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

[Peter Thorn]

Slim and Roger,

Alternatively, you could use a piece of light weight high-strength modern
sailing line -- to add a little separation so the boom end block doesn't
chafe on the boom end.  3/16" Vectran would be good for 4,750 pounds load
http://layline.com/llf/llpages/htmlp2origa/page2.htm     Think that would do
it?
Harken now recommends attaching some of its blocks with line for strength,
flexibility and weight savings.

PT




> 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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> >
> >
>
>
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