[Rhodes22-list] Short Cycle Fatigue Failure In StainlessSteel Tang

Thu May 6 19:05:34 EDT 2004

Slim,

How about replacing the bolt & tang with a threaded SS eyebolt into the end
of the boom & a shackle to the mainsheet?

Roger Pihlaja
S/V Dynamic Equilibrium

----- Original Message -----
From: "Steve Alm" <salm at mn.rr.com>
To: "Rhodes" <rhodes22-list at rhodes22.org>
Sent: Thursday, May 06, 2004 3:28 PM
Subject: Re: [Rhodes22-list] Short Cycle Fatigue Failure In StainlessSteel
Tang

> Roger,
>
> As always, very complete remarks.  The bolt is not tight and the tang
> rotates freely.  But when lowered, the end of the boom is about 18" from
the
> traveler and just slightly higher.  The tang normally points down with a
> slight angle aft.  When sheeted in tightly, the force is leveraged against
> the bolt alone and is easily bent until it's pointing at the traveler.  It
> didn't take much effort to bend it back either.  According to your info,
> this suggests a substantial amount of damage to the crystal structure.
It's
> hard to imagine all the disastrous possibilities if it were to break--like
> you say--just when I need it the most.  So it must be replaced.  I can't
> help but think there's a better piece of hardware for this than a rigid
> tang.  Maybe just a padeye on the end of the bolt?
>
> Thanks for your help,
> Slim
>
> On 5/6/04 7:22 AM, "Roger Pihlaja" <cen09402 at centurytel.net> wrote:
>
> > Slim,
> >
> > How tight do you keep the bolt that secures the tang to the end of the
boom?
> > The bolt is supposed to be sufficiently loose that the tang can rotate
> > smoothly about the bolt.  As long as the tang can rotate to keep the
loads
> > aligned down the C.L. axis; then, it should never bend.
> >
> > Assuming your tang is a piece of electropolished stainless steel like on
> > Dynamic Equilibrium, the fact that you did bend it is a cause for
concern.
> > The nickel/chromium stainless steel alloys like 304 SS and 316 SS have a
> > nasty propensity to strain harden.  That means that once the piece has
been
> > loaded enough to cause permanent deformation (i.e. a permanent bend);
then,
> > the crystal structure of the metal has been damaged.  When you bent it
back
> > straight, you further increased the amount of damage.  The only solution
> > would be to heat the metal above the so-called "recrystallization
> > temperature", for about an hour.  Assuming the bend was not too severe,
the
> > recrystallization temperature for a 300 series stainless steel alloy is
> > about 450 deg C.  This time/temperature treatment will permit the
metal's
> > crystal structure to "heal" itself.  If you either don't want to do that
or
> > don't have access to a furnace; then, you might consider replacing the
tang.
> > The damage to the crystal structure also causes the metal to become
> > embrittled.  Brittle failure is a cumulative damage sort of phenomena.
> > Brittle microcracks may have been initiated by bending the tang and then
> > bending it back.  Now, normal sailing loads will cause the microcracks
to
> > coalesce into macrocracks and cause the cracks to grow until the tang
can no
> > longer handle the load.  The presence of salt crystals in the air or the
> > water that splashes on the tang will accelerate this phenomena.  The
final
> > failure will occur in a spectacular brittle manner when the tang is
heavily
> > loaded.  i.e. The tang will fail with loud "kerpow" just at the moment
when
> > you needed it most!
> >
> > The whole phenomena I've described above is called "short cycle fatigue
> > failure" (SCFF) & we studied it as well as how to design pieces/parts to
> > prevent it in my ME451, Machine Design class, last semester.  SCFF is
> > usually an insidious hidden problem.  The tang will probably look just
fine
> > right up to the moment it fails catastrophically under load.  SCFF is
also
> > best described in terms of statistics, rather than in absolute terms.
In
> > other words, how "perfect" was the tang's crystal structure to begin
with,
> > did the bend just happen to load up some of these built-in crystal
structure
> > defects & initiate microcracks, and now how often will normal sailing
place
> > loads on the tang that will cause these microcracks to grow?  All three
of
> > these questions can best be answered in terms of statistical
probabilities &
> > the final SCFF will be some complex function of all three factors.  The
> > final question is, "How serious would it be if the tang failed under
load at
> > a critical moment?"  Only you can answer that question.
> >
> > If it were my boat; then, I would either heat treat the tang or replace
it.
> >
> > Good luck!
> >
> > Roger Pihlaja
> > S/V Dynamic Equilibrium
> >
> > ----- Original Message -----
> > From: "anima13" <anima13 at bellsouth.net>
> > To: "The Rhodes 22 mail list" <rhodes22-list at rhodes22.org>
> > Sent: Wednesday, May 05, 2004 9:00 PM
> > Subject: RE: [Rhodes22-list] First Reef Question
> >
> >
> >> Slim,
> >> I did that the first year single handing and never bent it back!
> >> Has been fine and going into 5th year.
> >> Anne
> >>
> >> -----Original Message-----
> >> From: rhodes22-list-bounces at rhodes22.org
> >> [mailto:rhodes22-list-bounces at rhodes22.org]On Behalf Of Steve Alm
> >> Sent: Wednesday, May 05, 2004 12:19 PM
> >> To: Rhodes
> >> Subject: [Rhodes22-list] First Reef Question
> >>
> >>
> >> I had a wonderful sail yesterday.  Temps in the mid-sixties, sunny and
> > winds
> >> 13 gusting to 21.  I was single-handing and had lots of fun washing the
> >> rails.  Rummy woulda been proud.  8-)  I had the boom in the lower
> > position
> >> (first reef) which is something I rarely do.  I suppose I should do it
> > more
> >> often when winds get that high--I guess I just prefer the headroom.
> > Anyway,
> >> with the boom down and when close-hauled, I noticed I had bent the tang
> > (?)
> >> ...the ~4" piece of steel at the end of the boom where the main sheet
> >> attaches.  When the boom is down and sheeted in tightly, the tang bent
> > back
> >> towards the traveler.  When I was done sailing, I bent it back to its
> >> original shape.  Obviously, I don't want to keep doing this because
> >> eventually it'll break.  Has anyone had this problem or found a
solution?
> >>
> >> Slim
> >>
> >> __________________________________________________
> >> Use Rhodes22-list at rhodes22.org, Help? www.rhodes22.org/list
> >>
> >> __________________________________________________
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> >>
> >
> >
> > __________________________________________________
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>
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