i guess hans is talking about orientation for a strength perspective so ive really go no experience on that
but it doesnt seems really that much difference with regards to stiffness
Keen,
No, you were on topic. And, forgive me for taking this opportunity to drift off topic a bit.
Indicator Threads
Though I rarely do repairs for others anymore, when I was doing them for extra cash there were two kinds of damage
that I found particularly interesting; a completely snapped board. and a ‘bottom’ stressed cracked board. I was particularly
interested in boards that had snapped relatively close there mid-section – particularly in the state of the stringer, if one
was present, and one usually was.
You can tell if wood has been worked, it becomes pulpy, for lack of a better term. A given piece of wood that has been worked
beyond certain limits looses its strength rapidly. Upon casual visual inspection, the signs that a piece of wood has been worked
are not always obvious. Nevertheless, if the ends of the broken stringer tended to show a sort of pulpiness, and not all did, I
tended to wonder if the failure could have been attributed to, if not potentiated by, prior working. For me, reading the foam was more
difficult, and as for the glass, more still, then again I wasn’t all that sure what to look for to begin with.
Basically the same for bottom stress cracks. But I wasn’t all that convinced that such cracks meant ultimate failure had occurred.
Of course with time and continued use, water seepage/damage would change the ‘working’ properties of the materials
and probably lead to ultimate material and structural failure. Anyway, I saw such stress cracks as basically a release of
fracture energy and not necessarily a failure – i.e. something had behaved the way it should, in particular the way a composite
should, or to put it another way, the board was often still sound. The fact that such cracks often lead to other problems was another
matter, and it was that which had to be addressed.
…. A Need For More Information – Indicator Threads
My solution, and its wasn’t so much a solution but a way of getting more information, was to find some way of incorporating
‘indicator threads’ into the glass. In particular, I felt three types of threads might would be appropriate, each a different color,
each breaking at a given level of strain (or stress, either approach is appropriate). The first would break under modest strain,
the second, somewhat more, the final very close to the ultimate failure strength of a glass fiber. The threads would be sparsely
incorporated into the glass weave. To tell if a given thread had broken would require a magnifying glass.
I don’t believe this is a novel approach. I don’t know if it has been used in the composite industry, perhaps it has been
considered and rejected for some obvious practical reason. At the time, I simply didn’t have the resources to pursue the idea.
I do know that just touching fiberglass breaks fibers and weakens the cloth – in general the less it’s touched, the stronger it will be.
So I wasn’t too convinced as to how practical the idea was just from that consideration alone.
Also, because a composite matrix often shrinks as it cures, there was the chance that virtually most of the threads, at least those
close to the points of attachment, would break as the board cured (differential strains set up during the cure, not all of them compression.)
In a nutshell, if the idea was feasible – it called for a good Material Engineer/Scientist.
However, it would have been nice to have such indicators, even if there were only of one kind. They might tell a builder a lot about
how and where loads are being developed, and could possibly used to gauge the state (health) of the composite during its lifecycle.
Anyway, my apologies for the diversion, and thanks for responding to my questions. You too Hans, sorry for the diversion.
kc
Quote:
? are we in agreement then? E-glass is isotropic, while Carbon and Aramide are anisotropic. Orientation of CF or Kevlar is very important. But orientation of a E-glass woven mat is unimportant.
No we are not!
The molecular structure has nothing to do with it!
When we lay all fibres in one direction then we get a marcostructure that could be called anisotropic! (do you understand?)
We do not use one plate of glass that is the same in all directions, we use fibres.
In the transversal direction we get “matrix-fibre-matrix-fibre-m-f-m-f-m-…” so the chain breaks on the matrix!
in the longitudinal direction we get long fibres that are glued on a long side (how do I have to explain in english?), not just on top, so it wont be the matrix that breaks first.
Orientation of FIBRES is always important!
Do you see that A will be stronger than C? Only in A the fibres break, in B and C the matrix breaks and then the molecular structure of the fibre really doesn’t matter!
PS: When the transversal break point of CF or Aramide is lower than that of the matrix (what I doubt) then the orientation of that fibres would be even more important than the glass fibres. But the orientation of glass fibres stays very important.
I didn’t consider the wood during cyclic loading. Do you think boards designed with a stringer fail because the wood gets worn out. If so would a well designed stringerless board or pvc stringer last longer?
Have you given any thought as to what your indicators would be?
I think you would want to match up the stiffness and have different strengths.
