Whether damage is localized or delocalized in a composite, the composi
te's fracture toughness when localization occurs can be controlled to
a much greater extent than hitherto exploited by properly choosing the
composite's internal geometry. Delocalization and high toughness are
both favoured by building in systematic defects and lock-up mechanisms
. Widespread defects make available arbitrarily many sites at which en
ergy may be absorbed by non-linear behaviour. Lock-up mechanisms cause
local hardening following local damage, which drives subsequent damag
e to initiate elsewhere, possibly leading to damage delocalization. In
brittle-brittle composites, these mechanisms may be the best hope for
achieving toughness values similar to those of alloys. Illustrations
are taken from recent research into woven composites with three-dimens
ional reinforcement and new work on model composites.