Fatigue Demo

ANSYS Fatigue Module 10.0

Predicting Fatigue Life with ANSYS Workbench

Demonstration

ANSYS Fatigue ModuleDemonstration

Goals: To have a basic idea of the fatigue capabilities available within the ANSYS Fatigue Module.

Problem setup: We have a portion of a suspension system that we wish to do a fatigue evaluation on. We have already solved the problem in Design Simulation, we now wish to perform an Fatigue Evaluation on this analysis.

Demonstration Note: The steps in performing an Fatigue Evaluation are entirely common to any linear static structural analysis. There is no need to use this particular model in your in-field demonstrations. You can add these steps to any demonstration model you chose.

Demonstration


To get started, right click on the Solution Folder and select:

Insert -> Fatigue -> Fatigue Tool

Demonstration


Next, although perhaps not first in a real analysis, add some results.

For this first part, we will add Fatigue Life, Fatigue Safety Factor and Equivalent Alternating Stress. To do this right click on the Fatigue Tool and select:

Insert -> Life

Fatigue Tool -> Insert -> Safety Factor

Fatigue Tool -> Insert -> Equivalent Alternating Stress

Demonstration


Now we need to make some decisions. If you make no decisions, the program will use the defaults. But it makes a nicer demo if you select a few different input decisions.

The possible input decisions are based upon which Fatigue analysis type you chose, Stress Life or Strain Life.

The Stain Life approach is widely used at present. Strain can be directly measured and has been shown to be an excellent quantity for characterizing low-cycle fatigue. Strain Life is typically concerned with crack initiation, whereas Stress Life is concerned with total life and does not distinguish between initiation and propagation.

In terms of cycles, Strain Life typically deals with a relatively low number of cycles and therefore addresses Low Cycle Fatigue (LCF), but works with high numbers of cycles as well. Low Cycle Fatigue usually refers to fewer than 105 cycles. Stress Life is based on S-N curves (Stress – Cycle curves) and has traditionally dealt with relatively high numbers of cycles and therefore addresses High Cycle Fatigue (HCF), greater than 105 cycles inclusive of infinite life.

Demonstration


If you decide to perform a Stress Life analysis, then these are the types of decisions you might make.

Demonstration


If you decide to perform a Strain Life analysis, then these are the types of decisions you might make.

Note: They are very similar to Stress Life with a few variations.

Demonstration


Because it is much easier to understand, we will pick Stress Life, however in a demo you should say something like:

“The ANSYS Fatigue Module supports both Stress Life and Strain Life fatigue analysis. Because this part has to undergo many cycles, we will use the more common Stress Life approach to this High Cycle Fatigue analysis.”

Demonstration

ANSYS Fatigue ModuleStress Life Demonstration

For Stress Life the ANSYS Fatigue Module supports 3 types of loading, indicated by the .

Click Zero Based and see the graph changes, then click Fully Reversed again.

Loading TypeConstant amplitude, proportional loadingConstant Amplitude, non-proportional loading

Non-constant amplitude, proportional loadingBin SizeNon-constant amplitude, non-proportional loading

Demonstration


For Stress Life the ANSYS Fatigue Module supports 5 different types of Mean Stress Effects, indicated by the .

Click the Mean Stress Theory box and see the list of options. Click the various options and see how the graph changes to indicate the implications of the selection, when finished, click None again.

Mean Stress EffectsGoodmanSoderbergGerberMean Stress Curves

Mean Stress DependentMultiple r-ratio curves

None

Demonstration


Click the solve button and then select each of the outputs. This is what the Safety Factor display should look like.

Demonstration


Expand the Geometry tree and scroll down.

Demonstration


Click on Part 67, then select Structural steel area, select the button to the right and then select “Edit Material.”

Demonstration


The previous selections should have brought you to the Engineering Data tab. Click on Alternating Stress.

Demonstration


Here we can examine the S-N curve (Stress-Cycle curve). We can also see that this data is based on zero mean stress. This data is what was used for our Stress Life analysis.

When the Fatigue Module calculates the Fatigue Safety Factor, it is not simply a factor of the cycles, but is a nonlinear calculation that iterates to provide a true indication of the Safety Factor as in the structural sense. Therefore, if the Fatigue Safety Factor is 2 for a part, then the analysis indicates that it can withstand twice as much load for the specified Design Life.

Demonstration


Click on the Simulation Tab, then scroll down to the Fatigue Tool and select Life and then select the box in front of Minimum Life.

By placing a “P” in front of Minimum Fatigue Life, it becomes a Workbench Parameter, which can now be optimized with DesignXplorer.

Demonstration


Conclusion: The integration of Fatigue Analysis inside the ANSYS Workbench allows for extremely quick fatigue analysis that can be added to anyone’s design process and allows for optimization with the inclusion of Fatigue results such as Minimum Fatigue Life.

Fatigue Demo

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Transcript of Fatigue Demo