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.