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Fretting Fatigue

Tutorial

Version 6

Fracture Analysis Consultants, Inc

www.fracanalysis.com

Revised: August 2012

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Table of Contents:

1.0 Introduction .......................................................................................................................... 3

2.0 Tutorial: Fretting Fatigue Analysis ..................................................................................... 3 2.1 Step 1: Saving the ANSYS Model and Results .............................................................. 3

Step 1.1: Select ANSYS Results File ..................................................................................... 4 Step 1.2: Read ANSYS Results for First Load Case .............................................................. 4 Step 1.3: List ANSYS Results ................................................................................................ 4

Step 1.4: Save ANSYS Results Listings ................................................................................. 5 Step 1.5: Archive ANSYS .cdb File ....................................................................................... 5

2.2 Step 2: Importing the ANSYS Data into FRANC3D ...................................................... 5

Step 2.1: Read ANSYS .cdb into FRANC3D Fretting Module .............................................. 5 Step 2.2: Filter FE Data .......................................................................................................... 7

2.3 Step 3: Computing Fretting Nucleation Cycles............................................................... 8

Step 3.1: Select Fretting Crack Nucleation ............................................................................ 8 Step 3.2: Set Nucleation Model Parameters ........................................................................ 10

Step 3.3: View Nucleation Data........................................................................................... 10 Step 3.4: Save Nucleation Data ........................................................................................... 12

2.4 Step 4: Modeling Discrete Cracking ............................................................................. 12

3.0 Command-Line FRANC3D Playback ............................................................................... 13 3.1 FRANC3D GUI Playback .............................................................................................. 13

3.2 FRANC3D Command-Line Playback ............................................................................ 14 4.0 Discrete Crack Growth from the Edge of Contact ............................................................. 15

4.1 Local and Global .cdb files ............................................................................................ 15

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1.0 Introduction

This manual contains a fretting fatigue tutorial example for FRANC3D Version 6 and ANSYS.

A tutorial example using a dovetail fretting test rig is provided here. A user is expected to know

how FRANC3D and ANSYS operate before following this tutorial.

2.0 Tutorial: Fretting Fatigue Analysis

This tutorial describes the fretting fatigue analysis procedure using the menu and dialog items

described in Section 11 of the FRANC3D Reference Guide. An ANSYS finite element model of

a fretting test rig is used here. The GUI commands that completed are saved to a session file that

can be played back from the GUI or from a command-line version of the FRANC3D software.

This tutorial assumes that the user is capable of creating and analyzing an ANSYS model that

includes contact. This tutorial uses the ANSYS Classic ADPL interface.

2.1 Step 1: Saving the ANSYS Model and Results

We start from a completed ANSYS analysis of a fretting test rig. The model is shown in Figure

2.1.1. The analysis solution contains results for two load cases corresponding to a minimum and

a maximum loading. The specimen (darker blue colored piece in Figure 2.1.1) is pulled

downward and then partially unloaded; the load ratio (R) is 0.1. There is contact between the

specimen and the loading platens; the coefficient of friction is 0.3. We assume that the model

has been analyzed and that the results exist for the next step(s).

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Figure 2.1.1 ANSYS model of fretting rig.

Step 1.1: Select ANSYS Results File

Assuming that the model is open in ANSYS, make sure the results exist. If the results file

needs to be read, select General Postproc from the ANSYS main menu and then select Data

& File Opts. This will prompt you to select the appropriate .rst file. Select the file name or

type it into the field and select OK. You can use the Results Summary menu item to verify

that there are two load case results.

Step 1.2: Read ANSYS Results for First Load Case

Use the Read Results menu item to select the results for the first load case.

Step 1.3: List ANSYS Results

Use the List Results menu item to list results. Alternatively, type the command in the

ANSYS Command Prompt field at the top of the ANSYS interface. To increase the number

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of digits that are output for the results, use the following in the Command Prompt field:

/format,,g,20,12

List the results for stress, strain, displacement and contact using the following commands:

prnsol,s

prnsol,epto

prnsol,dof

prnsol,cont

Step 1.4: Save ANSYS Results Listings

For each of the result listings, save the data using the File and Save As menu item for each

listing. Save the stress to a file with a .str extension. Save the other results using the same

file name but with extensions of .stn, .dsp and .con for strain, displacement and contact,

respectively.

Repeat steps 1.2 - 1.4 for the second load case. Differentiate the file names for the results

based on the load case number.

Step 1.5: Archive ANSYS .cdb File

Save the ANSYS model as an ASCII .cdb file. Use the Preprocessor menu and select the

Archive Model menu item and then select Write. A dialog will be displayed that allows the

user to specify the cdb and iges file names. Only the cdb file is needed for FRANC3D. The

iges field can be disabled by choosing to write only the DB All finite element information.

2.2 Step 2: Importing the ANSYS Data into FRANC3D

The file listing is shown in Figure 2.2.1. Note that the ANSYS db, rst and .s01/02 files are not

needed for the FRANC3D fretting analysis.

Step 2.1: Read ANSYS .cdb into FRANC3D Fretting Module

Starting from the main FRANC3D menu, select the Fretting menu and then select Read model

and results, Figure 2.2.2. The dialog shown in Figure 2.2.3 is then displayed. Select the

fretting_rig.cdb file and select OK. The next dialog prompts us to select the results, Figure 2.2.4.

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We check the P&Qmax and P&Qmin boxes and select the files for the ANSYS stress listings

(the .str files). We do not have to select the .stn, .dsp and .con files. As long as those files exist

in the same folder with the same file name prefix, they will be read automatically.

