Consequeces of "Knife Edges" with Countersunk rivets 5

[stresscalcs]

There are quite a lot of comments about countersunk rivets, their strength allowables and how deep you can countersink in the forums.

However, I can find virtually nothing about the 'consequences' of actually having a 'knife edge'.

Most of us will probably know the picture of head shear off from Bruhn and have heard from others "you cannot do that!".

Does anybody have or know of any references to documentation/tests showing the effects of the knife edge situation?

Any information would be appreciated.

 

[GrandpaDave]

Try looking in the stress man's bible, MIL-HDBK-5 or the latest version of MMPDS which replaced the handbook a few years back. There are lots and lots of allowables from test data for countersunk rivets in various sheet materials and thickness. Hope this helps ... And old stress man who just joined this forum for fun.

 

[stresscalcs]

As one old stressman to another old stressman. Welcome.
I am sure you will have fun joining the in the various interesting discussions that come up.

Unfortunately I am not looking for allowables. I am looking for documentation of the consequences of a knife edge existing.
This is most likely going to be known by people (old stressman like ourselves) who have experienced the problem.

Nonetheless, thanks for the response and stay tuned in.

 

[ESPcomposites]

But doesn't the allowable (as a function of the sheet thickness) directly demonstrate the knife edge effect (at least the static effect).

Brian

http://www.espcomposites.com

 

[stresscalcs]

I don't believe that the static allowable for a countersunk rivet with a knife edge will be too much different from a value for a rivet without a knife edge and 0.4mm cylindrical hole length.
I have to check, for the situations where rivet head depth exceeds the countersunk depth, are the lower allowables you find caused by the bad formation of the rivet head. It is not clear to me if the excess countersink depth is accommodated in the second part of the joint or not, such that the head is formed correctly.

Fatigue values might show more significant differences.

In bearing, statically the knife edge situation offers more bearing area and the rivet shear area is unchanged or could be seen as increased.

The question is, can the knife edge actually damage the rivet at the interface between the two joined items sufficiently to cause a noticeable joint detrimental effect?
Does it depend on materials being used?

Does any tension load developed in the countersunk rivet (which we never evaluate as it is automatically covered by test allowables) caused by the head shape increase with the knife edge?
Seems unlikely.

Does the first loading of a joint with a knife edge simply "dull" the knife edge to the equivalent of more than a 0.4mm cylindrical hole length anyway?
Consequence is a "normal" joint static strength but loose rivets?

Thus I ask, has anybody got any references to the real life effects of knife edges.

 

[GrandpaDave]

I miss read your question... sorry about that. I can't help with specific questions on knife edge situations other than design to avoid those situations especially when fatigue and crack initiation plus growth are a concern (Fracture Analysis). Can't help you there. Need to catch up to folks that like doing science projects... NASA, USAF labs, universities, etc. Good luck.

 

[SWComposites]

typing "countersink knife edge effect" into
google.com
scholar.google.com
brings up several papers/documents in the first few pages which might be useful. Have you looked in Bruhn or Niu's Airframe Stress Analysis book for data/info?

all data that I have seen is proprietary, but

In bearing, statically the knife edge situation offers more bearing area
> not necessarily; the effective bearing area is not the full projected area of the head, and the stress tends to be concentrated at the cylindrical portion or towards the shear plane in a knife edge case.

The question is, can the knife edge actually damage the rivet at the interface between the two joined items sufficiently to cause a noticeable joint detrimental effect?
> yes; particularly in fatigue
Does it depend on materials being used?
> yes

Does any tension load developed in the countersunk rivet (which we never evaluate as it is automatically covered by test allowables) caused by the head shape increase with the knife edge? Seems unlikely.
> depends on the joint configuration; if there is a high eccentricity (thicker sheets in single shear without any backup support structure to resist the moment), then a pull thru failure is more likely with a knife edge case, and this is not covered by the published allowables.

Does the first loading of a joint with a knife edge simply "dull" the knife edge to the equivalent of more than a 0.4mm cylindrical hole length anyway?
> yes if the bearing load is high enough, the hole is likely to deform, but since the material is somewhat constrained under the head the deformation is not going to be a simple formation of a cylindrical portion. And this is likely going to lead to fatigue issues

Consequence is a "normal" joint static strength but loose rivets?
> a not quite normal static joint, but "loose" rivets, which is not a good thing.

> unless the static bearing loads are quite low compared to the allowable, allowing a knife edge condition is probably not a good idea, as it will likely lead to fatigue issues - cracking, hole wear, fastener pull-thru, leakage, etc.

 

[edbgtr]

stresscalcs,

The knife-edge (K-E) condition, sometimes known as the feather-edge, is primarily a fatigue problem.

The rivet/fastener allowable joint strength tables either in the public domain, or in company manuals, vary depending on their application. Manufacturers of military hardware will often provide static joint “allowables” for conditions where the plate thickness is a little smaller than the fastener head. I use the term fastener because the condition applies to driven (bucked) rivets as well as for Hi-lok type fasteners. For military hardware such K-E allowables permit damage repairs of a temporary nature to be used for a short duration under combat conditions. For public domain data, like the MMPDS (Mil-Hdbk-5), allowable values vs. plate thickness can be plotted out to reveal the nature of the failure of the joint. The allowable values at low plate thicknesses usually have a linear trend and can be extrapolated towards the thinner plate material end to produce an intersect with the fastener head depth.

The reason the term feather edge is also used is because if the edge left behind after the over-zealous countersinking is inspected under a magnifying-glass, the sharp edge will look like the ruffled edge of a bird’s feather. This means that before any load is applied to the K-E joint there are small crack initiations present. Crushing these edges under the first significant load will not close them up, they will be compressed and when the load is released a certain amount of springback will occur, thereby putting said edge under a residual tensile stress. This material condition then has the tendency to grow a crack from this point.

The image of a plate cutting the head off a fastener is possible where the plate material is harder than the fastener material (BB or XBC rivets). 7075-T6 sheet IMHO would have a tough time cutting off the head of a HL11VF (XF) Titanium Hi-lok.

The latest Peterson & Pilkey(s) contains the results from a study into the SCF for CSK fasteners. According to the study, the maximum stress concentration occurs at the junction between the CSK slope and the cylindrical portion. Those who work with the Hi-lok, -tigue, -lite fasteners will know that much work has gone into preparation of the hole at this junction in order to obtain long fatigue life from such joints.

As a matter of interest, it should be noted that certain companies allow a cylindrical length of down to 0.2 mm (0.008”) for MRB purposes, depending on the static margin of the fastener row. Design guidelines are one thing, but production “reality” is another. For the company in mind, they cover themselves by having done their own testing to justify such a relaxation of the rules.

Ed.