> Nick, BEWARE!!! Long post.

> The tests I was refering to were the Bougeon Bros. test results.

> Agreed

> The test showed that the thinner panel didn't always fail first. So both
> panels fail at roughly the same point, and a catastrophic failure only
> occurs when both layups and the core failed. The difference is that the
> 1/4 inch strips seem to fail long before the point of catastrophic
> failure. This would mean that it is the epoxy and glass that is taking all
> the force. A 1/8 inch strip will flex about 4 times more than the 1/4 inch
> strip. The epoxy and glass will still take the same amount of force. So
> the 1/8 inch strip will not break until closer to and maybe after the
> failure point of the epoxy has been reached. Therefore, you could
> theoretically eliminate all failures except scratches, or at the other
> end, catastrophic failures. (Also if the 1/8 strip is still intact the
> composite is now stronger than the broken 1/4 strip and could have a
> higher failure rate. The trick is to get the strip and the layup to fail
> at the same time. Thicker strip/thicker layup-thinner strip/thinner layup,
> probably to an "ideal" ratio for the core material.)

While the thinner strip flex farther before failing, they do so with less force. The pounds per inch of deflection are higher for the thicker panels. While the thinner panels flex more, they do so while absorbing relatively less force. At the same force, the stress on the thinner panels will be greater because they are bent more.

> You have far more experience here than I do. All I can go on is what
> others have said and one picture of a catastrophic failure. (Don't worry,
> he lived.) I would be very interested in the details of various damage you
> have witnessed.

Most of what happens comes from running over rocks. If the rock has a sharp point, it will try to cut a groove through the outer cloth. Not much to do about this but add more cloth or use some less easily cut material.

Bruises come from a wave droping you on a rock, or by somehow getting forced into a hard object with significant force. The outer surface gets pushed in and the glass is damaged. This happens because the wood is compressed, which could be called a failure, but after the fact the wood is fine, it is the glass and resin which are in bad shape. Sharp objects may puncture the glass. Whether this is due to a failure of the core is a matter of perception. If the glass were more rigid locally, the wood would not compress. If the wood did not compress, the glass would not be subjected to as much stress. Thinner would may not compress as much, but the amount of compression appears relatively small (1/32" - 1/16")

In the one holed boat I have seen, the glass failed in compression before the force punctured through and blew out the class on the far side. There did not seem to be much wood-to-glass bond failure.

In impact seems to crush the epoxy, causing the resin-to-glass bond to fail. Once the glass is no longer supported by the resin it does not have much compression strength. If the force is a point load it can then crimp the grain of the wood, causing another compression failure. It is then up to the inner layer to

> Once again, I don't have much experience with kevlar, but my understanding
> is that even if the epoxy fails the fibers remain intact to provide a
> measure of structural support. Bullet proof vests use this property
> without epoxy.

Bulletproof vests operate under a competely different principle from the skin of a kayak. If you soaked the vest in epoxy, it probably wouldn't work as well. Kevlar by itself in epoxy is quite brittle.

> Hit that same balloon and egg with a hammer and the results would reverse.
> So we would have to decide what we are building for. I must admit that I
> have been thinking in terms of the impact being like a hammer blow, but it
> could be thought of as a pencil push which would change things. (More
> thinking!!)

Now we need to decide whether what we hit is hammers or pencils. Probably some of each.

> Agreed.

> On this point we disagree, which is why we are having this discussion. I
> only have experience in fiberglass, SMC and various plastics used in the
> automotive trade, and respect your many years of experience in kayak
> building. I have seen many many failures of fiberglass and SMC,but they
> don't deal with a core material. One interesting thing I have seen
> regularly though is a very flexable urethane or similar plastic ( they use
> so many)used as bumper covers on cars that when subjected to a collision
> are only scuffed or slightly torn when the metal reinforcement behind them
> is destroyed. Hmmmm. This is kind of like those indestructable rotomoulded
> whitewater kayaks, to the extreem.

In those bumpers, the metal would be in fine shape if the plastic hadn't given way and the pastic would not have been any good at all without the metal backing it up. The plastic was not really subjected to the same force as the metal. It is primarily there for cosmetic purposes.

> I agree that this would also reduce damage, as long as the failure points
> of the materials was close.

> I had trouble with the idea until someone in another post mentioned
> toughness. This would, I think more aptly describe this concept, than
> strength. Chromoly is much stronger than mild steel but when it fails it
> has very little strength. Mild steel gets stronger as it fails (work
> hardening). If I was going to build something I knew would eventually have
> a catastrophic failure (cars), I would build it with the steel, not for
> strength, but for toughness.

It is the wood, not the glass that is the tough material in the composite. Wood can be cycled indefinitely without degradation. It will take a high load, flex and spring back undamaged. Epoxy and glass are relatively fragile, tending to crack, fatigue and otherwise degrade with loading. Hurricane Floyd is currently out doing mechanical testing of materials. Although a lot of trees will break, proportionally more steel and fiberglass objects will sustain catastrophic damage. Wood has weaknesses such as not being strong across the grain, and the fiberglass and epoxy help address those problems, but don't sell the wood short with regards to its properties.

The main reason for adding any fiberglass to the wood at all is because wood is not very strong across the grain. The glass serves to hold the wood together across the grain. Unidirectional cloth running across the grain will provide most of the strength required and you could use half the weight of glass. The fibers running with the grain don't do much beyond provide abrasion resistance which is probably reason enough to keep them there.

> Thankyou for a very interesting discussion, and we haven't even touched on
> kayak shapes as a component of strength! HaHa. Just kidding, I am just
> very curious about this idea. Ian