Hole edge distance 1

[midsidenode]

thread2-153469
the reference thread was discussion about edge distance. 'Questions is: so how do determine a bearing allowable for a hole that has e/D < 1.5?

 

[GBor]

I don't believe the question is as straight forward as it sounds. It depends on the criticality of the part (fatigue and damage tolerance required?, so material toughness much be considered), the loading (stress toward the edge), material (ductile, brittle). Below 2D, things get a bit trickier.

 

[737eng]

For a joint you have to check:
Shear strength of fastener
Bearing of material
Tear-out of material due to short edge margin
Tear-out between fasteners
Tension failure
Transitional failure (head popping or bending, pull through, everything that can only be tested)

If you have an edge margin below 1.5, your joint will most likely be critical for tear-out of the material and you will have to calcuate the tear out strength using the shear allowables of the material. Note that a short edge margin condition is not only a static issue but more likely a fatigue and crack growth issue even if the short edge margin condition is in line with the load direction or normal to the load condition.

 

[SWComposites]

You can estimate the tear out strength as noted above, but you need test data (static and fatigue) to get an accurate design allowable for the short edge margin for a particular joint configuration.

 

[midsidenode]

737eng wrote: "If you have an edge margin below 1.5, your joint will most likely be critical for tear-out of the material and you will have to calcuate the tear out strength using the shear allowables of the material". yeah - that's kinda what i thought - right now, i'm just concerned with strength margin for this area. fatigue forthcoming. thanks

 

[WKTaylor]

midsidenode...

There are cases where SED is perfectly acceptable... such as for thick material [T>0.5D] and lugs.

What is important to remember is that the devil-is-in-the-details:

A. Drilling/reaming and positional/angular accuracy. How good is Your hole and how close are Your measurements??? Will NDI be able to detect critical cracks, pits, chips, nicks, scratches, etc?

B. Bare or Bushed? plain or countersunk?

C. Interference or cold-expansion [which generates a hoop tension and radial compression stress field].

D. Pin bending and other off-axis loading.

E. Scale effects relative to material or fastener allowables: Alloy/temper, thickness [at heat treatment and any cold/hot working] and form [plate, sheet, drawn bar, extrusion, forging, casting, etc]. Also: is the adjacent edge, square/sharp... or are sharp edges radiused or rounded-off.

F. Technician's workmanship and post inspection verification [NDI, etc]? Would You trust the "team" to build a spaceship... or to build bird-house like Laurel and Hardy? I'm serious... that famous cartoon in Bruhn [Who really determines the strength of a bolt?]is REAL!

G. Are the original material certification test results [Ftu, Fty, Fcy, e, KIc, etc] available? If NOT, is a sample available for testing? See MMPDS for typical examples of "scale" affecting allowables.

H. What is the criticality of this situation? One-hole-out-of-hundreds... or one-of-one??? Loss of aircraft... or just a maintenance nuisance?

I. Is an FEM available for static and DADTA?

J. Are corrosion, fretting, galling or scratching, scoring going to be potential factors. If so, what kind of coating is required? bare? conversion? anodized? plated? primed? sealed? SFL? etc. What are the possible effects of the coating? none, embrittling, lubricating, etc???

K. Are



Regards, Wil Taylor

 

[jvanantonio]

midsidenode,
For edge distances below 1.5D the shear tear-out and bearing faillure are closely related. You can use lug analisys methods for determining the load-carrying capability. As you know the lug analisys methods are based on empirical data. You can take a look into the books of Bruhn or Niu, or in your company hadbooks.

Regards

 

[rb1957]

for me a key condition is how much load is directed towards the edge ? and how heavily loaded the part is, and how many other fasteners are around (one isolated SED, or the only loadpath ?) ...