Bolt holes, burr and fatigue life effects

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wktaylor

Well-Known Member
OH YEAH... 'WM'... even though I know You are kidding...

RE: "Only half the rivets need to hold, so that cuts down workload."

Question… Which half-of-the-rivets in a row should 'hold'... front half, back half, middle half, every-other, groups of '3s'... etc?

Recently my organization encountered a ridiculous problem... we discovered that one/more replacement critical upper skin panels LH & RH wings on our many Depot maintenance jets... since 2012... were installed using highly non-standard/un-documented methods to speed-up production. A high majority of the rivets were irregularly bucked... every discrepancy imaginable.

Us engineers were asked: what is safety margin if 1/2 of the rivets were installed unsatisfactorily [other than fuel leaking]. Since riveting has become so standardized over the decades... and the 'acceptable limits' were tested and validated long-ago... and since rivets are accessible for detailed visual inspection... there was never any issue [reports, standards, etc] regarding wide-scale substandard workmanship... other-than rules such as "1-in10 questionable rivet installs... randomly scattered and no [3] clustered together".

The only answer we all had was this is a critical airworthiness issue.... going back 8-years worth of Depot maintenance. Mandatory removal and replacement of millions of discrepant rivets, under urgent circumstances, will cost millions of s, is warranted. EVEN the 'good' rivets have to be replaced since they have been disproportionately stressed... absorbing load/strain that 'bad rivets' should have been carrying.

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TuweetyBird

Member
HBA Supporter
Google Navair NA 01-1A-1....just happened to remember this guy as I was reading through here, plenty of PDFs available...be safe.

wktaylor

Well-Known Member
USAF => 1-1A-1 Engineering Handbook Series For Aircraft Repair - General Manual For Structural Repair [co-numbered 01-1A-1] Technical Orders

USAF => 1-1A-8 Engineering Manual Series - Aircraft and Missile Repair - Structural Hardware [co-numbered 01-1A-8]

wktaylor

Well-Known Member
BTW... TuweetyBird for grins...

In the 1980s [~4-years], I was USAF lead structural/mechanical engineer [SA-ALC, Kelly AFB] for the T-37 Tweetybird and the A/OA-37B Dragonfly. Great experiences... lots of mishap training, though...

SamP

Member
Thanks for everyone's input. My brain is slowly "grokking." Just trying to build the right model in my brain so all the factoids I pick up fit together.

Interestingly, I learned as I read through materials that while differences in clearance may affect how much load a particular bolt takes, oversized holes don't affect ultimate failure or maximum load, but does affect fatigue failure. I also learned that maximum load drops dramatically if your don't drill perpendicular to the surface.

From my understanding, bolt joints are designed to hold pieces together, and that friction between the two pieces is the predominant factor, not shear strength of a bolt. That friction is created by the bolt preload (how hard you are cranking down on the nut). I also know that the preload on bolts are pretty low for aircraft applications, implying that it is the bolt getting loaded in shear that is doing much of the work. Is there a contradiction there?

Edit: I just read that some shear joints are designed for the bolt itself to take the shear, and not the friction between the plates. It would seem that the aircraft joints are designed according to this So maybe no contradiction.

There is also what entails joint failure. Is it the bolt shearing (then oversizing shouldn't affect the failure since it's the same bolt), or maybe slippage / deformation. There is rabbit hole I'm definitely going down...

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Matt G.

Well-Known Member
No contradiction. In reality, the friction does do some of the work, but in analysis of aircraft structure it is always neglected (perhaps I should say almost always...someone here probably has an exception I have yet to encounter in my career), and assumed 100% of the load is transmitted between the two halves of the joint through fastener shear.

The failure mode of the bolted joint will depend on a lot of things, such as the materials being joined together, their thicknesses, the type, material, and diameter of the fastener, the thickness of the stackup of parts in the joint, and fastener spacing and distance from a free edge, among other things. Oversizing will increase the bearing strength of the sheet at that location, as well as the fastener shear, but having one larger fastener can cause issues in a fatigue critical area. If it is significantly oversized and the fasteners are close together, net area across the joint may become an issue, depending on the structure and loading.

The nice thing is that if you stick with recommended design practices and choose fasteners and materials that are commonly used for aircraft, there are joint allowables that have been developed by test that have many of these things built in.

robertbrown

Member
Doing mod & repair work on large transport aircraft, a lot of poor workmanship would be uncovered as structure was disassembled. Someone remarked that instead of analyzing for a "rogue flaw" that was a .010" long crack in a fastener hole, we should assume that the rogue flaw was a large chisel mark in the hole. I would say that workmanship on the wing was better; critical holes there were classified as "Fracture & Fatigue Critical" and had more stringent inspections.

gtae07

Well-Known Member
This mechanical engineer sees a bunch of individual issues that are taught to mechanical engineers:
I wish those things were taught to aerospace engineers, too. Unfortunately it seems the AE schools (or at least mine) see fit to leave that kind of "practical stuff" for employers/industry to teach. Gets in the way of theory and research, you see...

Reading accident reports for cases caused by minor manufacturing defects is sobering...

robertbrown

Member
I wish those things were taught to aerospace engineers, too. Unfortunately it seems the AE schools (or at least mine) see fit to leave that kind of "practical stuff" for employers/industry to teach. Gets in the way of theory and research, you see...

Reading accident reports for cases caused by minor manufacturing defects is sobering...
I had a boss who had been an AE student when he got a job as a draftsman. He couldn't manage working and going to school at the same time so he dropped out. At that time, they didn't have any problem with promoting draftsmen to design engineers and he finished up his career as an engineering supervisor. He said that it had worked out well because most of the AE's wound up in preliminary design or aerodynamics while more ME's got to work with the aircraft structure and he preferred being close to the aircraft. They don't tell you all that in school. We had several engineering coop students who changed majors after working with us but I'd rather blame that on pushing STEM on everyone regardless of individual personalities than on us being like Dilbert's office.