Learning Forum

[Roy Roffler]

Hi there

I have a question concerning the fatigue tool of Ansys Workbench:

I already figured out how to define S-N-curves for different levels of mean stress in the technical worksheet of Ansys Workbench. So, to make sure I understand what the fatigue tool is doing, I defined three S-N-curves: One for a fully reversed loading, one for a zero based loading and one where the maximum load is zero. As an example, lets say the endurance limits (the amplitudes) are as follows:

• Mean stress = 0 MPa       ==> endurance limit = 200 MPa
• Mean stress = 180 MPa   ==> endurance limit = 180 MPa
• Mean stress = -220          ==> endurance limit = 220 MPa

I then simulated a test geometry and tuned the load so the maximum stress is 200 MPa. I made a fully reversed fatigue assessment (10e9 cycles) and got a safety factor of 1 as expected.

I then tuned the load so the maximum tensile stress is 360 MPa (mean stress of 180 MPa plus the amplitude of 180 MPa) and made a zero based fatigue assessment (10e9 cycles). After playing with the mean stress theory used, I finally got a safety factor of 1 as expected. So far so good.

But when I tune the load so the maximum compressive stress is -440 MPa (mean stress of -220 MPa minus the amplitude of 220 MPa) and make a zero based fatigue assessment (10e9 cycles) I do not get a safety factor of 1 (but lower) no matter what mean stress theory I choose!

How does Ansys interpret or uses the specified S-N curves? It seems like Ansys is using some type of mean stress correction curves (see image) but not the values I deliberately specified for different mean stresses.

Obviously, I expect Ansys to figure out by itself, which areas of the part have positive, negative (or no) mean stresses, even if the analysis is defined as zero based, even though the compressively loaded area of the part only sees negative stresses / compressive stresses.

Thanks in advance,

Roy

[Daniel Shaw]

On an node-by-node bases, the Mechanical Fatigue Tool (FT) calculates the alternating stress and mean stress for the specified loading.  If "Mean Stress Curves" is specified as the Mean Stress Theory, the FT uses the S-N curve for that mean stress along with the calculated alternating stress to determine the number of allowable cycles.  If the calculated mean stress lies between specified mean stress S-N curves, the FT interpolates between the curves.  

The mean stress correction curves shown in the FT graphics window are just generic curves shown for visualization.  They are not the actual curves used in the analysis.  The actual S-N mean stress curves used in the analysis can be viewed in Engineering Data.

Why do you have a S-N curve with a large negative mean stress?  The effect of negative mean stress is usually ignored.  Can you show an image from Engineering Data of your curves

[Daniel Shaw]

I ran a test case using various mean stress curves.  The FT is behaving as expected for zero mean stress and positive (tensile) mean stress.  There appears to be a bug when using a mean stress curve with a negative mean stress.  I recommend not using the FT with negative mean stress curves until the bug is resolved.

 

[Roy Roffler]

First of all, thanks for your reply! To be honest, I think I solved my problem. But let me explain in detail, hopefully this helps others as well:

 

First, concerning the Question: "Why do you have a S-N curve with a large negative mean stress?":

If I have a part that is cyclically loaded in a zero-based manner, that means the load is applied and relieved periodically, but never inversed, some regions may see only compressive stresses (an no tensile stresses ever). A lot of material models assume higher fatigue loads for such compressive forces (which intuitively makes sense to me). And in order to consider that fact, I do need S-N-curves with negative mean stress values (since the compressively loaded area has a negative mean stress). Otherwise, the calculated safety factor is too low for such areas.

 

Concerning "The mean stress correction curves shown in the FT graphics window are just generic curves shown for visualization."

Good to know! Just a bit confusing that they show these (meaningless) curves...

 

Now concerning "There appears to be a bug when using a mean stress curve with a negative mean stress."

That was my first thought: A bug. But I guess the details are very crucial! In order to consider the S-N curve with negative mean stress, I have to assess the ABSOLUTE first principal stress (or absolute von-mises stresses). Because in the compression area, the first principal stress is not relevant but the third one. The one with compressive stress. By choosing ABSOLUTE first mean stress, I assume Ansys takes the first mean stress with the highest number, independent from the sign (+ or -). And then I get a safety factor of 1 for the compression side as well (in my test model described!

 

Now I hope I'm not just thinking I understood everything... 😬

Is it possible to upload an archive of my test model here? Would only be 228 kB...

[Daniel Shaw]

Thanks, but I do not need your model.  I have a test model.  I agree that the FT appears work  properly if Absolue Max Principal is the specified Stress Component is ABS believe the behavior is a bug because it is not well explained.

[Daniel Shaw]

After some additional review, I believe that the Mechanical Fatigue Tool (FT) is working as expected with negative mean stress S-N curves.  I had some flaws in my original model that I corrected.

If the specified Stress Component is a normal stress, absolute maximum principal stress, or signed von-Mises, the FT maintains the sign of the mean stress. If the dominant stress that produces the mean stress is negative, the FT uses the appropriate negative mean stress S-N curve. 

If the specified Stress Component is maximum principal stress or von-Mises, the FT does not maintain the sign of the mean stress, because those stresses are always positive.  Thus, even if the dominant stress that produces the mean stress is negative, the FT uses the specified Stress Component (which is always positive) and uses the appropriate positive mean stress S-N curve.

I checked the FT results against Ansys nCode DesignLife results.  They are producing equivalent results for each of the stress components.

How the FT handles absolute maximum principal stress can be a bit confusing, because the AbsMaxPrin stress is always positive.  The FT is able to maintain the sign of the underyling principal stress.  So, if it uses S3, it maintains the negative sign for the mean stress.

It was an interesting question that required some digging on my part to verify, but I do not believe that filing a bug report is needed.

[Roy Roffler]

I agree that filing a bug report is not needed. However I think a more detailed documentation about what the FT-tool actually does would help a lot...

Nevertheless thanks for your input and help!