Guidelines for bearing edge distance KDF calculation

[EKounis]

Hi all.

I am looking for some guidelines (or re-assurance) on how the bearing allowables are calculated in case of low edge distance\margin (e/D < 2).
[ol a]
[li]is it common practice to extrapolate using the bearing values given for e/D = 1.5 and 2.0 for values of e/D < 1.5? MMPDS suggests testing, but how is this commonly handled?[/li]
[li]in cases where only one of the e/D=1.5 and 2.0 is provided, is it safe to assume Fbru=0 for e\D=0.5 and use this point to interpolate?[/li]
[li]finally, if both e/D=1.5 and 2.0 are given but describe a behaviour which, if extrapolated would predict Fbru=0 for e/D < 0.5 do we neglect the e/D=2.0 value and interpolate between 1.5 and 0.5?[/li]
[/ol]

Thanks a lot!

 

[SWComposites]

You cannot extrapolate the e/d 1.5 and 2.0 values. Period.

For a protruding head fastener you can do #2. For flush head you should assume Fbru=0 when edge of c’sink touches edge of part.

 

[SAITAETGrad]

Best go back to the drawing board.

Start with a review of shear tear out. Read Bruhn Article D1.10.

Typically, for edge distances ratios below 2 you have a combined shear-out / bearing failure mode.

Also, remember the basic airworthiness requirements.

Sec. 23.601

General.

The suitability of each questionable design detail and part having an important bearing on safety in operations, must be established by tests.

Sec. 23.613

Material strength properties and design values.

(a) Material strength properties must be based on enough tests of material meeting specifications to establish design values on a statistical basis.
[(b) Design values must be chosen to minimize the probability of structural failure due to material variability. Except as provided in paragraph (e) of this section, compliance with this paragraph must be shown by selecting design values that ensure material strength with the following probability:
(1) Where applied loads are eventually distributed through a single member within an assembly, the failure of which would result in loss of structural integrity of the component; 99 percent probability with 95 percent confidence.
(2) For redundant structure, in which the failure of individual elements would result in applied loads being safely distributed to other load carrying members; 90 percent probability with 95 percent confidence.
(c) The effects of temperature on allowable stresses used for design in an essential component or structure must be considered where thermal effects are significant under normal operating conditions.
(d) The design of the structure must minimize the probability of catastrophic fatigue failure, particularly at points of stress concentration.
(e) Design values greater than the guaranteed minimums required by this section may be used where only guaranteed minimum values are normally allowed if a "premium selection" of the material is made in which a specimen of each individual item is tested before use to determine that the actual strength properties of that particular item will equal or exceed those used in design.

Most OEM design manuals have specific instructions for analysis of reduced edge distance. If you are resourceful, you can find these.

 

[WKTaylor]

CAUTION. This 'analysis' can be made very complex by actual fastener installation practices, as follows.

Solid pins clearance fit

Solid pins transition [net] fit

Solid pins light interference fit

Solid pins high interference fit

Solid driven aluminum rivets

Solid driven steel/CRES rivets

Each of these fastener installations create practical issues in stiffness and/or hole wall strain and or hole-wall 'bulge'... that can blow-out Your numbers.

Edge margin of 1.5D is sorta a lower boundary for reliability... and NO VOER SIZING for practical reasons cited.

1.7D was the design guide for the ancient aircraft I work on

2.0D provides [generally] good bearing-tear-out for fastener Dia.

2.0D+0.030" is a 'good design limit for new aircraft... allow modest over-size without falling below 2.0D... assuming accurate hole positioning relative to free-edge.

2.0D + 0.060" is a 'good design limit for new aircraft... allow modest over-size without falling below 2.0D... allows for inaccurate [sloppy] holes positioning relative to free-edge.

Just Saying...



Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[EKounis]

Hi all and thank you for your insightful comments.

It is clear that a design with e/D < 1.5 is far from optimal. Nevertheless, there are such instances due to e.g., oversized fasteners being installed during a repair on a joint which was already marginal. What I get from this discussion is that such cases always have to be substantiated by tests to establish the bearing margins of the joints with the reason being that the behavior of the bearing allowables is not at all linear to e/D (or at least not linear outside the 1.5-2.0 e/D region). Come to think of it, I don't remember seeing plots of Fbru vs e/D for any material; I find that quite surprising to be honest. If anyone has come across any, I would be grateful if he can share it. Just curious to see how the curve looks like..

Regarding shear\tear-out the situation is much more straightforward and that is why I did not bring it up in the first place.

Thanks again.

 

[WKTaylor]

BTW... if I recall the MIL-HDBK-5/MMPDS bearing allowables derivations... to further 'fog' this question...

Fbru, Fbry [1.5D, 2.0D EM] values were determined using a 'solid steel pin', in a very close-tolerance fit hole with guide-plates stabilizing the test article plate... and without benefit of any 'clamp-up' [no crushing friction forces] as would be generated by typical bucked or squeezed solid rivet, or high strength threaded fasteners, installs.

NOTE.
I have witnessed the high positive effect of a large diameter [3/16, 1/4-D] stop-drill-hole at a crack-tip with a tightly installed solid rivet or protruding tension Hi-Lok pin/collar in the [transition/interference-fit] stop-drill hole... etc. There is real benefit in the added circumferential/radial pre-stresses... and the added crushing/clamping-friction forces around the hole [thru the cracked-sheet] in retarding crack initiation/growth on the opposite side of the [deburred] stop-drill-hole.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[edmeister]

Having dealt with the occasional E/D < 2,
Have you entertained the possibility of 'pulling' the fastener hole
- provide a jig so that a larger diam hole is not lined up with the original centre but offset a distance to still allow a better margin.
- may require some talented employees & possibly some machining efforts to build the drill block.