Materials for engineering, 3rd Edition - (Malestrom)

212
Materials for engineering
10 –3
da/dN (mm cycle –1 )
10 –4
10 –5
PES
L
T
Matrix
20% wt% GF
30% wt% GF
direction
0.5 0.6 0.7 0.8 0.9 1.0 1.5 2.0 2.5 3.0 3.5
∆K 1 (MPa m 1/2 )
6.22 Log(da/dN) versus log(∆K) curves of PES composites tested at a
frequency of 5 Hz.
with their volume fraction. It is also apparent that the effect of the fibres is
greater for cracks growing normal to the mould fill direction than for those
growing along the mould fill direction.
The fatigue behaviour of a metal matrix composite is also usually superior
to that of the unreinforced matrix, particularly in the case of unidirectional
fibre composite systems. Three types of S–N curves have been reported, as
illustrated in Fig. 6.23, whose shape depends upon the mode of crack
propagation. If the matrix crack grows in a direction perpendicular to the
fibre axis, the sigmoidally-shaped S–N curve marked A is observed. If interfacial
debonding takes place by the crack progressing in a direction parallel to the
fibres and if the fibres are perfect, then the endurance limit will correspond
to the static strength and the S–N curve will be horizontal (marked B). The
majority of fibres will not be perfect, however, but will possess local weak
points, so that a mixed type of fracture occurs and the S–N curve will be a
mixture of types A and B.
6.4.5 Environmental effects in fibre-reinforced
composites
We have already discussed environmental effects upon metals and upon
polymers. Most practical reinforcing fibres (with the possible exception of
the aromatic polyamides such as Kevlar) are unlikely to be affected by