Composite Materials Research Progress

Damage Variables in Impact Testing of Composite Laminates 243
Considering the two-order difference in laminate thickness, data scattering is fairly limited.
Data for the thinner laminates are the most dispersed and show the lowest values. In [35], the
impact force was shown to depend on the flexibility of the laminate: values of Fpeak decrease
with increasing laminate flexibility.
A general trend for Fpeak can be envisaged. In [36], Found et al. proposed a relationship
between Fpeak and the square root of the impact velocity, i.e. between Fpeak and the impact
energy to the ¼ power. The proposed relationship (dotted curve in Figure 1) well interpolates
the experimental data.
Interpolation by two straight lines also appears rather good, allowing to point out that, for
impact energies above 40%-50% Pn, the rate of increase of Fpeak with increasing impact
energies slows down, with the value of Fpeak approaching an asymptote. The asymptotic trend
of Figure 1 well agrees with the idea of a maximum value for Fpeak [17,25]. In this respect,
data of Figure 1 seems to suggest that no real damage threat is associated to impact events for
which the impact energy is below 40%-50% of the laminate Pn. A concept of impact threshold
energy has been put forward by many researchers [35, 37-39]. This threshold has been
defined as a measure of the ability of a composite laminate to resist initial strength
degradation [35].
DD
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
E i/P n
GVP90_12.31 GE90s_8.00
GE90m_8.00 GE45_4.50
GE90s_4.00 GE90m_4.00
CE60_1.75 CE90_1.55
CE60_0.85 CE90_0.75
CE60_0.40 CE90_0.35
Figure 2. DD values plotted against non-dimensional impact energy E i/P n.
Figure 2 reports data for the DD, which appear fairly more dispersed, apart from the data
points of the three thicker laminates (GVP90_12.31, GE90s_8.00; GE90m_8.00) that are
basically overlapping. As a general rule, it can be said that the DD increases for increasing
impact energies and shows notably higher values for thicker laminates. In this respect, it is
important to note that high values of the DD do not imply severe damage within the
laminates. Indeed, the DD is a measure of the percentage of impact energy absorbed by the
laminate whereas no distinction is made on the absorption mechanisms as it is the case for the
DuI. High values of the absorption energy Ea can indeed be desirable, for example in crash