[Rhodes22-list] Bow pulpit Construction Materials and Tubular metalobjects

[Roger Pihlaja]

Ed,

The 1 inch OD rod in 316 SS would be about 16X stiffer than the 1 inch OD
rod in FRP composite.  The 316SS rod could experience 30-40% elongation
(bending or stretching) before breaking.  The FRP composite rod would fail
at about 0.4-2.3% elongation.  So, using the average values of 35% & 1.35%;
the 316SS rod is about 35 / 1.35 = 30X more ductile than the FRP composite
rod.  For example, let's say we are going to point load both rods due to
some sort of collision & the force of the collision is going to force a bend
into the rods.  Because the rods are both round, the centerline of the rods
is called the "neutral axis" and it experiences 0 stress during the bending.
The maximum tensile stress (pulling apart) is experienced by both rods on
the outside of the bend.  The maximum compressive stress (pushing together)
is experienced by both rods on the inside of the bend.  The material on the
outside radius of the bend will start to fail when:

(pi * (r + 0.5)) / (pi * r) => 1 + ([% Ultimate Elongation] / 100)

Where:
pi = 3.1416
r = the radius of the bend at the centerline of the rod (inches)
0.5 = the difference in the radius of the bend from the centerline to the
surface of the 1 inch OD rod (inches)
[% Ultimate Elongation] = 35% for 316 SS & 1.35% for FRP composite

Solve for r:

r <= 0.5 / [% Ultimate Elongation]    @ tensile failure on the outside
radius of the bend

Plug in the average values of [% Ultimate Elongation] for 316SS and FRP
composite and we get:

316SS:  r <= 1.43 inches
FRP composite:  r <=  37.0 inches

The FRP composite rod will start failing in tension when the forced bend
radius gets tighter than about 37 inches.  The 316SS rod will accept a bend
radius as tight as 1.43 inches before failure starts.

In addition, by bending that much, the 316SS rod would absorb much more of
the kinetic energy of the collision, thus "protecting" the rest of the boat
from damage.  The required load to bend each rod will be about 16X greater
for 316SS vs. FRP composite.  Since the kinetic energy in the collision is
proportional to V^2, the required collision velocity will be about (16)^1/2
= 4X greater for the 316SS rod vs. the FRP composite rod.

Ed, in this case, the difference is so great that statistics is not a
factor.

If the MacGregor's mast has been permanently deformed by bending or denting;
then, it has been strain hardened.  Even if the mast could be straightened
in some sort of hydraulic press, the old bend would probably always have a
"wrinkle" in it and would less ductile than the rest of the mast.  i.e. A
weak point.  The strain hardening could be removed by heating the entire
mast above the metal's recrystalization temperature, which for Al 7075-T6 =
about 150 deg C for several hours.  At this point, you would have an Al
7075-T0 mast, which would be very soft, very ductile, and not very strong.
The entire mast would then have to be put thru the so-called "T6" heat
treatment process to get it back to the original strength and ductility.
Then, the mast would have to be refinished and all the fittings and other
pieces/parts reinstalled.  It's probably cheaper to buy a new mast.

Roger Pihlaja
S/V dynamic Equilibrium

----- Original Message -----
From: "Kroposki" <kroposki at innova.net>
To: "'The Rhodes 22 mail list'" <rhodes22-list at rhodes22.org>
Sent: Sunday, February 22, 2004 8:23 AM
Subject: [Rhodes22-list] Bow pulpit Construction Materials and Tubular
metalobjects


