Subject: Re: [harryproa] Re: masts
From: "Rob Denney" <proa@iinet.net.au>
Date: 11/6/2006, 8:11 PM
To:
Reply-to:
harryproa@yahoogroups.com.au

An excellent post.  Many thanks for the research.
 
regards,
 
Rob
----- Original Message -----
From: Mike Crawford
To: harryproa@yahoogroups.com.au
Sent: Tuesday, November 07, 2006 5:44 AM
Subject: [harryproa] Re: masts


  After much research, I agree with both Peter and Rob.

  In summary:  epoxy cracking likely occurs in first tension on the leading edge, while structural failure of the mast likely occurs in compression on the trailing edge.  The cracking sound will probably be more of an issue for carbon and less of an issue for kiri/glass.   Also, it appears that  Rob's cold vacuum process for creating spars is superior to the heated autoclave process, and carbon and kevlar laminates are better than aluminum in terms of long-term fatigue.

  The rest of this long post contains some of the links and explanation.  It's not definitive, but at least it has some real data.

---

EPOXY PROPERTIES

  I originally made two faulty assumptions:  a) epoxy stretches more than it actually does, and b) epoxy is isotropic, i.e., it has the same modulus in both compression and tension.  The first assumption stemmed from the fact that the flexible epoxies used in some repair work are different from the stiff epoxies used to create masts.  The second assumption came from some research refers to epoxies as being quasi-isotropic, which I misinterpreted.

  Thanks to Peter for pointing out that epoxy has different modulii for tension and compression.  Unfortunately it took me over half a day to get actual numbers to verify this.

  As far as stretch goes:

    http://www.tech.plym.ac.uk/sme/MATS324/MATS324A5%20CFI.htm

    Epoxy only elongates to 2% before cracking, while carbon fiber can elongate up to 10% before failure, and E-glass to 3.37%.  As the leading edge of a mast stretches, the epoxy will crack before carbon fibers or E-glass fibers will break.

  As far as modulus goes:

    http://www.rpi.edu/locker/38/001238/pdfs/load.pdf
    http://www.google.com/search?q=cache::www.rpi.edu/locker/38/001238/pdfs/load.pdf
    (same as above, but html version)

  Epoxy alone has a modulus that is 17% greater in compression than in tension, and an epoxy/carbon composite has a modulus that is 21% greater in compression than in tension.  Both epoxy and epoxy/carbon composite will stretch more under a given force in tension than they will shrink under the same force in compression.

---

MAST CRACKING

Scenario A: flexible unstayed carbon mast

  If the mast is designed to be flexible, the smaller tension modulus means that the windward side of the mast will stretch more in tension than the leeward side will shrink in compression, and epoxy's earlier failure due to strain means that the stretching is going to crack the epoxy while leaving the carbon fibers intact.  These cracks will weaken the mast, but won't themselves represent a structural failure because the undamaged carbon is still very strong.

Scenario B: stiff unstayed carbon mast

  If the mast is stiff enough to keep the epoxy from elongating past its strain limit, cracks won't occur in tension on that windward edge.  Instead, cracks will develop from compression in the leeward edge as the matrix starts to buckle.  This is not necessarily better than the flexible mast, just different.

Scenario C: kiri/glass mast

  Because of wood's inherent flexibility, and the lower modulus of fiberglass when compared to carbon, the cracking is likely in the epoxy on the windward edge.  I don't have a modulus or strain data for kiri to test this.

---

MAST FAILURE

   When a failure does occur, it will likely happen on the leeward edge under compression.  This is because the fibers under tension require no support, and therefore don't put buckling stress on the matrix, while the fibers under compression will eventually buckle once the epoxy fails to keep them in column.  Worded differently, the compressed side is stiffer than the stretched side, but ultimately more susceptible to failure.

Scenario A: flexible unstayed carbon mast

  This mast will provide warning before failure, with the leading edge cracking before the trailing edge buckles. That's definitely a plus.  However, the cracks will weaken the mast over time  The more cracks there are in the epoxy matrix, the less it will be able to keep the fibers from buckling, and the more likely it will fail in compression.  Moral of the story: avoid stresses that cause the cracking sound.

