From: bill@extro.ucc.su.OZ.AU (William Logan Lee)
Subject: Re: Rivet popping
Organization: /etc/organization
Date: Sun, 22 Nov 1992 16:22:28 GMT


From bill@extro.ucc.su.OZ.AU (William Logan Lee)

I said:
 A non-penetrating hit on a bolt or rivet could still shear the
 fastener off and create a lethal secondary missile.
 
In article <BxB8D7.KKA@law7.DaytonOH.NCR.COM> Dan Sorenson <viking@iastate.edu> writes:
>	Now this got me to wondering, and while trying to imagine a hit
>on a bolt head sending the bolt flying about inside the tank, my mind
>went back to the movie Das Boot, where the rivets in the bulkheads were
>being sheared and flying about the ship.
>
>	Question: does it happen?  I can't imagine how a rivet, unless
>it's just incredibly brittle, would shear like that.  If it was just
>sort of cut apart by flexing bulkheads, the bulkheads should have
>caved in once their structural integrity gave way enough to allow this
much flexing.  Has anybody seen references to this?

Ignoring the question of whether or not there were rivets or not on German
submarines, I will attempt to explain why popping rivets can become lethal
missiles, and other trivia:

Whenever you rivet two plates together, you generally try to rivet them
hot (red hot rivets).  This makes the forming of the head on one end much
easier, but also, when the rivet cools, it contracts, drawing the plates
closer together.  Done properly, it will leave the the shank of the rivet
under tensile stress after rivetting.  Much of the strength of the rivetted
joint comes from the friction between the plates, rather than the shear 
strength of the shanks of the rivets.  If the plates are loose (not held
together by the rivets under tension) then you rely on the shear strength
of the rivets, rather than an even distribution of stresses in the plates.

Under ideal conditions, stresses are distributed in a rivetted joint so
that the failure occurs in the element designed to fail, or at any element
in the structure if there is no designed failure point.  Unfortunately,
when some structure is fabricated, things are never perfect.  Holes are
drilled unevenly; too close together, too far apart, holes too large, etc.
When the rivets are set in place, they can be incorrectly set, maybe
set whilst too cold, or maybe some surface contamination prevents the
the plates from mating perfectly.  Holes may be redrilled because of
misalignment problems, elongating rivet holes.

When the structure is put under stress, there are always going to be rivets
under more stress than others.  If a rivet fails, then a combination of
released strain energy (from that previously stored in the rivet), pressure
from one side, and springing of the joined plates (that were previously
under compression by the rivet, as well as by the deformation of the plates
under some stress like an explosion).  Armour plate is very tough and is
thus also springy. (As an aside, I worked once at a coal mine workshop.
They repaired mine machinery like underground miners which were protected
with high tensile plate steel.  When one of these plates was bent, it had
to be straightened out in a hydraulic press before it was replaced on the
machine.  The plate was set up on the press and everyone left the
hanger-sized workshop so that the press could be operated by remote control.
If something had given way, you would not have wanted to be near that press.
And this was only for 1/4" or 3/8" steel plate.) So when a rivet fails,
there is potentially a lot of energy available to convert this stress
into the kinetic energy of the rivet fragments.  The structure may not
fail if the strength of the structure  remains essentially uncompromised.
A failure of a single rivet should not cause failure of the entire
structure, with the stresses previously contained by that rivet being 
transferred to the remaining rivets around it.  In the case of a nearby
explosion, this is assisted by the stresses being of a temporary nature.
Under a constant pressure, you would indeed see a sucession of rivets
failing, as each breaking rivet increased the stress on its surrounding
neighbours, until the stress causes all remaining rivets to fail.

Once you have sprung a rivetted joint (lost some rivets or made them loose),
the strength of that structure is greatly decreased as you then only have
the strength of a limited number of rivets' shanks to withstand stresses
(instead of the contributions of all rivets and friction between the two
plates).

Bill Lee

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