Stress life durability analysis

Estimated time to complete: 20–30 minutes

You will learn how to:

  • Create a Durability solution process with one static event and one transient event.

  • Solve and display fatigue life results.

  • Modify the transient event, solve, and display fatigue failure index results.
1: Event-based durability workflow

Step

Summary

1.

Assign fatigue material properties.

The material assigned to the part of the model where you perform the durability analysis must have the appropriate durability and fatigue material properties.

If you do not assign fatigue material properties, you can override the referenced material when you create durability events.

2.

Create and solve the structural analysis solution.

The type of structural analysis solution that you create determines the type of events you can create during the durability analysis.

Because the results file needs to contain either stress or strain result sets for performing the durability analysis, you must specify either stress or strain results in the output requests.

3.

Create the Durability solution process.

A Durability solution process can contain multiple static and transient events.

4.

Create durability objects.

Durability objects define durability solver settings that this software uses when solving durability events. Because you can reuse durability objects in different durability events, you can define durability solver settings once, and use them in multiple events.

5.

Create events.

Depending on the structural analysis solution, you can create static events or transient events.

For each event, you specify the following:

  • The output results that you want for strength, fatigue life, and fatigue safety.

  • Methods for calculating strength safety, fatigue life, and fatigue safety.

  • Methods for calculating stress damage propagation.

In the case of a transient event, you also specify the part of your loading history on which you perform the durability analysis.

You can select only a portion of your model to perform the durability analysis.

You can override the material specified in the file for either of the following reasons:

  • If all the appropriate durability and fatigue material properties are not set in the FEM file.

  • If you need to perform what-if scenarios.

6.

Solve for durability analysis results.

You can solve each event and the durability solution process separately, or you can solve the durability solution process and all events at the same time.

The durability solution process results cumulate damage and life results from all the active static and transient events. Worst case safety factors are taken from all active events.

7.

Post process durability analysis results.

You can view the results you requested for each event separately and cumulatively under the durability result node in the Post Processing Navigator.

2: Open the Simulation file

  1. On your desktop or the appropriate network drive, create a folder named durability_muffler.

  2. Click the link below:

    Download the part

  3. Extract the part files to your durability_muffler folder.

  4. Start Simcenter 3D.

File

  • Open

  • Look in

    durability_muffler

  • Files of type

    Simulation Files (*.sim)

  • File name

    muffler_sim.sim

  • OK

     

The following nodes are created in the Simulation Navigator.

 muffler_sim.sim

 muffler_fem.fem

 CSYS

 Selection Recipes

 Groups

 Fields

 Modeling Objects

 Regions

 Simulation Object Container

 Constraint Container

 Load Container

 Solver Sets

 Static Solution

 Transient Solution

 Temperatures

 Simulation Objects

 Constraints

 Subcase - Direct Transient 1

 Loads

 Results

 Structural

3: Reset dialog box memory

The options you select in dialog boxes are preserved for the next time you open the same dialog box within a given session. Restore the default settings to ensure that the dialog boxes are in the expected initial state for each step of the activity.

File

Preferences→User Interface

  • Options

  •   Reset Dialog Memory

  • OK

     
4: Examine material properties

The durability solver can compute results only when the appropriate material properties are set.

Manage Materials (Home tab→Properties group)

  • Materials

    Aluminum_2014

  • Inspect

    The Isotropic Material dialog box opens.

  • Strength

  • Examine the Yield Strength and Ultimate Tensile Strength values.

    These properties are used to evaluate the static strength safety factors and margins of safety. You can display the property values as functions of temperature. The yield strength of Aluminum_2014 at 20°C is 394 MPa.

  • Durability

  • Examine the values in the Fatigue Strength, Fatigue Ductility, and Cyclic Parameters groups.

    • The Fatigue Strength Coefficient and Fatigue Strength Exponent values are used to evaluate high-cycle and low-cycle fatigue results.

    • The Fatigue Ductility Coefficient and Fatigue Ductility Exponent values are used to evaluate low-cycle fatigue results.

    • The Cyclic Yield Strength, Cyclic Strength Coefficient, and Cyclic Strain Hardening Exponent values are used in constitutive relationships to convert between stress cycles and strain cycles while computing fatigue damage and fatigue life results.

  • Close

     both dialog boxes
5: Examine the results of the structural solution

In the Simulation Navigator, observe the following nodes:

  1. Static Solution — This is the node for the Simcenter Nastran SOL 101 Linear Statics – Global Constraints solution that will be referenced by the static durability event.

  2. Transient Solution — This is the node for the Simcenter Nastran SOL 109 Direct Transient Response solution that will be referenced by the transient durability event.

