Composite Materials Research Progress

246
DuI
2.5
2.0
1.5
1.0
0.5
0.0
Maria Pia Cavatorta and Davide Salvatore Paolino
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
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
Figure 5. DuI values plotted against non-dimensional impact energy E i/P n.
Figure 5 reports data for the DuI plotted against the non-dimensional impact energy Ei/Pn.
By looking at the DuI values at penetration (Ei=Pn), thicker laminates appear to exhibit a
more ductile behavior. However, considering the elevated heterogeneity of the laminates
under study (in terms of type of fiber and matrix, orientation and percentage of fibers as well
as laminate thickness), it is the Authors’ opinion that caution must be taken in ranking
laminate performance. It should also be reminded that for the thicker laminates, penetration
and perforation thresholds do not coincide but are quite distant from each other. Significance
of Figure 5 is to show that, by extending computation of the DuI to impact energies below Pn,
the DuI can be used as a damage variable. In particular, data on Figure 5 show that for impact
energies up to 40% Pn, the amount of Epropagation is almost null meaning that the main energy
absorbing mechanism is matrix cracking. Above 40% Pn, and especially in the case of thick
laminates, the contribution of Epropagation becomes more and more important. This implies that
Fpeak occurs at a value of displacement significantly lower than the maximum displacement
reached by the laminate before dart rebound. As the impact energy increases, contribution of
delamination and of fiber breakage to the energy absorption mechanisms becomes more and
more important.
Figure 6 reports data in terms of the DI. By taking into account the value of the maximum
displacement, the DI is more of a damage variable than the DD was. Indeed, Figure 6 shows
that at very low impact energies the DI is almost null to then increase monotonically for
increasing impact energies. Up to impact energies of about 40-50% Pn, signaled by graphs of
Fpeak and DuI as energy thresholds, the difference in the value of the DI for different
laminates is quite limited. Above this threshold, the difference in DI data for different
laminates increases significantly. Data on Figure 6 also show that DI values do not saturate to
one at Pn, thus allowing to monitor the range of the penetration process. The effectiveness of
the variable in distinguishing between the penetration and the perforation thresholds can be
evinced from Figures 7 and 8 which report DD and DI data for two different laminates.