a. Micrograph of sample A.
b. Micrograph of sample B.
Figure 7. Very high velocity impact fragment fracture surfaces.
hackle marks in fracture surfaces would primar-
failure mechanisms, one taking over the other, as
ily indicate an impact induced damage, and prolif-
the velocity increases. For example, in the nonper-
eration of hackle marks on our ballistic fragments
forated low velocity impact on the multiple-ply
clearly indicates this trend (Fig. 7 a, b).
laminate, the Hertzian indentation was the major
The hackle marks present a periodic formation
form of damage, whereas at higher velocities shear
of sigmoidally shaped ridges and troughs running
plug-outs with extensive compressive failure and
roughly parallel to the fiber direction and located
delaminations were the main mechanisms.
in the matrix region between them. There is a spa-
The fractographic features and its relation to the
tial periodicity perpendicular to the direction of
failure are of interest. Very little information on
the fibers, which is the first suggestive observa-
the genesis of hackle mark formation is available
tion. The second observation is that the wave-
in the composites literature, except that a large
length of these hackle marks appears to increase
number of researchers have observed their pres-
as the distance between the neighboring fibers in-
ence related with brittle failure (Morris 1982, Ri-
crease (Fig. 7a). The wavelength of the hackle mark
chards-Frandsen and Naerheim 1983). It has been
is of the of the order of interfiber distance. Some-
shown by Purslow (1986) that in static failure, the
times, the ridges appear to be canted at an angle
presence of the relatively stiff fibers increases the
to the fiber axis.
influence of shear stresses on the resin fracture and
It is appropriate now to examine the phenome-
hackle mark development under such conditions
non of wavelength proportionality to fiber spac-
will be suppressed. Figure 8, reproduced from the
ing at some depth. Conceivably the wavelength
work of Dutta and Taylor (1989), shows almost a
proportionality to fiber spacing would result if it
total absence of any hackle marks from transverse
were associated with or determined by a disturb-
tensile fracture under quasistatic loading over the
full thickness of a seven-ply AS-4/3501 laminate.
ance that propagates in the plane of two adjacent
fibers from one fiber surface to the other at a fixed
However, Purslow observed that at relatively high
angle to their length direction. Then this propor-
strain rate of deformation and fracture, matrix
tionality would be reflected back from the other
cleavage and hackle formation are common. Thus,
6
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