 Static Solution 

 Transient Solution 

 Temperatures

 Simulation Objects

 Constraints

 Subcase - Direct Transient 1

 Loads

 Results

 Structural

  Simulation Navigator

  • Static Solution

  • Results (under Static Solution)

 Post Processing Navigator

  • Examine the stress results of the Static Solution.

  • Structural→Stress - Elemental

  • Von-Mises

  • Note:

    Notice that the maximum stress of Static Solution (~528 MPa) is higher than the material yield strength. Therefore, the Smith–Watson–Topper fatigue life criterion will be used for the static durability event that references results from this solution. This criterion needs both stress and strain results and is typically used for low cycle fatigue analysis.

  • Load the Transient Solution results.

  •  Structural (under Transient Solution)

  • Animate the stress results of the Transient Solution over all iterations.

    Note:

    Notice that when the stress results of Transient Solution are animated over all iterations the maximum value (~280 MPa) is lower than the material yield strength. Therefore, the stress life criterion will be used for the transient durability event that references results from this Response Dynamics solution process. This criterion is typically used for high cycle fatigue analysis.

Return to Home (Context group)

6: Create a Durability simulation
Note:

If necessary, right-click in the Ribbon bar and turn the Durability tab on.

Durability

  • New Durability Simulation (Durability group)

  • OK

     Durability dialog box

The Durability 1 node is created in the Simulation Navigator.

 Durability 1

Note:

The Durability 1 solution process node will contain both a static event and a transient event that you will create.

7: Create a strength durability object

The strength durability object defines requests for stress criterion, stress type, and strength output. Both the static and the transient event will use the same strength durability object.

Manage Durability Objects (Durability group)

  •   Edit

  •   Yield Stress (under Stress Criterion group)

  • Verify that Stress Type is set to Von Mises.

  •   Margin of Safety

    Because you want to maximize strength output, you request both the strength safety factor and margin of safety results.

  • Factor of Safety

    1.2

  • OK

     Strength dialog box

Note:

Do not leave the Durability Object Manager dialog box.

8: Create fatigue durability objects

The fatigue durability object defines fatigue life settings, fatigue life output requests, fatigue safety factor settings, and fatigue safety factor output requests. The static event will use the fatigue durability object called fatigue static and the transient event will use the object called fatigue transient.

  • Object Type

    Fatigue

  •   Edit

  • Name

    Fatigue static

  • Number Of Occurrences

    1e5

  • Verify that Fatigue Life Criterion is set to Smith Watson Topper.

    The Smith-Watson-Topper fatigue life criterion uses a strain life approach and solicits both the stress and strain results from the Static Solution solution.

  •  

  •   Event Damage

  •  

  •   Failure Index

  • OK

     Fatigue dialog box

  •   Clone

  •   Edit

  • Name

    Fatigue transient

  • Number of Occurrences

    1e9

  • Fatigue Life Criterion

    Stress Life

    The stress life criterion is selected because the Transient Solution stress results are in the linear elastic range and because a high number of occurrences (109) is specified.

  • OK

     Fatigue dialog box

Note:

Do not leave the Durability Object Manager dialog box.

9: Create axis search durability objects

The axis search durability object defines how the durability solver determines:

  1. The critical loading path of the structure.

  2. The most likely direction along which cracks could form and propagate.

Because fatigue is a surface phenomenon, damage values are calculated only on the free faces of elements and the plane stress assumption is used.

The static event will use the axis search durability object called axis search static and the transient event will use the axis search durability object called axis search transient.

  • Object Type

    Axis Search

  •   Edit

  • Name

    Axis Search static

  • Element Face Stress Axis

    Bi-Axial

    When you set the Element Face Stress Axis to Bi-Axial, the forces acting perpendicularly to the crack propagation direction contribute to the calculation of crack growth.

  • OK

     Axis Search dialog box

  •   Clone

  •   Edit

  • Name

    Axis Search transient

  •   Maximum Damage (under Stress Axis Direction Search Method)

    When you select the maximum damage method, the durability solver calculates damage in every direction on element free faces to determine the primary loading direction, its damage, and life values.

  • Search Resolution

    30

  • OK

     both dialog boxes
10: Create a static durability event

Static Event (Durability group)

The existing static solution Static Solution is detected and selected in the Static Solution List.

  • Strength

  • Strength 1

  • Fatigue

  • Fatigue static

  • Axis Search

  • Axis Search static

  • OK

     Static Durability Event dialog box

As the solve options durability object was not defined, the Solve Options 1 durability object is used. The software creates one durability object of each type with default values when you first open the Durability Object Manager dialog box.

The Static Event 1 node is created in the Simulation Navigator.

 Durability 1

 Static Event 1

11: Create a new excitation

Simulation Navigator

  • Static Event 1

  • New Excitation

  • Pattern Name

    ForceExt+Z

  • Subcase 1

  • Full Unit Cycle

  • OK

     Load Pattern dialog box

The ForceExt+Z node is created in the Simulation Navigator under the Static Event 1 node.

