: My comment about the glue was along the lines of a joke. Don't take offense
: at the following.

: There was a very interesting experiment described on the wooden boat BB:
: http://media4.hypernet.com/cgi-bin/UBB/Ultimate.cgi?action=intro

: The idea is to take 2 wooden rods, apply a layer of fiberglass to 1, support
: the rods at the ends, and apply a load at the center.

: By selecting the correct amount of fiberglass it is possible to make the
: fiberglassed rod break first. I think what this proves is that the wrong
: amount of glass was can be used. In any case it should lead one to pause.

: Your tests may show something along the same lines. It is well known that
: fiberglass has a much lower compressive strength than tensile strength (I
: think the ratio is 3:4 but it depends on the resin). If the glass is
: failing then the failure should always be on the compression side.

: It is possible that: In tests along the grain the wood is failing and causing
: the outer glass to fail along with the inner glass. It only looks like the
: glass fails due to differences in "E".

: In my tests using 1/8" wood strips and 12oz of glass failures are always
: in the compresssed glass. The wood becomes compressed (about 10-25%), the
: glass fails in compression, the wood fails in compression, and the rest of
: the glass fails in tension.

: This may shouw you can design for the type of failure you want to have
: happen.

: REGARDLESS of HOW the failure occurs: It occurs because of all the materials
: not because of any one material. None of the materials should be singled
: out as providing any of the strength (or weakness). They work together -
: wood, cloth, resin, and glue.

George,

I like jokes. No offense taken at your comments (yet). For the benefit of other readers, I would like to point out that I am not interested in defending any particular position, but that I enjoy discussing the details of these types of problems. This could very easily become tedious for those less interested in how things break. I am sure that they will find other, more interesting posts to read.

I could not find the wooden rod experiment, but I imagine it is very much like a common problem given to undergraduates studying mechanical behavior. Given: two rods of equal diameter and length. Rod A is steel, yield strength 855 MPa. Rod B is aluminum, yield strength 505 MPa. The two rods are deformed in parallel (tension). Which rod will yield first? The steel rod will yield first because the strain at yield is less for the steel rod than for the aluminum rod. This also means that the steel rod is carrying more of the load than the aluminum rod - unequal distribution of load.

In my mind, there were two primary points demonstrated by the bend tests that we (Nick and I) did. One, typical wood strip/fiber composites used in kayaks do not behave like ideal sandwich panels. Two, the properties of these composites are strongly dependent on wood grain orientation. The main consequence of idea one is that the wood core carries a significant portion of the load on the structure (in bending). Point two is a consequence of point one combined with the anisotropic properties of wood. Perhaps these points are intuitive to some, or appear trivial, but I work with experimentalists…

Regarding your comment that if the glass is failing then the failure should always occur on the compression side: it is true that the compression strength of glass fiber composites is lower than the tensile strength. However, the failure mechanisms are different in compression than in tension. In tension, failure occurs when the glass fibers break, generally a catastrophic event. In compression, fiber buckling occurs with yielding of the matrix (epoxy). The early stages of compression failure in composites are generally not catastrophic, the structure remains in one piece. Many of our samples in which the wood grain was parallel to the principle stresses showed complete fracture on the tensile side and limited damage on the compression side. The tensile failure was clearly recorded as coincident with the first dramatic drop in the load curves. It was not clear if the wood failed before, at the same time, or after the glass failed.

I agree that it is possible to design for a given type of failure. I also agree that the materials - wood, cloth, resin, and glue - work together. That is the essence of composites. A direct consequence of this is that we certainly can single out which of the materials, under a given mode of loading, is providing [any of the] strength (or weakness). Otherwise, we could not design for a given type of failure.

Thanks for the constructive comments.

Sam