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

[Todd Tavares]

Roger,

   Very good explaination.  I suppose this heat treatment and/or replacement would be recommended for the tangs for the backstay chainplates as well?
 
Todd

----- Original Message -----
From: "Roger Pihlaja" <cen09402 at centurytel.net>
Date: Thu, 6 May 2004 08:22:46 -0400
To: "The Rhodes 22 mail list" <rhodes22-list at rhodes22.org>
Subject: [Rhodes22-list] Short Cycle Fatigue Failure In Stainless Steel Tang

> 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
> >
> > __________________________________________________
> > Use Rhodes22-list at rhodes22.org, Help? www.rhodes22.org/list
> >
> 
> 
> __________________________________________________
> Use Rhodes22-list at rhodes22.org, Help? www.rhodes22.org/list

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