Theory of compsite sandwich - how boards break

Most folks will never break a board. I've built some ridiculously UNSOUND boards to test the limits of construction.

This is one such broken board.

Gnarly! How light was the construction on that board?

It's hard to see, but boards often break from the rails inward. In such instance, the buckle stops either at the stringer if it's in the middle, or 3/4's or so across - unless the board snaps in two.

In typical surfboard construction, rails are lapped to provide break strength, but also ding resistance.

The board in the picture above had a 1/16" stringer and 1 pound density foam - crazy unsound structurally. Although it was great fun to watch James sink as the board collapsed

From a composite construction standpoint, one of the design criteria will be to make the rails structurally sound. Higher density foam (5 pound or above is considered structural, 1 pound isn't ) or significant layers of laps will achieve this.

If the core is low density, say 1 or 1.5 density, a high density skin, can replace extra layers of glass or a heavier glass/resin schedule.

This is Divinycell, it's H45 which is a 3 pound density foam. The skin, can prevent heel dents, as well as, spreading the load over a greater area.

Hey TXSurf. 1 pound EPS, 1 layer of 4oz on the bottom and 2 layers of 4oz on top. Crazy light James had a different description for it after if folded under him

This is my next build. 1.5 pound core, 5 pound rails, 1/8" basswood stringers and a high density skin on the deck.

"James had a different description for it after if folded under him"

he he LMAO

Yeah, me too

The "blank" above was destined for a CNC machine, but I'm not going to have access to one for a bit, so I hacked into it at home. This is a way for folks that don't have access to a hotwire or a machine like myself.




After cutting the kerfs, a quick pass with the planer brings it down close.

Shear forces!

Another high density foam is corecell. Corecell is an SAN foam, whereas the divinycell is a crosss linked PVC foam. Both are considered structural foams. Corecell also comes in different densities. The foam in the foreground is 5#, the less "stiff" foam in the background is 3#.

Shear movement and shear forces are different concepts. Super stiff boards, like Surftech boards or any stiff composite sandwich construction will still fail under load because they dont tolerate shear movement. The shear force is there on any board, but can't be absorbed by shear movement in a sandwich structure that doesn't flex. In an over simplification the force must be redirected until it finds the weak link.

Resistence to shear is the force which helps the object return to shape, also if shear
forces are redirected because features are designed in to limit shear in some areas,
then its possible to turn shear force in another direction. That is to say create twisting or
buckling, hence the need for panel flex also. Again, oversimplified: loads can be redirected in a controlled way.

As demosntrated in the picture above, catastrophic failures give a lot of insight into
where the loads are being directed. One of the benifits of pushing the boundries of
lightness is finding the limits, combined with the extreme nature of what can go wrong.

What I have learned through the catastrophic failures is that everything that flexes will undergo shear forces, but will not necessarily undergo shear movement. Allowing movement, within a relative range, prevents failure. Maintaining shear movement within a relative range will also prevent failure.

This is the concept of this next build. Greg Loehr of Resin Research proposed the concept back in 2005, I think. A few issues that, I believe, have been adjusted are the "balsa" stringers, replaced with basswood, mostly. The interior reinforcement of 3/4 oz 'glass is mostly replaced with 2 oz and finally, the deck skin mostly overlaps the rails.

SD your are some mad scientist working in his lab muwahaaa ha

Too funny. Thanks Bob, it does feel a little like creating a monster, at times. What was it that Matt S called his one board - a Franken-something-or-other. That sure feels like the process.

Word on the street is that you had a few creations of your own at the Lake Mead gathering? I'd love to see pictures!

Dr. Surfdad it looks like your rocket is flying to another dimension
How long are Scaled Composites, Skunkworks, JPL, NASA and others on your client list?

Too funny Petr. I represent clients before the IRS and NOTHING is more complicated than THOSE folks!

Too funny Petr...I was just saying that I represent clients before the IRS and THOSE folks are more complicated and secretive than any of the above!

How to build a perimeter stringer close tolerance blank. I first hotwire in the rocker. You can 'wire the deck also, but I prefer to leave it square so that I have a flat surface to rest on while I work the bottom.

The key is shown here:

In the picture above, the outside vertical surface is 1/8" basswood. In the picture, you'll see that the basswood stringers have the rocker cut in. This is done, by wrapping the wood around the foam and marking it and then cutting it with a razor knife. When you wrap the foam with the stringer material, the bend or curve inward affects the length and amount of the curves. This is why you can't use the rocker template from the hotwire.

