Biomedical Engineering Reference
In-Depth Information
7.12 Percentage variation in modulus of elasticity of glass and glass-
ceramic matrix composites containing CNTs from the literature. Dotted
lines 1, 2 and 3 show the calculated elastic modulus of three glass
matrix composites with three-dimensional randomly oriented CNTs of
elastic moduli of 1000GPa, 500GPa and 100GPa; an elastic modulus
value of 70GPa was chosen for glass.
elastic modulus of the composites is expected. However, since the focus of
inorganic material reinforcement is usually toughness, relatively few data
are directly available (Fig. 7.12) and they are generated by disparate
techniques depending on sample size (three- and four-point bending tests,
ultrasonic techniques and instrumented hardness). There are some reports of
an increase in composite stiffness, for example a 38% enhancement in silica
stiffness at 7.5wt% CNTs, though matrix crystallization may play a role,
particularly where large increases are observed at low filler loadings (Guo
et al., 2007a, Loo et al., 2007). Improvements in stiffness of CNT-
borosilicate glass matrix composites at low CNT loadings (2wt%) have been
noted in the absence of crystallization effects; however, at higher CNT
loadings (3wt%), agglomeration and other consolidation issues lead to a net
reduction in performance (Thomas et al., 2009). In other cases, monotonic
reductions in stiffness have been attributed to inhomogeneous dispersion of
CNTs, insufficient densification or degradation of CNTs during processing,
in both glass (Boccaccini et al., 2007, Thomas et al., 2009) and glass-ceramic
matrix composites (Mukhopadhyay et al., 2010).
For three-dimensional randomly aligned short fiber composites, a
modified rule of mixtures (Krenchel's rule (Krenchel, 1964)) incorporating
