Hole-pin couplings happen very often in mechanics.
Therefore having to work on it frequently, I realized that many times PrePoMax gives wrong values (by a lot), not only as values, but also as distribution (which is even more strange).
I think I have identified the cause of some (or all) problems, but I am waiting for your confirmation.
I saw that it happened to others so I thought I would write this little report based on what I found out.
I have isolated the problems by creating very simple models so that you can easily validate my conclusions and do your own testing.
Below is a brief description of the problems:
In the folder here linked ( 02_Problem with Pin and Hole same diameter (Created in PTC-Modeling) ) a very simple system of beams connected by pins. The model is created with PTC-Modeling (we will see later that in my opinion this information is very important) by a friend and the hole has the same size as the pin (other important information).
For this model I am not interested in identifying whether the stress values are more or less correct (this is not its purpose) or whether the FEM model was created correctly in terms of mesh dimesions, but only to have confirmation that the blatant errors I get of stress distribution are confirmed by other users and whether the causes I identified can be agreed upon.
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Subfolder “01_PIN Contact model_Pin and Hole same diameter and Adjust NO”
Below are the images:
The pins (first image) are “squeezed” and the stresses within the hole are uniform instead of being placed only along the face of the hole subject to compression.
I believe here the problem is with the 3D modeling software (PTC Modeling). Those who use the software know that PTC-Modeling has its own whole unit management system, so when you export an object you never get a whole measurement (I give an example: I export a 10mm diameter pin, this is not exactly 10mm but for example 10.00025). Basically I think PrePoMax does an “interference fitting” in this case (I demonstrate this in the following subfolders). -
Subfolder “02_PIN Contact model_Pin diameter less than Hole and Adjust YES”. I have the same model, but the pins are 0.2mm smaller then the hole diameter.
You can see a marked improvement in the stress distribution around the hole and the lack of the “squeezed” effect on the pins. This made me think that the problem was with the PTC environment (I also have similar problems when measuring components in dwg drawings from this software).
However, the distribution is not correct. The problem, after many attempts, I think is in the contact setting, which in this model involves “Adjust”=”YES”. -
Subfolder “03_PIN Contact model_Pin diameter less than Hole and Adjust NO”. The model is similar to the previous one (pin diameter smaller than 0.2mm), but “Adjust” set to “NO”
Below are the results:
The results are well representative (at least as a distribution) of the behaviour of the structure.
So I think there are two problems, one is importing the .step file from some softwares and the other is defining the “Adjust” parameter in the contact.
Personally, I would prefer “Adjust” to be set to “NO” by default given the problems it creates, but I leave that consideration, once the problem is validated, to those who know more about it than I do.
