Need help w/ EPS strength project for Sways

General Discussion
1

I started out enthusiastic about calculating the “relative” break strength of varius EPS foam densities and foam thicknesses. I had seed Loehr’s cool 4 pound EPS board at the trade show and I started thinking, why use a stringer ever?

I am an engineer, but my memory of strengths of materials in school is a dim one. so bear with me.

My thesis was that there is high density foam of sufficient width and thicknes that could be used in place of a conventional stringer that would equal, possibly exceed, the break strength of a conventional wood or PVC stringer. My goal was to come up with a nomigraph or chart or some sort of guidance that would help us design our boards making use of the newer EPS foam densities. For instance, maybe a 4 inch wide strip of 4 pound foam would equal a 1/8 inch strip of PVC. The possibilities of shaping without having the extra step of glueing in a stringer could be a real time saver for shapers. Just glue up a big block of 2 pound EPS with a 4 inch strip of 4 pound and you could hot wire the whole thing saving several steps.

I ran into a few problems finding the data I needed and in interepting the data I found. Many manufacturers have data on their foam via web sites, but they call it different things. And some data was too hard to find. And I have a real job. And I have a wife. And a kid or two.

I’d really like to put a project team together to do this. Anyone interested? I’m thinking we bat the idea around a bit here then divide the work up, get it done and post.

2

I’d like to help, as I am going into engineering, and stuff like this always looks good to colleges. I can’t help on any financial stuff though, I can’t even buy stuff to test out myself, but anything I can do I will help out with.

3

damn- another good question. hell, that’s why I stick around here.

Now, Greg, perhaps you want to consider how to measure ;

  1. a foam blank or test section
  2. a foam plus denser foam blank stringer or test section
  3. #1, glassed
  4. #2 glassed
  5. #1 glassed plus a wood or pvc stringer
  6. break points /failure points of #1 glassed
  7. break points or failure points of #2 glassed
  8. Break points or failure points of #5
My own thinking is that conventional stringers are a basicly bad thing. They trigger structural failures in foam/glass structures as their flexibility is less, along with their fracture strength: they break, the fractures of the stringers bust the glass loose from the foam and trigger fast delams which lead to board breakage. Alt B, when the shear between stringer material and basic foam gets to a certain point, delamination is triggered in the lighter material and the same problems ensue.

I would be very interested to see your results. I would suggest two sets of tests, one under steadily increasing loads and one under impact or shock loads.

best regards

doc…

4

Greg, thats one noble goal. Ive thought about the same stuff.

FWIW, Ive concluded two things:

  1. shapers want that shaping reference line a stringer offers

  2. wrt failures, its about minimizing stress concentrations

I’ve heard stringerless eps boards made correctly are not easy to break…maybe even more reliable, as Doc notes…Im sure Greg L already knows a lot of what you need to know.

5

HI GREG I AM A COMPOSITES ENGINEER AND HAVE DONE A LOT OF TESTING ON THIS WHAT I HAVE FOUND OUT IS THAT MOST BOARDS BREAK FROM THE RAIL FRIST AND THEN THE DECK AND THEN THE STRINGER , THE STRINGER MAINLY CONTROLS THE FLEX OR THE BOARD UNLESS THE STRINGER IS VERY WEAK AND HAS VERY LITTLE FLEX

A UNIPLY STRINGER SEEM TO WORK THE BEST FOR THE PRICE BECAUSE IT HAS A HIGH FLEX TO BREAK STRENGTH IT WILL FLEX MORE THAN THE FIBERGLASS RAILS WILL

THE VERY BEST IS A COMPOSITES STRINGER IT WILL OUT FLEX THE FIBERGLASS RAILS AND YOU CAN ENGINEER THE FLEX PATTERNS OF YOUR SURF BOARDS BUT THEY ARE NOT CHEAP TO BUILD I HAVE BEEN USING THEM IN MY OWN SURFBOARDS FOR 15 YEARS NOW YOU CAN ADD ABOUT $100 TO THE BOARD BUT IT IS WORTH IT TO ME I USED MY OWN FOAM THAT HAS MORE FLEX THAN ANY FOAM OUT THERE

