Composite Materials Research Progress

248
DD, DI
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Maria Pia Cavatorta and Davide Salvatore Paolino
y = 0.80x + 0.21
R 2 = 0.90
y = 0.58x - 0.01
R 2 = 0.98
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
E i/P n
DD DI
Figure 8. Comparison between DD and DI values for impact tests on GE45_4.50 laminates.
Results for Repeated Impact Tests
Apart from monitoring the range of the penetration process, the DI has proven to provide
important pieces of information in case of repeated impact tests, a loading conditions of
particular relevance in marine applications [4,30-32,38-39,41-42]. Figures 9-14 report data
obtained on two different laminates (GVP90_12_31, CE90m_4.00) tested under repeated
impacts. The two depicted impact energies were selected to represent tests of no laminate
perforation within 40 impacts and tests of laminate perforation. Figures 9-10 reports data for
Fpeak. As it can be observed, for impact energies that cause no perforation within test duration,
values of Fpeak slightly increase in the first few impacts to then reach an asymptote. On the
contrary, for energies that cause perforation, values of Fpeak decrease impact after impact as a
consequence of damage accumulation. For a given laminate, initial values of Fpeak reported in
Figure 10 are obviously higher than those of Figure 9 due to the higher impact energy used in
the test (Figure 1), while values just before perforation are lower than the asymptotic values
of Figure 9 due to the significant damage induced in the laminate. With respect to the initial
values of Fpeak, it is worthwhile noticing that for the 25 J tests and the 98 J test performed on
the GVP90_12.31 laminate, the maximum in Fpeak is not reached at the first impact. This
effect has already been observed in the literature. In a series of repeated impact tests run on
carbon/epoxy composite laminate, Wyrick and Adams [22] commented the initial increase in
Fpeak as the result of the compaction process of the thin layer of unreinforced resin at the
impacted surface. At low impact energy levels, damage to the fibers near the surface is
minimal and the compaction process provides a harder surface for the next impact. In this
respect, it is worthwhile noticing that this initial increase in Fpeak was observed when the
impact energy was below 40% Pn, once more confirming the existence of an energy threshold
level.