Page 287 - Engineered Interfaces in Fiber Reinforced Composites

268 Engineered interfaces in fiber reinforced composites

toughness in mode I1 delamination which compriscs toughncss contributions from
matrix fracture, fiber-matrix interface debonding, frictional work due to sliding
between the opposite fracture surfaces as well as any fiber fracture and fiber
bridging. On the other hand, the failure mechanisms taking place in pull-out tests is
much simpler and idealised, and the experiment gives only the interface debond
toughness.

6.4.3. Crack growth resistance (R-curve) behavior in transverse fracture

6.4.3.1. R-curve behavior
LEFM of composites uses a simplified model of classical homogeneous isotropic
materials on a macroscopic scale, and assumes that crack propagation occurs when
the local stress exceeds the finite allowable critical strength which is measured on the
materials with notches. Many researchers including Konish et al. (1972), Ellis and
Harris (1973), Owen and Bishop (1973), Mandell et al. (1981, 1982) and Alexander
et al. (1982), have demonstrated that LEFM principles can be employed to
characterize the fracture toughness of short fiber composites by determining the
critical stress intensity factor, K,, with different specimen geometry. Fiber reinforced
composites, however, generally show a substantial amount of stable crack growth
before instability, even in composites with unidirectional continuous fibers, and the
fracture toughness increases with crack extension before it reaches a plateau value.
Therefore, a single parameter such as Kc is not totally appropriate to characterize
the whole fracture behavior and the concept of crack resistance curve (Le. R-curve)
has to be adopted.
Usually, an R-curve is represented by one of the fracture parameters: stress
intensity factor, KR; potential energy release rate, GR; contour integral, J; and crack
tip opening displacement, 6, as a function of crack growth, Au, including the length
of damage zone and any real crack extension. Comprehensive reviews on the crack
resistance behavior and its analysis and measurement of various engineering
materials, including fiber composites and cementitious composites, are given by Mai
(1988) and Cotterell and Mai (1996). Our discussion on R-curve behavior of fiber
composites presented below is focused mainly on transverse fracture.
Following the early report on R-curve determination for randomly oriented
glass-epoxy and glass-polyester systems (Gaggar and Broutman, 1975), many
workers (Agarwal and Giare, 1982; Morris and Hahn, 1977; Kim, 1979; Bathias
et al., 1983; Ochiai and Peters, 1982; Wells and Beaumont, 1987; Solar and
Belzunce, 1989) have studied the R-curve behavior for various types of composites.
The effects of fiber concentration, specimen thickness and width, and test
temperature and material have been specifically considered on the fracture
toughness of short glass-epoxy composites using the R-curve approach. In
particular, Wells and Beaumont (1987) developed a R-curve model based on the
energy absorbed due to the microfailure mechanisms in polymer matrix composites,
including off-angle fracture and delamination for cross-ply laminates in addition to
those described in Section 6.1 for unidirectional fiber composites. Reasonable
agreement is obtained between the predictions and the established data for the R-