[Rhodes22-list] Short Cycle Fatigue Failure In Stainless Steel Tang

[Steve Alm]

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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