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

[Steve Alm]

Roger,

Sounds like a workable solution.  Thanks.

On 5/6/04 5:05 PM, "Roger Pihlaja" <cen09402 at centurytel.net> wrote:

> 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
>>>> 
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>>> 
>>> 
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> 
> 
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