Has the stress limit (yield point) been reached?

• Are the properties dependent upon strain rate?

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Specifying Other Physical Properties

You can reduce computation time by defining a Mechanica model based only on free

Pro/ENGINEER surfaces, datum curves, and points. Clearly, such models are an

approximation, but require far less computer resources, a consideration for very

large simulations.

Using the following combination of elements, properties, and model subsets, you can

make your simulation model more flexible and effective.

Using Physical Properties

• Shell Elements and Properties — You can assign shell properties to the outer

surfaces of solids or to free Pro/ENGINEER surfaces. Use shells when the

thickness-to-link ratio is less than 1:10, but greater than 1:1000.

Shells provide quick and accurate results for sheet metal type models, with far

fewer elements than the corresponding solid model.

You can use solid and shell elements together in a mixed-mesh model. Be aware

that analysis of such models is slower than for pure shell models.

You can define shell properties to be as simple as a defined thickness. Mechanica

can infer the definition from your shell pairs definition, or you can specifically

define the property. When you specifically define a property, you can also define

laminate layups based on orthotropic or transversely isotropic materials.


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• Beam Elements and Orientations — You can use beams and shells together

often. An example would be a grain silo, where you are interested in the overall

behavior of the walls and supports when the silo is filled—but not in specific

stress results, where you would be interested in a support/wall connection.

Once you have assigned beam sections, you must use beam orientations to align

your standard or sketched sections correctly. Use beams when the ratio of the

largest cross-sectional dimension to its length is less than 1:10.

• Spring and Mass Elements — You can use these elements with all solid, shell,

and mixed-mesh models.

Use spring elements and an associated stiffness property to pass linear and

torsional loads from one portion of your model to another. You can define springs

either between two points or between a point and ground. The associated

property defines both linear and torsional stiffness. Use spring orientations to

align the directions inherent in the stiffness property.

Use mass elements to replace a solid component or feature, if you are not

interested in the specific internal behavior of the feature, but only the larger

effect of its mass and inertias on other portions of your model. A mass property

describes the component's physical characteristics and serves to lump them at

the point-applied mass element. Using mass elements instead of solid elements

greatly reduces the size and computational requirements of your model. Mass

elements are only important for modal analysis, such as buckling and dynamic

analysis types, and for body force loads, such as gravity and centrifugal loads.

Mass elements are typically used in modal dynamics problems to account for

lumped masses.

• Two-Dimensional Model Types — If your model's geometry is an extruded or

rotated cross-section, or flat or thin, you can use 2D model types. To create the

2D model, Mechanica uses a specialized subset of the three-dimensional

elements.

o Use 2D plane strain in Structure or Thermal when the out of plane strain (or

temperature gradient) is negligible. This is applicable to geometry with

extruded cross-sections. All geometry must lie in the XY plane, so you may

need to create a coordinate. Mechanica assigns 2D solid or 2D shell

elements to the geometry.

o Use 2D plane stress Structure or Thermal when the out of plane stress (or

temperature gradient) is negligible. This is applicable to thin, flat geometry.

All geometry must lie in the XY plane, so you may need to create a

coordinate. Furthermore, you must assign a thickness for this type of

model. Mechanica assigns 2D plate elements to the geometry.

o Use 2D axisymmetry in Structure or Thermal when all entities (geometry,

materials and boundary conditions) are symmetric about the same axis of

revolution. All geometry must lie in the positive X half of the XY plane for

2D axisymmetric models. Mechanica assigns 2D solid or 2D shell elements

to the geometry.

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The prescribed temperatures, convection conditions, constraints, heat and

structural loads you use must be able to idealize from three dimensions to

two dimensions, and you must apply them to the cross-section model.

Applying Loads and Other Boundary Conditions

Use loads to specify any external influence or environment in relation to your model.

Your objective is to create a load to simulate a particular real-world condition.

Mechanica provides a variety of loading options. Typically, you apply loads to

geometric entities. However, you can apply certain loads, such as gravity,

centrifugal, and temperature loads, to the entire model.

Here are some examples of different load applications:

• Bearing loads simulate the load of a shaft or bearing on its mounting hole.

• Gravity loads simulate the effect of gravity on an entire structure.

• Temperature loads simulate the effect of thermal variation in the environment.

You can import temperature distributions from Mechanica Thermal.

• You can also transfer load values calculated by Mechanism Design.

Use constraints to restrict or enforce movement of one or more geometric entities.

You can constrain an entity so that it can only move in certain directions, or you can

restrict its movement entirely.

Applying Loads

Consider these aspects of applying loads to your model.

• Preparing to Apply a Load — To prepare for applying a load, you often add

regions, or create datum curves or datum points. If it is not convenient to define

load vectors or distributions with respect to the default coordinate system, you

should also define the appropriate Cartesian, cylindrical, or spherical coordinate

systems. You can also choose from several different distribution and spatial

variation methods when applying certain loads.

