High concentration stress in a static analysis, fatigue analysis

[Anonymous]

Hello everyone,

I have a problem for the fatigue analysis that I am doing and I hope that you can help me resolve it.

I'm using Natran in CAD 2020. I attached my assembly (too big to be attached so it's compressed) and a picture of my problem.

When I do the analysis, the software result is that my assembly will not resist a single cycle. I tried to find why and the reason that I found is the high concentration stress in the static analysis in one part of my assembly (see picture).

The load in the analysis is -37 688 N in the vertical axis. It is divided between all the plate in the middle of the assembly. The constraint is the faces of the H-beams (the one that look like a H).

The result of the static analysis is a Von Mises stress of 2 297 MPa. This results is obtained with a very refine mesh. The more I refined the mesh, the higher the result without sign of convergence. 

My question is, am I correct in assuming that this is a singularity? If so, is there a way to remove it or at least to remove it from the calculation? Is there a way around it to get results from the fatigue analysis? 

Thanks a lot in advance.

Florian 

[John_Holtz · Accepted solution]

Hi Florian,

Yes, the increasing stress when the mesh is made finer is an indication of a singularity (especially if the high stress is confined to one element. If the stress were high in reality, it would occur in a volume, so more elements would be in that volume and have high stress as the mesh is made finer.)

There is no automatic way to remove the result from the stress analysis. In theory, you could edit the results and change the value, but then you would need to know what value to change it to, and then you would need to remember than you manually changed the results. It is better to change the scale of the legend and ignore the result at the singularity.

The same is true for the fatigue life. The life is low where the stress singularity is high, so change the legend scale and ignore the value.

The legend scale can be set by editing the contour ("Results > Options" on the ribbon) and changing the input on the Contour Options tab.

(Edit. FYI. The assembly file, .iam, is not sufficient to open the model. The part files, .ipt, are also needed to open a model.)

[Anonymous]

Hello John,

Thank you for your answer. 

Changing the legend will work.

Is there a specific reason for a singularity to appear? 

Is there a way to reduce this singularity by changing the geometry? I tried adding a fillet or a chamfer but it is not enough to reduce the stress in this area.

I didn't know the part files were needed, thanks for the information.

Florian