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[TowerTalk] Bolt in Hole (45 Top)

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Subject: [TowerTalk] Bolt in Hole (45 Top)
From: (Dick Weber)
Date: Sat, 3 May 1997 21:09:09 -0500
Comments on Stress Risers and Filling Holes

Holes do create stress risers which means there is a local amplification of the 
stress around a hole due  the disruption of shear flow. The higher local stress 
can then cause the initatition of a crack which will then grow. Crack growth is 
usually initiated by a cyclic loading which causes the material to have 
alternating compressive and tensile stresses.  A stress riser causes the local 
stresses, both compressive and tensile,  to be amplified thus causing a fatigue 
failure in the ID of the hole to happen several orders of magnitude earlier in 
time. The fatigue failure (cyclic plastic flow) in the ID of the hole creates a 
crack which has a much higher stress concentration at its tip than the original 
hole did. So the problem is now worse and, as a result, the crack grows 
quickly. Cyclic stresses (plus and then minus) cause fatigue much quicker than 
loads which may vary between two levels of compressive stress or two levels of 
tensile stress. A rivet can create a "bias" or stress preload by puttiing the 
ID of the hole into a comressive state. When the cyclic loads acts the two 
stress levels may now be a higher compressive (old tensile level +preload) 
stress and a lower level which is also compressive (old tensile + preload). The 
preload, if set correctly, can move the tensile stress (negative value) upward 
and make it a low level compressive stress. Doing so can greatly increase the 
fatgue life of the hole and diminish the effect of the stress riser. 

For this to work,  the "thing" that fills the hole must preload the ID of the 
hole. A rivet can do this by expanding outward. A bolt can not because it is a 
loose fit.


Dick / K5IU / PE


 aod;aresiiveeh  if teh rivet is upset properly.

propoerlyucprelaodepreloadRivets can change the  e etesStresses around a hole 
filled with a rivet can limit the stress es   cause fatigue much quickerr that 
laodaods The main case key to much  tariondiurds urtiguegn   cracka Crack 
growth can be greatly reduced by preloading the materail around the ID of the 
hole to a level such that the alternating load now causes can not resuklt in a 
tensile stress, but two levles of compressive stresses. One high and one lower. 
eelssgive stress alternates between a higher and lower level of to such that it 
bicircunlThe initaiation cile "plus" and a likely propagate.

A few years ago, in a different context, the question of holes as
stress risers and centers of fatigue failure was raised.  Since then,
I've seen a reference to the idea that a filled hole has much less
susceptibility to fatigue than an empty hole.  This reference was 
regarding riveting of aircraft skins.

It makes sense that a filled hole can't deform inward, so the idea of
filling tower-leg holes with a nutted bolt seems a good one.  The filler,
whether it's a bolt in a large hole or a rivet in a smaller one, should
be a tight fit or the effect is lost.

Re Rohn 45, I prefer the flat top sections that bolt onto a standard
section.  Besides giving a place to stand and work (assuming the mast
you're strapped to is trustworthy!), it's a more flexible logistics
situation.  The thick-wall sleeves on the top plate reduce the chance
of fatigue failure at the bolt holes, which in any event are filled by
the bolts.

Cheers de Dave, W6NL (ex-QHS)

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