Another view of the stringer material being glued up.

I typically glue the rails and stringers in separate pulls, mostly because I don't have enough hands. For pulling the stringers down, I set the vac. at 12 inches. You could also use the offcuts and some bar clamps to glue up the stringers, but I just find the vacuum so much easier and uniform for glue-ups.

WEIRD! Sorry for the seeming double post, Petr. I hit post and it wasn't there! So I posted again.

Greg L just posted up a revision to the above formula. He's using a 1/42" bamboo veneer. I'm not quite sure how the board in the one photo with the tan polyu rails turned into the one with the white rails. Perhaps they painted the rails before bagging on the veneer. The bamboo looks interesting.

Yep Jeff secrets about money is same but different story than with with technology.

Today I was jumping on Arbour longboard sk8 desk with bamboo veneer. Nice pattern.
http://www.tactics.com/prodimg/79709

I've seen bamboo fabrics and laminates, but I'm not familiar with the properties. I'll need to check into them.

Back to the build, I've completed the outline, deck and bottom rocker rough cuts.

Show once asked me how I got the EPS off of me after shaping - I told him that Judy would chase me down the road with a broom and most of the EPS would come off. We are high tech now.

Dang, Judy over did it. What happened to all of your long locks of hair!!

In between the debate, I finished shaping this blank.

The stringers are the biggest issue with shaping this blank, the blank is curved along the rails and using a hand plane along that curve proves challenging.

I don't need to seal the polyu rails, but I did want to seal the interior EPS. I should have used some masking tape along the polyu to prevent the spackle from migrating over the rails.

This blank weighs just under 1.5 pounds. I'm striving to get the final laminated board around 3 pounds. It won't be indestructible, but I believe it should last a season. I believe that with a high density skin on the deck, this construction will last several seasons and the weight should be just below 4 pounds.

To bad I dont live near by I would be over every night. haha. Nice work SD you should start your own webpage.

Yesssss and since I live in Ut you should send me all your secret plans,Yes all of them you know you want to. And I will send you my taxes (jedi mimd tricks)

Here is what I managed this year. The 2 on the right I tried Hula-dek on , I like it

Thanks Bob,

I love that mini-longboard/noserider design of yours, how did that ride?

I like Huladek for coverage in between the regular EVA, but find it too slippery as the main traction. Did you like it?

This construction process developed by Greg Loehr has recently generated a ton of interest over at Sway's. It's serious gearhead orientation, but worth a view for those interested in composite construction. The thread

I asked the folks from Warvel to make me a wakesurf specific blanks back in May and they gave me the run around for several months. Maybe now, they'll be able to start the production process.

The concept that Greg is offering up is a bamboo laminate over the foam, not dissimilar to balsa, except it's in wider sheets.

The folks at Inland use a similar concept in their boards, although I believe that is "aussie pine". It's readily visible on their "woody" model boards.

In the thread above, Greg is positioning this build methodology as a means for the small local shaper to offer multi-density composite sandwich construction that is both light and exceptional tough, ALL allow custom construction.

Jeff, that new board looks like its coming along nicely! That vacuum pic is a classic!

And those Bamboo boards!!! Beautiful!!! I want a wakesurfer with a true bamboo veneer (wonder what that would set me back)

You like the way Judy keeps me in line? I like the look of that bamboo veneer, also. I think they located a source without the paper backing and I haven't been able to price that. If you can find a 4' x 5' section, I don't think attaching it is all that hard. The sailboard builders are the inspiration for that...I've been reading up on some of their advances. Interesting stuff.

I'm going to attach a Divinycell H45 deck skin, at some point today. The H45 is a 3 pound foam and will effectively prevent heel dents on this board. The H45 is hard to come by in the 4' x 8' sheets. I think the most commonly available dimensions are 32" x 48". I had to be a bit creative to get it to fit my 4'5" board.

The H45 is very easy to work with, you can cut it with scissors or a razor knife.

I will attach the skin with some 2 oz eglass underneath the skin. I could probably get away with an even lighter 'glass, but this piece weighs a total of 1 oz, so what am I saving by going lighter?!

