5.2 Thermoelastic Data
In this section, the data processed according to the procedure indicated
in section 4, are presented.
Fig. 8 reports for three imposed
stress values, the(S1_filt)N /(S1_filt)N0 normalised thermoelastic signal values.
The sample at 50%UTS exhibits a widespread lower signal except in some
localised areas where the signal is high in presence of the transverse
cracks. The samples at 60-65%UTS, at the sameN/Nf value present a similar behaviour of the
thermoelastic signal. In this case, the thermoelastic maps present a
more evident effect of the transverse cracks due to the higher values of
the imposed stress.
By resuming, the normalised thermoelastic maps at the initial stage of
the test present a characteristic behaviour related to the stress/strain
of the laminate, as previously described. During the cycles, the damage
produced by the transverse cracks determines a stress/strain
redistribution within laminae that, in turn, leads to a stress/strain
increase or decrease with respect to initial conditions. It follows
that, the thermoelastic signal increases in correspondence of the cracks
and decreases in the rest of the gage area due to the reduction of the
load carrying capability of the laminate. In this regard, the minimum
thermoelastic signal (2nd percentile) was adopted as
the proper metrics for describing the stiffness degradation.