Preload in shear joints
Preload in shear joints
(OP)
Hey everyone,
I was wondering if aircraft manufacturers are using preload tensionning to enhance fatigue life into shear joints, What I mean by preload tightening is instead of introducing a torque to induce a preload ( a lot of scattering, non accurate...), they introduce directy tension by using more accurate toolings ( hydraulic tensioning, strain gauges...)
Thanks
I was wondering if aircraft manufacturers are using preload tensionning to enhance fatigue life into shear joints, What I mean by preload tightening is instead of introducing a torque to induce a preload ( a lot of scattering, non accurate...), they introduce directy tension by using more accurate toolings ( hydraulic tensioning, strain gauges...)
Thanks





RE: Preload in shear joints
RE: Preload in shear joints
other than the practical difficulties of doing this, one thing to consider is the effect this has on the structure around the joint, you'd be introducing tension and compression stresses.
next you need to consider the reduction in shear capacity of the bolt, for static loading.
then there's bolt bending.
this idea runs counter to everything we do in manufacturing (to avoid manufacturing/assembly stresses).
RE: Preload in shear joints
RE: Preload in shear joints
In other industries, with critical tension loaded joints, they do use more exacting methods of generating a preload. This can be done via bolt extension and in certain cases a "torque to yield" can work (though you have to be careful with that).
Brian
www.espcomposites.com
RE: Preload in shear joints
Tightly fitted together parts with interference fit fasteners provide ideal shear joint construction... such as solid rivets, or interference fit Hi-Loks, Bolts, etc
CAUTION... here is where tension preload comes in to play...
The stack must remain tightly clamped together. Thin sheet metal has relatively low eccentricity [sheet centroid to sheet centroid] and the retaining force of solid rivet heads/tails, or the relatively low tension preload of shear head/collar Hi-Loks, or shear-head bolt/shear-nut combos, will keep the parts tightly clamped together... even as loads approach ultimate, especially in the end-fastener [sheet peel, induces fastener tension].
As stack thickness increases for lap shear so does eccentricity... and so does end fastener tension loads. This is where I, and knowledgeable stress guys, will introduce high-clamp-up tension rated fastener systems [still in interference fit]. At some point of load eccentricity, even tension rated bolts can fail heads. NOTE: Tight clamp-up/interference is essential to ensure fasteners see minimal bending loads at ultimate.
NOTE. One way to relieve this eccentricity situation is to introduce a double-shear joint... which may not be practical in all cases.
You haven't lived until You have seen shear head/collar Hi-Loks zipper-off heads, or distort/crush collars, and shear joint pulls apart in tension-shear failures.
Regards, Wil Taylor
Trust - But Verify!
We believe to be true what we prefer to be true.
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RE: Preload in shear joints
wktaylor,
for high eccentricity single lap shear joints, what is the preload tightening method (or control method) used to verify the preload installed? Is it still torque tightening?
RE: Preload in shear joints
RE: Preload in shear joints
RE: Preload in shear joints
RE: Preload in shear joints
Yeah perhaps it does reduce the shear capacity of the bolt but in my case it isn't very important because the bolt is dimensioned so that the plates fail before. And yes the original post was about tensioning the shear bolt to increase his fatigue resistance (transmit some load by friction between plates)
RE: Preload in shear joints
you're straying onto thin ice when you could on friction for shear load transfer.
ok, you've designed the joint to be critical in bearing, and now you've found a fatigue problem with the joint, yes?
i would do several other things before a grasped at preloading a shear joint. i'd start with increasing the section size, maybe going to a double shear joint, but i suspect these are non-starters (since they're obvious, and probably the joint is designed and now you've got to "show it good"). next i'd try an interference fit bush.
so what really is the problem ?
RE: Preload in shear joints
Friction in aircraft structures, is a highly unreliable force, and is NEVER, counted into structural equations. We design all joints as if the spaces in between fasteners is "0" friction... and load transfer only occurs thru fasteners.
Clamp-up forces from fasteners ONLY ensures that the intended shear joint geometry remains stable and viable for the life of the joint. IF the joint becomes loose, for any reason... such as fastener component crushing or failure [head/nut/washer yield, debris in the joint, failure to properly pre-load torque, etc, etc], uneven fastener loading [poor hole quality control], shear-bearing yield, etc... then all fatigue calculations go-out-the-window for a host of reasons: uneven/unintended fastener locding, fretting, corrosion, scoring, etc.
Regards, Wil Taylor
Trust - But Verify!
We believe to be true what we prefer to be true.
For those who believe, no proof is required; for those who cannot believe, no proof is possible.
RE: Preload in shear joints
RE: Preload in shear joints
if you Really want to transfer load over the fitting faces (similar to friction), then you could serrate both faces so load would be transferred by the serration teeth.
RE: Preload in shear joints
RE: Preload in shear joints
Companies have spent many decades, hundreds-of-millions of $$s and millions-of-man-hours in R&D to develop exactly what You are asking about: highly fatigue resistant shear joints. That design/assy data, developed privately by companies, will be proprietary to the individual companies... and will be access-controled. NOTE. There is some data that has been developed and published by the FAA; however, that data will have restricted usefulness without a broader context [such as design manuals].
CAUTION. Due to the technology critical aspects of this data, I suspect that most of corporate developed data [and design documents] would be considered ITAR restricted [= International Traffic in ARms]... at least in the USA... and would ONLY be transferraable with significant corporate/USA-Govt oversight/restrictions on it's distribution/useage.
Regards, Wil Taylor
Trust - But Verify!
We believe to be true what we prefer to be true.
For those who believe, no proof is required; for those who cannot believe, no proof is possible.
RE: Preload in shear joints
sure preload tension control is well understood, and applied to tension joints.
but you're been talking about shear loaded joints ?
RE: Preload in shear joints
RE: Preload in shear joints
I agree with the group that this is BAD idea because of the variations of friction and eccentric loading on the sheets. It would scare me to death to think about what would happen if we depended on interface friction in an aircraft.
Ultrasonics is little used on airframe bolts because the low L-D ratios generally give such limited elongation that the results end up lost in the gage error.
RE: Preload in shear joints
a better approach would be to bond the shear faces together with say a room temperature cure epoxy.
but why the interest in this ? you have a good static design (critical in bearing), but a poor fatigue result ?
sounds odd to me, but ...
RE: Preload in shear joints
rb1957 Interest is in performance obviously, bonding is also a good solution but it's different from what I'm interested in
Sorry for my english, I perhaps don't use the right words to express what I mean
RE: Preload in shear joints
but i don't see how preloading the bolt will improve the fatigue performance of the joint, presumably it's the lug that's critical in fatigue, not the bolt ?
i guess we're not saying it cannot be done, sure the technology is well understood for preloading a bolt.
i thin kwe're all saying that it's a pretty bad idea, at least a pretty odd (unusual) idea. i think it'd be an unusual joint that is designed to be critical (statically) for bearing and then it has a fatigue problem. if i did encounter this joint, i'd use an interference fit bush to improve the fatigue performance.
but it sounds like this is just a thought exercise. can we end on "this is a bad idea" ?