Scenario B: stiff unstayed carbon mast

  For a stiff carbon mast, this means that there won't be the benefit of hearing the micro-cracks in the leading edge as the mast gets stressed.  That's a bummer if the mast has not been designed to be strong enough to handle the dynamic loads involved in a huge gust capsizing the boat.  If the mast is strong enough, the issue then becomes making the boat strong enough to handle the stress, while also surviving the knockdown.  Many folks would rather have the mast fail.

Scenario C: kiri/glass mast

  Failure will likely be in compression because wood does better in tension than compression, and a hollow mast will eventually buckle in compression when subjected to too much force.  Fortunately, the cracking should be less of an issue with the kiri/glass mast.  First, the kiri itself is going to provide more resistance to compression than the glass/epoxy skin.  Second, even if the epoxy is cracked, the glass will still hold tension, and will still help prevent the kiri from buckling.

---

VACUUM-BAGGING VERSUS AUTOCLAVE

    http://www.gmtcomposites.com/the_autoclave_myth.htm

  Autoclaved prepreg masts are stronger and stiffer in tension and compression, while vacuum bagged masts are better in shear.  Six on one side, a half-dozen on the other.  However, vacuum-bagged masts can be made in one long piece, regardless of length.  Autoclaved masts, on the other hand, must be spliced together from sections that fit in the autoclave.  For a small mast, this doesn't matter.  For a large mast, this means joints between sections, and that's less desirable than a single piece.

---

FATIGUE

    http://www.tech.plym.ac.uk/sme/MATS324/MATS324A5%20CFI.htm

  "Under a static load of 50% ultimate stress, the probability of survival for carbon/epoxy, Kevlar/epoxy and glass/epoxy over a 30 year period are 99.99%, 99.8% and 22% respectively.  Under a load of 40% ultimate stress, the survival probability for glass/epoxy is 97%"

    http://www.cstcomposites.com/505_carbon_spar.htm

  "Fatigue resistance orders of magnitude better than Aluminium  .Life Span - pretty well indefinite with normal sailing loads apart from normal wear and tear."

  Not that anyone needed a reason to like carbon.  I've just seen fans of aluminum insulting carbon's ability to resist fatigue.  I suppose early designs could have had problems, but that would seem to be a design or manufacturing issue, not a materials issue.

       - Mike




Peter Southwood wrote:
Hi Rob,
I dont see why the resin should crack because the carbon is stiffer. If the resin cracks it is because it is stressed beyond its maximum strength. The stiffer the fibres, the less likely the resin is to reach failure strain before the fibres. More likely the resin cracks because it is too stiff for the fibres and does not have a matching strain at failure.
Cheers,
Peter
----- Original Message -----
From: Rob Denney
To: harryproa@yahoogroups.com.au
Sent: Friday, November 03, 2006 7:23 AM
Subject: Re: [harryproa] Re: masts

G'day,
Yes, but the stiffer it is, the more cracking there will be.  My understanding of this is that the noise is the resin cracking as the carbon is so much stiffer.  With glass and kiri, there may be less cracking, or it may just happen at a higher load.  Regardless, I think you will get plenty of warning.  Maybe test a piece of kiri and glass by bending it until it starts to crack, then see how much more it will take before it is visibly damaged.  This may give you some idea of how far you can go past the cracking stage.
 
regards,

Rob 
----- Original Message -----
From: Doug Haines
To: harryproa@yahoogroups.com.au
Sent: Friday, November 03, 2006 1:13 PM
Subject: Re: [harryproa] Re: masts

Rob,
 
Is composite glass/kiri?
 
Doug
 


Rob Denney <proa@iinet.net.au> wrote:
If it is composite, you will hear a lot of creaking and cracking before it breaks.  I would work on one of them doing the lifting as in a capsize you will probably have dumped one of them.
 
regards,

Rob
----- Original Message -----
Sent: Thursday, November 02, 2006 12:13 PM
Subject: Re: [harryproa] Re: masts

Robert,
 
I'd like to sense a failure ahead of the crack, split whatever, so as to salvage the mast and simply add more glass around the outside. But that would be a fairly satisfactory level of strength if they - cause there are two of them - raise the hull.
 