BECAUSE I FOUND THAT THE EPS AND POLY-U FOAM WOULD SHEER AND CAUSE THE BOARD TO BREAK WE EVEN HAD SOME BOARDS WHERE THE FORM CORE BROKE AND THE BOARD DID NOT BUT DID DELAM WHERE THE BREAK WAS, THE GLASS AND THE STRINGER FLEXED MORE THAN THE FOAM

I HAVE DONE MORE TESTING AND SPENT MORE TIME AND MONEY THAN I EVER WANT TO REMEMBER

IN ALL IF YOU THINK OF A BOARD TO FLEX TO ASORB IMPACT RATHER THAN BREAKING I THINK IS THE KEY AND IF YOU GET THE FLEX AND REBOUND RIGHT ON A BOARD YOU WILL HAVE THAT MAGIC BOARD EVERONE IS ALLWAYS LOOKING FOR IF YOU HAVE ANY QUESTION YOU CAN E-MAIL ME AT GOD BLESS

6

Everyone, thanks. My pesky job is limiting my time this morning. I’ll get back with a plan and share my data so far, this evening. We can kick it around a bit and then go to work. I’ll try to put togethe a little work plan and division of labor. Looks like we have some very talented vounteers (that I can hide behind. ha). thx

7

Hey would you each PM me with your email addresses. I’d like to send you my EXCEL spreadsheet with the data I had been rounding up. Have a look at it and let’s try to agreed on the parameters of interest and how we are going to analize the problem. I had envisioned a desk top analysis only, but if you want to take this to the gargage, then we could talk about some test panels.

BTW, I’m finding modulus of Elasticity for a lot of different woods and PVC, but expressed in a variety of units. also, you will see the data is scarce for EPS. Let me hear from you so we can all look at the same starting point. Then we can come back here to the forum, discuss it and divide up the chores.

8

I’m not sure if this helps any, but we make boards with flat sheets of EPS. Since the sheets we get are only 1" or 2" thick, we have to combine 2 sheets to get the right thickness. But, we’ve added a piece of woven bamboo between the 2 sheets as a horizontal stringer. The boards are covered in either the same bamboo veneer, 1/8" balsa, or a very thin wood veneer. There is a layer of 4oz glass under the wood lam and we then glass it with 4oz bottom and 6oz on top. We use epoxy resin for all the gluing and lamination, and a vacuum bag to laminate the wood onto the foam as well as create a rocker. We don’t use the bag to glass the board.

I’m riding boards up to 8’ that don’t have the traditional stringer, just our horizontal woven bamboo. I have had several opportunities to get a real good pounding with my boards, and they don’t show it. Only the boards with the thin wood veneers have any signs of pressure dents. The balsa and the bamboo boards are so hard that they don’t get pressure dents, instead I get big bruises and bumps.

I just bought a custom PU/PE fish with a very nice hard glass job. After one day of dropping in to 6’ to 8’ faces (free falling several times), I have several noticeable pressures on the deck. I’ve ridden the epoxies in the same situations without getting pressures. I’ve even hit the boards with my hand very hard to show people how tough they are. They freak out when I do that. A PU board would have a pressure dent, not these.

I think the strength with the EPS boards is in the outer skin because we use ridiculously soft foam (less than 1lb), but these boards are so strong. I laminated a 2lb EPS board (which is pretty hard foam) without a wood skin and it’s very soft by comparison. Much like a PU board.

My brother has posted details about our unusual board building techniques.

http://www.swaylocks.com/forum/gforum.cgi?post=248295;search_string=oneula;#248295

Aloha,

Sharkcountry (Oneula’s brother) AKA Harry

9

Shark, thx. Thanks for the info.

