How pitch affects the joint strength?
How pitch affects the joint strength?
(OP)
This is very general question, there is a guidline which says 4D pitch requirement but I was wondering how does it affects the joint strength?
Thanks
Thanks





RE: How pitch affects the joint strength?
0.190 D was not used because installers are ham handed enough to break them. 0.250 D meets that test.
RE: How pitch affects the joint strength?
4D < pitch < 8D are typical bounds.
RE: How pitch affects the joint strength?
- PITCH is the distance between fasteners (in line with the load).
- WIDTH is the distance between fasteners (perpendicular to the load). This is the distance that plasgears and rb1957 are referring to.
For the WIDTH, the basic failure modes are net section failures and bearing/fastener failures. If the width is too far apart, then you will have high bearing/fastener stresses. If the width is too close, then you will have a net section type failure (i.e the "swiss cheese" that plasgears referred to).
You can run some simple checks for "net section" and bearing and fastener type modes to come up with joint optimization. This depends on the material system and the allowables (i.e. metallic vs. composite). The numbers above are reasonable, but are not written in stone. The other thing to consider is that you might want to have a larger width in the event that there is an error during manufacturing (misdrilled holes, etc.). You may also want to bias it towards a more graceful bearing type failure mode by increasing the width.
For the PITCH, it is not nearly as critical (as should be obvious right now). However, if the joint has multiaxial loading or shear, then it becomes the width. The pitch itself would have a small affect to the fastener distribution, which is usually more important in fatigue of metallics and static strength of composites.
Brian
www.espcomposites.com
RE: How pitch affects the joint strength?
Brian
www.espcomposites.com
RE: How pitch affects the joint strength?
RE: How pitch affects the joint strength?
An aside, I've never heard the term "width" as ESP is defining it. I can see the benefit of making the differentiation, but haven't seen it done. Is this a common thing?
RE: How pitch affects the joint strength?
Have a look at Peterson, Chart 4.32. For an open hole, at 4D, the effect is about 1%. At 3D is at about 2%, which is pretty minor.
Direct comparison of the pin loaded hole is more challenging since the width is very important to the Kt (although there is still little "interaction" amongst Kt's). I am in the process of writing a series of technical papers that is closely related. Aside from the standard data, I have built FEM's, some of which go all the way to use contact.
As far as "width" goes, there may be other terms (spacing, etc.). I wanted to better distinguish the two parameters since they were being mixed together a little bit. I have been on a Boeing program for the last several years and that is what they use. I have not checked to see if Lockheed or others use different terms.
Brian
www.espcomposites.com
RE: How pitch affects the joint strength?
that'll make for interesting (?) reading, seriously (??). is this to be published ?
"spacing" is the term i use for "width".
RE: How pitch affects the joint strength?
Yes, I plan on publishing the results. This is related to a closed form bearing-bypass solution that I developed. Part of the problem has been the discrepancy between the bearing Kt reported from test data (i.e. Peterson) versus that is which is commonly used in analysis programs (BJSFM).
Paper 1: Demonstrate the bearing Kt difference between test, cosine distribution (series solution based), cosine distribution (FEM based), contact solution (FEM based).
Paper 2: Difference between a finite "unit" strip versus the as installed configuration with multiple holes (FEM based)
Paper 3: Closed form bearing-bypass solution that uses any of the selected curves from Paper 1 and combined with the effect of Paper 2.
I originally went down this road as a means to allow analysts to simply solve bearing-bypass problems for my book, though I am not sure I will implement the results (I want stay as "standard" as possible)
http://www.espcomposites.com/book.html
Hart-Smith also has a closed form approach which is pretty good, but somewhat dated.
Brian
www.espcomposites.com
RE: How pitch affects the joint strength?
RE: How pitch affects the joint strength?
RE: How pitch affects the joint strength?
I suspect you are referring to the distribution of fastener loads in the joint? That is what it seems like.
I think I was less than clear (too close to me). What I was referring to was the local stress field AFTER you have already established the bearing and bypass loads.
Brian
www.espcomposites.com