993. Shear Loading in Fibrous Composites

Paper

Abstract

Fibrous composite materials have demonstrated significant weight savings when applied to aerospace structures and non-aerospace commercial applications. The basis for weight savings typically is due to either high specific strength, high specific modulus or in the case of dynamic systems, high specific fatigue strength. In general, applications that show the most promise are applications in which primary loads are directional and the high performance fibers can be oriented to their best advantage. However, since fibrous materials are laminated from individual plies the high performance fibers do not cross between plies. Although the in-plane properties can be extremely favorable the limitation due to this lamination results in low interply shear properties. Even in-plane shear properties can be substantially reduced from optimum (~45o) due to requirements for multidirectionally oriented plies for structures subjected to combined loading. Therefore, when looking at specific shear moduli and strength of fibrous composite materials, the comparison with structural metals is not nearly as spectacular. This is not to say that composite materials are still not advantageous but rather that a clear understanding is necessary of not only these shear limitations but also the differences in shear properties for the three orthogonal axes of a laminate.
A discussion of these three orthogonal shear properties is presented along with the differences in laminate construction which affect these shear properties. From a micromechanics standpoint these differences are clear. However, the present test techniques utilized for determining shear properties are often misunderstood or misinterpreted. The shear loading tests commonly used and an analysis of their individual merits and problem areas are discussed. Correlation between tests and analytical predictions are also presented.