Biomedical Engineering Reference
In-Depth Information
the dispersed phase serves to impede crack growth. In this case, it
acts as reinforcement. A number of methods, including deflecting
crack tips, forming bridges across crack faces, absorbing energy
during pullout and causing a redistribution of stresses in regions,
adjacent to crack tips, can be used to accomplish this [96]. Other
factors to be considered in composites are the volume fraction of
(a) dispersed phase(s), its(their) orientation and homogeneity
of the overall composite. For example, higher volume fractions of
reinforcement phases tend to improve the mechanical properties
of the composites, while continuous and aligned fibers best prevent
crack propagation with the added property of anisotropic behavior.
Furthermore, the uniform distribution of the dispersed phase is also
desirable, as it imparts consistent properties to the composite [64,
94, 95].
In general, composites might be simple, complex, graded and
hierarchical. The term “a simple composite” is referred to the
composites those result from the homogeneous dispersion of one
dispersed phase throughout a matrix. The term “a complex composite”
is referred to the composites those result from the homogeneous
dispersion of several dispersed phases throughout one matrix. The
term “a graded composite” is referred to the composites those result
from the intentionally structurally inhomogeneous dispersion of
one or several dispersed phases throughout one matrix. The term
“a hierarchical composite” is referred to the cases, when fine entities
of either a simple or a complex composite is somehow aggregated to
form coarser ones (e.g., granules or particles) which afterwards are
dispersed inside another matrix to produce the second hierarchical
scale of the composite structure. There is another set of four types of
composites: (i) fibrous composites, where the fibers are in a matrix;
(ii) laminar composites, in which the phases are in layers; (iii)
particulate composites, where the particles or flakes are in a matrix;
and (iv) hybrid composites, which are combinations of any of the
above. Still other classification type of the available composites is
based on the matrix materials (metals, ceramics and polymers)
[63].
In most cases, three interdependent factors must be considered
in designing of any composite: (i) a selection of the suitable matrix
and dispersed materials, (ii) a choice of appropriate fabrication and
processing methods, (iii) both internal and external design of the
device itself [66]. Furthermore, any composite must be formed to
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