I’m studying for mechanical engineer (catholic university of Lovian, Belgium), my exams are coming.
The course I am studying fot the moment is “material engineering”.
I just rounded off the polymer and polymer-composite part.
And related it to surfboards. Here are my theoretical thoughts, feel free to react ;)!
First some terms and facts to make my theory clear:
I-lam: the inner sandwich laminate (closest to foam)
O-lam: the outer sandwich laminate (on the outside)
L-or: lenght oriented fibres
W-or: width oriented fibres
C-or: Cross oriented fibres
material | Specific Strength (10³ Nm/kg) | Specific Stiffness (10^6 Nm/kg) | Belgian price (euro/m²)
E-glass | 570 | 140 | ~5
S-glass | 770 | 140 | ~40
Aramide | 750 | 190 | ~40
PAN-Carbon | 400 | 210 | ~50
Now my thoughts about using these materials in Sandwich
Aramide is superior in impact resistance. So we Edit: better use it as place specific reinforcement.
Carbon is superior in stiffness. If we use it we use it in the I-lam because of its weak impact resistance.
E-Glass is cheap and is always better to use if the others arent nessesary.
BAD COMBINATIONS:
Ar+C in 2 different layers on each other (I-lam or O-lam):
C is stiffer than Ar so the strenght of Ar is not used. Only its impact resistance is used.
not optimal in my eyes.
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.
Not optimal.
GOOD COMBINATIONS:
O-lam: Ar I-lam: E-glass:
Uses the strength of Ar and E-glass is strong enough for the sandwich (Impact resistance).
Strong and not too stiff (see remarks for more flex)
edit: like “Rikds” sais (in a comment under this) Aramid is bad in compression. I forgot that but that makes this construction not optimal too. Aramid has to be used as impact reinforcement or on bottom.
O-lam: E-glass (eventually Ar reinforcements)
I-lam: C L-or + E-glass W-or (a weave you can buy like that):
Very Stiff. Better version of #2 from bad. When C breaks E-glass will break after it.
O-lam: C L-or + Ar W-or (existing weave)
I-lam: E-glass:
Very stiff and impact resistant. With cost equally to one C-layer.
REMARKS:
If you want more flex you can substitude all Ar with S-glass.
The use of an C-or Glass layer reduces the twist.
For windsurf there are specific reinforcements needed, but you can find this elsewhere.
I do not believe in full double sandwich this is invented to make construction easier. This has to be used for place specific reinforcements only.
The use of epoxy is a must!
I hope it is useful to know where to use what. In sandwich this really makes a difference.
Have you applied any of this to your boards? Also, what have you done about core materials? When you say you want an inner lam with a particular material, how does this relate to the shear resistance of the core materials?
It stops the production of surfboards, but increases my knowledge about materials and mechanics ;).
Next year I will have fluidum mechanics!
I haven’t applied this yet. This is just an existing construction method (sandwich) but I explain the use of the materials.
The core material does not make a big difference. Just like every compsand 10-15kg/m³ EPS foam an HD PVC foam or balsa sandwich core.
This is just the theoretical use of materials. Finding out if it works is not really possible. Or it will need many experiments and complicated finite element analyses.
I wrote this because I see people using carbon fibres where it is not really needed to use them or use them for purposes where these materials do not give any advantages, sometimes even disadvantages. Using carbon in a normal construction is gives only disadvantages for example.
My next board will be a windsurf board (85L waveboard). A perfectly material engineered one ;).
I will use I-lam Carbon L-or and glass W-or + an C-or glass layer (or carbon tape) and O-lam glass + Aramide reinforcement in high stress areas. Under the feet I’ll build in a double sandwich (just to make a thicker sandwich) with Aramide in the I- and O-lam.
A 3mm HD PVC (+100 kg/m³) deck and bottom sandwich. Maybe a bottom sandwich of just some HD EPS or XPS (+40kg/m³) but that needs some more research.
But that project will start when I have a vacation without “herexamens” (do not know the english name but dave will understand ).
