Introduction

Composites reinforced with discontinuous fibers are categorized here as short-fiber composites. The fiber aspect ratio (length/diameter = l/d) is often used as a measurement of fiber relative length. Depending upon the dispersion of fibers in the matrix, the relevant d values may include those of the filaments, strands, rovings, as well as other forms of fiber bundles. Although discontinuous fibers such as whiskers have been used to reinforce metals and ceramics, the majority of short-fiber composites are based upon polymeric matrices. Discontinuous fiber-reinforced plastics are attractive in their versatility in properties and relatively low fabrication costs. The concern of the rapid depletion of world resources in metals and the search for energy-efficient materials has contributed to the increasing interest in composite materials. Discontinuous fiber-reinforced plastics will constitute a major portion of the demand of composites in automotive, marine and aeronautic applications.

A discontinuous fiber composite usually consists of relatively short, variable length, and imperfectly aligned fibers distributed in a continuous-phase matrix. In polymeric composites the fibers are mostly glass, although carbon and aramid are also used; non-fibrous fillers are often added. The orientation of the fibers depends upon the processing conditions employed and may vary from random in-plane and partially aligned to approximately uniaxial.

The understanding of the behavior of short-fiber composites is complicated by the non-uniformity in fiber length and orientation as well as the interaction between the fiber and matrix at fiber ends (Chou and Kelly 1976, 1980).