• Load Distribution — Entity and pressure loads allow you to choose from several

distribution and spatial variation methods. The spatial variation option lets you

define a non-uniform load on a geometric entity. To do this, you can use either

interpolation points or you can define a function of coordinates. For example, if

you want to create linearly-varying hydraulic pressure or tapering load, you can

select interpolation points on a curve or surface.

Furthermore, use Total Load at Point (TLAP) surface distribution to apply a

load on a point in space that is distributed over an area. For example, if you

were investigating a door hinge, you could apply a load at the door's center of

gravity, which would represent the door's weight , and you could omit the

geometry of the door. Mechanica will resolve forces and moments acting on

the hinge.


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• Load Sets — Every load you add in Mechanica is a part of a load set. Load

sets provide a logical means of organizing your analyses. You can select one

or more load sets for a particular analysis. Because convergence is performed

for each load case, specifying more than one load set, rather than combining

loads into one set, will increase computation time. The computation time,

however, will be significantly less than if you run the analyses separately.

Using more than one load set also allows you to combine and scale load case

results in postprocessing.

You define the time or frequency dependency of a load set for dynamic

response analysis on the Analyses and Design Studies dialog box. You can

also transfer load values from a Mechanism Design analysis to represent

structural loads, joint reactions, or body loads.

Before running an analysis, use the Review Total Load menu to check whether

the load values at a particular location are what you expect.

Note: You do not need a load set if your model contains prescribed

displacements, or if you are only running modal analyses.

• Heat Loads — You can use heat loads to define local heat sources or sinks

in your model. You specify a heat load as the heat rate (Q), where a positive

value represents a heat source and a negative value is a heat sink. You can

make the heat load time-dependent.

Applying Constraints

In a Structure model, when you add constraints, you restrict the translation and/or

rotation movement of a part in reference to a Cartesian, cylindrical, or spherical

coordinate system.

When you define a constraint, you indicate whether the degrees of freedom along

each of three translation or rotational axes is free, fixed, or has prescribed non-zero

values. Use constraints to:

• Simulate real-world mounting — For example, you can constrain the radial

and axial direction at a bolt hole to allow rotation about the hole. This requires a

cylindrical coordinate system with the z-axis aligned with the hole's axis.

• Enforce displacements — For example, you can move on part a specified

distance into another in a contact analysis.

• Analyze a section of a cyclicly symmetric model — The section simulates the

behavior of the whole part, reducing meshing and analysis time. The geometry,

materials, and loads must be cyclicly symmetric. For example, if you want to

analyze a section of a fan blade subject to pressure and centrifugal loading. It is

worth noting that the symmetry surfaces need not be planar.

• Analyze a section of a reflective or planar symmetric model — Use this

type of constraint to impose symmetry conditions.

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• Eliminate rigid body motion — For all analyses, you should, at minimum,

specify constraints to prevent rigid body motion. An insufficiently constrained

model will give an error during run time. There are two exceptions to this rule:

o unconstrained modal analysis

o presence of spring-to-ground elements

You can also use rigid connections to tie degrees of freedom between portions of a

model that are disconnected.

There is no limit to the number of constraint sets you can have in a model. However,

you can only include one constraint set in an analysis. Therefore, make sure that

your constraint set contains all constraints needed for your analysis.

You can apply constraints to multiple geometric entities, such as points, curves,

surfaces, and regions. Be aware that point and curve constraints can introduce

singularities that produce infinite stresses or flux in both solid and shell models.

Excess or redundant constraints can also introduce singularities. For example, a bar

undergoing tensile loading will shrink slightly in diameter due to Poisson's effect. If

the bar is fixed in all degrees of freedom at one end, the constraint will restrict the

contraction and you will see unrealistically high stresses.

When constraining Theraml models, you define any combination of prescribed

temperatures and convection conditions.

Taking Advantage of Planar Symmetry

Symmetry allows you to analyze only a portion of the geometry. This has obvious

benefits for solution time and resource requirements, but can also help eliminate

rigid body motion. In addition to geometry, the properties, loads and constraints on

your model must be symmetric.

• Constraints — For planar or reflective symmetry, constrain all degrees of

freedom that permit deformation across the plane of symmetry. The following

example illustrates how a model can be approximated with a one-quarter section

using planar symmetry:

Full Model


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a b

Dx, Ry and Rz are fixed in a, and Dz, Rx and Ry are fixed in b. Note that solid

elements do not have rotational degrees of freedom, so it is not necessary to

constrain Rx, Ry and Rz in this case.

• Constraints in Thermal — Thermal models require symmetry conditions, but

these already exist by default. No constraint in Thermal implies that the

boundary is a perfect insulator (heat will not transfer across the plane of

symmetry).

