CATIA Analysis and Simulation: Representing the Strength of a Joint

3D Design Volume Model
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Hello

You have to imagine a square base box closed with a lid by 4 screws in each corner.

In this cover, a gasket groove with an O-ring located on the inside of the 4 screws.

I would like to do an RDM analysis on my cover to see the stresses applied by the force of the seal.

I have done some tests on the seal, and I have determined the pressure exerted by it when it is compressed by 30%. Which corresponds to my case.

I also know the tightening torque of my screws.

I started by representing my screws, and then applied the pressure found in my testing in my gasket groove. But this is not representative of reality since I get a displacement greater than my joint diameter...

So I then recessed my cover at the level of the screws, created a rigid virtual spring part in my gasket groove still with my pressure, then I created an imposed displacement at the level of my recesses (as if I was tightening my screws). In this case, my lid simply moves on the Z axis as if it were simply compressing a spring... There is no "twisting" effect of the lid as in reality.

Do you have any opinions on my work? Or a more correct idea?

Thank you for your help.

 

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Hello

We can discuss Catia in abstracto since, whatever the software, the rules of mechanics are the same: only the implementation in the software is different.

I am thinking of reading to you that you have to put your terms, in the other direction, not to say backwards.

When you say " I have determined the pressure exerted by this one when it is compressed by 30% ."

I would rather say that it is a compressive strength of your joint. From this point of view, it acts like a spring whose resistance increases the more it is compressed. The pressure is that of the four clamping screws.

After you say that you " recessed my cover at the level of the screws, created a rigid virtual spring part in my gasket groove still with my pressure, ".
I don't think I would have done that! 

No need for a virtual part that will deprive you of information on the deformation of the lid (yes ! there will be significant information (see below)
I would have constrained in simu by appropriate contacts the lid with the box (the lower body). I would have applied a force to the lid with a distance limit indicating the equivalent of " no penetration " between the lid and the lower body.

After how to know the good resistance of the joint. If the material is correctly defined for your gasket (if it is not an elastomer that is too exotic) then the gasket simulation will appear to be in direct relation to the force (and not the pressure) exerted by the screws on your cover.

The more force you put in, the more compressed your joint will be. It's the delta force between little and a lot compressed that goes.

1°) tell you the minimum and maximum force exerted on your joint.

2°) the deformation of the joint as well as the deformations of the cover and the lower part which will be located on the vertical partitions. Indeed, the force exerted will tend to be on the partitions because the corners are the least deformable parts.

Saying that your joint is compressed by 30% is probably inaccurate. The deformation can only be measured by the result of the simulation and the compression taus is not at all the same if you have a round toric or a square or rectangular toroid. The latter deforms much less than a round toric which, crushing more, deforms relatively more.

One last point for your simulation, you don't have to use screws since, as we know from the literature the standards on the forces exerted on the underside of a washer by a screw, you just have to define an area (on the top of the cover) corresponding to the surface of the washer and apply the force of the abacus. The standard represents the maximum force if you set a lower force and increase it gradually and iterationly up to the max : then you can see almost dynamically how your joint deforms and how it deforms your box.

Hooa!  to start chatting 

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Hello and thank you for your very complete answer and for your time.

Indeed, reading you, I realize the lack of precision in my request. We must talk about the compressive strength of the joint. For the latter, everything is well defined, I am in O-ring with a diameter of 1.8mm 70 Shore A. As for the 30% compression, this is of course in the "perfect" case where the final assembly corresponds to my CAD rib chain. My final goal is, as you said, to determine, depending on the force exerted by my screws and the compressive strength of the resulting gasket, to determine the stress applied to my cover, which is made of ABS with 2mm walls.

I will therefore follow your approach.

Now, what I was showing my shortcomings in Catia simu, when you say "I would have applied a force to the lid with a distance limit indicating the equivalent of " no penetration " between the lid and the lower body." 

Similarly, I quote you " all you have to do is define an area (on the top of the lid) corresponding to the surface of the washer". I knew how to draw this area in Creo but on Catia I couldn't find the function. I went through an extrusion function in "Part Design" which distorts my result since it creates a concentration of stress in the wrong place...

Thank you again for your help.

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I hope that you have taken all of my remarks in a positive way. :-) Even if you may have perceived my approach as critical, which is not the case at all. You were asking for an opinion so I told you things based on my experience in simulation and in real life.

I don't know Catia as I said in the preamble : but the approach to achieve the result seems to me to be quite identical.

When you say "I went through an extrusion function in "Part Design" it's the right approach because if you make an extrusion of 0.0001mm it's enough to define an area (in Solidworks we use "separation line"  which avoids having a mini extrusion.

The fact that the washer exerts its force only at this place and creates "concentration of stress in the wrong place" and well in real life the washers will create the force (the pressure in this place and zero elsewhere). The force is then distributed over the rest of the workpiece according to its flexibility or high rigidity. This can be seen very well in the results of a simulation and even better when you do topological optimization you can physically see the forces and deformation propagating mm by mm.

Kind regards

PS: I hope you will achieve the result you want. Keep us informed ;-)