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

[Roger Pihlaja]

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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