• Loads — In symmetry models, loading must be symmetric also. In some cases,

you must reduce the load magnitudes by the same amount as the geometry. For

example, when analyzing a half section, you must halve some loads as well. In

general, you must scale total loads (point, curve, and surface total loads, total

loads at a point, bearing loads, and total heat loads). Loads that are a function of

area (pressure, heat load per unit, force per unit area) or volume (centrifugal,

gravity, global temperature, volumetric heat load) do not need to be scaled.

• Properties — In general, only isotropic material properties should be used with

symmetry unless the material orientations of an anisotropic material are, in fact,

symmetric.

• Modal Analysis — It is important to realize that symmetry in modal analyses

will only compute the symmetric modes. In general, this means that you should

not use symmetry for this analysis type, or for any other types that access modal

results (buckling, dynamic time, frequency, random, and shock).

Controlling Solution Quality

Mechanica uses powerful adaptive convergence algorithms, which automatically

refine the solution to give converged results of known quality.

Convergence is an extremely important issue in finite element analysis. It is the best

way to minimize mesh discretization errors (errors due to element size and shape).

In conventional finite element programs, you must refine a finite element mesh

(adding elements in areas where the errors are high). Then you must compare a

sequence of results until changes are minimal.

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Mechanica refines the model by automatically increasing the polynomial order of

selected element edges until errors are acceptably low, using one of the following

two methods:

• The default convergence method for most types of problems is SPA (Single-Pass

Adaptive), which is the most efficient choice for general problems. In the first of

two solutions, Mechanica calculates results and estimates errors, based on

continuity of stresses over element boundaries—these errors are then used to

predict the polynomial order required for a converged solution. The software then

performs a second run that produces the final converged results. During each

pass, Mechanica calculates single element stresses and superconvergent

stresses, and reports the stresses as a percentage error.

• An alternative method called MPA (Multi-Pass Adaptive) is available in cases

where SPA does not apply, or when you require more convergence feedback. In

this method, the software adapts the polynomial orders across element edges

until global or user-defined measures no longer change within a user-specified

tolerance or until the maximum polynomial level is reached.

In a variation of MPA, the solution converges using one or more measures,

either local to a part of the model or global over the whole model. Use this

method when you require a very accurate result at a particular location, but it

is vital that one or more global measures, such as strain energy, maximum

displacement, or energy norm, is included to capture the behavior of the part

as a whole.

Mechanica stores a convergence results summary in the .rpt file. Take care when

using point loads and constraints, sharp internal edges, and other singularities that

could cause theoretically infinite results. These entities can have detrimental effects

on the errors reported and the computer resources required.

Simulations You Can Perform

Mechanica can perform different types of analysis that simulate different types of

real world situations. Defining the correct type of analysis depends on the physical

nature of your modeling problem, and on the type of results you are seeking.

In Structure, you can obtain results for stress or strain, displacement, structural

frequencies, or mode shape in a model subjected to structural or thermal loads,

using one of the following analysis types:

Static Analysis Provides information on how a part or assembly

behaves when it is subject to a constant loading.

Modal Analysis Provides information on natural frequencies and relative

displacements in your model.


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Prestress Analyses Provides information on stresses for structures that are

subjected to a pre-load.

Buckling Analysis Provides information on how much of a load a part can

withstand without buckling.

Contact Analysis Provides information on stress and strain conditions in

an area of an assembly where two parts contact each

other, but are not rigidly connected.

Dynamic Analysis Provides information on loads in your model that will

vary over time or frequency, or base excitation

conditions.

Fatigue Analysis Provides information on your model's susceptibility to

fatigue damage when subjected to a time-varying load.

In Thermal, you can obtain temperature or heat flux results in a model subjected to

thermal loads, using these analysis types:

Steady-State Thermal Provides information on temperature and heat

flux distribution in your model.

Transient Thermal Provides information on changes in temperature

and heat flux, starting from an initial condition,

measured over a time interval.

Defining the Correct Type of Analysis

Define an analysis based on the kind of results you want to obtain.

• Use static analysis when you are looking for displacement, stress or strain

results, when the load does not change over time, or when the load frequency is

less than approximately one-third of the structure's lowest natural frequency).

Use the large deformation analysis option to account for geometry nonlinearity.

• Use modal analysis when you are looking for natural frequencies and mode

shapes of a constrained or unconstrained structure.

• Use prestress analysis if the structure is subject to a preload condition.

o A prestress static analysis simulates how a prestiffened or prestressed

structure affects your model's deformations, stresses, and strains.

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o A prestress modal analysis determines the natural frequency and mode

shapes of your prestressed model.

• Use buckling analysis to calculate the critical buckling load in situations where

your model may fail by buckling under a compressive load.

• Use contact analysis, instead of static, when there are one or more regions of

contact and the contact area changes as the structure deforms.

