Page 333 - Engineered Interfaces in Fiber Reinforced Composites

314 Engineered inlerfaces in fiber reinforced composites

121
Cure Temp: 177OC
0- 0
0 121OC 0
8- 0
-0 0
6- 0 0
n 0
4- 0 0
C)
0 0 0
2- cb
O0 %%
, * ?o.,, . ,
uv

Fig. 7.21. Maximum deflections of [Oi/9Oi], AS4 carbon fiber-3501-6 epoxy matrix composites plotted as
a function of temperature. After Crasto and Kim (1993).

cure temperature. The laminate returns to a completely flat sheet at a temperature
slightly above the cure temperature, and produces a reverse curvature with further
increase in temperature. The lamination residual stresses can be reduced, to some
extent, within a given temperature range by selecting an optimum stacking sequence
and ply orientation (Ishikawa et al., 1989).

7.5.1.3. Other sources of residual stresses
In addition to those induced thermally, there are other sources of residual stresses.
These include mechanical residual stresses, and those induced by phase transfor-
mation in composites based mainly on ceramics, and crystallization in semi-
crystalline thermoplastic composites. Mechanical residual stresses are present
mainly in ductile metal matrix composites due to the difference in flow stress
between the components (Metcalfe, 1974). This type of residual stress becomes
important when the composite is highly loaded to plastically deform one or both
components. Phase transformation in certain ceramics and ceramic composites
accompany significant volume changes, while the transforming component is
mechanically restrained. As a result, the surrounding material is locally strained by
this volume change. A good example of the phase transformation is partially
stabilized zirconia (ZrOz) which contains small amounts of MgO, CaO or Y203, and
undergoes a 3.25% volume expansion during cooling below approximately 1000°C
due to transformation from the tetragonal phase to the monoclinic phase (Porter
and Heuer, 1977). A very high fracture toughness can be achieved if the particle size
and processing conditions are carefully controlled.