> Roger:
> You lost me.  Are you saying that a 1" stainless steel rod is
> stronger than a 1" fiberglass rod of the same shape :-).
>
> If that follows, then would a stainless steel rod also be safer?
> Would the margin of safety be statistically significant?  What are
> breaking points in pounds (or whatever the metric measurement is)?  What
> is the statistical difference?
>        Yesterday, on Lake Hartwell, a guy was attempting to launch his
> MacGregor 26, which he bought on Friday.  In changing launch sites, he
> was moving his boat with the mast up.  The mast hit a limb and
> completely bent over.  Can we say with reasonable certainty that he
> cannot straighten his tubular aluminum mast?  Can we now say that he has
> a metal pretzel?
>                         Ed K
>
>
> -----Original Message-----
> From: rhodes22-list-bounces at rhodes22.org
> [mailto:rhodes22-list-bounces at rhodes22.org] On Behalf Of Roger Pihlaja
> Sent: Sunday, February 22, 2004 5:15 AM
> To: bestpestcontrol at earthlink.net; The Rhodes 22 mail list
> Subject: Re: [Rhodes22-list] Bow pulpit
>
> Barney,
>
> The material properties of FRP composites (fiber reinforced polyester or
> "fiberglass") do not lend themselves to building long "skinny"
> structures
> like bow pulpits.  For example, Young's Modulus for 316 stainless steel
> is
> about 28,000,000 psi while Young's Modulus for a typical FRP composite
> is
> only about 6,500,000 psi in the axial direction (oriented along the
> fibers)
> and 1,800,000 psi in the transverse direction (oriented at 90 deg to the
> fibers).  Since the bow pulpit is too "stupid" to know which way the
> fibers
> should have been oriented during lay-up for any given load situation,
> such
> structures must be designed using the lower value for Young's Modulus.
> Young's Modulus is a measure of the inherent stiffness of a material.
> Therefore, FRP composites are only 1.8E6 / 28E6 = 0.0643 X as stiff as
> 316
> SS.  In order to have an FRP composite bow pulpit that was acceptable
> stiff,
> the legs and railing would have to be so thick that there would be an
> excessive amount of windage up on the bow.
>
> In addition, FRP composites are not ductile like metals.  In other
> words,
> when an FRP composite is stressed beyond its yield point, there is some
> internal damage.  Some of the glass fibers break and the chemical bond
> between the polymer matrix and some of the glass fibers fails in shear.
> This damage is cumulative and irreparable.  The next time the damaged
> FRP
> composite is loaded, it yields at a lower value of stress than before &
> more
> internal damage occurs.  However, since the damage is internal to the
> composite structure, it may not be visible on the surface.  This
> cumulative
> damage can progress to the point where the bow pulpit might fail when
> someone merely leaned on it.  In contrast, when a metal is stressed
> beyond
> its yield point, it can deform and bend a lot before failure.  After the
> deformation, the metal is said to be strain hardened & the yield stress
> is
> actually greater than before the incident.  This is why it's nearly
> impossible to straighten a piece of tubing back to its original shape
> after
> bending.  In a collision situation, the 316 SS bow pulpit might come
> away
> bent.  But, as long as there were no visible cracks and the mounts were
> not
> pulled out of the foredeck, the bow pulpit would still be safe to lean
> on.
> With an FRP composite bow pulpit, you wouldn't know unless you ran an
> ultrasound nondestructive test on it.
>
> Bottom line - for long "skinny" structures like bow pulpits that have to
> withstand shock loading & have a safety function, FRP composites bad -
> metals good.
>
> Hope this helps.
>
> Roger Pihlaja
> S/V Dynamic Equilibrium
>
> ----- Original Message -----
> From: <bestpestcontrol at earthlink.net>
> To: <rhodes22-list at rhodes22.org>
> Sent: Sunday, February 22, 2004 12:32 AM
> Subject: [Rhodes22-list] Bow pulpit
>
>
> > from Barney-- Has anyone seen a fiberglass bow pulpit on a Rhodes 22?
> >
> >
> >
> > it looks to extend about 2 ft in front of the bow. Im thinking of
> installing one on my rhodes
> > __________________________________________________
> > Use Rhodes22-list at rhodes22.org, Help? www.rhodes22.org/list
> >
> >
>
>
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