Composite Materials Research Progress

184
Giangiacomo Minak and Andrea Zucchelli
for a general crisis of the laminate. In fact the PII,1 is followed by a small series of strain
energy storing phases, PI,2 and PI,3, and a combination of sub-function of type II, III and IV
highlighting a great material damage.
The QI3 is characterized by a sentry function that mixes the behaviour of the studied UD
and AP laminates. At the test beginning is visible a combination of sub-functions of type I
and of type II with a predominant strain energy storing phase. This phase is characterized by
the sequence of the following sub-functions: PI,1, PII,1, PI,2, PII,2 and PI,3. A delamination
failure is revealed by the slope change of f corresponding to PII,3. After this failure a reduced
energy storing capability is revealed by the PI,4 and the following drop PII,4 is due to the
failure of the weakest ply inside the laminate. This first ply crisis is followed by a subfunction
of type I, PI,5, that has a smooth trend due to the previous material damage. The
damage corresponding to the sub-function PII,6 is due to the crisis of one of the stronger plies.
After this laminate crisis the energy storing phase represented by the PI,6 is due to the stresses
redistribution between the undamaged plies that are now working as springs in parallel.
The analysis here described has been used to perform a quantitative estimation of the
laminate damage and in particular the sentry function of each laminate has been used to
perform a discretization of the stress-strain curve. As an example of this analysis we report
the case of UD laminate type.
f
40
35
30
25
20
15
10
5
f Stress-Strain
Drops
0
0
0.000 0.003 0.005 0.008 0.010
Strain
0.013 0.015 0.018 0.020
Figure 11. Stress and f diagram versus strain, the most key drops of f are highlighted by means of gaps
diagram.
The analysis of the sentry function in figure 11 allows the identification of 10 drops on
the basis of which the stress-strain curve was divided in order to calculate the Elastic
modulus.
2500
2000
1500
1000
500
Stress (MPa)