PSI - Issue 28

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Haibao Liu et al. / Procedia Structural Integrity 28 (2020) 106–115 Liu H et al./ Structural Integrity Procedia 00 (2020) 000–000

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4. Results and discussions 4.1. Effects of the impact velocity

To study the effects of the impact velocity, the round-nosed steel impactor, using velocities of 2.40 m.s -1 (giving a 15 J impact energy) and 4.16 m.s -1 (giving a 45 J impact energy), was employed to strike the composite specimens. The load as a function of time curves, and the load as a function of displacement curves, are shown in Fig. 5a and 5b, respectively. The composite specimens when subjected to both impact velocities reveal an oscillating response in these curves at the initial stage, followed by impact damage initiation and propagation. The results show that, when the impact velocity increased by 73%, the maximum load increased by about 75% (i.e. from 4.7 to 8.3 kN). Compared to the lower velocity impact (i.e. 2.40 m.s -1 ), the higher velocity impact (i.e. 4.16 m.s -1 ) resulted in a slightly shorter duration event time of 6.2 ms but a significantly higher out-of-plane displacement of 9.8 mm. This shows that the maximum out-of-plane displacement was relatively more sensitive to the impact velocity. This is apparent as a 8% decrease in the duration of the impact event time but a 63% increase in the maximum out-of-plane displacement.

(a) (b) Fig. 5. Loading response obtained from the specimens tested at 15 J (i.e. an impact velocity of 2.40 m.s -1 ) and 45 J (i.e. an impact velocity of 4.16 m.s -1 ) energy levels using the round-nosed steel impactor: (a) load as a function of time curves and (b) load as function of displacement curves. (Results from three replicate tests for each test condition are shown.) The composite specimens, tested at different impact velocities, were inspected using the portable C-scan device. The delamination contours are shown in Fig. 6, where the colour of the delamination contour varied from red to blue as a function of the increasing depth through the thickness of the composite test specimen. It can be seen that the higher velocity impact caused significantly more damage in the composites than the lower velocity impact. Further, when the impact velocity was increased from 2.40 m.s -1 to 4.16 m.s -1 , more delamination is observed near the rear surface of the composite, whilst the delamination near the front face did not show a significant difference.