Experimental results
DIC Analysis
The DIC technique (as discussed in section 2.4) is used to obtain the out-of-plane deflections and the in-plane strains on the back surface for all the five panels. The speckle pattern is applied onto the back face of the panels (fig. 5) which are subjected to shock loading. The high speed images captured using two Photron SA1 cameras are analyzed to get the back face deflections from the DIC as shown in fig. 6. Experiments have already been done to compare the back face deflection from the real time transient image and DIC to verify the accuracy of the DIC results. The error between the maximum deflection from DIC and real-time transient images is 4% [12]. The DIC results are within the acceptable error limits and so the DIC results can be used to better understand the failure and damage mechanism in the panel. The full-field DIC analysis for the five different glass panels is shown in fig. 6.
The figure shows the real-time deflection of the different panels for the first 600 ^s. The lamination of the sandwiched glass panel improved the blast mitigation property of the laminate and also resulted in delayed deflection and damage propagation. For a better understanding, the center-point of this full-field analysis was chosen and out of plane deflection and in-plane strain data were extracted at this point. The center point deflections in all the five panels are shown in fig. 7. The sandwich glass panel has a maximum deflection of 18 mm prior to complete fracture, whereas at the same time, the laminated sandwich glass panel shows a deflection of 9mm and no through hole formation. The tempered glass panel has a maximum deflection of 8 mm prior to fracture, the wired glass panel has 6 mm and the plane glass panel shows a deflection of only 2 mm. The deflection-time history for the laminated sandwich glass panel is shown in fig. 8. This shows that the laminated sandwich panel has a maximum deflection of 28 mm and recovers back to a final deflection of 16 mm. The other important point is that it experiences fragmentation and cracking in glass panel, but the protective film is able to contain the shattered glass pieces from flying off. Also, the in-plane strains on the back face of the five different glass panels tested are shown in fig. 9. The sandwich glass panel has a strain of 5% before fracture initiates and at the same time the laminated sandwich glass panel only has a 1.7% strain (there was no through hole formation at this time), whereas in the case of the tempered glass panel it is 2%, 1% for the wired glass panel and 0.01% for the plane glass panel before fracture. The in-plane history for the laminated sandwich glass panel is shown in fig. 10. It shows that the laminated sandwich panel has a maximum in-plane strain of 6% after which it recovers to 3%. This time-deflection and in-plane history shows that the laminated sandwiched glass panel behaves in a more ductile manner as compared to the other glass panels.
Figure 6: Time-deflection history of the back face for: (a) Plane Glass, (b) Tempered Glass, (c) Wired Glass, (d) Sandwiched Glass, & (e) Laminated Sandwiched Glass Panels
Figure 7: Time-deflection history of the back face for five glass panels
Figure 8: Time-deflection history of the back face for laminated sandwich glass panel
Figure 9: Time-in-plane strain history of the back face for five glass panels
Figure 10: Time-in-plane strain history of the back face for laminated sandwich glass panel
Macroscopic post-mortem analysis
The result of post-mortem evaluation of the shock loaded glass panels is shown in Fig. 11-13. The post-mortem analysis of the clear glass and tempered glass panels have not been shown as they lost the structural integrity and shattered into pieces. The post-mortem analysis of a wired glass panel is shown in fig. 11. The panel shows a large amount of fragmentation but in comparison to the clear and tempered glass panel, which shattered completely, it retained structural integrity. The postmortem image of the sandwiched glass panel is shown in fig. 12. There is heavy fragmentation on both the front and back face as seen in figs. 12(a)-12(b). The PVB interlayer is able to withhold a substantial amount of these fragments from flying off. The post-mortem images of the laminated sandwich glass panel are shown in Fig. 13. It is evident from the post-mortem images that there is substantial fragmentation in the case of the laminated sandwich glass panel. However, the protective film is able to capture these pieces and prevent them from flying off. Also, there is no cracking in either of the layers (both on the front and back face of the glass panel) of the protective film.
Overall, it can be concluded that the laminated sandwich glass panel has better blast mitigation properties than the other four panels. The clear glass panel and tempered glass panel have the worst blast mitigation properties and are shattered into pieces when subjected to the shock loading. The sandwiched glass performs better than the wired glass panel, but it still has fragmentation and shattered glass pieces flying around. The fragmentation in the case of the sandwich glass panel is lower as compared to that in the wired glass panel. Also, the diameter of the through hole formed in the wired glass panel is larger as compared to that in the sandwich glass panel. This improvement in the blast response of the sandwich glass panel can be attributed to the PVB interlayer which helps in withholding some of the shattered glass pieces from flying off.
Figure 11: Post-mortem evaluation of Wired Glass Panel (a) Front view; (b) Back view
Figure 12: Post-mortem evaluation of Sandwich Glass Panel (a) Front view; (b) Back view
Figure 13: Post-mortem evaluation of Laminated Sandwich Glass Panel (a) Front view; (b) Back view
Conclusions
Five different panels are subjected to a controlled shock loading using a shock tube. The high speed photography and DIC analysis is applied to obtain the out-of-plane deflection and in-plane strain on the back face of all the five panels. 1. The macroscopic post-mortem analysis and DIC deflection analysis shows that the sandwich glass panel has less damage due to blast loading as compared to the wired, tempered and clear glass panels. The PVB interlayer increases the flexural rigidity of the panels, and results in less damage when subjected to the shock loading.
2. The area of the through hole formed in the case of the sandwich glass panel was smaller as compared to that in the case of the other three glass panels. This will minimize the blast overpressure entering in the buildings and thus lower the damage inflicted as compared to the wired, tempered and plain glass panels.
3. The application of the protective film (XO-ARMOR®) on the front and back face of the sandwich panel further improves the blast mitigation property of the sandwich glass panel.
4. The laminated sandwich glass panel has fragmentation and cracking in the glass panel but the protective film is able to withhold the shattered glass pieces from flying off. Also, there is no through hole formation in the case of the laminated sandwich glass panel. This prevents the blast overpressure from entering the building and thus restricting the damage because of the overpressure.
Overall, the laminated sandwiched glass panels with PVB interlayer and protective film on both the faces has a better blast mitigation properties as compared to the other four panels.