I see people using carbon fibres where it is not really needed to use them or use them for purposes where these materials do not give any advantages, sometimes even disadvantages. Using carbon in a normal construction is gives only disadvantages for example.
You forgot the one reason that counts most to some companies… marketing. Consumers can be suckers for fancy carbon bits where they can see them. Look at how many aftermarket carbon fiber car parts sell that have no function, and in fact just add weight to your car. A carbon dash that goes on top of your existing one is only cosmetic… zero practical function… yet many companies exist because suckers buy the “look” - they’d rather look cool than be cool.
Don’t put aramid anywhere near a surface you have to sand! It doesn’t sand and cannot be made to! Although it’s good for abrasion resistance, for impact control it should be positioned so that it goes into tension (it can stretch a bit more than the other fibres without breaking). It is really poor in compression. Very critical where it is put in a laminate stack.
You are right. Aramid is bad in compression (I have no nubers about compression strength), and I forgot to count it in. So I made 2 little changes in the tekst.
It is always better to not to sand on your fibres (even if it is not aramide). You have to make make a sanding coat of resin+microballons, superb sanding coat! So sanding is not really the big problem.
#1 My thought are not a scientifical study. It is just the use of courses in reality.
The test results are coming from tables of my professor, they can variate from brand to brand. It is the order that counts (I hope this is english ). So I haven’t done any analysis at all. This is just my thoughts about what to use and what not to use.
#2 Surfboards are very difficult to analyse on stress. Nobody really knows what forces act on surfboards. Specially windsurfboards. In the case of surfboards (without sail) there is a lot more compression on the deck than bottom (vice versa for tension). In case of windsurfboards the high stress areas are at the fin, mastrail(impact) and footstraps(impact).
#3 in the proportion of matrix/fibre it is as everybody sais: “the more fibre the better”
I do not think I can really learn you things at the moment you didn’t knew already.
One thing that isn’t often mentioned is that E-glass is isotropic. Therefore it doesn’t really matter which way the fibers are oriented as long as the area fraction of fibers remains similar. This is much different than CF or Kevlar, these materials have much higher stiffnesses in the longitudinal direction making fiber direction a critical component of design.
Another thing that isn’t mentioned nearly enough is moment of inertia (I). I’m not talking about swing weight, rather moment of inertia used in equations to calculate bending, displacement, buckling, and etc loads. Like the example of an I beam being much stronger than a solid bar of the same cross sectional area due to an increased Moment of inertia. With this in mind a thicker surfboard should resist buckling more. As a surfboard experiences many forces both globally and locally, compression of the deck must also be considered and again a sandwich construction resists this compression more because of higher moment of inertia. This stuff is all common sense. But I feel it is important to look at the issue analytically in order to avoid silly trial an error
Sometimes I wish I knew how to do all those advanced simulations that ME’s do. That along with my materials engineering degree would make me spend even more of my time devoted to theoretical surfboards.
The way glass fibres are oriented is important. The orientation doesn’t matter when you use glass MATS. But when you use woven glassfibres it really matters! There is no way fibres can have any strength in the transversal direction, then the strength depends on the matrix strength, remember that the chain breaks in the weakest link first!
About the moment of inertia :). That is important indeed but in my explanation it doesnt matter a lot. I took it in count when I said that you sould use the carbon in the inner lamination because the the outer lamination gets more force so it helps the carbon fibres more in strength because they get flexed a bit more then the carbon (carbon is stiffer), otherwise the carbon gets more load and breaks faster. When the sandwich gets thicker the skin gets more I and that gives more impact resistance.
But if you really want to use I properly, you really need to make calculations. And because we don’t know the forces on our board there is no need to bring it more in count than that.
And yes it stays a bit trial and error. But we try to minimise that ;).
@KC
I know 4 languages (dutch, french, english and a bit german)
But non-english born people need to learn more languages in this world ;). And specially when they are born in a three language country like Belgium.
Look it up E glass has the same modulus and strength in the longitudinally and transverse directions.
