Page 99 - Engineered Interfaces in Fiber Reinforced Composites

82 Engineered interfaces in fiber reinforced composites

The ENF specimen is essentially a three-point flexure specimen with an embedded
delamination located at the mid-plane of the laminate where the interlaminar shear
stress is at its maximum when loaded. An almost pure shear prevails at the tip of
the mid-plane delamination (Gillespie et al., 1986). The major difficulty in
designing a pure mode I1 specimen is in preventing any crack opening without
introducing excessive friction between the crack faces. The compliance method
given by Eq. (3.10) can also be used here to determine the mode I1 strain energy
release rate, Gllc. The relationship between C and a is much more complicated for
the ENF test than for the DCB test. Although many equations have been
proposed, the one based on the classic beam theory (Carlsson et al., 1986) has been
most widely used
+
2~3 3a3
E‘ (3.31)
8Cbh3 ’
where E’ is the effective Young’s modulus and L, the half-span length. Therefore, the
expression for G1lc is obtained for small values of (E’E/Gs)(h/~)2, where G, is the
interlaminar shear modulus (Carlsson and Gillespie, 1989)

(3.32)

One of the disadvantages of this geometry is that unstable crack propagation occurs,
producing only one value, or only part of the propagation values, for tough
materials. For this reason, ELS specimen has been favored as it promotes more
stable crack propagation. For the ELS test, the compliance equation is given by

L3 + 3a3
8 CBh3
E’ = ~ (3.33)
and the corresponding expression for G1lc is given

(3.34)

In Eqs. (3.33) and (3.34) for the ELS specimen, L is the total length of the span. In
general, interlaminar fracture toughness in mode I1 may also be derived from a
mixed mode test, among which are the cracked lap shear (CLS) test, shown in Fig
3.33, which was originally developed for testing adhesively bonded metallic joints,
the end notched cantilever beam (ENCB) test (OBrien, 1985), and the cantilever
beam enclosed notch (CBEN) test (Benzeggagh et al., 1985). Using the CLS
specimen, the force-displacement (P - S) curves may be obtained for various crack
lengths and dC/da be determined. The substitutions of P and dC/da in Eq. (3.16)
directly give the interlaminar fracture toughness, G1-11~. Alternatively, G1-11~ may
also be calculated from the expression (Russell and Street, 1985)