I read everything your brother posts. Good stuff.

Chime in when you want. I just distributed the data so far to the guys who sent there emails.

TEAM:

Our first task is to look over what I have gathered, and decide where we want to go with this. We’ll keep it democratic and go as far as we can. If anyone has a burning desire to take a tangent in addition to the goals of the group, God bless you. We will try to help if you want it.

In the article I forwarded called “Foam Core Materials in the Marine Industry”, I have noted a formula at the bottom of the first page that calculates relative strength based on geometry and E Mod. I was going down the path of finding the data for various materials and had planned to find an equivalent flex rigidity or flex strength for say, 1/8 inch fir and see how wide and thick 4 pound EPS would need to be for the equivalent strength. I’d love to see if high density EPS could replace a conventional stringer. If not, it would be very useful to know that for say a 2 1/2 inch thick board, you needed a fir stinger at least X inches deep. Or PVC or whatever. You know that there will be many opinions but if we could set down some guidelines, you could see that when you work outside the guidelines, you increase break potential. I’d like to know how PVC stringers compare with various woods. In fact, I question the value of PVC except as a navigation mark for shaping. But smarter people than me think different.

Perhaps we will do the calcs and find we want to assemble some test panels to confirm. Or whatever?

Where can we go with this?

Doc, I know your time is limited, but feel free to jump in.

10

it’s not only the composition of the material but the alignment of the grain or weave such as vertical vs horizontal vs 45 degree offset vs compound 45 degree overlay that can play a major role too. There’s alot of components to mess around with.

Last a foam

corecell

divinycel

balsa

balsa scrim

honeycomb

carbonfiber

kevlar fiber

basalt fiber

pvc

woods

and all the multicomposites that can be created by altering layers in different directional angles of the items above.

This could get very complicated compared to figuring out the diff between a t-band constructed stringer and one that wasn’t.

Also where are you going to place it?

Down the middle?

Along the outside?

Inset from the outside?

A curved fleur(sp) design?

exoskeleton?

ribbed skeleton?

Like my old mathmetician classmates at scholl used to jokingly say…

“Damn engineers… Always think they can solve the worlds problems with the right book… prove that theorem boy, prove that theorem then you can at least understand the problem eventhough you may not be able to solve it” … or something to that effect… You know math geeks… it’s all about the chalkboard…

Best of luck…

Getting a headache think of all the possibilities…

Last year CMP was asking me the same question…

It sure would be nice to have a magic composites scale book that you could use to figure how much of what material you needed to put togethor to use to obtain a certain flex or strength requirement… Hey but he’s in the medical field so it made sense what he was asking.

11
Quote:

Doc, I know your time is limited, but feel free to jump in.

Thanks, Greg, appreciate that consideration. An old man with several jobs kinda has less time to play than he’d like.

Now, let me make a modest suggestion ( ohhh gawd, not another one of those ) that might make it a skosh easier.

You see, I dunno that testing, say, a 1/8" fir stringer and seeing how much thickness of, oh, 4 lb/cu ft EPS will replace it is really gonna give us much. The 1/8" fir stringer in what? and glassed how? Using what adhesive?

But what we can do is come up with, as oneula well says, a whole bunch of measurements of everything from basalt cloth to simple foam.

So, my proposal:

Lets standardise on a ‘test article’: a chunk of a combination of materials, glassed ( or carbon fibered, or kevlared, or bamboozled) of a length, width and thickness we all agree on. Test it for deflection under load and ultimate break strength, both steadily increasing load and and impact loads ( also of agreed opon standards ) .

This gives us a database- you use materials X, Y and Z, it’ll be stronger than A, B and C but weaker than E, F and G. Being as there’s no such animal as a ‘standard surfboard’ so that we can intelligently predict what will be equivalent strengths, the best we can do is comparative testing on bits of core material, reinforcing material and skin material.