 Durability 1

 Static Event 1

 ForceExt+Z

12: Create a transient durability event

Transient Event (Durability group)

The existing transient solution Transient Solution–SOL 109 is detected and selected in the Transient Solution List.

The existing subcase Subcase – Direct Transient 1 is detected and selected in the Transient Solution Subcase List.

  • Strength

  • Strength 1

  • Fatigue

  • Fatigue transient

  • Axis Search

  • Axis Search transient

  • Data Control

  • Verify that the Decimation Order is set to 1.

    The decimation order value controls the number of stress/strain data points that are used in the durability evaluation. The default decimation order of 1 means that every data point is used; a value of 2 means that every other point is used.

  • Solve Options

  •   Create Solve Options

  • Name

    Solve Options transient

  •   Select (under Use Elements)

  • Method (Top Border bar)

    Elements by Group

  • any element that is on the face shown.

    1721 elements are selected. These elements are in the Joint group.

  • OK

     both dialog boxes

The Transient Event 1 node is created in the Simulation Navigator.

 Durability 1

 Static Event 1

 ForceExt+Z

 Transient Event 1

13: Solve the Durability simulation

Solve Durability Simulation (Durability group)

  • OK

     Durability Solver dialog box

  • Wait for the job to finish. This could take a couple of minutes.

     the Information window

The following three result nodes are created in the Simulation Navigator.

 Durability 1

 Static Event 1

 ForceExt+Z

 Static Event 1

 Transient Event 1

 Transient Event 1

 Durability 1

  • The Static Event 1 result node contains the set of results you specified in the Static Durability Event dialog box.

  • The Transient Event 1 result node contains the set of results you specified in the Transient Durability Event dialog box.

  • The Durability 1 result node contains the set of results that cumulates the results from the two event result sets.

In general, when a Durability solution process contains multiple active events, the durability results are calculated as described in the following table.

Cumulative durability results set

Values calculated

Strength Safety Factor

Minimum value of all events

Margin of Safety

Minimum value of all events

Fatigue Safety Factor

Minimum value of all events

Fatigue Failure Index

Maximum value of all events

Fatigue Damage

Algebraic sum of damage due to each event

Fatigue Life

Reciprocal of Fatigue Damage value
14: Display fatigue life results

  Simulation Navigator

  •   Static Event 1

 Post Processing Navigator

  • Static Event 1

    The following result nodes are created in the Post Processing Navigator.

     Durability 1

     Durability 1

     Static Event 1

     Fatigue Life – Element-Nodal

     Fatigue Damage – Element-Nodal

     Fatigue Safety Factor – Element-Nodal

     Strength Safety Factor – Element-Nodal

     Margin of Safety (%) – Element-Nodal

     Fatigue Failure Index – Element-Nodal

     Transient Event 1

    You can use the following table as a guide to evaluate the durability results.

    Durability results set

    Failure when value is

    Strength Safety Factor

    < 1.0

    Margin of Safety

    Negative

    Fatigue Safety Factor

    < 1.0

    Fatigue Failure Index

    > 1.0

    Fatigue Damage

    > 1.0

    Fatigue Life

    < 1.0

  • Fatigue Life – Element-Nodal

  • You will invert the color bar scheme to get a more meaningful representation of fatigue life results.

    Results

    Edit Post View (Post View group)

  • Legend

  •  Invert Spectrum (Color Control group)

  • OK

     Post View dialog box

  •   Post View 2

  •  New Template

  •   Transient Event 1 under Durability 1 node

    The Transient Event 1 results are loaded in the Post Processing Navigator.

  • Transient Event 1

  •   Fatigue Life – Element-Nodal

  •   Post_View_2 under Templates node

  • The durability results of the Transient Event 1 result sets are only computed on the elements that are in the Joint group.

  •   Durability 1

    The Durability 1 results are loaded in the Post Processing Navigator.

     Durability 1

  • Durability 1

  •   Fatigue Life – Element-Nodal

  •   Post_View_2 under Templates node

Notice that for each result set, some regions of the muffler have fatigue life results that are less than one. Therefore, further analysis and design changes are required.

15: Display fatigue failure index results

  Post Processing Navigator

  • Fatigue Failure Index – Element-Nodal under Transient Event 1 node

Fatigue failure index values should be less than one for the model to withstand the stress history from the transient solution. In this case, some elements have fatigue failure index values that are greater than one. Therefore, further analysis and design changes are required.

Design Intent:

Examine other durability result sets.

Modify your model to withstand the provided stress histories.

16: Save and close

Save and close your files when you are finished.

File

  • Save→Save All

File

  • Close→All Parts