Bearing-Bypass Interaction Basics 1

[jpaero]

I have been trying develop some intuition when it comes to bearing-bypass interaction in composites. I would like to see a comparison of bearing+bypass stress distribution for metals vs that for laminates. Just to make it simpler for understanding, I was hoping to see the plots if the laminate had only 0deg plies, another laminate with only 90deg plies and yet another with +-45deg.

Would you know any sources?

 

[SWComposites]

Unlikely to find those plotted anywhere. Besides, no one would use those layups in a joint area.
You could create stress distribution plots using the BJSFM code, if you can track down a copy.

 

[Burner2k]

ESPComposites have free excel spreadsheets including one for BJSFM. I personally have not used the BJSFM one (although I have a copy of it) but I think it would be a good choice for you to get a feel. For metals, you can do a quick & simple hand calcs.

I hope you are getting an idea of how BJSFM/bearing-bypass works in composites before trying out automated tools...else it would be garbage-in garbage-out situation.

 

[rb1957]

I doubt you will find anything (practical) that has only 0 (or 90) degree lay-ups.



another day in paradise, or is paradise one day closer ?

 

[crackman]

jpaero

Hi, I would recommend looking at and using Hart Smiths codes (a4ej/k). See afwal-tr-81-3154. I find it more useful but thats just preference.

good luck

 

[rb1957]

everyspec has that AFWL-TR ... dig around for both volumes

another day in paradise, or is paradise one day closer ?

 

[WKTaylor]

This might be useful... if in public domain...

AFWAL-TR-81-3114 EFFECTS OF BEARING/BYPASS LOAD INTERACTION ON LAMINATE STRENGTH

Regards, Wil Taylor
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[jpaero]

Thank you all for your responses. I'm realizing that unlike metals, there is no simple way to get a hang of bolted joints or notches in composites.

 

[SWComposites]

When we finally finish it and get it published, probably in 2023, CMH-17 Volume 3 Rev H will have a new ~ 100 page chapter on bolted joints in composites.

 

[Ng2020]

Are you familiar with the concept of the 'characteristic dimension' of a laminate/discontinuity?
It is one means of developing a strength criteria at notches/holes, so not directly related to bearing bypass interaction, but it's a function of hole/notch size and layup.

 

[Compositepro]

There is so little off-axis strength in a uni-directional laminate that bearing strength is meaningless.

 

[SWComposites]

What the academic textbooks leave out is that characteristic dimensions are actually not constants, but are a function of loading, diameter, temperature, layup, thickness, open vs filled holes, and a bunch of other variables. And that the char dim approach does not work at all for bearing strength.

 

[Ng2020]

...aa are bearing/bypass allowables...I think the post originator is interested in the effect of layup alone.
I'm not suggesting CD is a substitute means to compute joint margins, but rather as another means for seeing the influence of layup at a strain concentration.

 

[jpaero]

I did have a read of the Whitney& Nuismer paper where they introduce this concept of char.dim but even there they don't speak bout the origin of this so called interaction. What I am interested in is a stress plot which shows how the bearing stress is interacting with the bypass stress. As far as I can see, the bearing stress is more of a contact stress (@0deg) and the bypass stress is a net-section stress (@90deg). I don't understand how and where do they interact. Apologies if this wasn't clear in my original post.

 

[SWComposites]

The interaction of a bearing load on a hole is related to the far field reaction of that load. If the bearing load is reacted in far field tension, then there is a tensile stress concentration at the side of the hole. When a bypass load is superimposed that is aligned with the bearing load, the stress concentration at the side of the hole is a combination of that from the applied bearing and bypass loads. If the bearing load is reacted in compression, then there is no stress concentration at the side of the hole from the bearing load. Now if the bearing and bypass loads are not align, it gets very complex.

 

[jpaero]

Thank you @swcomposites!!!That is the kind of explanation I was hoping for!

 

[Joe Moore]

To have a better understanding of bearing bypass curves realize that it is just an interaction curve. A good analogy is the Von Mises Criterion Failure curve.

 

[ESPcomposites]

I am not sure that is a good analogy. A bearing-bypass curve is an interaction curve, but it takes multiple test data points to fit it. Also, the curve itself it a function of various factors (so it takes quite a bit of empirical data to generate a set of curves that could be used for practical implementation. Conversely, you can generate a Mises curve via one test data point and the rest is a physics based solution. The main distinction is that a bearing-bypass solution is predominantly empirical while a Mises criterion is predominately physics based. This is perhaps the major distinction between strength analysis of composites (purely physics based failure criteria are not very useful) and metals (purely physics failure criteria are useful).

Brian

http://www.espcomposites.com