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.
73,
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.
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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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