• When the applied load is not quasi-static, you must use dynamic analyses to

account for the inertia of the model. The type of dynamic analysis you run is

determined by the input forcing function.

• Use fatigue analysis to calculate the fatigue damage, fatigue life, safety factor,

and life confidence for a structure subjected to large numbers of repeated load

cycles. Before you run a fatigue analysis, you must run a static analysis for the

same model.

In Thermal, you can use these analysis types:

• Use steady state thermal analysis to calculate the equilibrium temperature and

heat flux distributions of a model subjected to a heat load, known temperature,

and/or convection boundary conditions.

• Use transient thermal analysis if you are interested in temperature and heat flux

time history from an initial condition.

Static Analysis

Perform a static analysis when the loads and other boundary conditions on your

model will not change over time, or the load frequency is less than approximately

one-third of the structure's lowest natural frequency.

Typically, static analyses produce interesting results for stress and displacement:

• If the stress in a part exceeds a certain value, the part may fail. Interpreting

stress results depends on the type of material and the nature of the loading. For

example, most engineering materials are ductile and thus will yield prior to

fracture. Von Mises stress is generally considered most accurate for predicting

ductile material failure. Maximum Shear (Tresca) theory may also be used for

ductile materials. Since brittle materials fail in fracture, you may want to use the

Modified Mohr theory in such cases. Composite structures generally have

different modes of failure, and thus require different theories for predicting them,

such as Tsai Wu, Maximum Stress, or Maximum Strain. If the loads are cyclical in

nature (that is, they are applied and removed many times, like an automobile

running over a rough road surface), the part may fail at a lower stress due to

fatigue.

• The displacement results produced by Mechanica indicate how the structure will

deform under the applied boundary conditions. Most designs need to be both stiff

enough to perform a job and strong enough not to break.

Mechanica offers different types of static analyses to address special needs.


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You can produce reliable stress and displacement results from a static analysis by

making sure you:

• Model the loads and boundary conditions as realistically as possible.

• Ensure that the geometry of the Pro/ENGINEER model accurately reflects the

geometry of the real part in areas where high stresses may occur.

For example, if you suppress a round in an area where Mechanica finds a high

stress, the reported stress will be much higher than in reality (since theory

predicts that an inside corner will produce an infinitely high stress or singularity).

Additionally, Mechanica will require more computer time and resources to

calculate these unrealistic stresses.

Mechanica provides p-adaptive technology to produce analysis results:

• Use the Single-Pass Adaptive (SPA) method wherever possible—this is the most

robust and efficient way to obtain reliable results in the least time.

• Use the Multi-Pass Adaptive (MPA) method to converge on the quantities you are

particularly interested in, using predefined or user-defined measures.

Static Analysis Types

Depending on the structure of your part, its loading, and whether you are measuring

stress between two parts, you will want to consider performing various types of

static analysis:

• If your part is thin and subject to compressive loading, it may buckle rather than

fracture. In this case, you can perform a buckling analysis.

• If the displacements are large with respect to the size of the part, you may need

to consider a nonlinear effect called large deformation. Most static analyses use

"small deformation theory," which tends to underestimate and in some cases

overestimate the deflections—for example, where a part is thin and flexible. In

this case, you can perform a large deformation analysis.

• If you are interested in the stresses that occur where two parts contact and

transfer loading, you will want to perform a contact analysis.

• When it is important to include the effects of prestress in an analysis—for

example, when a part has a thermal pre-load applied to it, you can perform a

prestress static analysis.

Modal Analysis

Perform a modal analysis when you want Mechanica to calculate the

• natural or resonant frequencies (eigenvalues) of the model

• relative displacements of the geometry when the model is vibrating at these

frequencies (modes or eigenvectors)

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Typically, engineers need to know these natural frequencies for situations where the

structure is exposed to excitation forces. In this case, the part may resonate and fail

if the excitation frequencies coincide with the natural frequency of the structure.

Even if the excitation frequency is offset from the natural frequency, it may still be

close enough to produce undesirable vibrations.

In modal analysis, it is important that you accurately model the mass and stiffness

distribution. Small features that are important in static stress analysis (to avoid

stress concentrations) are not as important in a modal analysis.

When deciding what factors to include in your analysis, you will want to consider the

following aspects of your model:

• The mass of the supported part

• The number of modes of vibration you want to calculate

• The frequency range you want to select

• The constraint state of your model

• Sometimes, it is also important to include the effects of prestress in an analysis—

if you are, for example, studying the vibration of a structure under a tensile load.

In this case, you can use prestress modal analysis.

• In most cases, symmetry conditions should be avoided in modal analysis since

only the symmetric modes will be captured.

Mechanica provides p-adaptive technology to produce analysis results:

• Use the Single-Pass Adaptive (SPA) method wherever possible—this is the most

robust and efficient way to obtain reliable results in the least time.

