Issue 49

R.V Prakash et alii, Frattura ed Integrità Strutturale, 49 (2019) 536-546; DOI: 10.3221/IGF-ESIS.49.50 541 the specimen in this region happened with reduced energy levels. Further, the heat flux in the specimen does not pass it on to the surface, hence the peak temperature rise in region A3 could be less. However, in regions A1 and A2, where there was no indication of ply-failures, the energy absorbed by the specimen was relatively high, which resulted in higher temperatures at the time of final tensile fracture. Based on the above, one can say that a good correlation between failure events (as inferred through F-d plots) and temperature evolution exists. Thermal imaging technique could identify the on- set of damage initiation as well as non-homogenous damage distribution across the gage length of the specimen. Figure 4 : The composite graph of Force vs. time and Temperature vs. time for the un-impacted specimen (the temperature response at the bottom segment (A3) is plotted). The temperature response during static strength tests of post-impact post-fatigue specimens is shown in Figs. 5 and 6. It may be noted that the presence of matrix cracks and minor delaminations in the specimen in the case of impact, fatigue damaged specimens could obstruct the passage of heat flux to surface from the bulk of the specimen. As a consequence, the temperature rise during tensile pull-out tests is not as significant in impacted specimens compared to the previous case of un-impacted tensile specimen. The peak temperature response at the onset of failure of the material is indicative of the failure mode. Between impacted specimens of different impact energy levels, the temperature rise at failure is not significant except for 23 J CA (flat) and 23 J Lo-Hi specimens. This could be due to reduced levels of damage caused to the specimen by 23 J impact. In the case of 23 J CA hour glass specimens the entire impact damage and subsequent fatigue damage was concentrated at the middle minimum width (35 mm) region and hence the minimum temperature rise is seen (ref. Fig. 5). In the case of 23 J Lo-Hi, the multiple major peaks in the temperature profile can be seen as the load drop (ply failure) occurs in two major steps as seen in the force-displacement curve [see Fig. 2 (a) for legend 23 J LH]. The temperature response of 23 J CA flat specimens with 45 mm uniform width is presented in the Fig. 5 for a comparison of specimen shape effect – which suggests that the temperature rise in hour glass specimen is much lower compared to flat specimen. This could be due to the combined effect of average temperature that was measured over the window of 25 mm x 35 mm and localized damage present in the hour glass specimen. Amongst the specimens tested after impact with higher impact energies (35 J and 51 J), the residual strength of 35 J Hi-Lo was comparatively high as seen in Fig. 2 (a) (as well as in Tab. 1); the temperature rise during tensile pull out had a distinct peak as seen in Fig. 6. For other specimens, the impact and post-impact fatigue damage is severe and is distributed over the specimen gage section and hence the temperature change at the onset of tensile failure (ply brakeage) is negligible. Fig. 7 presents the typical temperature–time response and corresponding load-time response for 23 J and 35 J impacted specimens. The temperature rise is due to the energy dissipation during the fiber breakage and fiber-matrix debonding during tensile pull-out, and this indirectly indicates the severity of the damage present in the specimen. Further, the temperature rise is proportional to the load drop at the onset of failure.