This may or may not work, and would be very difficult to set up because E (youngs modulus), UTS (Ultimate tensile strength), %elongation, CTE (coeficient of thermal expansion), bond strength, and others would be factors in the snap of these indicators.
I think you would need to
Match E, CTE, and bond strength
and have a much lower % elongation, and a lower UTS in your indicator
anyone else have some input?
Paul,
Very nice.
Failure analysis and monitoring is one of the few low cost tools available to a builder, but (obviously?) an important one.
kc
Yes A is stronger than C, but when do you ever use unidirectional woven cloth? Since a two directional woven cloth will always have the same effective length in one direction as it does in a perpendicular direction regardless of the orientation of the cloth.
Quote:
The molecular structure has nothing to do with it!
Now I’m confused, what were you getting at with your previous post with the molecular drawings?
I don’t have time now but if you are still confused I will show you the math tomorrow.
Something else to blow your mind the critical fiber length in an epoxy/e-glass matrix is only a few mm. That means that if you could lay down chopped mat with the same volume fraction of reinforcement. You could create a composite with effectively the same stiffness and strength without the use of woven cloth.
Keen,
I wasn’t really interpreting the ‘worked’ ends of the stringer as the cause of the failure, just symptomatic of a failure,
in particular that the board had been ‘worked’ in a certain fashion.
Interesting thoughts, nevertheless.
kc
Quote:
I didn’t consider the wood during cyclic loading. Do you think boards designed with a stringer fail because the wood gets worn out. If so would a well designed stringerless board or pvc stringer last longer?
I haven’t noticed wood ‘wearing out’. . . not in surfboards, and not in trees either. Wood is designed to flex repeatedly without fatigue. .
If any of you manage to make a glass and resin tree which performs as well as a wooden one then please let me know
Some basic tensile strength numbers so that ya’ll can decide for yourselves:
(values are approximate)
#2 EPS: 25 psi
Dcell H60 (similar to surfboard PU foam): 250 psi
Balsa: 1,000-3,000 psi
Laminating Epoxy: 10,000 psi
E-glass: 500,000 psi
That’s 500,000 pounds per square inch. A Boeing 747-100B weighs 373,300 lb empty.
Note typical eps/epoxy boards have much lower foam core strength but higher fiber content. Are eps/epoxy boards more snap resistant?
Do you have any pictures of these pulped stringers? Looking back at my previous post and reading roy’s makes me question the validity.
Do you think that the wood is worked along with the foam/fiberglass/epoxy or does the composite loose its strength and takes the stringer with it?
Keen,
On Pictures
I described the edges as pulpy, ‘for lack of a better term’, or softened. No pictures - wish I did have some.
On Working
Perhaps there are materials for which it is not the case, but my guess is that virtually any stiff, if not solid, material can
be worked to the point of failure.
For a given solid material there are always limits to the amount of strain it can handle and still return to its original state
when the load producing the strain is removed - i.e. return to its original state without permanent deformation. Surprisingly,
for most materials, the ultimate strain - that level of strain that a material can be subjected to prior to permanent damage) is
usually pretty small, commonly less than 1%, but there are exceptions.
Working usually refers to taking the material up to and just a little beyond its ultimate strain, repetitively. The damage
accumulating with each cycle. Unless the material somehow manages to repair itself, e.g. its living tissue, or be
repaired in some other way, the damage will accumulate. The now weakened weakened material is likely to fail until
failure under conditions of lower stress than would be have been expected if the material hadn’t been worked.
Working also tends to flatten, at least change the initial, or low strain, section of a material’s stress/strain response
– i.e. you’ll see a greater strain for a given stress as the material is worked.
On stringers
It is my impression that virtually all of the strength of a composite structure like a surfboard, unless poorly
designed structurally, can be attributed to the composite. I’m inclined to believe that most modern stringer
applications in composite structures tend to be used structurally e.g. controlling characteristics such as flex
or stiffness, vibration, etc. rather than with the objective of contributing to ultimate strength, though they can
be used in this way.
The problem here is that, the proper analysis is not that of ultimate strength, but that of durability and stiffness
characteristics. In terms of ultimate strength, putting together a composite isn’t going to give you something of
greater ultimate strength than any of its individual components, nor is it going to give you something that has
the ultimate strength of the sum of the ultimate strengths of its components. What a composite might give you
is something that is less prone to ‘catastrophic’ failure, or more precisely, something that is more durable,
- something with a superior ability to handle handle ‘cracks.’
If you look at a the surface of a surfboard under a microscope you’ll see a plethora of micro-cracks. However,
the neat thing about composites is that they can, in general handle such cracks -i.e. they are stopped, by the
very nature of the composite structure from growing into larger, more critical cracks. Of course, not all composites
are equal in this ablity.