• Use the Multi-Pass Adaptive (MPA) method to converge on the quantities you are

particularly interested in, using predefined or user-defined measures.

Convergence plots of calculated entities are only available with MPA.

Mass of a Supported Part

When you are simulating the modal vibration of a part that is supporting another

part—for example, a bracket—you must take into account the mass of the supported

part. If the supported part is much stiffer than the bracket, you can assume it will

vibrate as a rigid body, and you need not consider its stiffness.

In such a case, you can substitute a mass element for the supported part. Mass

elements must be connected to the rest of the part with elements, usually beams.

Because these beams should behave in a rigid manner, you should increase the

stiffness or the dimension of the section and give them a zero material density.

Review the modal shape results to ensure that the beam elements are not bending.

In a similar way, you can substitute spring elements for flexible components.


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Constraints and Modal Analysis

In modal analysis, your model can be constrained or unconstrained.

• When it is constrained, its geometry features are fixed in place or in particular

directions.

• When it is unconstrained, you can consider it to be vibrating while floating in

space, without any tethering features.

If your model is very flexible compared to its mountings, use an unconstrained

analysis. If it is a stiffer structure—for example, attached to ground—use constraints

to simulate the fixtures and supports.

Constrained modal analyses are required for all base excitation dynamic problems.

You can use unconstrained modal analysis for dynamic time, frequency, and random

analyses, provided a load set is specified.

Dynamic Analysis

Perform a dynamic analysis when:

• the magnitude, direction, or position of the applied load varies over time, or

• the structure is experiencing base excitation or cyclic load.

As a result, the structural response to a dynamic load varies over time or frequency.

You can use the following types of dynamic analysis:

• Dynamic Frequency Analysis — Use a dynamic frequency analysis when the

loading is periodic, such as in rotating machinery, where you know the time

frequency variation of the loading.

• Dynamic Time Analysis — Use a dynamic time analysis when the load is

transient, and you know the time variation of the loading. An example would be

an impulse load on a structure.

• Dynamic Random Analysis — Use a dynamic random analysis when the

loading can be defined in a statistical sense. An example would be the loading

experienced by an automobile driving on an uneven road.

• Dynamic Shock Analysis — Use a dynamic shock analysis when loading is

transient and unknown. An example would be the maximum response due to

shock spectra on a water tank structure. You would usually use this type of

analysis for situations where either response spectra or design spectra are

available. The results of a dynamic analysis are often too conservative, but they

cost less to obtain than running a dynamic time analysis. Dynamic shock provides

a quick and approximate way to get a conservative solution.

Mechanica uses the modal method for obtaining dynamic response and uses output

from a predefined modal analysis. This means that the response is expressed as a

summation of the free-vibration shapes. You can use a previously executed modal

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analysis. The accuracy of a dynamic response analysis depends upon the number of

modes used, which you can specify when defining the modal analysis.

To define dynamic response analyses effectively, you need to be aware of certain

guidelines. Also, be aware that you typically perform dynamic time, frequency, and

random analyses using a specific series of steps.

Guidelines for Using Dynamic Analysis

Following are some guidelines that you should consider when running dynamic

analyses:

• To produce accurate results from a dynamic response analysis, you must know

the necessary modes to be included in your analysis. The number of modes you

include depends upon the nature of the input load and the analysis type:

o For all Dynamic Analysis types with base excitation, request enough modes

to ensure that the industry standard of 80% total effective mass

participation is reached.

o For Dynamic Frequency with frequency-dependent loading, ensure that the

highest natural frequency specified in the modal analysis is larger than the

highest frequency of the applied load. Furthermore, some recommend

including all modes from half the lowest operating frequency to twice the

maximum operating frequency.

o For Dynamic Random analysis, include enough modes to cover the entire

frequency range of the Power Spectral Density (PSD).

o For Dynamic Time analysis with a time-dependent load, you must compare

the results from several analyses to determine the dependence on the

number of modes. Use 80% total effective mass participation if you are

using base excitation as a starting point. Furthermore, the mode shapes

you request must be representative of the deflected shape of the part as if

the loads were static. For example, in a dynamic analysis of a flat plate

subject to bending loads, ensure that you have included out-of-plane mode

shapes.

• You need to consider several issues concerning base excitation when you define a

dynamic shock analysis. These include:

o Dynamic Shock uses response spectra as the forcing function. Mechanica

uses this response spectra as a weight factor to multiply each individual

modal shape and then add them together. Mechanica uses one of two

methods to add the modal shapes—the Absolute Sum method or SRSS

method.

If the frequencies of major contributing modes for your model are not very

close together, the SRSS provides a better approximation method. In this

case, the Absolute Sum method overestimates the maximum response.

o Be sure to include enough modes to capture the response spectra

frequency range.


