Mechanical Requirements of the Fiber-Matrix Interface

Abstract

THE GREAT STRENGTH enhancement and resistance to fracture which occurs when one combines high strength fibers with a low strength, low modulus polymer matrix is well known. The high strength and toughness of the resulting composite material, to a great extent, is dependent upon the many polymer glass interfaces which exist and their ability to deflect cracks propagating normal to them. The presence of these large number of interfaces, however, results in problems which may partially overcome the advantage of their existence. Sufficient adhesive strength must be developed at every point along the polymer-fiber interface so that the maximum stress can be transferred from the polymer matrix to the fiber reinforcement. The critical fiber length or length of fiber required to achieve this maximum stress is thus dependent upon the interfacial strength. A void or an air pocket existing at the interface will cause a stress concentration regardless of the stress state; in addition, this unsupported length of fiber (i.e. the length of the void parallel to the fiber axis) will be subjected to buckling when compressive stresses exist in the fiber. A poorly bonded area at the interface will cause rupture of the interface at very low stresses and the resulting discontinuity will act as a stress concentration.