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Fastener separation stress

346gnu
12-Amethyst

Fastener separation stress

Hello,

Creo3.0 M030

Just checking, Can someone confirm a positive separation stress means surfaces are trying to separate?

I get the following in the log from both the studies below:

' The fix separation option for the fastener is preventing the parts from

  separating at locations where they should separate. Hence, the results

  for this analysis may be inaccurate. You should turn off the fix separation

  option for the fastener and, if desired, introduce a contact interface

  between the parts that are connected by the fastener.'

Thanks

A model with preloads only (100kN per bolt), ZERO external loads, no contact region (time constraints) ...

   

Fastener1_sep_stress:90
Fastener2_sep_stress:75
Fastener3_sep_stress:196
Fastener4_sep_stress:105
Fastener5_sep_stress:62
Fastener6_sep_stress:91
Fastener7_sep_stress:65

Trying to pull the parts apart with a very big load ...

   

Fastener1_sep_stress:2485
Fastener2_sep_stress:1816
Fastener3_sep_stress:3370
Fastener4_sep_stress:2104
Fastener5_sep_stress:1837
Fastener6_sep_stress:1987
Fastener7_sep_stress:2095
Fastener8_sep_stress:2027

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3 REPLIES 3
346gnu
12-Amethyst
(To:346gnu)

Hi Steven,

I prefer always to use contact as I so not like the separation springs as they can mislead; particularly if a hole is partially separated.

I don't use account for stiffness as it is slower when one has to do several analyses starting from the same preload; it is better to perform a preload study and modify the preloads by hand. (this has always been my criticism of this functionality, fastener preloads are not automatically updated)

I only began using the fasteners with separation springs in this case for speed (time constraints). But I got stupid answers. Because I cannot trust the answers and have reverted to contacts where the answers are much slower but realistic. This is a connection where I know there will be no separation (preload is sufficient).

The model I showed in my first post (and have attached here) was just to test. The sign of the separation stress should change if the components separate and I now believe there to be a bug.

I haven't had time to try other versions/cut and the model attached is one I will submit to ts.

Creo3.0 M030

Regards

346gnu
12-Amethyst
(To:346gnu)

I submitted a call

346gnu
12-Amethyst
(To:346gnu)

Steven,

Yes I agree they are updated and the account for stiffness check box does exactly what it is intended to do.

BUT the software keeps the required preload values secret.

... unless I have missed it.

I want to know these values and update the input stiffness. If I don't do this, every time I run the model with account for stiffness on I have implicitly and extra run while the software works out what the fastener preloads should be. Every time.

To make the point,, this doubles the analysis time. Once is acceptable. More is not.

Therefore if more than 1 run with external loads is likely, it is cheaper to carry out a stand alone preload analysis and update the preloads by hand.

It is a nuisance when there are 64 fasteners.

What I want is a check boxes or config options that allow me to see what the software has calculated as the required input preload and/or automatic updating of the preloads to take human error out of the equation. I mentioned these to Tad D before the functionality was implemented (here on this forum somehwere).

Besides, knowing the required preload inputs to get the desired preloads at each fastener helps us debug a model and when debugged tells us a lot about the local stiffness of the structure. It is remarkable how local features change the amount someone assembling parts has to turn the wrench,

bfn

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