Composite Materials Research Progress

Damage Variables in Impact Testing of Composite Laminates 253
perforation, the increase is on the contrary very rapid. Differently from the DuI which keeps
to a constant low level up to a few impacts before perforation (Figure 12), the DI allows to
monitor the initial phase of steady damage accumulation helping foreseen perforation. The
initial slow decrease of DD values from the first to the second impact is followed by a
constant phase up to the 17 th impact, after which the DD increases rapidly and reaches a value
of one at perforation. Likewise the DuI, DD data are not very sensitive for predicting laminate
perforation as, apart from the last 2-3 impacts, the constant phase of Figure 14b does not
differ from the asymptotic trends of Figures 13a and 13b, where no perforation is achieved.
Also, DD values at the first impact are about the same, regardless of the level of impact
energy.
Conclusion
Impact test data obtained on different laminates are used to compare damage variables which
have been proposed in the literature over the years. To this aim, definition of the two energy
contributions used to compute the DuI has been extended to analyze impact tests with
rebound.
In single impact tests performed at different impact energies, data for Fpeak and DuI point
out the existence of an impact energy threshold at about 40-50% Pn, below which the energy
absorption mechanism is mainly matrix cracking. Graphs of Fpeak vs. impact energy show a
bi-linear trend with a change in slope around the energy threshold; while values of the DuI are
almost null below the energy threshold to then increase quite abruptly up to penetration.
Interestingly, the energy threshold is about the same for all the laminates analyzed in the
study, whose thickness varies from tenths to tens of millimeters. DI data increase
monotonically for increasing impact energies and show very limited scattering up to the
energy threshold. DD values and data in the Master Curve give no indications on the laminate
damage tolerance; rather, they provide a measure of the absorption capability of the laminate.
Results show that thicker laminates are characterized by a higher efficiency of energy
absorption.
Main advantage of the DI variable is the possibility to distinguish between the
penetration and perforation energy thresholds. The distinction is essential when dealing with
thick laminates, for which the impact energy that causes laminate perforation can by far
exceed the penetration energy. In the range of the penetration process, the DI effectively
monitors the impactor moving deeper and deeper into the laminate.
Also in case of repeated impact tests, the DI provides important pieces of information.
For impact energies that cause no laminate perforation within test duration, the DI stays at a
constant low value throughout the test, owing to a negligible damage accumulation besides
initial laminate indentation. For impact energies that cause perforation, the DI shows an initial
phase of linear growth with the number of impacts, owing to a steady accumulation of
damage. A few impacts before perforation, the DI starts raising quite abruptly, helping
foreseeing laminate failure.
Results for Fpeak show that it maintains a constant value when perforation is not achieved
while it decreases rapidly otherwise. However, graphs of Fpeak versus impact number do not
signal any change in the rate of damage accumulation. DuI values are almost null throughout
the test when no perforation occurs. Low and constant values also characterize tests at higher