topic Re: Reaction forces seem incorrect in Fusion Support Forum

Reaction forces seem incorrect

colin.mcdonald — Fri, 20 Oct 2017 23:29:25 GMT

Hello all:

The results showing for reaction forces seem incorrect in Mac version 2.0.3620.

Please see the attached screenshot -- theres two 750N loads in the up direction, I would expect something in the same magnitude down at the fixed face of the assembly. Instead I'm seeing a set of zero's.

I am doing something obviously incorrect?

(MacOS 10.12.6, Fusion version 2.0.3620, f3d file attached.)

Cheers, C

Re: Reaction forces seem incorrect

innovatenate — Sat, 21 Oct 2017 22:59:15 GMT

You are right, there is something that doesn't look right with the Reaction Forces.

One of the bodies in the design is completely free. If you take a look at the DOF view, you can see that one piece is completely free. Originally, I thought this was going to be the issue.

When I use a different contact, additional constraints, linear mesh elements, materials, and/or a simplified model, I'm still getting some odd results when I use the Inspect > Reaction Forces command or review the Reaction Forces in the Results Report. I've even tried making a simplified version of the problem to reproduce the issue, but I'm failing there as well. This leads me to believe there is something very specific to this design, but I'm not sure what it is.

I've reported this to development (reference ID ARRO-8875). I or someone else will circle back as soon as I learn more from the report and let you know as soon as I learn more. Thank you for reporting this to us!

Re: Reaction forces seem incorrect

colin.mcdonald — Mon, 23 Oct 2017 17:00:03 GMT

Hi Nathan:

... thanks for the reply.

After some more testing, I discovered that turning off "remove rigid body modes" fixes the problem.

Q: why?

Cheers, C

Re: Reaction forces seem incorrect

innovatenate — Mon, 23 Oct 2017 18:28:48 GMT

I'm don't believe this is the expected behavior. There's a pretty good explanation of what this option does in the help with some interesting notes about when and when not to use this option. In this case, I would have assumed that it was the correct option to enable, but perhaps I'm missing something.

One piece of advice that came back already is that using the NLSS (non linear static stress) solver worked normally as well.

Thank you for sharing your findings, I've passed this information along to development. I or someone from the dev team will circle back as soon as I can confirm if this seems like expected behavior or not.

Thanks,

Re: Reaction forces seem incorrect

colin.mcdonald — Mon, 23 Oct 2017 21:38:32 GMT

... sounds good. Thanks.

Re: Reaction forces seem incorrect

shekar_sub — Wed, 09 Mar 2022 16:16:32 GMT

Hi Nathan and Colin,

The recommendation from development team is to use

1. Remove Rigid Bodies setting to be OFF

2. Bonded instead of separation contacts

After that I'm getting the 1500N reaction forces on the face. Thanks

Best regards

Shekar

Re: Reaction forces seem incorrect

colin.mcdonald — Wed, 09 Mar 2022 17:53:45 GMT

Hi Shekar:

... thanks for the reply.

Since 2017 (!) I've been kinda ignoring that Fusion can return reaction forces.
I remade the test file from scratch (properly modeled this time!), and ran a set of simulations on it to see if I can get the <separation> contact and the reaction force.

In the end, it seems that simply toggling-off <remove rigid body modes> lets the "reaction force calculation" work: a cloud-based simulation happily ignored DOF problems, and was able to calculate reaction forces with a "seperation" contact.
See the attached screenshots and f3d-export for the details ...

What is it about <remove rigid body modes> that stops the "reaction force calculation"?

Thanks again.

Cheers, C