Page 257 - Plastics Engineering

240 Mechanical Behaviour of Composites

for reinforced plastics are in the range 5-50 MN m-3/2, with carbon fibre
reinforcement producing the higher values.

3.18 Impact Behaviour of Reinforced Plastics
Reinforcing fibres are brittle and if they are used in conjunction with a brittle
matrix (e.g. epoxy or polyester resins) then it might be expected that the
composite would have a low fracture energy. In fact this is not the case and
the impact strength of most glass reinforced plastics is many times greater
than the impact strengths of the fibres or the matrix. A typical impact strength
for polyester resin is 2 H/m2 whereas a CSWpolyester composite has impact
strengths in the range 50-80H/m2. Woven roving laminates have impact
strengths in the range 100- 150 kJ/m2. The much higher impact strengths of the
composite in comparison to its component parts have been explained in terms
of the energy required to cause debonding and work done against friction in
pulling the fibres out of the matrix. Impact strengths are higher if the bond
between the fibre and the matrix is relatively weak because if it is so strong
that it cannot be broken, then cracks propagate across the matrix and fibres,
with very little energy being absorbed. There is also evidence to suggest that
in short-fibre reinforced plastics, maximum impact strength is achieved when
the fibre length is at the critical value. There is a conflict therefore between the
requirements for maximum tensile strength (long fibres and strong interfacial
bond) and maximum impact strength. For this reason it is imperative that full
details of the service conditions for a particular component are given in the
specifications so that the sagacious designer can tailor the structure of the
material accordingly.
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