Surprisingly, in a composite, cracks, by themselves aren’t necessarily a sign that something is about to fail, often
its the contrary - something has done its job. But that doesn’t mean that they [cracks] shouldn’t be monitored.
kc
Some references:
J.E. Gordon, The New Science of Strong Materials (1976) ISBN 014 020 904?
J.E. Gordon, Strucures or Why Things Don’t Fall Down (1978) ISBN 014 021 961 7
Quote:
If you look at a the surface of a surfboard under a microscope you’ll see a plethora of
micro-cracks.
Only if you are looking at a surfboard with micro cracks, and not if you are looking at a surfboard without micro cracks
By the way have you looked at a variety of surfboards under a microscope ?
I very much doubt that my boards are a plethora of tiny cracks, and just wanted to add that there is nothing more high tech than a tree.
Theory has it (and testing proves it) that flat unidirectional cloth is stronger in tension than woven cloth. Where the strands bend over eachother in weaving results in a “crimp angle”. The higher the angle the greater reduction in strength. So non woven cloth is better.
I’ve done a fair bit of testing composites and one of the most noticeable effects when doing a tensile test of materials containing woven fabrics is the early debonding of the matrix around the “crimps”. This is because the fibres are trying to straighten out as they are loaded. Unidirectional fibres do not do this (assuming they are straight to start with) they just load up until catastrophic failure.
You can see this effect in fatigued samples as well. Problem is that cloth used for boards is so fine (and the background is often white) that this is almost impossible to see. It might appear over an area as a sort of “milkyness”.
Wood is fantastic in fatigue (lets hear it for Tom!). Early GRP racing dinghies became uncompetitive after only a few seasons as microcracking in the unsupported flat panals of the planing hulls resulted in a material that slowly got soggy. The ply boats continued to work fine and many people went back to them. That all changed when foam cores came along!
There is loads of info out there relating fibre angle to strength and stiffness. As has already been said the outside skin of a bending panal is in tension and the tensile strength of that skin (and its modulus) will affect the stiffness, bias cut cloth is not as stiff.
" For those composites with fibres aligned in the same direction, the tensile properties can differ markedly with the angle of applied forces to the fibre direction. Thus the tensile strength at right angles to the fibre direction can be about 1/6th of that in the direction of the fibres. The tensile strength at just 10° from the fibre direction may be as low as ½ that in the fibre direction. The material is thus said to show anisotropy. Composite made with randomly orientated fibres do not show this effect, however their tensile strengths are much less."
Quote:
That means that if you could lay down chopped mat with the same volume fraction of reinforcement. You could create a composite with effectively the same stiffness and strength without the use of woven cloth.
I don’t think so. And I don’t have to do any calculations to know that. I’ve done the tests panels, and it’s not as stiff and it’s not as strong. There is a reason why you don’t see chopped mat being used for surfboards.
Ditto.
Esp with thin surfboard laminates.
What’s funny about these discussions is that there are volumes of books written on this subject, and yet people try to summarize it all into one small all encompassing sentence or paragraph.
Plain old woven fiberglass cloth is a small miracle. I’ll take the tight woven high “crimp angle” stuff any day. Think of it as the ‘butter’ of composites. Its easy to make and makes things taste better.
Quote:
O-lam: Ar and I-lam: C:Same as previous + When C breaks Ar is still intact and your board will still be strong but loses its stiffness.
You are assuming that carbon fiber doesn’t contribute strength and aramide doesn’t contribute stiffness. You are also not taking into consideration the material properties in a matrix (epoxy). And who designs surfboards to function when one part fails. The emphasis should be on what is happening before something fails (in the case of flex), or how quickly or not it fails.
The text books are good for aerospace applications and things that don’t flex and are made to be durable. But everything gets thrown out the window with surfboards (and also snowboards and skis). Sailboards, I think aren’t designed to flex so your theories might apply. But if you want to make something strong and stiff you are not going to get any better than carbon fiber. There is a reason why aerospace uses carbon fiber and not aramid. There is a reason why sailboat masts are made with carbon fiber and not aramid.
Like I said before, when it comes to flex, you’ve got to throw away the text book, because there is nothing in there about flex. Stiffness is not a measure of flex. There is a reason why carbon fiber is not used very often in surfboards (besides the cost, of course). The flex is different from E-glass. And aramid is different also. This is a concept that is very hard for engineering types to fathom.
