Composite Materials Research Progress

238
Maria Pia Cavatorta and Davide Salvatore Paolino
initiation point for delaminations and fiber breaks which may dramatically reduce the local
and or global laminate stiffness thus affecting the load-time response. The literature also
acknowledges that more damaging energy absorption mechanisms (such as delamination,
fiber pull-out, fiber/matrix debonding and fiber fracture) follows matrix cracking and that
they significantly reduce the strength and stiffness of the laminate. Considering the
importance of damage assessment, there have been several attempts in the literature to look
for measurable test quantities that could be correlated to the damage process [6, 12-19].
Under low-velocity impact loading conditions, the time of contact between the impactor
and the target is relative long. Even though vibratory load responses from the composite
sample, the impactor and the specimen supporting fixture are common features of impact
loading history, the load history can still yield important information concerning damage
initiation and growth [20]. Several authors have used the force-time history to compare the
structural response from impact tests: in particular, values of the First Damage Force (FDF)
[14-21], the Delamination Threshold Load (DTL) [6] or Hertzian Failure Load (Ph) [15] as
well as of the Peak Force (Fpeak) [2-3, 17, 22-24] have often been used to rank laminate
performance. Identification of the FDF poses no troubles as it corresponds to the first load
drop which can be detected in the load-time history. However, comparison of laminate
performance on such basis can be risky since the level of laminate damage associated with the
first load drop may be quite different for a given laminate tested under different impact
energies, or for different laminates tested at a given impact energy. In this respect, definition
of the DTL and of the Ph appear more suitable for ranking laminate performance as they are
intended to identify a more specific damage condition, that is the initiation of significant
damage. In the case of the DTL, significant damage is defined as predominately delamination,
while for the Ph energy absorption mechanisms other than matrix cracking are considered.
The DTL and Ph do not necessarily correspond to the first load drop; rather, they are
associated to the load drop at which a significant change in the slope of the forcedisplacement
curve may be detected and which signals a change in laminate stiffness.
Experimental determination of these load thresholds, which are shown to vary with the
laminate thickness to the 3/2 power, may prove helpful for damage tolerant design: no
significant damage threat is associated to impact events for which Fpeak is below the laminate
DTL or Ph. On the contrary, for impact events for which Fpeak is above the laminate DTL or
Ph, a damage threat exists, even if no information can be obtained on the final amount of
cumulative damage that will occur.
Difficulties and possible ambiguities in determining the DTL or the Ph have often led
researchers to use Fpeak instead, considering it as the turning point between rather limited and
more significant forms of damage. In [17,25], Liu suggested that for any composite laminate
there exists a maximum value of Fpeak. When the impact energy is such that Fpeak is below this
maximum value, the laminate suffers indentation and local matrix cracking, whereas when
loaded by the maximum Fpeak significant delamination starts to take place.
The idea of Fpeak as the signaling point of significant damage initiation is the basis of the
dimensionless parameter introduced in 1975 by Beaumont et al. [16] called the Ductility
Index (DuI). The DuI, which is proposed as a useful tool for ranking the impact performance
of different materials under similar testing conditions, is defined as the ratio between the
propagation energy Epropagation and the initiation energy Einitiation and it is given by the
expression: