I am know an engineering mechanical teacher in a high school. My initial job was to design industrial parts and process, from car châssis to mold for infuse plane wings, and all kind of parts, metallic and plastic. Change Job after a long disease, french teachers have time + allow me to leave 2km from the beach.
For hand lam epoxy on pu : a stiffer resin with uv filter, RR KK can be à good one. 2x4 top 4 bottom. On eps, no less than 2lb, all good epoxy resin, 6x4 top 6 bottom. Minimal Schedule for short to mid size boards. Blanks stringered with thicker stringers of light flexible wood.
For vaccum bag or infusion go with a light high modulus plastic fiber, kevlar or dynema with sglass over and paint finish. Schedule dépend of blank density, but light density blank with thicker skin is better. Infusion is only a cleaner way to vacuum bag composits. Both need consumable to be effectively done with fiability.
vaccum bag a thin layer of glass is a lost of time and money, go with hand lam and keep vac bag for what is good.
Quality of how material are used is one key of composits. A sander can kill a thin lam of best materials in a second. Fill coat must be done correctly as cleaning finish shape if you want a strong and light hand lam.
It would have been amazing to have you as a high school teacher…lucky students.
Always appreciate your recommendations, your input and willingness to share on this forum is outstanding.
Curious why you recommend S glass OVER the plastic?
I would do the reverse in order to protect the S glass with the ballistic material from dents dings. Utilizing it to carry compressive loads that the plastic does not.
Perhaps it doesn’t actually matter? You said in an earlier post that it gets it’s impact properties from high tension strength not fiber stretch. So over or under may not make a difference in this case.
Can you also explain to me the stringer’s structural benefits and when a stringer is/isn’t beneficial. Without using the word “back bone” ;o)
Most of my testing thus far has ignored those “wooden things” as I don’t fully understand their purpose other than being a stiffening member in very lightweight laminations and all of my laminations are on the stiff side.
A stronger build comes from matching elasticities in order to avoid damage from shear. The bond between inelastic and very elastic will separate when the two dis similar materials are flexed.
The foam core is very elastic. Against the foam should be your most elastic fiber. Then as you build outward, the fibers should progressively become less elastic.
At least that’s what an engineer who designs crash helmets told me.
I’d like to highlight the power of the stringer. A stringer does two things:
Increase the buckling resistance of your lam due to the better bonding strength between glass and wood. So the board’s strength will increase significantly.
It increases the stiffness of the board by connecting the top and bottom lam with less elasticity than foam.
I don’t really know if it’s like high school. My students have 16 to 20 years.
Plastic fiber as interior layer because plastic fiber have high tensil strengh and low compressive strengh. When you push on skin interior layer take tensil stress and outer layer compressive stress. Glass have a decent compressive strengh even if resin is more important for compression. Real Sglass is a tough fiber.
If you want to increase abrasion resistance is good to put a layer of plastic fiber as last coat but with an elastic resin.
A surfboard take mostly to kind of stress. First is bending stress, wich snap the board, second is pressure ding more or less penetrate (dings, dents etc).
Bending stress intensity is really variable, if you are at the wrong place the wrong time a lip can load lot of energy to board. Plus board experiment fatigue wich reduce strengh. The way a bended sandwich panel break is well known, compressed layer buckle where stress is maximum then wrinkle or delam from foam because of shear, skin loose is stiffness then all panel break. To increase strengh the first way is to increase stiffness of skin, it’s sandwich skin purpose. An alternative or additional way is to localy increase skin stiffness where needed with stringers.
Against ding, dent, you need to increase skin stiffness with tough material. Many way to do it but cost weight. For surfboards you can live with dents but dings (penetrate) resistance is a real bonus. BIC molded tech (school board) are a good exemple for a relatively soft material (thick layer of mid modulus stretchable plastic) with lot of ding resistance.
Assigned all snap resistance to exterior skin is slightly dangerous because if skin is dinged board became britlle. Commom problem with sandwich boards (surftech an firewire) and carbon rails. Does the stringer must connected top to bottom according to the I beam theory ? Yes !.. but if there is a foam core…
Stiffness material gardiant is an important theory with structural materials, but in our case foam is so light and elastic and shear stress less resitant relative to skin that there is no real problems: if skin give way foam have no chance to survive. Only need to have a bond shear strengh higher than foam shear strengh.
Hollow is a valide concept for me. The frame have to keep shape and to stabilize skins, hollow i see have too strong (stiff) lenghwise frame but It´s an other question.
kevlar inside is the way to go for me: work in tensil, protected from uv, keep skin in one piece if hardly impacted. Carbon is good for stiffness and fatig ue strengh, if you need that why not. S2 glass outside.
Deck side have to be stiffer than bottom side because deck supported surfer weight and pressure. Bottom normaly not LOL.
I have a few general composites-related questions, and maybe some example situations that I would appreciate your thoughts on.
Would you suggest any basic guidelines for properly matching the modulus of resin to the modulus of reinforcement? As a starting point, at least. Like if the reinforcement has an elongation of x amount, then use a resin with an elongation of . . .
On a different note, here’s a scenario. I’ll try to keep it as brief as possible. Suppose I am glassing the bottom of 2 boards (stringerless). One is glassed with 4-oz warpglass. The other is glassed with 4-oz plain weave, but with the fibers on a 45 degree bias. If everything else is equal, would you expect one board to be stiffer than the other?
My understanding of these things is that fibers only work in tension. Therefore the board’s resistence to flexing/buckling comes largely from the fibers on the board’s bottom resisting the stretching under load. If I’m understanding this properly, then having fibers on a bias would potentially create a laminate with a higher tensile strength (at least in the length-wise direction) than a laminate with the fibers at 0/90. Am I on the right track?
For resin it´s simple, resin must have an higher élongation to break than fiber. Traditionaly, fiber in composit is a reinforcement, it increase strengh of a resin wich keep shape. But fiber have strengh in only one direction, in axis of fiber. So if you use a fiber that is stiffer and stronger than resin, it will take load when stress in axis and you expected that resin don´t break before fiber because it can ´t stretch at least the same than fiber. For fiber it´s finally simple too, you must have fiber in axis of stress. Mostly Two kind of stress for Surfboards: lenghwise overall bending stress and surfacions impact bending stress. Because Surfboards are sandwich, when bending, fiber in axis of lengh take tensil in on side and compression in other. For tensil no problem but for compression fiber can take load only if resin can keep them in axis, glued on foam, because fiber buckle in compression. Fiber off axis plays near nothing for those stress, they are good for surfacing impact bending stress, those who make dents.
So for your two boards, one with fiber at 45° will be far flexible and will snap quickly, the other with warp glass will be stiffer and will break too but under more load.
It’s worth pointing out that in classical laminate theory fibre properties decrease at cos^4 of the angle of the load. So anything more than 5 degrees off axis doesn’t do very much wrt to the primary loads.
That is way for me you need at least 1/2 or more of fiber in axis. At 90° it’s good for impact strengh end sprade some load when buckling initiate. A light 2,5 to 3 oz flat glass at 45° will be à good add but i’m not able to find one…