Carbon Nanotubes in Acrylic Bone Cement - Biologically Responsive Biomaterials for Tissue Engineering - page 182

8.3.4 Polymer Matrix Composites

The mechanical success of any polymer composite is governed by the successful
transfer of load between the matrix and the reinforcement. This transfer of load is
dependent upon the volume fraction, dispersion, orientation of the reinforcing
phase, host matrix-reinforcement interface and individual mechanical properties of
the phases that are present [ 29, 38, 99 ] . Within fi bre-reinforced composites four
main microstructural regions exist: (1) the matrix, (2) the fibre, (3) the interface and,
in some composites, (4) the interphase. An interphase may be present if a mechani-
cal or chemical interaction takes place between the polymer matrix and the reinforc-
ing phase. Examples include adsorption of the polymer onto the surface of the
reinforcing agent in particulate-reinforced polymers, inter-diffusion of the compo-
nents during blending and chemical reactions at the polymer/fibre interface [ 74 ] .
Fibres aligned in the direction of applied load are particularly effective at reinforc-
ing composites. Corresponding mechanical properties which effect failure perfor-
mance may be identified, with a complex interaction between individual phase
properties, the interfacial strength between the host polymer and the reinforcing
fibres and the composite microstructure. Polymers are the most common form of
composite matrix and are often reinforced with a low fraction of fillers such as
glass, CF or aramid. This results in composites of high specific strength and modu-
lus as the low levels of additives allow a more homogeneous dispersion [ 12 ] . Of
these three reinforcements, CF-polymer composites often exhibit the best resis-
tance to fatigue failure due to superior mechanical properties as well as the higher
thermal conductivity of the CF, which assists in the dissipation of heat during cyclic
loading [ 79 ]. In compression, the mechanical performance of fibre-reinforced com-
posites is dependent on the interaction between the host polymer matrix and the
fibre. For optimum reinforcement, the matrix would provide lateral stabilisation to
the fibre preventing subsequent buckling. Alternatively, the tensile behaviour is
governed by the tensile strength of the fibre additive [ 38 ]. Fatigue failure in polymer
composites is commonly characterised by a gradual reduction in stiffness [ 80 ] .
Without reinforcement, fatigue failure typically occurs perpendicular to the applied
load. In contrast, the presence of fibres generally results in a diffuse damage zone
due to the combination of a number of subcritical failure modes and crack shielding
mechanisms. In general, crack propagation through fibre-reinforced polymers may
be considered as a multifaceted interaction between the polymer matrix, the fibre
reinforcement and the associated interface/interphase regions. A combination of
mechanisms may occur and subsequently, fibre inclusions may impede crack growth
by three main mechanisms [ 54, 78 ] :

1. Debonding of interface/interphase between fi bre and matrix—as a crack
approaches, failure of the interface occurs serving to blunt the crack tip and
reduce crack propagation.
2. Crack bridging—transferring load across a given matrix crack, reducing the
crack.
3. Fibre pullout, subsequent to crack bridging, may also absorb energy due to matrix
deformation and/or interface friction.