I dropped a steel rod from equal heights, through a pipe (for
control) onto the two samples. SInce the rod was controled by the pipe,
the rod consistently landed on the end. It was 1/2 " diameter steel.
[img_assist|nid=1066736|title=Rod Drop Test|desc=|link=none|align=left|width=283|height=497]
The Steel rod of 405 grams was dropped from increasing heights onto
the sample. The sample was placed on a scrap of PU surfboard blank. US
Blanks red if it matters.
Both samples did well, with a slight lead in strength to the Sample where I seperated the two Cerex layers.
It took a drop height of four feet before either sample was penetrated. Lower heights produced crazing in the surface.
TESTING RESUMES
A similar sample was made, this time with the addition of an Aramid Scrim. An example of an Aramid fiber is Kevlar. A scrim is a woven netting. In this instance the scrim strands were 1/2 inch appart.
The sample was laminated to a US Blanks Red off-cut. Impact testing on a complete surfboard would be cost prohibitive.
When the same rod used in earlier tests was dropped, I was unable to puncture the laminate. The highest drop was eight feet, the limit of my reach. The sample did dent, but no fracturing happened. The laminated sample remained watertight.
Designed and controled flex. Current popular design theory is to control flex by adjusting thicknessof the blank, and stringer width and wood species. So, generally a more flexible board will be thinner, and not float as well. I'm working on controling flex with the laminate schedule.
Durability. Standard surfboards don't last long enough. Great for an industry who survives on reproducing replacement surfboards, but not so good for the customer. Marketing price point equilibrium would be making a board that lasts twice as long, and costs twice as much. Competitive advantage would be to charge twice as much for a board that lasts three times longer, or a board that costs 50% more, but lasts twice as long.
Cosmetics. The board must be attractive. Some products like the Aramid scrim aren't attractive to the customer. Either the reinforcement will need to be disguised, or part of the marketing image. I haven't done an isolated test of the scrim, so I don't know what it does to flex yet. If it was placed under the feet, and kept flat, and not wrapped onto the rails, then flex would not be altered significantly. Flex is more affected when the plane of the reinforcement is perpindicuar to the load. A swiming pool diving board flexes when you jump on it. It will not flex if turned up on end.
please do a pointy nose on that rod so you ll see that the experiment dramatically change
-better do hand lay ups and vac bag stuff to compare (after the impact with the pointy rod not squared nose…)
1 with 2 layers of 4oz premium eglass
and the others
-How do you control the thickness of the foam?
I tell you that this is more important than anything, because a non overshaped blank very near the crust with 2 layers of 4oz, reaaally holds abuse and still pretty light
Yes a foam blank that is not over shaped will last longer than a blank that is overshapped.
Loads are measured in pounds per square inch. Puting a point on the tip of the projectile rod would reduce the impact area, and change the test. The point of testing is to compare which material fails under less load, when measured in pounds per square foot. Of course pointing the rod would puncture easier, but so would using a heavier rod, or dropping it from a greater height.
With laminates of equal weight per square yard, Nylon without fiberglass punctures easiest. Then comes fiberglass alone. Then comes fiberglass reinforced with nylon. Then comes aramids.
There are many materials to be tested. Each will have a trade between strength, cost, resiliency and flexibility, and weight.
For instance, polypropylene’s advantage is that it is inexpensive, and lighter than water. It’s negatives are that epoxy doesn’t bond with it, and that is isn’t very strong. If epoxy doesn’t bond with the fiber, then only a portion of its strength is used. Glass doesn’t bond well with epoxy well either, but better than polypropylene.
The easiest test for bonding is to wet out a piece of the material, and when cured, peel it off. Epoxy spilled on polypropylene sheeting will practically fall off. No bond at all. Epoxy spilled on windowpane glass can be removed with osme effort and a razor blade. A better bond. Epoxy spilled on Nylon bonds quite well, and will not be removed without great effort. Epoxy spilled on wood is also difficult to remove.
When you turn the diving board so that width has become thickness, I believe you are really altering the thickness which is changing the flex – rather than angle of orientation.
I believe stringers create an I beam with the composite skins to decrease flex and increase longitudinal strength.
I have been toying with these considerations for my 0.5-0.625" thick street decks. You have to increase shell strength significantly to reduce flex. Strength in the bottom skin (tension) is critical to prevent the deck from breaking, at least for street decks. Pretty sure flex is the directly proportional to the cube of thickness.
What I ment to say with the diving board is like this Take a wood stringer, and flex it in the flat direction - quite floppy. Bend it in the other direction, like a knife blade it is stiff.
With surfboards, carbon on the deck does little for global stiffness. Carbon on the rails stiffens greatly.
Carbon rails should be more like channel beams than thin horizontal sheets. The diving board on its edge has greater functional thickness.
A 0.5" thick baltic birch ply deck with 6-oz Xglass covered with 6-oz plain weave carbon cloth on the bottom, no composite skin top, had way too much flex. I had to add one layer of 4oz Sglass to the top and 2 layers of 4-oz Sglass to bottom to make it stiff enough for the ride I wanted: increased weight significantly.
I still want it stiffer – I will be looking at vertical plywood stringers in the plywood deck next to reduce the glassing schedule/weight using 4-oz Uni-axial carbon on the bottom (with a single layer of Skinz underneath) and a single layer +/- 45 6-oz glass on top for the stringered build (next up).
I am going to try multiple 0.5-0.75 thick Baltic Birch plywood stringers to get vertical cross-grain.
I got into a heated exchange about lapped rails on street decks with a screen name of Enemy Combatant at silverfish. It is true that tickness has to be greater than 0.5" for lapped rails to add major longitudinal strength. However lapping will add some strength. But lapping 0.5" rails is a bitch when you are hand lamming like I do. (I do not have work space let alone storage space for a vac setup and I want to limit my builds to what I can do by hand in an un-used bedroom, personal preference for an old dog.)
Besides doing it by hand has made me get more innovative with structural design to get what I want. Not there yet. What I have learned for street decks should help with surfboards.
I’ve got something under vacuum tonight. Not really a stress test sample, more of a figuring out something sample. I’ll post tomorrow, we can all have a good laugh.
Glad samples are cheap. I couldn’t imagine screwing up complete boards with some of this. Some stuff works and some stuff really doesn’t.
Hey Everysurfer, keep up the R and D, it’s been a good read. In the original Cerex thread, kiterider mentioned something about using fibreglass flyscreen instead of something more exotic like the scrim you’ve got. At the time I thought it sounded a bit odd, but the other day I was in the hardware store and just happend to walk down the flyscreen aisle, so I checked it out. Turns out that despite what I was expecting, I couldn’t tear it and I also couldn’t permanently distort it. That’s when I realised what kiterider was on about. If you can be bothered, it would be interesting to apply your standardised testing to a panel with flyscreen in place of the scrim to see how it compares. I know it’s not really a modern fibre (and is definitely amateur hour), but would be interesting nonetheless.
Hi Paul, there is a better mesh than flyscreen. If you ever go into a building supply store, plasterers have a fiber mesh that thay put in stucco to keep the cracking down. I did some of that last night. Go into Home Depot tile department. Pick up the sheet mosaic tile, and turn it over. On the back there is a mesh that holds the small tiles together. Same stuff. Home Depot doesn’t sell it, but a plasterers building supply does.