The one issue I do have here, is that I'd like to balance the resin and 'glass by weight. Attempting to accurately measure out 1 oz of epoxy, by weight, is beyond my capabilities! I'll most likely wind up mixing 2 oz of epoxy as I'm using a fairly cheap 1:1 ratio epoxy for this part.

I trimmed the H45 back a little, just because I am liking the industrial look of the powder blue against the tan.

What approximately 1 oz of epoxy looks like Don't forget to zero your scale with the bucket empty.

How to saturate a 4.5' x 2' piece of 'glass with about 1 oz of resin.

Whenever I am attaching a skin, I flip it upside down and lay out the laminate on TOP of what is the bottom of the skin and then flip all of that over on top of the core, after smoothing and making sure I have good contact.

Then I bag that skin down on the core while the epoxy cures. Should be ready around noon. You'll see the stray pieces of masking tape on the skin, that's just to keep it in place as I slide the whole thing into the bag.

Out of the bag and ready for boxes and 'glass.

smart

Thanks Petr.

I am trying to keep my ratio of resin to 'glass at 1:1. My top laminate is a single layer of 4 oz, and the total fabric weight was 3 oz. I was able to wet out the fabric, using the "squish it in a bucket" method above, but the 'glass is extremely dry for lamination purposes. In order to get it to stick, I bagged the external top 'glass on.

My estimate is that this board currently weighs 38-39 oz (I also installed the boxes ). If that's true that will leave me 9 to 10 oz to play with and have the board still at 3 pounds finished weight.

Final weight, with fins and traction, came out to 3.75 pounds! I'm hoping that it stays in one piece.

It didn't break!!!!!!! Actually not even a pressure dent so the construction process works, IMO.

One of the issues, now, is that the design, while working well for James when constructed of straight EPS didn't do as well with these various components. The nose, which has a pretty heavy piece of polyu foam, dug into the wake. Also, with the reduction in weight, the fairly heavy fin pod left the board somewhat unbalanced, or at least tail heavy. New materials and construction, will need new designs.

New board, same basic construction. I am bagging the external bottom layer and testing a theory.

Paint adds a ton of weight. What I wanted to test was a version of molding where the graphic/artwork (such that mine is) is applied to the mold surface and then it is transferred to the resin as it cures.

In this picture you can see the paint applied to a piece of Mylar that will also hopefully give me a "shiny" bottom lamination and a texture from the "bunched" breather. We'll see.



The bunched breather that I hope will leave a slight texture.

It seems to have worked according to plan! I'm going to try the same thing on the top using some form of rough plastic that would mimic traction on the surface - I'll still add eva, but this will be in place of rolling on Hula Deck.

Jeff's Surf Non-Stop Lab never sleep ... and is full of ideas

For one of my new projects I was thinkig about simple thing - eps core and rails covered with this carbon strips which catched my eyes longtime before (and they were on sale too)
http://www.havel-composites.com/newsimages/uhlik_sklo_paska.jpg

I saw it in one board showroom in France. (It was actually board from shaper which has gas mask in logo and is shaping for quiksiver - can't remeber his name now.)

This carbon strip was "simply" treated over rails during glassing to the shape of parabolic stringer. i.e in the center part of rails was strip spreaded wide and towards the nose and tail it was continuously narrowed but not cutted. Simple and nice. Hope I used proper words to describe it

I never had this strip in my hands so I dont know how much you can spread it or not.
Also I am not sure about proper glassing methods to do it. Probably board is not sitting flat on rack but on its rail so gravity helps you. And they use brush insted of squeegee with strokes lengthwise with rails. One rail each time.
If they form strip before soaking or after, even if these small carbon strings soak well thru I really don't know.

Hey Petr,

I'm not sure I'm following - the carbon fiber is used in place of wood stringers?

nope Jeff, it is just simply wraped around rails

Got it - I've done that with a balsa railed board that had corecell skins. The Carbon looks sick, but boy is it stiff. I had to bag it on. I tried laminating it by hand and it just opened back up.

Newest board - 2 pounds 14 oz with some "artwork" such that it is and a finished gloss coat to boot!

Surfdad, Every couple of weeks I pop in and read your posts. I love following your work. Figured I'd drop a line. Thanks!

Yep Jeff is our "Shapers Guru" #1

Jeff that shortboard was from shaper Mark Phipps http://www.markphipps.com/PhippsSurfHome.aspx they had his older one with full balsa parabolic rails and this one was newer with carbon strips. I guess that this guy knows the stuff

Hey Craig! Thanks for following along and the kind words! Petr, you are too kind, sir!

