PSI - Issue 14

T Sreekantha Reddy et al. / Procedia Structural Integrity 14 (2019) 265–272 Author name / Structural Integrity Procedia 00 (2018) 000–000

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It is clear from the figure that only 3mm and 5mm laminates were completely perforated at 100J and 150J impact energy respectively. All the remaining samples were partially perforated and rebound of the impactor was observed. Absorbed energy, rebound energy, perforation energy and contact duration values of all the impacted laminates are given in table 2. From the table it can be seen that at an equal impact energy, absorbed energy is decreased and rebound energy is increased with increase in thickness. This suggests that lower thickness laminates have absorbed more energy by undergoing various failure modes like matrix crack, indentation, delamination, bending displacement, fibre breakage etc. to varying extents. As the thickness is increased, the laminate will have adequate bending stiffness consequently the rebound of the impactor take place and absorbed energy is decreased. Therefore this decrease in energy absorption with increase in thickness resulted in decrease in damage area. From the table 2 it is also evident that contact duration is decreased with increase in thickness at all impact energies. It suggests that rate of energy dissipation increases with increase in thickness.

Table 2. Comparison of absorbed energy and contact duration at three impact energies Thickness (mm) Impact energy (J) E a (J) E r (J) E f (J)

Contact duration (ms)

3

50 100 50 100 150 50 100 150 50 100 150

42 90

8 --

-- 10

8.7 17.7 4.6 6.4 14.5

5

34 89 144 34 76 131 39 75 127

16 11 -- 16 24 19 11 25 23

-- -- 6 -- -- -- -- -- --

3.5 4.2 5 2.6 2.8 3.2

7

10

3.3. Damage evaluation

3.3.1. Visual observation Fig.5 shows the comparison of different thickness laminates impacted at 100J energy. In all the laminates, two types of damage regions namely fibre breakage area and delamination area were observed. But the extent of these damage areas are found to be thickness dependent. From the figure it is clear that 3mm laminate is completely perforated by breakage of fibres in warp and weft directions. Delamination between the plies was not observed. Damage in 5mm thick laminate is found to be different, where delamination is also formed in addition to the fibre breakage. However, complete perforation is not occurred. As the thickness increased to 7mm, very little fibre breakage is observed due to increased stiffness of laminate. Hence the energy is dissipated transversely and resulted in more delamination area. When the thickness of laminate is increased further to 10mm, damage has taken place in the form of delamination only. Area of delamination also decreased as compared to that of 7mm laminate.

Fig.5. Photographs of different thickness laminates impacted at 100 J energy