Workshop 1: Fatigue : Stress-Life · 1 © 2015 ANSYS, Inc. ANSYS Fatigue Module Training – WS 1...

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1 © 2015 ANSYS, Inc. ANSYS Fatigue Module Training WS 1 16.0 Release The Fatigue Module of ANSYS Mechanical Workshop 1: Fatigue : Stress-Life

Transcript of Workshop 1: Fatigue : Stress-Life · 1 © 2015 ANSYS, Inc. ANSYS Fatigue Module Training – WS 1...

1 © 2015 ANSYS, Inc. ANSYS Fatigue Module Training – WS 1

16.0 Release

The Fatigue Module of ANSYS Mechanical

Workshop 1: Fatigue : Stress-Life

2 © 2015 ANSYS, Inc. ANSYS Fatigue Module Training – WS 1

Goal: • In this workshop our goal is to perform a Stress-Life analysis of the

connecting rod model (ConRod.x_t) shown here. Specifically, we will analyze two load environments: 1) Constant Amplitude Load of 4500 N, Fully Reversed and 2) Random Load of 4500N.

Goals

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Start Page

1. From Analysis System, select “Static Structural”

2. Import the Geometry:

a. RMB on Geometry >Import Geometry>Browse…

b. Browse to the file “ConRod.x_t” to open it

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Preprocessing [1]

4. Set the working unit system:

• Units > Metric (m, kg, N, °C, s, V, A

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Preprocessing [2]

6. Apply loads to the model:

a. Highlight the connection rod surface shown…

b. RMB > Insert > Force

c. Change “Components” and enter a magnitude of - 4500N for the “Z Component”

a.

c.

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Preprocessing [3]

7. Add supports to the model:

a. Highlight the bolt holes shown

b. RMB > Insert > Cylindrical Support

c. Set Radial = Fixed, Axial = Free, Tangential = Free

d. Highlight the face on the connecting rod shown…

e. RMB > Insert > Fixed Support

a.

c. d.

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Solution / Results

8. Solve the model:

• Click Solve

• View the Results :

a. Highlight the “Solution” branch

b. RMB > Insert > >Deformation >Total

c. RMB > Insert > Stress > Equivalent (Von-Mises)

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Fatigue Tool [1]

9. Insert the Fatigue Tool:

a. Highlight the “Solution” branch

b. RMB > Insert > Fatigue > Fatigue Tool

10. Specify fatigue details :

a. Specify a Fatigue Strength Factor (Kf) of 0.8 (material data represents a polished specimen and the in-service component is cast).

b. Specify fully reversed loading to create alternating stress cycles.

c. Specify a stress-life fatigue analysis (No mean stress theory needs to be specified since no mean stress will exist – fully reversed loading).

d. Specify that Von Mises stress will be used to compare against fatigue material data.

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Fatigue Tool [2]

11. Add results to the Fatigue Tool:

a. Highlight the “Fatigue Tool” branch

b. RMB > Insert > Safety Factor.

c. From the Details of “Safety Factor” window, set the “Design Life” to 1e6 cycles.

d. RMB > Insert > Fatigue Sensitivity

e. Under the Details of “Fatigue Sensitivity” window, specify the following:

Lower variation” of 50% (an alternating stress of 2250N)

Upper variation of 200% (an alternating stress of 9000N).

f. RMB > Insert > Biaxiality Indication

12. Click Solve to view results.

c.

e.

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Results [1]

Safety Factor

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13. Highlight and plot the “Fatigue Sensitivity” result for a minimum base load variation of 50% and a maximum base load variation of 200%.

Results [2]

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Results [3]

14. Find the sensitivity of available life with respect to loading for a maximum base load variation of 400%. • Note : must re-solve to obtain the new Fatigue Sensitivity results.

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Results [4]

15. Highlight and plot the “Biaxiality Indication” result. • Note : The stress state near the critical location is not far from uniaxial (.1~.2), which gives an

added measure of confidence since the material properties are uniaxial. Recall, a biaxiality of zero corresponds to uniaxial stress, a value of –1 corresponds to pure shear, and a value of 1

corresponds to a pure biaxial state.

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Fatigue Tool [1]

16. Analyze a random load of 4500N: • Note : Assume that we have strain gauge results that were collected

experimentally from the component and that we know that a strain gauge reading of 200 corresponds to an applied load of 4500N

a. Highlight the solution branch.

b. RMB > Insert > Fatigue > Fatigue Tool.

17. Specify fatigue details:

a. Specify a Fatigue Strength Factor (Kf) of .8 (material data represents a polished specimen and the in-service component is cast).

b. Change “Loading” “Type” to “History Data”

c. Click inside “History Data Location” to open “SAEBracketHistory.dat” containing strain gauge results over time

a.

b.

c.

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Fatigue Tool [2]

d. Define the scale factor to be .005 (we must normalize the load history so that the FEM load matches the scale factors in the load history file)

e. Specify Goodman theory to account for mean-stress effects.

f. Specify that a signed Von Mises stress will be used to compare against fatigue material data (use signed since Goodman theory treats negative and positive mean stresses differently).

g. Specify a bin size of 32 (Rainflow and Damage matrices will be of dimension 32x32).

005.gaugestrain 200

load FEM 1

gaugestrain 200

1000

1000

load FEM 1

lbs

lbs

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Fatigue Tool [3]

18. Add results to the Fatigue Tool 2:

a. RMB > Insert > Life

b. RMB > Insert > Safety Factor

c. Set the Design Life to 1000 cycles.

d. RMB > Insert > Fatigue Sensitivity

e. In the “Details” window for “Fatigue Sensitivity”, specify :

– Lower Variation of 50% (an alternating stress of 2250N)

– Upper variation of 200% (an alternating stress of 9000N)

f. RMB > Insert > Biaxiality Indication

g. RMB > Insert > Rainflow Matrix

h. RMB > Insert > Damage Matrix

i. From the Details of “Damage Matrix” window, set the Design Life to 1000 blocks

19. Solve

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Results [1]

20. View Results:

a. Highlight and plot the “Life” result.

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Results [2]

b. Highlight and plot the “Safety Factor” result for a design life of

1000 cycles.

If the loading history corresponded

to the loading experienced by the

part over a month time, the

damage and FS will be at a design

life of 1000 months. Note that

although a life of only 77 loading

blocks is calculated, the needed

scale factor (since FS @ 1000=.60)

is only .60 to reach a life of 1000

blocks.

Note, the “scale factor” (FS) is the

scale factor for the loading to make

it meet the life of 1000 months.

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Results [3]

c. Highlight and plot the “Fatigue Sensitivity” result for a minimum base load variation of 50% and a maximum base load variation of 200%.

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Results [4]

d. Highlight and plot the “Biaxiality Indication” result.

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Results [5]

e. Highlight and plot the “Rainflow Matrix” result.

Here, one can see from the

rainflow matrix that the

majority of the cycle counts

are for low mean stress and

low stress amplitude (range).

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Highlight and plot the “Damage Matrix” result.

Results [6]

Although, from the previous

slide, one saw that most of

the counts were for the low

mean and range bins,

these do not cause the

most damage at the critical

location, as shown in this

damage matrix. Instead,

the 'medium' stress

amplitude cycles cause the

most damage at the critical

location.