Figure 2.2.1 File listing of fretting rig model ANSYS files.

Figure 2.2.2 FRANC3D window with Fretting menu item (Read model and results) selected.

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Figure 2.2.3 Open File dialog for selecting the fretting model file.

Figure 2.2.4 Fretting results files selection dialog.

Step 2.2: Filter FE Data

FRANC3D reads the .cdb file as well as the results files and then displays the dialog shown in

Figure 2.2.5, which allows FRANC3D to filter out some of the data. For this model, we let

FRANC3D automatically determine the contact surfaces and retain all the material regions, so

select Finish.

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Figure 2.2.5 FE model data filter dialog.

2.3 Step 3: Computing Fretting Nucleation Cycles

The fretting rig model is displayed in the FRANC3D window, Figure 2.3.1. The two material

regions are drawn in different colors. The next thing that we will do is to perform a fretting

nucleation analysis.

Step 3.1: Select Fretting Crack Nucleation

From the Fretting menu, select Fretting crack nucleation. The dialog shown in Figure 2.3.2 is

displayed, which allows us to choose the fretting nucleation model. We will use the first model,

Equivalent Stress, so just select Next.

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Figure 2.3.1 FRANC3D main window showing the fretting rig model.

Figure 2.3.2 Fretting dialog to select fretting nucleation model.

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Step 3.2: Set Nucleation Model Parameters

The next dialog is shown in Figure 2.3.3. The Equivalent Stress model requires several

parameters. These are material dependent. For this fretting test, the material is Ti-6Al-4V,

which corresponds with the default parameters in the dialog; these parameters were obtained

from the literature. Select Next on this panel to display the panel shown in Figure 2.3.4.

Step 3.3: View Nucleation Data

The user can display the master and slave contact surfaces as well as the (computed) edge of

contact nodes based on the contact results. The fretting cycles and fretting parameter values can

be displayed as text or as color contours, Figure 2.3.5. These values can be plotted for both

master and slave surfaces to determine where crack nucleation is likely to occur. Note that in

some models, cracking could occur in either or both regions.

Figure 2.3.4 Equivalent stress fretting nucleation model parameter dialog.

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Figure 2.3.5 Fretting nucleation dialog for displaying fretting nucleation data.

Figure 2.3.6 Equivalent stress fretting nucleation parameter color contours - indicating crack

nucleation at the bottom edge of contact.

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Step 3.4: Save Nucleation Data

Select the Save to File button in Figure 2.3.6 to save the data to a text file. The format as

described in the FRANC3D Reference Guide is:

master surface

node x y z parameter cycles

19222 0.49648972 0.91291108 0.15 25 19283

20917 0.52414545 0.94056681 0.12 29 18453

20881 0.51283174 0.92925310 0.12 24 19898

Select Finish. Note that selecting Finish will cause FRANC3D to write the GUI command to a

session log file. The session files can be played-back using the Playback menu item in the

FRANC3D File menu or processed by the command-line version of FRANC3D. The command

line options are described in more detail in Section 3. The recorded session file is shown here:

FretModelImport(

model_type=ANSYS,

file_name='C: \fretting test rig\fretting_rig.cdb',

load_case_flags=[false,true,true],

results_files=['NONE','C:\ fretting test rig\fretting_rig_ls1.str','

'C:\ fretting test rig\fretting_rig_ls2.str'],

results_load_cases=[0,0,0],

retained_mats=[ALL],

color_regions=true)

FretNucleationCycles(

fretting_model_type=FRET_SEQ,

fretting_params=[0.43,52476,-0.6471,450.85,-0.03582],

do_averaging=false,

save_file='C:\ fretting test rig\predicted_nucleation.txt')

2.4 Step 4: Modeling Discrete Cracking

The next step is discrete crack insertion and growth from the predicting fretting crack location.

The standard FRANC3D/ANSYS Tutorial describes the process of inserting and growing a crack.

For a fretting fatigue model such as this dovetail test rig, the analysis is complicated by the

contact conditions. The discrete crack growth procedure for this model is described in Section 4.

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3.0 Command-Line FRANC3D Playback

This section describes the fretting fatigue analysis playback. The GUI commands are saved to a

log file as described in Section 2.3. These commands can be replayed either through the GUI or

in a batch or command-line version of FRANC3D.

3.1 FRANC3D GUI Playback

From the FRANC3D File menu, one can select the Playback option, Figure 3.1.1, which leads to

the File Selection dialog shown in Figure 3.1.2. The session (.log) file can be selected and

FRANC3D processes the commands from the file. Based on the commands shown in Section

2.3 (Step 3.4),the fretting model and results are imported and the fretting nucleation predicted.

Figure 3.1.1 FRANC3D GUI Playback menu item.

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Figure 3.1.2 File selection dialog for session (.log) files.

3.2 FRANC3D Command-Line Playback

The same session (.log) file used in Section 3.1 can also be replayed using the command-line

version of FRANC3D. A separate executable, called franc3d_batch(.exe) is delivered along with

the GUI version of FRANC3D. The FRANC3D license works for both executable files.

The command-line version is executed from a CMD window (in MSWindows, Figure 3.2.1) as:

C:>\FRANC3D\franc3d_btach.exe session01.log playback.log

where the playback.log file saves the replayed commands and parameters.

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Figure 3.2.1 MSWindows CMD window showing FRANC3D command-line execution.

4.0 Discrete Crack Growth from the Edge of Contact

This section describes the discrete crack insertion and growth from a crack that is nucleated at a

position indicated by one of the fretting nucleation models.

4.1 Local and Global .cdb files