Ductile reinforcements for enhancing fracture resistance in composite materials

Ductile reinforcements can supply fracture toughness to a polymer matrix by pulling out and by plastically deforming. In the case of metal reinforceme nts that are not in a toughened condition, there may be more toughening to be gained when the fibers remain in the matrix and plastically deform rathe r than pulling out. These fibers can be said to have an unused plastic pote ntial. When these fibers bridge a crack, their plastic deformation causes a rise in the force which is trying to pull out the fiber. Because of this, the shape of the fiber must be adjusted along its length if it is to remain anchored and contribute its plastic work. The use of anchored, ductile fib ers provides a new design axis that brings new possibilities not achievable by the current research focus on the fiber-matrix interface. This paper de scribes the experimental pullout of aligned ductile fibers from a polymer m atrix, and indicates the effect of the shape and embedded length of the fib er on the toughness increase of the composite. Anchored, plastically deform ing fibers are shown to provide a major improvement to the toughening. Even for unoptimized ductile fibers, the calculated toughening improvement equa ls or exceeds the toughening available from current short glass or graphite fibers. In addition, pullout values are obtained for fibers that are embed ded at an angle, simulating fiber bridging from fibers not perpendicular to the crack surface. These results further demonstrate the toughening effici ency of ductile fibers. (C) 2000 Elsevier Science Ltd. All rights reserved.