Page 363 - Engineered Interfaces in Fiber Reinforced Composites

344 Engineered interfaces in fiber reinforced composites

and latter laminates. The tensile normal stress is harmful as it opens up the free edge,
leading to delamination.
The presence of [ + 15'1 and [ f 45'1 layers in a laminate also changes drastically
the magnitude and sign of the interlaminar normal stress, oz, depending on the layer
stacking sequence. Typical distributions of the interlaminar normal stress, oz,
obtained near the free edge when subjected to an uniaxial tension are presented in
Fig. 8.13 (b) (Pagan0 and Pipes, 1971) for the laminates with stacking sequences
[ f 15O/ f 4S0],, [ 1 So/ f 45"/-1 So], and [ f 4S0/ f 1 SO],. It is clearly shown that the
[ f 15"/ f 45"Is laminate has the highest tensile stress concentration in the mid-plane,
due to the largest difference in the stacking angle. From design considerations,
stacking sequence should be selected which can result in low tensile or compressive
normal stresses under tension.
The influence of material and stacking sequence on failure of boron fiber-epoxy
matrix laminates was studied by Daniel et al. (1974), and is summarized in Table 8.3.
It is noted that the ultimate tensile strength depends largely on the stress
concentration and the volume fraction of [O"] plies. Laminates with a high fraction
of [OO] plies, but with sufficient number of [45"] plies have the highest strength among
those studied, due to the low stress concentrations. Laminates without either [0°] or
[45O] layers fail prematurely due to the delamination initiated from the free edges:
laminates without [45"] plies give the lowest notch strength, whereas those without
[OO] layers show the lowest unnotched strength (Daniel et al., 1974). The other
parameter which influences the interlaminar stresses is the ply thickness. Thick plies
tend to encourage higher interlaminar stresses, thus causing premature delamina-
tion. It is shown that the critical strain for the onset of delamination decreases with
increase in 90" ply thickness in the laminate, in particular when placed in the mid-
plane (O'Brien, 1983).

Table 8.3
Effect of laminate layup and stacking sequence on stress concentration and strength of boron fiber-epoxy
matrix composites containing circular holes under uniaxial tension".
~~
Layup Young's Measured stress Predicted stress Notched Unnotched Strength
modulus concentration concentration strength, u~ strength, uo ratio,
(GPa) factor factor (MW (MPa) uN/cO
[0°/900/00/900]s 115.2 4.82 5.80 194 61 7 0.314
[ODz/ k45°/00], 133.9 3.58 3.68 498 807 0.617
[ f 45°/002/00]s 127.3 4.02 3.68 426 807 0.529
[Oo/ ~45°/O"/90"], 115.2 3.34 3.45 29 1 669 0.435
[0°2/=k450/900]s 116.3 3.15 3.45 29 1 669 0.435
[Oo/ i~45~/90"], 79.5 3.08 3 .OO 180 457 0.394
[45°/900/00/-45"], 81 .4 3.1 3.00 213 459 0.465
[ + 45'/0°/ f 45'1, 59.3 2.46 2.45 206 378 0.546
[ k 45O/ i 457, 19.9 2.06 1.84 125 137 0.909
[45"2/-45°z1~ 20.2 2.55 I .84 115 137 0.833

dAfter Daniel et al. (1974).