So, we have a test section:

Let us agree opon an X, Y and Z dimensions, something that will work both in inches and in real dimensions. I would suggest fractions of a meter ( 39.37 inches ) so that it’ll work for everybody and the testing arrangement can be reproduced by everybody. Then, you make a test section with your favorite materials, futz with it, eventually breaking it, and record the results.

And, being an evil-minded old SOB whose heart is in the right place, and besides, the guy who suggests it gets to name the units - let me suggest that the unit we use in relative terms: the Paler, in honor of our founder and host.

If, let us say, a Clark blue urethane foam test piece, with 2x6 oz deck, single 6 oz bottom, 1/4" mahogany stringer, has an ultimate (breaking) strength of so and so, deflection under load of this and that, etc, etc - let us assign it a value of 1 Paler and all others are compared to that. 0.95 Paler. 2.31 Paler, that sort of thing. We can have a catfight about exactly what we use for the standard, but lets use that for the standard unit…

hope that’s of some small use

doc…

12

Sounds good to me, but how are we going to make sure that every thing is equal? My peice of oak stringer may not have the same grain or strength properties as one you get etc.

13

I just wanted to show the boards we made (mine). The Quad and bonzer don’t have stringers. 8’ #002 doesn’t have stringer either.



14

What’s up with the Gemini? Any links would be cool, that thing looks very different. What’s the point of the split nose?

15

I need to defer that to my brother. He made that for me as a Christmas present in 2004.

These boards have a deep concave down the middle about an inch deep at the nose and about a quarter inch or more at the tail. The board is like 2 hulls that are combined into a single board. They have a very complex bottom design and are fast, but strange looking. I thought these were the fastest boards I’ve ridden until I got a 5-fin fish from Greg Griffin. I think the way the Griffin fish has it’s wide point up front compared to the gemini with the wide point lower has a lot to do with the way the board projects out of turns. Plus less people make fun of your board (until they see the 5 fins). But, once you turn on the speed they get impressed.

I never did like the nose of the gemini, but that’s a part of the design. It’s supposed to have the nose plan shape of a longer board, so it turns differently than if the board had a rounded off nose. I think the rear fins are not setup just right, so my board can spin on a dime. My brother’s board has a little more bite.

http://www.alexandersurfboards.com

16
Quote:

Sounds good to me, but how are we going to make sure that every thing is equal? My peice of oak stringer may not have the same grain or strength properties as one you get etc.

Good question… and exactly the reason for all this.

If there’s a whole bunch of us making test pieces and testing 'em, then over time there will probably be several guys who test oak stringers. One with EPS 2 lb home depot foam, maybe, somebody else does it with urethane foam, a third with some exotic foams and mebbe a little carbon fiber -

The idea is, we wind up with a whole lot of data about relative strengths of assorted materials combinations. And if, lets say, you want to build a board using a particular materials combo that’s not listed, you could make a test piece, using the materials you want to use, test it and have a very good idea of how well your boad will hold up, flex, al kinds of stuff compared to the other materials combos you could use.

This same data - well, lets say you had a 6’3" fish that was really nice but you wanted to make one with more ( or less ) flex. Okay, what you do is check the materials in the one you have and then find a set of materials that flexes more under load. Maybe a lot more, maybe just a little more.

But you could have a fair idea of ( relatively speaking) how much more before you go and build the board. Rather than the kinda wild guess stuff we use now, that leads to some very weird occurences, ya know?

hope that’s of use

doc…

17

Doc,

Great idea. I suggest making deflection and under load and ultimate breaking strength separately calculated measurements, to keep as far as possible from from making “fruit cocktail”. A particular combination of materials might have zero deflection up to the middle of the breaking strength scale (wherever that turns out to be) then fail catastrophically. The two curves can always be graphed on the same coordinates to combine the two characteristics. Call one the “Paler” and the other something else (how about the “Barnes”?). Or call the defection under load the “sway” and the breaking strength the “lock”…

-Samiam