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o You can define your response spectrum either as uniform or as a function.

If you define the response spectrum as uniform, Mechanica assumes that

the X, Y, and Z values you enter in the Direction of Base Excitation area

represent both the magnitude and direction of the response spectrum. For

example, if an acceleration spectrum response is normalized to G's, the

vector you enter should contain not only the direction of the spectrum

response, but also the magnitude of gravity.

If you define the response spectrum as a function instead, Mechanica uses

the product of the X, Y, Z information and the function you define to derive

magnitude and direction. Thus, you can include some or all of the

magnitude information in the function if you prefer.

Steps for Dynamic Analyses

You generally perform dynamic time, frequency, and random analyses in four steps:

1. Set up the constrained or unconstrained modal analysis.

2. Set up the first dynamic study. The purpose of this is to sweep over the time or

frequency domain and report measures. You must specify these measures prior

to running the analysis. You must also specify the modal range, and specify non-

zero damping.

3. Graph the results from the measures and note where the peaks occur. These are

the intervals where full results should be specified.

4. Set up the second dynamic study. This time, specify the intervals for full results

based on the plots from the first run. Full results will enable you to create fringes

and animations.

Thermal Analysis

Perform thermal analyses to calculate temperatures and heat fluxes for your model.

Mechanica offers two types of thermal analyses.

• Steady-State Thermal — A steady-state thermal analysis calculates the thermal

response of a model subjected to heat loads, prescribed temperatures, and/or

convection conditions. For example, you could define a steady-state thermal

analysis to analyze the cooling fins in a lawn mower engine.

• Transient Thermal — A transient thermal analysis calculates temperature

variation in the model over a period of time.

In a transient thermal analysis, you calculate temperatures and heat fluxes in

your model at different times in response to specified heat loads, prescribed

temperatures, and or convection conditions. You can use transient thermal

analysis to find out:

o how long your model takes to cool down or heat up

o how your model might respond to a time-dependent heat load or bulk

temperature

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o what thermal stresses develop as a result of temperature changes

You can transfer thermal analysis results to Structure as MEC/T temperature loads to

determine thermal strains due to thermal gradients.

The following methods are available for convergence of the two thermal analysis

types:

• Steady-state thermal convergence methods support single-pass adaptive, multi-

pass adaptive, and Quick Check.

• Transient thermal convergence methods support single-pass adaptive and Quick

Check.

You can base MPA convergence on local temperatures combined with local energy

norms, local and global energy norms, or measures.

Fatigue Analysis

Perform a fatigue analysis to see if your model is susceptible to fatigue damage

when subjected to a time-varying load. Loads that fluctuate or repeat at regular

intervals can induce stresses that result in failure by fatigue. Repeated stress can

produce cracks and ultimately fracture in a part, even when stress levels are well

within the elastic range of the material. Because materials that are under a repetitive

or fluctuating load will fail at a lower stress level than those under a constant load, a

model subject to such stresses is a candidate for fatigue analysis.

Before you attempt to run a fatigue analysis, make sure that:

• Your model is a 3D solid or shell model.

• Your model has only isotropic materials associated with it.

• You have defined fatigue properties for the materials.

• You have defined a static analysis, since the software uses the stress results from

the static analysis along with the load factors you specify to determine the

loading variation for one life cycle.

Running the Solution and Viewing the Results

After defining the simulation model and the required analysis type, you are ready to

run the analysis through the Structure/Thermal numerical engine. When running

your analysis, Mechanica gives you several options for allocating your computer

resources.

You can run an analysis interactively or in batch mode. When the analysis is running,

you can monitor its progress through summary files and diagnostic messages.

When the analysis is complete, you can view results and make design changes as

needed:

• You can see predefined measures and user-defined measures, which Mechanica

summarizes, in the summary or report file.


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• You can create color fringes, x–y plots, and animation of deformed meshes in the

Mechanica postprocessor.

• You can use convergence and error estimate results to assess the accuracy of

your finite element solution.

Allocating Computer Resources

When solving an analysis, Mechanica allocates RAM to the solver engine, and writes

files to a temporary directory as well as a results directory.

The following strategies may improve performance or overcome computer resource

limitations:

• Ensure that the output files write to local directories rather than over a network.

• Select the solver you want to use (the direct solver or the iterative solver).

• Reuse an existing mesh from an earlier analysis of the same model, even if the

analysis is not of the same type.

• Change the solver RAM settings.

• Use results of an existing analysis. For example, a dynamic analysis requires

input from a modal analysis of the same model.

Histories of computer resource usage information such as CPU elapsed time, total

elapsed time, memory and disk space usage are available interactively and through

engine summary files to help you allocate and manage your computer resources.

