Page 273 - Engineered Interfaces in Fiber Reinforced Composites

2 54 Engineered interfaces in fiber reinforced composites

Fig. 6.10. Schematics of the dependence of total fracture toughness, R,, on fiber volume fraction of short
fiber reinforced thermoplastic composites at different loading rates: (a) static loading; (b) dynamic
loading. After Lauke et al. (1985).

should be multiplied with the fiber pull-out term. This reduces effectively the fiber
pull-out toughness and hence Rt. However, random orientation of ductile fibers,
such as steel and nickel wires, in a brittle matrix (Helfet and Harris, 1972; Harris
et al., 1972) may increase Rt due to the additional plastic shear work of fibers, as
discussed in Section 6.2.2.

6.3. Fracture toughness maps

Wells and Beaumont (1982, 1985) have related the composite fracture toughness
to the properties of the composite constituents using a ‘toughness map’ based on the
study of the energy absorption processes that operate at the crack tip in
unidirectional fiber composites. The microfailure mechanisms dominating the whole
composite fracture processes would determine which of the parameters are to be
used as variables. Having predicted the maximum energy dissipated for each failure
mechanism, a map is then constructed based on the available material data,
including fiber strength, modulus, fiber diameter, matrix modulus and toughness
and interface bond strength, as well as the predicted values of the debond length and
the average fiber length. By varying the two material properties while the remaining
parameters are being held constant, the contours of constant total fracture
toughness are superimposed on the map. These toughness maps can be used to
characterize the roles of the constituent material properties in controlling fracture
toughness, but they also describe the effects of testing conditions, such as loading
rate, fatigue and adverse environment on mechanical performance of a given
combination of composite constituents.