Biomedical Engineering Reference
In-Depth Information
4.4
Crack propagation, toughening mechanisms
In linear elastic fracture mechanics, K
IC
is a constant (equation 4.4). When a
crack starts to propagate, it cannot be stopped and will grow catastrophi-
cally across the whole specimen. In order to improve the performance of
brittle materials, various strategies have been devised for slowing down and
potentially stopping crack growth. In composite systems, the toughening
phases act either behind the crack tip, where they slow down further opening
of an existing crack by keeping mechanical contact between the crack faces,
or in front of the crack tip where, by causing residual stresses, they cause the
crack to deviate. By dissipating energy in this way, they serve as pinning
sites for arresting cracks. In nanocomposites, the first type is mainly used in
systems containing nanofibres and nanotubes. The second type uses hard
nanoparticles such as silicon carbide.
4.4.1 Toughening by contact mechanisms
The mechanisms by which elongated structures (whiskers, large grains,
nanofibres) can hinder crack propagation are crack deflection, crack
bridging, pull-out and the associated friction. These are often observed at
fracture lines and fracture surfaces and are schematically illustrated in Fig.
4.13. Duszova´ et al. (2008) observed these mechanisms acting in ZrO
2
4.13
Toughening mechanisms acting behind the crack tip between the
fracture surfaces: D, deflection; F, friction; B, bridging.