Reviewing Error Estimates and Results Quality

After you submit an analysis or design study, you can review the engine's progress,

while Mechanica is running in the background, by selecting the Info>Status and/or

Info>Diagnose commands from the Analyses and Design Studies dialog box.

For each analysis in the design study, Mechanica reports

• any warning and/or error messages

• key statistics on convergence and performance

• predefined and user-defined measures

• key statistics on convergence and performance

This information helps you determine whether the results are acceptable, if you can

improve them, or if you can run the analysis more efficiently by modifying computer

resources.

The summary report includes a table showing RMS stress error estimates from an

analysis and provides important statistics about the accuracy of the results. In

general, this error should be 5% or less.

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• For a static study, the table shows stress errors, which are estimated RMS values

for each load set, as well as its values as a percentage of maximum principal

stress.

• For modal analysis, the table shows stress errors for each mode as a percentage

of max modal stress.

Typically, high percentage values indicate that the analysis may not have converged.

Convergence failure can be caused by several factors:

• There may be singularity in your model.

• The maximum polynomial (P) order may have been reached while running a

multi-pass adaptive convergence analysis. In this case, Mechanica prints a

warning message in the report file.

After reviewing the report file, you can view the results of your analysis to further

gauge the quality of the results.

If an engine run ends in an error and you see a message:

• in the summary file directing you to use results to review a diagnostics display of

your model, you should display the highlighted entities associated with the error.

• in the summary or log file telling you that the model is insufficiently constrained

for the analysis, you may be able to locate your model's problem area by using

Mechanica entity ID numbers.

Assessing Results Quality

You can do several things to assess and improve your results.

• If you use multi-pass adaptive (MPA) convergence, you can plot a graph of

measure(s) versus the P pass to view how the convergence was reached. A

minimal difference between the last two passes (flattening of the curve) indicates

good convergence. Measures you might want to graph include are:

o Strain_energy or max_disp_mag for static analyses

o Modal_frequency for modal analyses

o Energy_norm, max_temperature, or max_flux_mag for thermal analyses

• For both single-pass adaptive (SPA) and multi-pass adaptive convergence, a P-

level fringe plot, available in independent mode only, showing a high P-level at a

few edges indicates possible singularity. Also, a plot of stress (von Mises,

principal stresses, and beam total stress) fringes allow you to pinpoint any stress

spikes in your model.

• If the analysis has not converged, you may

o Specify a higher P level in the analysis data form for MPA analysis.

o Use datum points, curves, and/or volume regions to force Mechanica to

create additional elements in a region with high P-order or stresses.


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o Remesh your model with a lower aspect ratio and/or allowable angles.

• You can remesh your model with a lower aspect ratio and/or allowable angles.

However, this will create many more elements in your model, slowing your

analysis.

Displaying and Interpreting Results

When an analysis has completed, you can study the convergence results to see

whether a satisfactory solution was achieved. The software reports the convergence

information in the .rpt file, a report file associated with the summary report that also

lists predefined measures, such as maximum stress and displacement, and user-

defined measures.

When evaluating your analysis results, you might take a minute to note whether

your results are close to what you expected. If not, first make sure that any failure

to converge or other error condition is not due to a simple mistake, such incorrectly

entering a load or material property. If the reported convergence errors are

acceptable, and the values of other significant measures are within an expected

range, you can proceed with viewing and interpreting results.

You can plot results using a variety of graphical methods, including point query,

fringes, contours (lines and surfaces), vectors, and graphs. You can use several

effects to better communicate the information, such as animation, shading, and color

smoothing. Additional methods for obtaining results information include:

• Cutting and capping planes — Use to slice through the model and view results

of a cross section. You can drag the cross section dynamically to gain improved

insight.

• Dynamic query — Use to dynamically display results values at any point in a

particular view.

After you generate results windows for display, be sure to take sufficient time to

understand and interpret the results from an engineering point of view. You can then

determine whether any design changes are necessary, and/or define additional

results measures and design parameters for sensitivity and optimization studies.

Improving the Design

Once you have performed a baseline analysis, the next step is to begin improving the

design. You can do this using sensitivity and optimization studies. However, before

you can set up an optimization or sensitivity study, you need to define design

parameters. Design parameters provide a means of defining what Mechanica will be

allowed to vary in the study.

The most common method of defining design parameters is to use driving

dimensions from the model geometry. These can be either feature or beam section

dimensions. You can use almost any dimension as a design parameter, except for the

number of instances in a feature pattern. When you have chosen the dimension,

Mechanica defines the upper and lower bounds. These bounds define the range

through which Mechanica can vary the dimensions in the study. You can use

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Pro/ENGINEER parameters as design variables, and all Pro/ENGINEER relations are

maintained.

