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.