Blind rivet tension allowable

[dcascap]

Hi,

I came across a guideline that ask for a tension allowable reduction k=20%. So the new tensile allowable would be 20% of what the vendor (as cherrymax) specifies for that blind rivet.

Have you encounter similar guideline? I understand that blind rivets should not be designed to be used in tension, but sometimes can be almost unavoidable. This reduction factor seems too conservative, even though I could be missing something...

Is maybe this guideline if you use the material allowable? or maybe is legacy? There is no much more info than the reduction factor on the guideline itself...

Let me know what are your thoughts about it, I'm curious what you think.

Thanks!

 

[SWComposites]

Where did you see this "guideline"?

Riveted joint strength (tension and shear) is often not driven by the fastener strength, but by the sheet strength. Hence why there are "joint strength" values in MMPDS (and company allowables documents). Vendor fastener strengths are based on tests with steel sheets to force a pure shank shear failure or fastener tension failure.

 

[dcascap]

Yes, that is true but this failure only refers to the fastener itself (its a calculation method in which Ft of fastener = 0.2Ft for blind fasteners, on the other hand, for Hi lok there is no tensile reduction) other joint failures are evaluated later on.

The guideline correspond to a handbook that compiles different methods and guidelines from an OEM and other aerospace companies (I don't want to specify more due to IP).

 

[SWComposites]

maybe it means 0.2 times the material tensile strength * area? a rivet is going to develop nowhere near the base material strength.

 

[WKTaylor]

dc...

WHAT TYPE OF 'BLIND RIVET ' are you actually considering/contemplating with this question...???

Does the BR conform to a procurement spec? shear-head? tension Head? alloy? formed-tail [bulb] style?

IF you have a genuine tensile load to worry about... perhaps you should consider a high strength alloy blind rivet with a tension head and a large formed tail/bulb... or a blind bolt.

 

[dcascap]

Thanks for your comments SW and WK, it does not specify any spec, only "blind rivet", I re-read the methodology, and it said that this reduction factor comes due to the fact that there is no mention on how much the fastener yields and deforms before it reaches ultimate load.
But in my opinion, even if it deforms it would still carry the load, of course, for limit load you should check if deformation of your fastener is detrimental for the structure, and if it is always check with the yield of the material, and have a good margin to it.

SW yeah if I use the material spec, then this factor would make a lot of sense as it's nearly impossible that the fastener can reach this load without failing first, but the guideline specifies fastener tensile allowable so I guess it is the vendor spec.

WK thanks also for the suggestion of what to use if I have a tension issue, it will probably help me in the future...

 

[Ng2020]

Dca. For ref, in Mccombs supplement to bruhn Appendix A, there is generalised guidance for estimating a rivet's tensile allowable for a specific sheet gage (secondary tensile load applications only) as 20% of the joint shear allowable.

Mccombs material was sourced mostly from Chance Vought from memory.

Possibly the author of your handbook has contorted this guidance...?

 

[SWComposites]

as 20% of the joint shear allowable.

and presumably this is the joint shear allowable, not the fastener shear allowable.

 

[dcascap]

That's a good one, probably there is a relation between them, I'll reread the Mccombs supplement to check, thanks!

 

[WKTaylor]

dca, guys... there is burr under my saddle RE this discussion.

As a fastening veteran since ~1985, I am aware of elements that would be relevant to this discussion... and have asked a few basic questions about... but I NOW believe there is a LOT you are not able-to, or willing-to, tell-us... withholding from us[?]... that is very relevant to the big question 'WHAT is your point for asking'??

Historically rivet usage... solid and blind... is confined to small diameters with incidental tension due to shear loading. IF there is a known tensile load component, then by good/standard design practices, bolts/nuts, lock-bolts or blind-bolts MUST be used for reliability.

Long ago and far away... 1940s into the early 1960s... there were tension allowables tables for common solid, and a few [if-any] blind, rivets until it became evident that loading a rivet in tension is a dangerous proposition: too many workmanship variables made this an unreliable crap-shoot. Rivet tensile data... with exception of fastener qualification per procurement specs... is all gone for good reasons.

PS: There were examples of WWII aircraft employing the practice of large solid-rivets in tension. BUT cost/time demands of mass-war production, specialized installation by highly experience assemblers, brutally short in-service life expectancy [5-years, 500-hours], etc... made this acceptable. AND THEN the entry into the jet-age where longer [5+years, 2500-hrs service lives] lives and different loading factors, demanded rethinking of fasteners/usage. AND it prompted fastening innovation that emphasized consistency and performance in differing environments: many 'types' came and are now gone that did not pass muster.

 

[rb1957]

it sounds like your "guideline" is an inhouse stress Analysis Handbook.

It sounds like most I've encountered where various other manuals are "shamelessly" plagiarized.
And in each copying, some meaning is lost. I've used the McCombs tension table.

My suggestion would be to run some tests so see if you can validate the 20% rule of thumb.

My other suggestion is to stay away from blind fasteners ! Particularly for tension loading, particularly for varying tension loads ...