Guys... Taylor’s Rule-of-Thumb to using rivets in a shear-tension application…
1. Use of driven rivets in shear-tension applications has a checkered history of success and failure. Obviously Bolts***, with washers and substantial nuts, can perform reliably and repeatable in shear, tension and shear-tension joints.
2. How to use rivets in tension-shear applications begs an understanding of the variables.
2.1 The formed head has very reliable geometry including head size, transition radius and grain flow.
2.2 The bucked-tail ["driven head"] has extremely unreliable geometry. Tail protrusion determines potential bucked-tail dimensions (driven perfectly): a short tail will result in a minimum diameter/height bucked “head”; and a long Tail will result in a large diameter/high bucked “head”. If driven incorrectly, You can have a wide variety of bucked head imperfections, the worst of which include: too shallow a head: toe-nailed head, off-center head, under-bucked diameter, cracking/chipping, etc…
2.3 Hole geometry, including roundness, size, roughness, hole-edges (burrs, excessive chamfer, etc) and depth of material stack-up can have a profound affect on rivet installation [driving] and mechanical performance [both shear & tension].
2.4 See MIL-STD-403 and NASM47196 for allowable installation and bucked-tail geometry [acceptance] limits. Very unnerving as to what can happen… and what is acceptable.
3. NOW, can rivets be used successfully in tension or tension-shear joints??? I believe they can, but ONLY with installation precautions and conservative analysis, as follows. NOTE: it is up to the designer and analyst to put “numbers” on the following guidance.
3.1 Installation precautions.
3.1.1 Shall not apply to reduced-head [shear] rated rivets.
3.1.2 Specify that rivet-heads be on primary tension side [bracket/clip, fitting, side]. The head geometry is a reliable size/shape and can be analytically evaluated for tension.
3.1.3 Rivet installation shall be per MIL-STD-403 and NASM47196. NO EXCEPTIONS, especially in bucked tail dimensions.
3.1.4 Hole depth [material stack-up] should be at least 1 x D. The tension-side member and bucked-side thickness MUST be at least 0.33 x D. If total thickness is less than 1 X D, then back-up bucked-tail with a 0.032 or 0.063 thick washer or plate [aluminum or steel, deburred]. This insures that the rivet shank provides high interference friction to resist “pull-thru” and “loosening” in shear-tension loading… in addition to the cross-section of the deformed bucked tail.
3.1.5 A minimum of (2) rivets shall be in tension-shear. NOTE: all rivets loaded in tension-shear should experience “balanced loading”, IE: the centroid of load and centroid of the fastener pattern (in tension) should be closely aligned to minimize eccentricity [unbalanced fastener loading].
3.2 Analysis (ultimate load).
CAUTION: use rivets in tension for lightly-loaded structural purposes ONLY. If failure occurs, then safety of aircraft cannot be compromised.
3.2.1 P (tension-ult) = ((nominal rivet Dia/2)**2) x (Pi) x (nominal depth of head per spec) x (alloy shear strength from NASM5674)
Note: Fsu is used as opposed to Ftu since head will be shear critical thru the fillet radius “upward” [also isolates from rivet-shank-shear inter-action].
Example for MS20470AD4-X
P (tension-ult) = ((0.125-in / 2)**2) x (3.1416) x (0.054-in) x (26,000#/SqIn) = 17.3 pounds-force
Example for MS20470AD6-X
P (tension-ult) = ((0.187-in / 2)**2) x (3.1416) x (0.080-in) x (26,000#/SqIn) = 59 pounds-force
Example for MS20470E6-X
P (tension-ult) = ((0.187-in / 2)**2) x (3.1416) x (0.080-in) x (41,000#/SqIn) = 89.9 pounds-force
3.2.2 Analysis shall verify a minimum tension margin of safety for ultimate load as MS = 0.5 [equates to a SF= 1.5 + 0.5 over limit load] for P (tension-ult). This “conservative” loading will provide added margin for “maintenance abuse”. Note: this is in addition a +MS for the rivet in shear [small decrease in shear due to a tiny cross-over from the head-in-tension will likely be insignificant in the “noise” of the analysis… unless MS = 0.0]
4. Comments.
4.1 I strongly recommend using at least an AD6 rivet or larger “just because”.
4.2 The low performance of rivets suggests “why” industry rarely uses rivets in significant tension.
4.3 A typical AN3-X ALUMINUM Bolt has a P (tension-ult) rating = 1,100#, per specification (although *I* would not stress it above 66% of that number… especially in a fatigue environment!!!). Use an aluminum “AN” nut with matching tension capability for a light weight Installation. Note: a tension-head Aluminum Hi-Lok and mating collar would also have good tension strength! NOTE: for this high load capability, an elliptical load distribution envelope for Shear-Tension allowables, should be used.
Bolts*** = male fasteners with hex/pan/flat/countersunk/etc heads (full or reduced size), NOT including male fasteners with full-threaded shanks [screws]. Lock-bolts with pins and swaged or threaded collars also “fit” this definition.
Regards, Wil Taylor
