Biomedical Engineering Reference
In-Depth Information
7.14
Percentage variation in fracture toughness of glass and glass-
ceramic matrix composites containing CNTs available from the
literature.
those of unreinforced glass/glass-ceramic matrices. However, there has been
some discussion in the literature about the validity of such measurements,
particularly in poorly consolidated materials. Single-edge V-notch beam
(SEVNB) and chevron V-notch beam (CVNB) techniques are a better guide
to absolute fracture toughness values. Nevertheless, comparative studies
have shown that both approaches can offer semi-quantitatively similar
trends for well-processed systems, although VIF tends to overestimate
CVNB values (Cho et al., 2011).
An increase in fracture toughness has been observed in all composites
developed (Fig. 7.14), although in some cases, a drop again occurs at high
CNT loadings due to inhomogeneous CNT dispersion. However, the extent
of improvement of fracture toughness, although significant, is limited; a
maximum increase of 150% has been observed using the indentation
technique (Guo et al., 2007a, Ning et al., 2004). This level of improvement in
fracture toughness, is not, in itself, expected to deliver structurally reliable
composite materials; conventional fiber reinforced composites (discussed
earlier) usually exhibit higher fracture toughness. Nevertheless, the
improvements observed using CNTs may provide useful benefits in
combination with other properties, or indeed in combination with
conventional fibers, to create hierarchical composites, as has proved
promising for polymer matrix systems (Qian et al., 2010). CNT-glass/
glass-ceramic matrix composites tested for fracture toughness include those