Doug

Robert <cateran1949@yahoo.co.uk> wrote:
Easy enough to check if the masts are strong enough. Load them up by
trying to lift the ww hull with a 100kg weight sitting on it.
I was looking at the characteristics of quality bamboo section and it
looks considerable better than glass weight for weight with 4GPa
youngs modulus and a breaking strain of 30kg/mm 2. It may be worth
considering for cheap masts. I am considering it myself for skinning
cores in areas where exra stiffness is wanted without the expense of
carbon. Hope carbon becomes more easily available and cheaper as it
is by far the best material.
Robert
-- In harryproa@yahoogroups.com.au, "Robert" <cateran1949@y...> wrote:
>
> --- In harryproa@yahoogroups.com.au, Doug Haines <doha720@y...>
wrote:
> >
> > Hi,
> >   
> >   Just meant swinging around on the water, like anchored in the
sea
> breeze comes in 20knots.
> >   
> >   I was wondering about general ideas about swinging around
coming
> from experienced multihullers out there. Like when you anchor a
> bigger boat out further where the wind is. What about two anchors
30
> degrees apart?
> >   
> >   Mast is same as boat - kiri strip and glass.
> >   I've tapered the tops to half dimensions. There is a round pole
> up a metre and a half into the mast that slots in the hull. I hope
it
> is all not going to break.
> >   Obviously it is heavy but is cheaper.
> >   
> >   Doug
> >   
> >   
> >  
> > Myriam & Youri <wangka@s...> wrote:
> >        
> >
> >     v\:* {behavior:url(#default#VML);}  o\:* {behavior:url
> (#default#VML);}  w\:* {behavior:url(#default#VML);}  .shape
> {behavior:url(#default#VML);}        st1\:*{behavior:url
> (#default#ieooui) }                Dear Doug,
> >   
> >   
> >     Dear Wangkas,
> >
> >     
> >
> >     Nice pictures and all, but where is a finished sailing boat?
> >               We know it takes a long time, but sometimes we have
> other worries on our minds … like carbon spars for classical yachts
> where we have to sort out two boats, masts, booms, sprits, poles
> etc,  we have to make molds and plugs for rudders, keels and bulbs,
…
> for the carbon parts we are achieving and testing  a construction
> method VAPM (vacuum assisted pressure molding that gives us
autoclave
> quality laminates (not only on the voids ratio but also on the
> compression side) in our pressure molds( compression +-5 Bars)
which
> can produce one piece parts, which is not possible by standard
> autoclave procedures. We are also currently testing a method for
> direct CNC mold cutting for parts construction …
> >
> >     Also, what do you hope to acheive with around beam?
> >
> >      They are not only rounded in length but also elliptical in
> section and will be constructed VAPM… less windage, better stress
> distribution, less wave interference …
> >
> >     Thanks again for your mast section, it is coming together.
> >               We are glad, how are you going to construct it ? …
> >
> >     The boat really swings around at the moment without masts up
> and I wonder if it can be helped? More a question for the cruising
> boaters - do you plan doing cruising Elementarries?
> >               What do you mean with swinging around ? I suspect
we
> will but that depends on the interest shown … for the moment we
have
> some asks for quotes from France, Switzerland and Germany …
> >   
> >   
> >   Best regards,
> >   
> >   
> >   Myriam & Youri
> >   Wangka bvba
> >   Belgium
> >   
> >   e-mail. info@w...
> >   web. www.wangkaboats.eu
> >   
> >   
> >   
> >
> >     
> >
> >     yaendenboom <wangka@s...> wrote:
> >
> >     Dear people,
> >
> > Our site has been updated.
> > We kindly invite you to have a look at:
> >     www.wangkaboats.eu
> >
> > Suggestions and criticism is welcome at:
> >     info@w...
> >
> > Thank you very much,
> >
> > Myriam & Youri
> > Wangka bvba
> > Belgium
> >
> >
> >
> >
> >
> >
> >
> >   
> >    Send instant messages to your online friends
> http://uk.messenger.yahoo.com 
> >
> > 
> >
> >  Send instant messages to your online friends
> http://uk.messenger.yahoo.com
> >
>




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