Composite sandwich considerations

Digging a little further into the compressive strength comparison of basswood and fiberglass especially as used in a composite sandwich, we found some interesting numbers. Basswood has a compressive strength of about 4,700 pounds per square inch when the force is applied parallel to the grain. That is to say, if the wood is on edge, like you would find in a stringer. The compressive strength of fiberglass is around 30,000 pounds per square inch! That’s 6 times greater than basswood. The vertical arrangement of a basswood stringer does keep the structure stiff during shaping, but when in use, if the fiberglass buckles due to being exposed to a force that exceeds it’s compressive capacity, the basswood stringer will snap almost instantly. In our last post we talked about placing wood inserts under the areas of high impact as part of our composite sandwich. We’ll have to do some additional research as fiberglass by itself is stronger, but we would venture the density increase is significant for a fiberglass only structure.

That is to say that a .04 inch thick piece of basswood is most likley significantly less weight than a .042 inch thick piece of fiberglass and epoxy resin. However, we are thinking that our little wakesurf board. It isn’t receiving 4,700 pounds per square inch upon landing from an aerial and so a wooden insert would easily exceed the maximum expected compressive forces. Or is it? Now the formula for determining the amount of force generated by an object requires a few factors that we really don’t know. One is the acceleration of the object when it hits the water, but we know gravity has an acceleration of 32 ft/sec per second. The other issue is how long it takes the rider to stop that downward acceleration once they hit the water. It’s less than a second, but how much less, who knows exactly! The shorter that time span, the more force is applied.

This makes sense to us logically, throw a ball into a solid brick wall there will be more force generated at impact than if you throw throw that same ball into a giant pile of feathers and we also know once the ball hits the bricks, it’s deceleration is DONE, where as with the feathers it might take some time.

So the formula for this is outlined as:

force x time = mass x (ending velocity – starting velocity)

Often times mass is described in pounds or slugs which is the same thing on earth, it’s different on other planets where gravity is different. Isn’t ”slugs” a cool term to use in a formula! If we assume a 10 slug ball is dropped onto a wood floor and it hits that wood floor at 96 ft/sec AND it takes .1 second to come to a complete stop we can calculate the force imparted on that wood floor, as follows:

force x 0.1 sec = 10 slugs x (0 ft/sec – 96 ft/sec)
force x 0.1 sec = -960 slug ft/sec
force = -960 slug ft/sec / 0.1 sec
force = -9600 slug ft/sec^2

You can see that the force imparted to the wood floor is substantial based upon the amount of acceleration and how stiff the floor was in resisting the impact. Now water, isn’t that stiff, but it does stop the rider when coming back down from an aerial and if the rider is skilled, landing on the face of the wake and sort of surfing down it can greatly reduce the forces generated. This is much like the various transitions in a half pipe, hitting on the downward slope rather than the bottom or the staging deck reduces the forces of re-entry. It also gives us an idea as to what we need to build to in terms of resisting that force. The 9,600 pounds per second squared, if it was concentrated on a single square inch would be dramatic and hard to build for. Although we know that our single layer of fiberglass can manage 30,000 pounds per square inch before breaking.

This is different than what it takes to bend or deform the fiberglass and that is part of beauty of fiberglass, it can bend and deform and NOT break. At the same time, a single layer of fiberglass in a matrix with epoxy is almost rubbery it has no real stiffness to it. Did we muddy the waters there? A rubber band has GREAT break strength and tensile strength, but ZERO stiffness. It also has great elongation properties. A toothpick doesn’t have much compressive or break strength, but it’s relatively stiff. Differeing materials have different attributes that can be used in ways that make best use of their “strengths”.

Now the wood on edge couldn’t manage this load, which is only a 10 pound object dropped from about 3 feet high. It’s considerable stiffer than the fiberglass of about the same weight, but NOT of the same thickness. That is to say if we compared a 1/8″ sheet of basswood 1 foot by 1 foot to a sheet of fiberglass in an epoxy matrix of the same dimension, the fiberglass sheet would be stiffer and heavier than the basswood. The reason being that the epoxy resin has a density close to 30 pounds per cubic foot, whereas the basswood is around 8′ish pounds. We would naturally expect the fiberglass and epoxy to be stiffer and stronger if it weighs 22 pounds more! So therein lies one of the issues, we have a weight critical application and we are trying to develop a composite of materials that best suit the environment and application, while also remaining as light as possible.

The point of that was to point out that the choice of wood to resisting the impact of a rider may not be a wise choice and also that horizontal stringers are completely ineffective at adddressing the sorts of loads we are discussing here when a rider lands from an aerial. However, we have been using Bamboo for part of our deck skin and also for a part of the wakesurf board that is internal to it’s construction that we haven’t disclosed yet.

So for now, we’ll end this post here and come back to a more indepth discussion of Bamboo in the not too distant future!

And we’ll keep on finding innovative ways to test! Now THAT’s some compressive forces.

Technorati Tags: composite sandwich, wakesurf board

6 comments

1 ping

dieta

January 13, 2012 at 6:20 am (UTC -7) Link to this comment

The Star45 is a 45″ long hard chine hull; semi-scale model of the full-size Olympic Star. This model is recognized by the American Model Yachting Association, AMYA. S45′s weigh 12 pounds (minimum), have 12 inch beam. They are easy to scratch build and sail. Well suited for building by by the novice builder and skippers looking for a classic looking model to race.
Val Jaksch

January 15, 2012 at 9:37 pm (UTC -7) Link to this comment

Hello there, just turned into alert to your weblog through Google.
Maggie Lesure

January 19, 2012 at 10:21 am (UTC -7) Link to this comment

Amazing article. I desire you up-to-date your blog significantly more often, I just cannot seem to be to acquire adequate of your blog. I preserved your blog in my bookmarks. Would it be feasible to do a guest post sometime?…
private jet charter

January 19, 2012 at 8:51 pm (UTC -7) Link to this comment

I really enjoy reading through on this website , it has got excellent articles .
the pileo diet food list

January 20, 2012 at 6:16 pm (UTC -7) Link to this comment

wonderful points altogether, you simply received a emblem new reader. What would you recommend about your submit that you made a few days ago? Any positive?
Traci

February 3, 2012 at 1:25 am (UTC -7) Link to this comment

It’s hard to find knowledgeable people on this topic, but you sound like you understand what you’re talking about! Thanks