George,

I was out for a week, so I am just now responding to this. From the thread below this one, it looks like there is a lot of catching up to do. Please forgive me if some of what I write here is remedial for you. These discussions are for the benefit of all readers, so I will try to fill in some detail.

The work we did was a preliminary investigation into the mechanical properties of wood/fiber-reinforced polymer composites. The intention was to fill some knowledge gaps and begin to understand the mechanical behavior of the layups that are commonly used in strip-built kayaks. Before testing, I did have reservations about treating wood/glass layups as an idealized sandwich construction where the core serves only to separate the two skins. However, that was not the sole reason for doing the tests.

At no time did I claim a connection between the failures and plate theory. What I wrote was that these layups do not behave like an ideal sandwich. The data shows that conclusively. There is a very important distinction here; an ideal sandwich and plate theory are not the same concepts.

I am not the least bit surprised to hear that our test results correspond with lamination theory. The reason for this is a concept that no lesson in laminated plate theory is complete without. The concept is orthotropy. A single layer (lamina) of continuous fiber-reinforced composite has mechanical properties that vary with orientation and are symmetric about three orthogonal axes (for this reason, it really is not proper to talk about things like Young's modulus when discussing composites). Laminated plate theory uses the concept of orthotropy to calculate stresses and strains within laminated composites. Wood is orthotropic. Therefore, it stands to reason that wood/glass fiber epoxy laminations may be treated with lamination plate theory.

I fail to understand why you feel this is bad news. In any field, agreement between theory and experiment is considered to be a good thing. However, the application of laminate theory is only as good as the assumptions that one must make regarding the loading conditions and part geometry. It is clear how to apply lamination theory to three-point bending (I assume this is what you did). The assumptions required to apply it to a kayak hull introduce a much, much greater element of uncertainty.

It is well known that first failure in a laminated plate does not always result in complete failure. That was clearly the case for our samples having transverse wood grain orientation. The tension side glass remained unbroken after failure. This can often be a good argument for the use of composites since it allows some margin of safety. However, we should keep in mind that laminated plate theory calculates stresses and strains. Failure theories (such as Halpin-Tsai or similar models) are used predict the allowable load on a structure. I am very skeptical of crack propagation calculations in composites.

You point out that there is large variation in the properties of wood between grades. I would (no pun) go further to say that there is significant variation within grades. There is also significant scatter in the properties of fiber-reinforced composites. Glass fibers are sensitive to surface defects that can arise from handling or environmental interactions. This is one possible explanation for the small difference between our test samples made with 4 oz and 6 oz glass.

The issues of quality and workmanship are real. Unfortunately, they are most often used as a 'magic card' when dealing with the concepts of structural integrity.

Earlier, you advised me that to be taken seriously I should stand behind my test results. I do. Your statement that our results agree with lamination theory and that you use lamination theory in your calculations, contradicts your earlier stance that our results were meaningless for application to kayak building. In addition, you have often referred to cases of kayaks sustaining impact damage as support for your views. Therefore, I find it notable that you choose to ignore posts from others describing impact damage that does not support the notion of catastrophic failure. Perhaps you should pay more attention to your own credibility.

Sam