Composite Materials Research Progress

196
Giangiacomo Minak and Andrea Zucchelli
In this case, referred to the back side of a medium damage level lamina, matrix-fibre
debonding is evident and the tensile fibre fracture surfaces are orthogonal to their axis and
widely separated.
Load-displacement curves for the three damage levels are reported in figure 26 while the
maximum load, maximum displacements and total strain energy recorded in each test can be
found in figure 27.
Acoustic emission analysis of the transversal load test
A parametric AE analysis was performed considering the acoustic energy, the cumulative
events and the cumulative counts per event. As an example, in figure 28 these three AE
parameters are plotted together with the respective load-displacement diagrams.
For the AE diagrams reported in figure 28, the ΩAE is defined in the displacement range
[0.7; 11.1] mm. It is interesting to notice the presence of the FFD that represents a phase of
the test during which no damages are induced in the laminate (i.e. [0; 0.7]mm). The diagrams
of cumulative events and cumulative counts versus the displacement are characterized by a
general monotonic increasing trend, a quite similar shape and, for each diagram, at the same
displacement value there are significant slope variations. In particular from figure 28A and
figure 28B, inside the ΩAE, it is possible to notice a first part of the cumulative events and
counts diagram characterized by low values and a smooth trend and dominated by AE events
with a low number of counts and a low AE event rate. In figure 28 this first test phase has
been identified in an AE sub-domain marked as Z1 limited, for the considered example, in the
displacement range of [0.7; 6.3] mm. Considering also the energy diagram, figure 28C, it is
possible to observe that only one event in Z1, at the displacement value of 3.6 mm, has an
appreciable acoustic energy (over 3.0 10 -6 J) and from the statistical analysis it was observed
that only 5 events have an energy over 1.0 10 -6 J, that can be considered typical for fibres
breakage in bending. The sub-domain Z1 is physically dominated by material adjustment
(especially fibre alignment) and by matrix deformation and matrix crack onset that are
typically related to AE events with a small number of counts and low energy content [47].
The presence of few AE events with a high value of energy (over 1.0 10 -6 J) can be physically
related to the breakage of some fibres as was previously noticed by the SEM image (figure
23) even if probably the energy content of these events in most cases should be low since
these fibres are not loaded in tension. So during this first test phase (Z1) no significant
damage is induced to the material, as it was noticed during the visual inspection of laminate
surfaces, a small hemispherical mark in the matrix is appreciable (figure 23 L-a & L-b), and
as pointed out by means of the SEM image, figure 24, a certain amount of fibres are broken.
After this first phase there is a considerable increase of the AE activity represented by
increased values of the slope in both cumulative event and counts diagrams. The test phase
characterized by a high AE activity presents two other slope variations that can be used to
define three other sub-domains: Z2 over the displacement range [6.3; 7.2] mm, Z3 over the
displacement range [7.2; 8.1] mm and the Z4 over the displacement range [8.1; 11.1] mm. It is
important to point out, as initially noticed, that all the slope variations in the cumulative event
and counts diagram correspond to the same indenter displacement values. This is mainly
related to the fact that, generally, the damage growth inside the material causes an increase of
the total AE activity (events with an increased rating and an increased number of counts per