Mike,

The stress state makes the distinction between strength in compression or tension, versus flexure strength. In a flexure test (three- or four-point bending), tension and compression stresses are not uniformly distributed. So in general, the failure stress does not represent the value that would be found in uniaxial tension or compression. In addition, the span-to-thickness ratio of the bend specimen determines the ratio of shear stresses to tension and compression stresses. If you choose the correct span-to-thickness ratio, the specimen will fail in shear. A short beam test is the basis for determining the interlaminar shear strength of composite laminates. For design, only uniaxial compression and tension properties are used.

A plain weave 40:60 epoxy to glass ratio is easily attained using hand lay lay-up practices described on this board. I recently measured the ratio for 6 oz plain weave and 4 oz plain weave samples that I made with the same techniques that I use to build boats. The values were: 6 oz. – 36:64 and 4 oz - 33:67.

Sam