Once you have defined a design parameter, it is important to see how its change

affects the topology of your model. You can do this by performing a shape animate

on the model. Shape animate allows you to choose a design parameter and sweep it

from its lower to upper bounds in a specified number of steps. This will point out any

parameter settings that render a model unable to regenerate, without the resource

expense of attempting an engine run. Alternatively, you can use Shape Review to

test a particular parameter value.

How do you ensure that your model will be robust enough to support large design

parameter changes? One way is to create a "flexible model". What this implies is that

a model can regenerate through a wide range of dimensional values. There are a few

ways to ensure this robustness.

• Keep individual features as simple as possible. Do not try to capture too much

detail in a feature sketch.

• Capture flexibility in your sketched sections. Once you have defined your sketch,

modify the dimensions using the drag dimension value functionality. In this

manner, you can quickly evaluate how flexible the individual sections are.

• Perform design studies early in the design process. The fewer features there are

in a model, the more flexible it will be. By performing these studies earlier in the

design process, not only will your models be more flexible, the time required to

run them will be dramatically reduced.

Determining Sensitivity to Design Changes

It is generally useful to understand the sensitivity of a model to design parameter

changes. Mechanica handles design parameter changes through sensitivity studies.

Sensitivity studies are an engineer's "what if" tool—for example, you may want to

understand the influence of a fillet radius on the stress in a part. There are two types

of these studies:

• Local Sensitivity — The information garnered by running a local sensitivity

study is the reaction of a measure to small changes in the design parameter. The

parameter is perturbed from its design study setting by a small increment. You

can think of the results produced as the slope of a graph, where the axes

represent the measure and the design parameter. The higher the value (slope),

the more sensitive the measure is to changes in the design parameter. A graph

with a slope of zero (the horizontal line) implies that changes in the parameter

have little or no effect on the design. If your model has a large number of

parameters, local sensitivity studies are useful in determining which variables to

include in an optimization run.

• Global Sensitivity — In a global sensitivity study, the design variable is actually

swept through its full range in a specified number of steps. The results from this

study produce a graph that depicts the value of selected measures versus the

value of the design parameter. If you select four steps for the global sensitivity


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study, the results will contain five data points, including the extra point from the

initial position.

It is possible to select multiple design parameters to vary concurrently in a

sensitivity study, but it is seldom useful since the individual contributions of the

multiple design parameters are not obvious.

Optimizing Your Design

Optimization studies are the final step in creating the optimal design. The most

common task for an optimization study is to reduce mass, while keeping the stresses

or displacements within certain bounds.

In an optimization study, you specify in the following order:

• a goal, such as minimizing mass

• limits, such as keeping displacements below a certain value

• the design parameters Mechanica will change during the study

It is important to note that when selecting the design parameters, you also specify

the initial value—as such, if you accept the default initial values, mass history and

other results often show a jump for the initial step. This is because the optimization

begins with all of the design parameters at their lower bound. It is often a good idea

to have the optimization begin with the design variables at their current value.

In addition to running an optimization to optimize a design that is currently within its

design limits, you can launch an optimization from territory where the limits are

violated. An example of this would be a design in which you know that the stresses

are too high, but you are not sure of the best combination of part thickness and fillet

radius to make the design work.

You can run optimizations without specifying a goal. When you run a goal-free study,

the optimization will stop as soon as the limits are satisfied. The converse is also

true—you can run an optimization without limits, with only a goal.

Although optimization runs can take considerable amounts of time and resources,

and cannot run under some circumstances, there are ways to reduce or avoid these

conditions.

Once the optimization run is complete, you can review the topology of the model,

tracking the steps Mechanica takes through the optimization iterations. To do this,

use the Optimize Hist command under the Dsgn Controls menu. At this point, you

can also accept the final, optimized, design of the Mechanica simulation model. When

you accept the optimized design, Mechanica sets the values of all the design

parameters to their optimal configuration, which also changes these values in the

underlying Pro/ENGINEER model.

You can also use optimization to correlate to experimental data. In some situations,

you may not know specific properties, but you have empirical data. You can run an

optimization study to determine what the properties should be in order to achieve

the measured deflection, stress, temperature, and so forth.

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Reducing Optimization Resource Use

Optimization runs can take considerable amounts of time and resources, but there

are a few ways to reduce these factors:

• The first is to limit the number of optimization iterations when defining the study

on the dialog box.

• You can also try starting the optimization with the values of the design

parameters as close to optimal as can readily be determined.

• Loosen the Optimization Convergence.

Why an Optimization Might Not Run

An optimization can fail or will not run if Mechanica cannot

• find a combination of values for the design parameters in which the design limits

are satisfied

• change the parameters in a way that improves upon the goal

• In this instance, the optimization may run if a different starting point is selected

for the design parameters. This is because it is possible to find local optimums,

which can be different from the global, optimal design. If time permits, it is also

a good idea to run an optimization from a few different starting points to ensure

that the results returned are for a global optimum.

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