Joint Allowables, Existing Fastener Locations 1

[MNLiaison]

Hello to All,

I am a relative new guy to the field with only one year worth of experience and I would like to know what other engineers out there use as a joint allowable (per SRM tables)in the following situation.

An external doubler is going to be installed on a fuselage skin where one of the repair rows is common to an existing production flush fastener row (i.e. stringer,frame, etc.). The typical repair would be to remove the existing flush fasteners, install countersunk repair washers in the skin, then install new flush fasteners through the doubler and remaining stack up. When determining the joint allowable between the doubler and the fuselage skin at the existing flush fastener row I run into a problem. I can easily determine the allowable for the countersunk fastener through the repair doubler but what allowable can I use for the underlying skin that has a countersunk repair washer installed?

Can I use the allowable for a countersunk fastener in the original skin thickness? Or do I use an allowable for a protruding head fastener in a thickness equal to the skin thickness minus the countersink height?

So far I have been on the conservative side and have gone with using the allowable for a protruding head fastener in the remaining thickness after subtracting the countersink height. I have enjoyed reading all of the threads on this forum for the past year and I finally made the decision to join.

Regards

 

[jetmaker]

MNLiaison,

To directly answer your question, the approach you are presently using would be the one that I'd take. Use the reduced height as the thickness and calculate the bearing allowable based on the "non-clad" joint data with protruding heads.

However, it is better if you could do without the repair washers as they are a poor fatigue detail, and offer further questionable static strength issues such as bolt shanking. Not knowing the specific configuration you are working with, it is tough to provide further guidance. However, look at maybe oversizing the row in question to improve the bearing area (if possible), or think about installing straight freeze plugs, downsizing the fastener through it to get appropriate wall thickness, and then oversize or add another row on your doubler to handle the strength loss due to downsizing.

Best of luck.

jetmaker

 

[Sparweb]

Another alternative that may work, depending on the details of your repair, would be to dimple the upper sheet into the existing countersunk hole. Joint allowables for a variety of dimpled rivet joints are published in Mil-Hdbk-5 (now reincarnated as AR-MMPDS-01).


Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout

 

[MNLiaison]

Thanks to jetmaker and SparWeb for their replies.

jetmaker: I find it interesting that the use of countersink repair washers are a poor fatigue detail. A quick check of a Boeing SRM (757) calls out countersink repair washers for all existing countersinks when performing an external skin repair. A check of the Douglas SRM (DC9) shows no call out for using countersink repair washers for external fuselage skin repairs. I have heard the philosophies can be quite different between the two manufacturers, case in point.

SparWeb: I had some experience with dimpling when I was a Co-Op and it proved to be a royal pain. We attempted to dimple 2024-T3 sheet for use on a DC9 B.U.T.E. door and every time the sheets would incur radial cracks emanating from the hole. We talked with the manufacturer of the dimpling machine and they stated that "dimpling is more of an art form". I had left to go back to school so I don't know the final outcome or solution of the problem. Therefore I personally would be very hesitant to call out dimpling of a repair doubler.

Thanks again.

Regards

 

[Sparweb]

I didn't say it was easy...

Dimpling only works on very thin sheet, so your trouble with it in the past on a door is no surprise (doors tending to be thicker than the nearby skin). Different sources give upper thickness limits of 0.032" to 0.040".


Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout

 

[CRACKKILZ]

The use of csk repair washers is standard practice on Boeing aircraft. I am not aware of any joint strength test data for this type of joint.

I believe your method for determining joint strength may be overly conservative.

Assuming the csk repair washer is bonded in place with some kind of adhesive, and assuming the csk repair washer has at least equivalent bearing strength to the original sheet, then I believe you could use the joint strength value of a csk shear head fastener.

Since this value is not based on actual tests, I would then knock down this value by 15%.

 

[737eng]

Great question, and one that I have had for many years. I agree that the best way to handle these situations is to try and oversize the fastener to clean up the countersink, stepping up a 5/32" fastener to a 3/16" or 7/32" will remove the countersink and give you a good hole that won't require a countersink washer. Remember that you don't want to end your repair on an existing fastener row for DT and fatigue reasons. Keep in mind that on BAC and airbus a/c (don't think the older doublas or MD aircraft chem-milled there skins), that the skin in the stringer and frame areas are typically "padded-up" in order to prevent knife-edging during production as well as to improve the joint allowable for the fasteners, and therefore, these repair fastener rows, typically will have a higher joint allowable than the repair fasteners installed in the adjacent thinner skin pockets.

CrackKilz, good to hear from you, hope all is well!

 

[jetmaker]

MNLiaison,

The problem with using csk repair washers is that you do not get the hole fill that you do with a properly installed csk fastener. Therefore, you must take a fatigue compared to the design scenario.

Furthermore, a risk that is run is possible fretting of the csk washer if the bond or sealant fails to remain in place during service.

When I worked as a Liaison Engineer at Boeing, repair washers on 'new' aircraft were avoided at most costs. If they were required, fatigue margins were checked to ensure no in-service issue. For aircraft in service, if the holes are zero-timed, I would not be too concerned about the fatigue issues of a repair washer.

Regards,

jetmaker