Mininum thickness of skins to use countersink rivets 10

[snappish]

Hello. There must be a minimun number for the thickness.
Have searched online and the MIL-HDBK-5H, found nothing.

Pleae help.

 

[Zapster]

Why must there be a minimum thickness?

The answer may be very dimple

 

[freckle]

Hello snappish,
When applying countersunk rivets, you'll want at least 0.2mm of the rivet's shaft standing. This is for keeping a proper allowable shear load.
When you know the height of the rivethead, you know the minimum thickness.
When the present thickness is lower than that, you may want to use countersunk rivets with a low head or apply dimpling like Zapster already mentioned.
You may also want to take a look at Bruhn, Section D1.18 - D1.22.

Good luck

 

[rb1957]

min thk = 1.5*CSK depth

 

[Bazzo]

Hallo
Airbus will tell you that you require a minimum cylindrical portion of 0,4 mm in the sheet. For example a 4 mm dia NAS 1097 has a head height of about 1,0 mm, so the minimum skin thickness is 1,4 for that fastener.
A cylindrical portion of 0,2 mm will reduce the fatigue life by 25%
(ref A300 RAG)

 

[A380istoobig]

As per Boeing SRM, the countersink depth must not exceed 60% of the material thickness. (ref 737 SRM 51-40-08 Figure 7). Anything greater, the material is considered knife-edged and a poor fatigue detail. However, most Boeing AR's and DER's will provide FAA approval to coutersink a maximum of 80% material thickness.

 

[RPstress]

If it's composite the rule is usually csk is a maximum of 2/3 the laminate thickness. In a pinch (conceessions, etc.) you can check the bearing strength of the remaining uncountersunk material against the applied shear, but it will make the pull-through strength awkward to deal with.

 

[Sparweb]

Going back to your research in Mil-Hdbk-5 - if the rivet you are using is in there, and the sheet you are using is on that table, then that test data is relevant. Most tables for csk rivets have a line somewhere in each column. That line is the "knife-edge" line. Above the line is a poor fatigue situation (unacceptable on pressurized skin), but if your structure isn't cyclically loaded, then it's a reliable static value.

What is the rivet/sheet configuration that concerns you?

Steven Fahey, CET

 

[snappish]

Many thanks to all.

SparWeb: What is the rivet/sheet configuration that concerns you?

It is to rivet the wingskin to spar caps and rib caps. And I do care about the wing's aerodynamics... But at the wing tip, 0.025" skin is used for weight considerations. That's my situation.

 

[rb1957]

so you've got choices ...

1) live with the knife condition ... it is a wing tip, so you can expect it to be quite lightly loaded,

2) add an extra thickness on the rivet line ... quite commonly done, machining from a thicker plate, bonding (hot bonding preferred) doubler strips,

3) dimpling the skin ... a bit of a past practice (done many years ago to save the complexity of drilling a CSK).

a fatigue test would validate the design; conservative calcs too.

 

[lchybik]

For recommendation see e.g. Niu - Airframe structural design:
A) Chapter 7.1 General design considerations (of fasteners and structural joints)
B) Chapter 7.7 Fatigue design considerations - especially Figure 7.7.3 (page 232) and other references.

 

[jgarth]

For a composite skin, rule-of-thumb for max csk depth is 70% of structural laminate thickness.

 

[Stache]

There are several FAA documents that have minimums for dimpling. In AC 65-15A Airframe and Powerplant Mechanics Airframe Handbook, Chapter 5 Aircraft Structural Repairs under countersinking and Dimpling it states;

“As a general rule, use the drill countersink method when the thickness of the material is greater than the thickness of the rivet head, and use the dimpling method on thinner material. Figure 5-47 illustrates general rules for countersinking. Note in figure 5-47A that the material is quite thick and the head of the countersunk rivet extends only about halfway through the upper layer of metal. Countersinking will leave plenty of material for gripping.

In figure 5-47B the countersunk head reaches completely through the upper layer. this condition is permissible but should be avoided.

In figure 5-47C, the head extends well into the second layer of material. This indicates that the material is thin and that most of it would be ground away by drill countersinking; therefore, dimpling is preferred. Dimpling will work best if the material is not over 0.040-in. thick.”

Another good reference AC is AC 43.13-1B Acceptable Methods, Techniques, And Practices—Aircraft Inspection And Repair, Chapter 4. Metal Structure, Welding, and Brazing, Section 4 references back to MIL-HDBK-5 for hole sizes.

The above AC’s is available on the FAA web site at:

http://www.faa.gov

search under the AC and type in 65-15 or 43.13-1B and are great reference books.