Issue 49

R.V Prakash et alii, Frattura ed Integrità Strutturale, 49 (2019) 536-546; DOI: 10.3221/IGF-ESIS.49.50 543 cooling response better. It is observed that in the case of transmission mode i.e. heating from back (HB), the peak normalized temperature (Tp) and the time lag (∆t) for reaching the peak temperature (on the surface facing the IR camera) from which point the cooling response starts is proportional to the extent of impact-fatigue damage that is present in the specimen. The pristine samples too showed a small time lag, which was proportional to the thickness of the specimen. The increase in peak temperature and the time delay to reach the peak temperature prior to start of cooling could be due to the presence of different modes of failures within the specimen and due to the presence of air gap which has a different thermal diffusivity that is trapped between the different layers of the laminate. In the case of reflective mode (HF) of active thermography, the fast exponential cooling rate in the initial part of the temperature response curve (say observed within initial 80 seconds) provides an alternate method to assess the extent of damage present in the specimen. Upon comparing the temperature response curves of post-impact fatigue loaded specimens with that of pristine specimen in both ISF HB and ISF HF configurations, the shift in the cooling response curves from that of the pristine can be seen, but no correlation can be made with the extent of damage. Figure 7 : The combined Force vs. time and Temperature vs. time plots (blue line) for the post-impact post-fatigue specimens (the temperature response at the middle region (A2) (shown by dark red color) is plotted. Fig. 9(a) presents the cooling response of impact damaged specimens with the impacted surface on the rear side of the IR camera. Similar to the previous case of ISF, the cooling response shows a time delay to achieve the peak normalized temperature in transmission mode (HB). Fig. 9(b) shows the histogram of damage volume estimated through image analysis of X-ray CT images for the three specimens considered for this analysis. Upon plotting the peak normalized temperature (Tp) as well as the time lag (∆t) for reaching the peak temperature versus the damage volume, a good correlation was observed as shown in Fig. 9(c) for the heating from back (ISB-HB) configuration. The large shift in peak temperature and time delay observed in the case of 51 J Lo-Hi may be due to the presence of major de-lamination in the specimens. Such a clear distinction in cooling response cannot be easily tracked from ISB-HF configuration data. From the above, it can be said that for the qualitative analysis of post-impact fatigue damage in composites the ISB configuration is preferred over ISF and the HB configuration may be chosen over HF as it gives more accurate results. Further, the use of active thermography with impact surface at back (ISB) and transmission mode (HB) of heating provides a good understanding of the damage state of the specimen, though more work is required to quantify the damage that exists across multiple layers of laminate.