Simulation Failure

[Anonymous]

Hello,

Can't get my linear stress model to work for my tooling design, have been through contacts and DOF but unsure if I've done it right. I have attached a Picture of the errors as well as the file itself.

Cheers,

Nico

[Marcin_Kosciolek]

Hi @Anonymous 

Your model is very complicated for FEA analysis because of the exact geometry of the thread, so it causes problems when calculating. We should always simplify geometry if it is possible (for example removing the thread from the bolt component). Tell me what outcome from the simulation you are interested in? Maybe we can simplify something.

[Anonymous]

Hi @Marcin_Kosciolek,

I am simulating a hydraulic pressure pack in between the plate at the bottom and held by the washer and nut at the top. The hydraulic pack will cause tension in the threaded bar and compression in the plate, doughnut load cell, washer and nut. This is why you can see an offset bonded relationship between the bottom of the doughnut load cell and the plate at the bottom. Thought it would be essential to keep the thread to see if it can withstand the load and to see what factor of safety it will achieve.

Cheers,

Nico

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Thank you.

[Anonymous]

Hi @Marcin_Kosciolek,

I have just tried simplifying to remove the threaded parts and I made sure there were no interfering parts. It still didn't work, attached screenshot of errors.

Cheers,

Nico

[Marcin_Kosciolek · Accepted solution]

Hello @Anonymous,

I have modified your model. I removed all threads, but the main reason the simulation failed was a small face in the load cell. Solver is not able to handle it. I removed that face and after that split it in order to apply force on the outer area of the new face. The model was calculated without any error and the computation time is now very reduced.

Also, I think we don't need separation contact between plate and load cell. I added separation contacts between bolt, load cell, and washer.

I applied frictionless constraint on the cylindrical face of the load cell. I assumed, in reality, it can only move along the bolt and other directions of movements are not allowed. But you have to decide it is ok for this design, I don't know how it looks in real. This constraint pretty sure will stabilize the model, but here we have mainly tensile loads, so the model should be stable by nature.

If you want to be more accurate regarding the strength of the bolt, you can change the diameter of the bolt which corresponds to the "tensile stress area". Below I attached this expression. From that, you can extract the "working diameter" of the bolt using the formula for a surface area of a circle.

Here is the link to the modified model:

https://a360.co/2V14f4A

[Anonymous]

Hi @Marcin_Kosciolek,

Thanks so much for your help, this simplified version is more than sufficient for what I need and I will now be able to play around with the design and change a few things. If I run in to any more trouble I'll reply again in this thread.

A few questions however; any particular reason why there is such a low Safety Factor at a couple small incident faces on the Hex-Nut? Is this potentially just an error from fusion 360 due to the mesh in this small chamfered area?

I designed this to be a very safe and strong tooling for the job its needed for and throughout most of the design the Safety Factor analysis outcome is what I expected; a very high Safety Factor (maybe even too high lol), however in a couple of unexpected places the Safety Factor drops as low as 1.785 (a very rapid change from 6+ to this value). These faces are at the outer chamfered edge of the Hex-Nut, not where you would expect the maximum Stress to be as this is not even in contact with the washer and thus not directly taking load. I would have expected the maximum stress to be at the incident edge of the Hex-Nut at the inner-most diameter at which it meets the washer (thus where the Hex-Nut resists against the load through the washer), or possibly at its outer flat edge where it is still in contact with the washer also.

Thanks so much again and I must ask if you would be okay for me to reference your name and email address in a project report (for university) and offer you due thanks and credit for your help.

Cheers,

Nico

[Marcin_Kosciolek]

Hi @Anonymous 

I am really happy the problem is solved!

Regarding the values of safety factor/stresses on the outer edge of the nut. It is totally normal behavior. It is caused by singularities. They appear in sharp edge corners, the chamfer of the nut is a good example. What's more, if you start to decrease the size of the mesh, values of stresses will be increasing to infinity. It is not a bug of the software, it is a feature of FEA method. In this case, we should not consider these values in the final assessment of our design. To be honest, we don't need this chamfer to FEA model, we can simplify that, and delete this feature from the design.

I really recommend discovering this blog. In my opinion, Łukasz explains FEA problems in a great way using simple examples and comparisons.

Here are the links to the problem of singularities:

https://enterfea.com/stress-singularity-an-honest-discussion/

https://www.youtube.com/watch?v=9l9ZngKZRPs

I am very pleased you appreciate my work. Thank you!

My name is Marcin Kościółek and my address mail is marcin.kosciolek@autodesk.com

Kind regards