I am trying to decide between using UD prepreg vs woven for a surfboard deck. I am leaning towards using a quad axial prepreg (one UD layer of 0,90,45,-45) as opposed to two layers of bi axial woven (one woven 0,90 and one -45,45 From my understanding:
UD being noncrimp has higher stiffness than woven
UD being flat has less chances of having resin rich pockets resulting in a laminate less prone to cracking
So essentially a laminate with 2 layers of UD (0,90) would have higher stiffness and lower weight (less resin) than a woven ply (0,90). So it seems like UD would be the more sensible choice, so then why is woven used so commonly? I understand iwhen woven is used instead of one UD ply as woven has fibers in both directions but my question is arent 2 plies of UD (0.90) stacked on top of each other better than using a woven fabric?
the only “prepreg” kinda used in surfboards is cloth that is saturated on a wet out table and coming from an aerospace background I wouldn’t really refer to that as a prepreg.
Prepreg is basically frozen saturated cloth that doesn’t cure if not tempered.
I appreciate that you are interested in composite theory, and when working with fibermaterials thinking about the direction of the fiber layup is the path to follow.
So why woven?
Because it does the job.
It’s not the most delicate engineering and not the highest art, but surfboards are not starships.
Again, I like the thoughts, and you can also leave the pattern of 0/45/90 degree.
Fibers should be placed where they take the load best.
EXACTLY ! Fresh out of High School, I worked in the plastics department at Convair General Dynamics. That was 1959, and they were using some pretty exotic prepreg stuff, even then.
One issue you might encounter (and it’s a pretty big issue) with unidirectional fabric is how it’s going to drape and wrap on the overlaps when it comes to tight curves - particularly tricky for instance on a wide rounded nose. It’s difficult to explain but you will be trying to take a fabric that is designed primarily for flats and introducing it to some tight compound curves. Even cloth with a fairly loose weave might need some relief cuts in addition to the standard cuts for overlapping the left and right sides at the tips. Standard 90 degree weaves that are ‘tight’ are difficult enough… I would think that uni would be very tricky. That said, if you’re just planning to use it as an inlay on a basically flat area, it might work fine.
A question I have is regarding whether or not the pre-preg fabric has enough resin embedded in it to provide a decent bond to a somewhat porous substrate like surfboard foam. I think there is a reason why surfboard laminators typically ‘flood’ the laminate and squeegee off just enough to keep it light weight without introducing pin air. It might be arguable that giving the resin a bit of time to soak in to the blank will provide a better bond in the long run. Not sure if pre-preg has that much resin in it.
Thanks guys! It seems like I didnt word my question correctly or even the topic for that matter, I was comparing UD prepreg vs woven prepreg! so basically both prepreg and question being would i get better mechanical properties if I used 2 layers of UD as opposed to the common practise of one layer od woven.
As long as drapeability is concerned, I will be using very lightweight 4 oz sheets and will prpopose making relief cuts so hopefully that wont be a huge concern. Other than relief cuts, is there another way to improve drapeability?
All True. It’s really basic; but on a deck I like to combine “Warp” and Plain Weave E. Or 227 vector Net combined with E or Warp. So many less Techy alternatives that do a good job.
The world of high-tech composites is fascinating, and plenty of swaylocks members and other board-builders stay tuned in to the newest materials coming out, always looking for a surfboard-construction application.
Having said that, it would be advisable for you to first build a couple boards from start to finish using conventional materials - urethane foam/ polyester resin, EPS / epoxy - before you start looking to improve upon those materials. Take the time to understand why it’s generally done the way it’s done first. Many of your questions will be answered in the process.
I think prepreg requires heat and vacuum. I don’t think a foam core surfboard can handle the heat requirments of prepreg curing. Just doing a vacuum lam would add strength by keeping the resin to glass ratio close to optimum, but vacuum lams are hard enough to learn. Using the unidirectional fibers and getting it all wetout, layed out and then into a vac bag is a lot of work.
Look up the posts from Everysurfer and see where he’s gone with materials. You should talk to him.
