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Fastener Margin Checks

Fastener Margin Checks

Fastener Margin Checks

I was just introduced to fastener margin checks and have some basic questions on the topic.

From research, I've been doing fastener analysis through the following interaction equation:

MS = 1/(((Rt+Rb)^2 + Rs^2)^.5) - 1

where Rt = F_pullthru/Ptu
Rb = bending (somewhat involved process, excluded)
Rs = Fs/Psu

My questions are the following:
Where does this interaction equation come from? Is it simply that bending and tension are normal stress inducing loading conditions and therefore are coupled?
Does the equation above not account for bearing loads on the fastener or is that somehow accounted for in bending?
NAS documentation has columns for Double Shear LBG MIN (Psu) and Ultimate Tensile LBF MIN (Ptu) loads. How are these derived? How come when I simply multiply the ultimate tensile strength of a fastener it doesn't match the Ultimate Tensile load listed. Would it not be more simple to provide values for ultimate tensile and shear strength?

Thank you in advance

RE: Fastener Margin Checks

No, using the material strength properties is not accurate.
Fastener shear strengths account for some non uniform bearing loading, but most fastener shear tests are run in double shear with steel plates so for solid shank fasteners the difference is small. For blind fasteners and for rivets the difference can be large.
Fastener tension strength accounts for stress concentrations under the head and in the threads.
The interaction equation accounts for combined stress state due to sheat, tension and bending on the fastener. The interaction equation has been validated by empirical tests, most from long ago.

RE: Fastener Margin Checks

What SWC said,
Something else to note in the NAS specifications is that is just a minimum value for the batch to be accepted, based on sample testing specified by some other MIL spec (called a procurement spec). That's a separate rabbit hole and doesn't get you closer to allowable strength values for design. Use AR-MMPDS for threaded fasteners, rivets, and always check for a test chart for common types of Csk and Blind fasteners because their allowable strength will be dependent on sheet thickness.
Bearing is a separate issue - usually the fastener is harder than the sheet (or at least equal to) so bearing doesn't usually play a role in the interaction equation.
If you want to chase the origins of the interaction equations that's probably fun but I've never been there. Check out the "B" chapters in Bruhn and follow the breadcrumbs in the references section. If you can find the supplement by Bill McCombs you will learn much more.


RE: Fastener Margin Checks

Thank you both for the responses.

Just to make sure I'm understanding, material properties are not used as they do not account for stress concentrations and other potential interactions. Therefore, when doing fastener checks, I should find the corresponding fastener in MMPDS and obtain the allowables from there? I won't have access to MMPDS till Monday, but what you're saying is that the fastener allowables will change depending on substrate thickness for csk and blind fasteners? Wouldnt this apply to all fasteners and not just csk and blind fasteners? Im assuming it would be the shear strength that would primarily be affected.

SWComposites, can you expand a little on what you mean by "difference" when you're speaking on fastener shear. Is the difference referring to the additional bearing load? And why would this difference be larger for rivets and blind fasteners?

Also, when dealing with single shear, I've been halving the shear strength of fasteners to get a pseudo single-shear strength. Is this a valid method or should I go back and change this?

Thank you once again. The responses have been extremely helpful.

RE: Fastener Margin Checks

Blind fasteners do not have solid shanks.

By difference I mean calculated shear strength using material properties and actual tested joint strength.

Halving the double shear allowable is in general not valid. With single shear the bearing stresses are not uniform thru the thickness.

There is a reason MMPDS (and company design manuals) publish specific joint allowables - simple calcs don’t work in all cases.

RE: Fastener Margin Checks

Oh, and the strap material matters also. Single shear with high strength steel straps will produce shear values near half the published double shear values (assuming the fastener vendor is actually publishing minimum strength and not some wildly optimistic value). Aluminum straps will result in lower SS strength, and composite straps even lower. And strap thickness has an effect also.

Bottom line, if you want an accurate fastener shear value, test your specific joint configuration.

BUT, joints should not be designed to be shear critical.

RE: Fastener Margin Checks

I would emphasise the drop off in strength for thin sheets. MMPDS chapter 8 covers fasteners. Unfortunately you need early versions (like -01) for many typical fasteners; or MIL HDBK 5.

another day in paradise, or is paradise one day closer ?

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