Composite Materials Research Progress

198
Giangiacomo Minak and Andrea Zucchelli
event [47]). The slope variations in all diagrams are associated to events with a high energy
content. In fact, as shown in figure 28C, the transitions between Z1 and Z2 and then between
Z2 and Z3 are determined by two events with an energy content higher then 1.0 10 -6 J
probably caused by fibres breakage. It is also interesting to notice that in the sub-domain Z2
there is a considerable number of events with a high energy content (22 events with an energy
higher then 1.0 10 -6 J) probably related to the breakage of quite large number of fibres. This
fact was also confirmed by the visual and SEM analysis (figure 23 M-a & M-b, figure 25).
Similar considerations can be developed considering the sub-domains Z3 and Z4 with a
number of 27 and 100 events with energy content higher then 1.0 10 -6 J respectively, that
probably reveal the progressive fibres breaking process.
Besides the analysis of the AE information it is interesting to relate the AE diagrams to
the mechanical response of the laminate.
In particular it is possible to notice that both the first zone Z1 and the second zone Z2 end
at the two important load drops. In the sub-domain Z1 the load-displacement diagram has a
monotonic increasing trend with an increasing slope and this is a direct consequence of the
fact that the system stiffness is increased by the transition from the bending to the membrane
behaviour [35]. So this sub-domain characterizes the test phase during which no important
damage is induced and the main part of the mechanical energy is stored in the material as
strain energy, in fact only a small part of the mechanical energy is dissipated by fibres
adjustment or alignments and matrix crack onset.
After the first load drop, the sub-domain Z2 begins, where the load displacement diagram
is again increasing monotonically, but contrary to what is observed in Z1 the slope is
decreasing. This is related to the material damage corresponding to the first load drop. In fact,
as noticed by visual inspection and SEM analysis, after the first load drop the fibres breakage
and the brittle matrix leakage reduce the local resistance of the laminate. So in the subdomain
Z2 the strain energy storing capability of the laminate is reduced if compared with the
laminate behaviour in Z1.
In the sub-domain Z3 the load-displacement diagram is characterized by a monotonic
increasing trend with a consistent decreasing of the slope. After the second load drop
delamination and fibre breakages compromise the local out-of-plane resistance of the
laminate and the energy storing capability is significantly reduced. The third zone ends when
the load reaches a relative maximum value and then it decreases. The sub-domain Z4 is
characterized by a slowly decreasing trend of the load with the total penetration of the
indenter in the laminate and the AE event are mainly caused by the delamination, matrix
cracking and leakage, fibre breaking and bending-pull-out. At the end of the loaddisplacement
diagram there is a new increasing trend due to the contact of the support of the
spherical indenter with the laminate surface.
The physical evidence of the failure modes that happens during the loading history in Z4
can be reconstructed by visual and SEM inspection as shown in figure 23 (H-a& H-b) and
figure 24.
An example of implementation of the function f(x) for this case study is shown in figure
29 where the strain energy (Es), the cumulative AE event energy (Ea), the load and the f(x)
diagrams relative to the same test reported in figure 28 are shown.