You really have to do test panels to see what I’m talking about. The panels need to be the same measured stiffness regardless of weight. Once you have the panels, you just have to flex them and notice the flex characteristics like flex return and flex dampening. Believe me, there is no way to measure or describe these characteristics anymore than I could describe why one poet or artist is better than the next.
In my opinion, nothing beats E-glass for surfboards that are made to flex (also snowboards and skis). But if I just wanted to make the lightest and strongest (and probably horrible to ride) surfboard, I would get the highest modulus carbon fiber I could find.
BRAVO MEASTRO!
ITS OFFICIAL, FROM NOW ON, I DEFER ALL OF MY COMMENTS TO KENZ. 
I esp love the throw away the textbook and poet parts…I’d love the textbooks to explain ‘feel’ for me. Can they also explain why an off-key musical note sounds wrong?
e-glass=butter
@ kenz.
I think that most people here do not understand what I’m trying to say.
When you use aramide and carbon on the same side of the EPS-core then the carbon fibres will take the most load because they are stiffer, I never said that aramide takes none of the load. When you use Carbonfibres you better use glass fibres with it because the aramide fibres won’t be worth the cost. Exept if you want the super preformance but even more super expensive board.
I also said that if you want a flexible board you will need glass fibres (S or E) that’s you to decide.
If you want a light and unbreakable surfboard that flexes, just use S-glass!
In this topic I just want to give my thoughts about the use of aramide and kevlar. Where to put them and where not!
And sorry that this theory more useful for sailboards but I am a windsurfer and I thought this theory would be a bit interesting for you too.
Quote:
The text books are good for aerospace applications and things that don’t flex and are made to be durable.
Well I ain’t no “injuneer” but I know with certainty that if the aforemention 747-100B (great ol bird… I’ll get to that…) didn’t flex it would have been a very short flight indeed…
The newer generation aircraft like the 787 and A350XWB actually use copious amounts of epoxy and assorted funky fibres like carbon and aramide because they can engineer the flex and save weight to boot.
747-100b… used to have a bar in the upper deck so you could booze and snooze your wat across the pond… those were the days…
Quote:
Quote:O-lam: Ar and I-lam: C:
Same as previous + When C breaks Ar is still intact and your board will still be strong but loses its stiffness.
You are assuming that carbon fiber doesn’t contribute strength and aramide doesn’t contribute stiffness. You are also not taking into consideration the material properties in a matrix (epoxy). And who designs surfboards to function when one part fails. The emphasis should be on what is happening before something fails (in the case of flex), or how quickly or not it fails.
The text books are good for aerospace applications and things that don’t flex and are made to be durable. But everything gets thrown out the window with surfboards (and also snowboards and skis). Sailboards, I think aren’t designed to flex so your theories might apply. But if you want to make something strong and stiff you are not going to get any better than carbon fiber. There is a reason why aerospace uses carbon fiber and not aramid. There is a reason why sailboat masts are made with carbon fiber and not aramid.
Like I said before, when it comes to flex, you’ve got to throw away the text book, because there is nothing in there about flex. Stiffness is not a measure of flex. There is a reason why carbon fiber is not used very often in surfboards (besides the cost, of course). The flex is different from E-glass. And aramid is different also. This is a concept that is very hard for engineering types to fathom.
You really have to do test panels to see what I’m talking about. The panels need to be the same measured stiffness regardless of weight. Once you have the panels, you just have to flex them and notice the flex characteristics like flex return and flex dampening. Believe me, there is no way to measure or describe these characteristics anymore than I could describe why one poet or artist is better than the next.
In my opinion, nothing beats E-glass for surfboards that are made to flex (also snowboards and skis). But if I just wanted to make the lightest and strongest (and probably horrible to ride) surfboard, I would get the highest modulus carbon fiber I could find.
Boom!
This is why shapers must also surf. Thanks Kenz!! Even Lockheed gets the pilots up in here when designing their fighters.
Ok all fun aside, I think it takes both sides to come to meet in the middle. The engineers know materials and the methods behind it, and the pilot takes the application of it . . . both tweak it better.
Maybe set up a few boards like Hanz was proposing and then having someone surf them (and describe what is going on) and get specific, such as it flexes but bogs down in the flats, or feels solid in a bottom turn, but gets squirrelly when trimming on a powerful / fast waves.
Now we’ve got the hypothesis, time to test it!
Roy Stewart is right about the tree being the most complicated out of fiberglass, resin, or foam.
Humans can make all three, but not the tree.
“My first surfboard is anything but tree.”