Composite Materials Research Progress

200
Giangiacomo Minak and Andrea Zucchelli
monotonic with an increasing trend and it is characterized by four main parts: the first part
has an exponential trend, the other parts have nearly linear trends with different slopes. The
slope variations coincide to the sub-domain Z2, Z3 and Z4 limits. The cumulative AE event
energy is a discrete function that monotonically increases. The sub-domain previously cited
delimits some of the Ea diagram variations: the first part of Ea is characterized by low values
and a smooth trend (inside Z1), the second domain, Z2, is characterized by an Ea increasing
trend and slope and at the end of Z2 there is an evident gap in the Ea value. This gap, as it is
clear in figure 28C, is due to an AE event with a high energy content and from the mechanical
point of view is related to the second load drop: the material damage has reached a high level.
In the sub-domain Z3 and Z4 the Ea diagram is characterized by an increasing trend but no
particular behaviour can be noticed. On the contrary, it is interesting to notice that in the Z2
domain the Ea trend is increasing with a general slope greater than the Ea slope in Z3 and Z4.
In particular comparing the diagrams in figure 28 and the Ea diagram in figure 29B it is
evident that the AE activity in Z2 is characterized by an increased AE event rate than in Z1
and in Z2 the events have a mean number of counts and a mean energy content higher than the
ones in Z1. This analysis of the AE information indicates that the test phase coinciding to the
sub-domain Z2 is the prelude to the main material crisis.
Considering the f(x) diagram it is possible to notice the presence of three increasing
diagram parts (type I: PI,1; PI,2; PI,3), four falls (diagram of type II: PII,1; PII,2; PII,3; PII,4), two
constant parts (PIII,1; PIII,2) and one decreasing part (PIV,1). The three increasing diagram parts
of f(x) are limited in the sub-domain Z1 indicating that at this first test stage the material has a
moderate attitude in storing the mechanical energy. Analysing the f(x) diagram in the subdomain
Z1 it is possible to note that the first fall PII,1 that connect PI,1 and PI,2 is not related to
an important material failure: there is no slope variation of f(x) before and after the fall PII,1
(S1-=S1+). On the contrary, considering the second fall (PII,2) it is possible to note that the final
slope of PI,2 and the starting slope of PI,3 are different (S2->s2+). This fact is due to the first
important material damage and, as noticed during 28C analysis, it is related to the fibres
breakage. It is worth noting that that the simple analysis of the load-displacement diagram
does not single out this first damage even though it is important because it definitely
influences the material strain energy storing capability and indicates the displacement value at
which the damage significantly begins. The other three sub-domains are characterized by a
general decreasing and constant trends of f(x). In particular in Z2, after the third fall (PII,3), the
f(x) is characterized by an initial constant trend directly followed by a decreasing trend (no
falls connect PIII,1 and PIV,1). This behaviour can be explained by the fact that the cumulated
damage during the test phase in sub-domain Z1 and the first fall is great enough to
compromise the material strain energy storing capability. The fourth fall, PII,4, that follows the
decreasing trend of f(x) (PIV,1) is due to the final material local degradation. It is interesting to
notice that in sub-domains Z3 and Z4 the f(x) is characterized by a constant trend while the
load-displacement diagram reveals the presence of a local load maximum value. So even if
the load diagram indicates a residual stiffness of the laminate the f(x) clearly indicates that the
material damage is definitive. The constant behaviour of f(x) indicates that despite the local
load maximum the material energy release is continuous and great enough to compensate the
material strain energy storing attitude: the mechanical energy propagates the material damage.