I remember reading a thread on Sway's a year or so ago about a board where the shaper didn't run stringers at all, just the carbon on the rail. He did a cut lap and hand layup. I could never figure out how he got the stuff to stay around the rails!

I am just on it Jeff
search with Ctrl + F for "Yorky" user, he is doing something near to it

http://www.swaylocks.com/forum/gforum.cgi?do=post_view_printable;post=375933;gues t=19679931

but he is using 2 layers of uni-carbon stripes all around rails with some ballistic fabric
"its 2.5 x stronger then kevlar, as strong as carbon but more flexible and is not affected by uv rays. crazy color"
Dont know if his EPS is 1 or 2LB?

(Message edited by hawaj on October 16, 2008)

This has been a very good read Jeff. Here is a board from Dave Pearson that he is in the process of releasing. A stringerless board with a carbon fiber and kevlar fabric on the top. It was a very fun ride. This particular board was a 4'6" if I remember correctly. It felt great in the water and was very responsive.

The carbon rails sounds interesting.

I forgot to add the pic, its the board on the left in the pic

Props! Dave Pearson is a killer shaper. I'm glad to see him involved in the sport

Fin boxes curing.

you got special jig for C5?

No, I wish I had one. I'm sure that Future's offers a jig for the C-5, but I just freehand them. I'm in the process of building a new tool - as I get closer to finishing it, I'll post up. It's based on the concept of a pantograph...hopefully I'll be able to duplicate the fin placement without a struggle.

This construction did ok, but I think it places too much stress on the bottom for any longevity. Next build will be skinned top and bottom.

I sent Dennis out on it, no heel dents with Dennis' 250 pounds.

It was fun and quite responsive.

I was surprised that you put HD foam on the bottom and not the deck. When shaping a standard PU blank I thought that you were supposed to limit the amount of material that you remove from the deck by removing material from the foil. Thus reveling less dense foam on the bottom of the board and leaving stronger foam on the deck. This approach is supposed to help prevent heal dents. Right?

Am I confused Ed or did I mislead you?! This board JUST has HD foam on the deck, and just the glassed EPS exposed on the bottom. Your understanding of shaping a PU blank is absolutely correct.

The issue with this build now, is that you can feel the deck doesn't flex at all, but the bottom does. My fear is that I've displaced ALL of the stress to the weaker bottom, so the next build will have skins top and bottom. I'm sorry if I was clear in this.

On my other thread - the Surftech that will have skins top and bottom also, I just started with the bottom as that allows me to lock in the final thickness of the board, so that when I shape the deck, I can start from the bottom skin, subtract the thickness of the top skin and shape the deck down to that dimension.

I had to test it! This is probably no big deal on a 6 - 8 pound board, but this thing is 3 pounds!

I mis-read the following "This construction did ok, but I think it places too much stress on the bottom for any longevity. Next build will be skinned top and bottom. "

Oh, nice shoes. Did you get them at Surf Expo?

Yes I also remember that these shoes some celebrity was wearing at Surf Expo!

But I know better ones from Swaylock which put you in total shame Jeff

But you know what, these are shapers shoes!!! Endangered species now ....

Those are the best shoes ever! They offer several features:

1) I have several handy references for a laminate schedule with me at all times.

2) I never have to worry about them being stolen.

3) It's virtually impossible to lose me in a crowd, as folks sort of disperse as I walk by.

4) The Carbon Fiber reinforcing is extremely strong as evidenced by these shoes long life.

5) These shoes help uncomplicate my life, not ONCE have I felt the anxiety of "I'll ruin my shoes!"

6) They are an incredible at maintaining self-esteem. If anyone EVER talks disapringly about one of my boards, saying something like: That's the ugliest thing I've ever seen, I just smile, nod knowingly and point to the shoes. Ego intact!

Really, you guys have no idea how handy a pair of shoes like this can be.

Surfdad bulletproof sneakers.
What about to make them stealthly? Paint them black

http://i163.photobucket.com/albums/t319/Kristian_Maria/Pokracovani/Pokracovani2/245005_Theretheseshouldlastforever.jpg

Duct tape! I was laughing at your bulletproof sneakers - brought back memories of working with kevlar - I'd rather spend a week with my mother-in-law than attempt to cut and sand kevlar again!