Page 362 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 8. Improvement of interlaminar fracture toughness with interface control 343

tension. The testing technique to measure the free edge interlaminar fracture
toughness using mode 1 edge delamination test (EDT, see Fig. 3.34) is schematicaily
shown in Section 3.4.4. Free edge delamination is a direct manifestation of large
interlaminar stresses which are highly localized near the free edges. Fig. 8.13 (a)
illustrates the stress concentrations, in particular for the tensile stress component
through the laminate thickness, taking place near the free edge. The free edge
stresses arise from the mismatch in the laminar coefficient of thermal expansion
(CTE) and Poisson ratio between the adjoining layers (Herakovich, 1981). If there is
no mismatch of these parameters, the interlaminar stresses do not exist even if there
is a mismatch in elastic and shear moduli. Even so, delamination between layers of
the same orientation can take place if there is an interface moment arising from the
neighboring plies of different orientations or elastic properties. For example,
delamination at the free edge of a laminate [ f 45°/00/900], may occur in the mid-
plane, that is at the interface [9O0/9Oo], laminae where a large interface moment and
tensile stress in the through-thickness direction exist.
The magnitude and distribution of the interlaminar stress components vary
widely and depend upon the number of plies for each angle, ply thickness, stacking
sequence, the properties of the composite constituents and the nature of loading
(Kim, 1989). The lay up sequence, in particular, has a significant effect on the
magnitude and the direction of the interlaminar stresses (Pipes and Pagano, 1970;
Pagano and Pipes, 1973; Daniel et al., 1974; Wang and Crossman, 1977;
Herakovich, 1981, 1982; Wang and Choi, 1982; Kim and Soni, 1984; O'Brien,
1983, 1984; Heyliger and Reddy, 1985; Joo and Sun, 1992; Kim and Hong, 1992;
Xu, 1996). For example, the [90°/450/00/-450], laminate is less prone to delamination
than the [ * 45"/0"/90°], laminate because the through-thickness normal stresses
generated at the mid-plane are compressive and tensile, respectively, for the former

5

Fig. 8.13. (a) Distribution of interlaminar normal stress, I?=, and interlaminar shear stress, rz.,., in [OO/W"]s
laminate under axial tension. (b) Effect of stacking sequence on through-the-thickness distribution of
interlaminar normal stress, cZ. near free edge: stacking sequence: (- ) [ f I5"/ f 45"],; (- ) [ 15'/45'/-
45"/-15"],: (...---.) [*45"/* IS'],. After Pagano and Pipes (1973)