Page 255 - Plastics Engineering

238 Mechanical Behaviour of Composites

Limit = 1800 Limit = 100
I I
0 77 I

I
0 244 5
01 02 712
Limit = 0.09% Limit = 2.3%

- ____

0 0.172% I 0 0.07%! 0 0.12%
€2 Y12
Fig. 3.31 Stress and strain in the plies, Example 3.21

probably because the stress in the 2-direction is getting very close to the
limiting value.

3.17 Fatigue Behaviour of Reinforced Plastics
In common with metals and unreinforced plastics there is considerable evidence
to show that reinforced plastics are susceptible to fatigue. If the matrix is ther-
moplastic then there is a possibility of thermal softening failures at high stresses
or high cyclic frequencies as described in Section 2.21.1. However, in general,
the presence of fibres reduces the hysteritic heating effect and there is a reduced
tendency towards thermal softening failures. When conditions are chosen to avoid
thermal softening, the normal fatigue process takes place in the form of a progres-
sive weakening of the material due to crack initiation and propagation.
Plastics reinforced with carbon or boron are stiffer than glass reinforced
plastics (grp) and they are found to be less vulnerable to fatigue. In short-fibre
grp, cracks tend to develop relatively easily in the matrix and particularly at
the interface close to the ends of the fibres. It is not uncommon for cracks to
propagate through a thermosetting type matrix and destroy its integrity long
before fracture of the moulded article occurs. With short-fibre composites it has
been found that fatigue life is prolonged if the aspect ratio of the fibres is large.
The general fatigue behaviour which is observed in glass fibre reinforced
plastics is illustrated in Fig. 3.32. In most grp materials, debonding occurs