As Shakcountry said prepeg need too much heat for our foam to set. you need other kind off foam that not exist in our low density or may be varial foam… by flattening fiber with pressure, vacuum bag reduce thickness skin and so weight and flexural stiffness too. well done it optimize tensil by weight properties but surfboards fiber don’t break by tensil but buckle, which is a flexural mode under compressive forces. to increase buckling skin strength you have to increase skin stiffness… vacuum bag is needed when you do a thick lam with multiple layers and/or use light plastic fibers that float over resin like kevlar, PA, PP, PE etc, and carbon too.
main advantage of ud is low crimp which increase fatigue strength.
Thanks Lemat, your comment that surboards fail due to buckling and not tension makes a lot of sense. UD layers have better stiffness (non crimp) than woven ones so in essence they would buckle less, right?
UD carbon along the rails would help with buckling, but it also adds stiffness. Rail channels on the deck do the same. There are a lot of different styles for the rail channels to increase longitudinal strength. Warp cloth uses more strands of fiber running lengthwise to add longitudinal strength. Many ways to increase a board’s strength including heavier glassing. A super light core with a thicker complex skin will result in a stronger build. Everysurfer builds that way.
Surfboard fiberglass fabrics are designed for hand laminations onto the blanks. So the way they well wrap around the board’s rails are a big consideration. Once you move to vacuum bagging the lamination, you can go with many different types of fabrics that don’t work well with a standard hand lamination. Once you start in that path the time spent working on the board increases, so the labor costs increase, and the exotic fabrics usually cost more too.
The other thing is that if a product lasts many years, the industries wouldn’t be able to sell as many of those products.
With fiber axis lengthwise gain flexural stiffness by material stiffness, but ud lowcrimp is flatter at same weight so thinner so lost stiffness by thickness. flexural stiffness is a mis of material stiffness (young modulus) and skin thickness (quadratic moment of section). so you can play with both to increase flexural stiffness and so buckling strength. Each time you increase skin stiffness you increase board overall stiffness. A good example are modern windsurf board built with pvc sandwiched carbon skin on a super light eps foam. skins are really stiff and board too. They can experiment far more stress than surfboards with a far better weight/volume ratio. But when you make a surfboard like this near no one want to surf it except in perfect glassy waves. it’s like sport car suspension on every day road.
What Wide Awake and others have said. Prepreg is a bad idea. Wet out table and vacuum bag is the same thing. Otherwise you are going to try to get a non draping skin to wrap rails. It’s going to need a bag anyway. And then how to apply heat? A clothes iron? EPS would melt. PU might Brown. Just go wet out table.
The way you are proposing it, you have 4 layers of UD and 2 layers of fabric. For an accurate comparison, you need 4 layers of fabric.
You are correct that for the same laminate schedule, UD has less weight and higher strength/stiffness compared to fabric because the fibers do not have ‘crimp’. Also UD is less expensive than fabric. So if you are building a huge aerospace structure with thin margins, then it may make make sense to use UD. BUT…Fabric is much easier to work with on tight radius parts such as surfboards. Using UD will be very difficult to do on a surfboard without getting a lot wrinkles and/or overlaps from darting (those types of ‘defects’ could actually make the UD laminate version weaker than the fabric laminate if the fabric version can wrap and lay clean).
As others have mentioned, pre-preg will need a vac bag and heat. The only core likely to hold up to the needed temperature would be a honeycomb or Varial foam which seems to be a tweaked version of Rohacell (high temp foam made to cure with pre-pregs) or some other aerospace type high temp foam. Most pre-pregs are used with a mold - are you trying to make a hollow board off a mold?
The other thing to consider is that pre-preg is significantly stiffer with more memory than dry fiber. a surfboard shape is more inclined for wet layup or an infusion. with pre-preg you are going to probably need an 8 harness or similar weave to even have a chance at draping without a bunch of wrinkling or darting. not saying you cant do it, but given its costs and workability drawbacks it doesnt seem worth it?