I agree that if here are no fibers in the longitudinal direction the strength of the composite will be roughly the strength of the matrix. But in the case of 2 directional woven fiberglass since there will never be completely horizontal orientation of fibers and the volume fraction of fibers doesn’t change. The angle at which the mat is oriented has no effect on the properties. If you visualize the two directions of fiber like vector force’s components it starts to make sense. Keep in mind that if the woven mat has more fibers in one direction then this principal no longer holds true.
When I first heard this I didn’t believe it either. But after going through all the matrix math and seeing the data I started to believe.
Quote:
And because we don’t know the forces on our board there is no need to bring it more in count than that.
? That doesn’t sound like engineering. Estimate, refine, refine. I’m surprised I haven’t seen anyone estimate the shape of a surfboard as a cylinder Hahah
I’m still in my bachelor years and do not have the time and knowledge to write a whole finite element program for it ;).
I’ve never heard that the direction of E-glass doesn’t matter. In the shop the tell me each time that it matters (they always think that it is my first time working with it :D). But you I will believe you. And if it don’t matters you’ll never do wrong with orienting them (hmm, you will by spilling glass). I think I’ll ask the professor on my exam, the 23th of june. I hope I won’t forget.
E-glass has the same E-modulus in transversal an longitudal direction. But that is because of its molecular structure.
Glass molecules (actually cristals) are 3dimensional and that is the reason of the same E-modulus in all directions. Aramide and Carbon molecule are real long molecules (fibre like) you can speakwise pluck all molecules from eachother.
But you misinterprated that! Glass fibres on macro-scale are fibres but they arent on micro scale, where CF and Aramide are!
But the direction of the fibre matters for sure!
structure:
glass:
Aramide
Carbon
Can’t find a pic. But beleave me it looks like Aramide but without the non-C atoms
Working a material. I’m assuming you are talking about repeat stresses? Theoretically an epoxy fiberglass should never experience failure due to cyclic loading. But it does. From what I have read this is mostly due to interfacial de-bonding. Some of the bond between the glass and the epoxy comes from a mechanical interlock. As epoxy cures it compresses the fiberglass and conforms to the surface roughness of the fiber. As the epoxy ages it expands. Expands might be the wrong word. It relaxes its grip on the fiber. This is because while the fiber is compressed the epoxy is in tension. If this tension is too great there are all sorts of problems but that is a whole different can of worms. Back to the fiber in tension. The tension of the fiber is due to molecular interaction. Although slight as epoxy ages the residual forming stresses and strains relax. This is the natural process of high energy slowly going to a lower energy state (like iron rusting). Things that speed up this relaxation are increased temperature, which enables faster molecular movement, as well as educes strains due to CTE mismatch. Mechanical stresses can also speed up this relaxation. For this reason composites experience cyclic failure. By the way epoxy changes in other ways such as UV light. one thing at a time.
If you simply meant laying up the fiberglass or whatever I’m sorry for the long winded explanation.
All the composites that I have dealt with exhibit catastrophic failure. Someone else could probably tell you better than me.
Well that really is a great question. I imagine that the point varies greatly depending on the surfer. Sort of like a plastic fork versus a heart valve. What level of failure are you willing to accept. Tow surfers probably would be on the heart valve side while small wave belly boarders would be on the opposite. As far as fixing it goes, I don’t know probably replacing that section or reinforcing it. I’m probably not the best to ask there are more experienced people. Sorry that probably wasn’t much help to you.
Buckling. It should be noted that most boards don’t buckle. They micro buckle which is often due to flaws.
I think someone else should chime in on Casey’s 2nd and 3rd questions.
? 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.
yeah it the glass orientation doesnt seem to make much difference in the amounts i use for a surfboard
I agree with you on this! While I can understand the logic/concept of glass orientation I haven’t seen the difference in the real world. When I’ve taken pieces of dried fiberglass and tried bending it in different directions they seem equally stiff in any direction. However, Ken (airframe) talked about glass orientation being very important in his molds and on his carbon boards, but as you said - theres a lot more glass.
).
(dutch, french, english and a bit german)
