PSI - Issue 5

Dan Mihai Constantinescu et al. / Procedia Structural Integrity 5 (2017) 653–658 Mocian et al./ Structural Integrity Procedia 00 (2017) 000 – 000

657

5

The PS_PUR panel shown in Fig. 4 (b) has a rigid behavior due to the higher density of the polyurethane core and doesn't bend. Top skin is severely punctured and, for this panel, bottom skin fails also. The deformation concentrates towards the central part of the panel, and there are no delaminations at the interface of the core with the skins. As shown in Fig. 5 at higher speeds of impact of 4 m/s and 4.5 m/s the response of the two panels becomes more complicated, especially for the PS_PUR panel - Fig. 5 (b). In the first part of the impact, up to 2-3 ms the response of the panels at both speeds of impact is about the same. In Fig. 5 (a), for the PS_PE the force drops slightly from about 6 kN after an initial damage of the top aluminum skin and then, while the polystyrene core is deformed the force grows. At 4.5 m/s the force signal is cut-off as getting failure by yielding of the skin (zero slope at maximum force), followed by a small amount of stable damage propagation as the polystyrene core deforms in a ductile manner, then some vibrations of the striker are registered at the contact with the bottom skin and afterwards force drops to zero with some deep drawing of the top skin. On this last part the curves for both speeds overlap.

Impact PS_PE

Impact PS_PUR

14

14

12

12

4 m/s

10

10

4 m/s

8

4.5 m/s

8

4.5 m/s

6

6

Force [kN]

4

4

Force [kN]

2

2

0

0

0

2

4

6

8

10

12

14

16

0

2

4

6

8

10

12

14

Time [ms]

Time [ms]

(a) (b) Fig. 5. Response of sandwich panels at higher speeds of impact: (a) polystyrene sandwich PS_PE; (b) polyurethane sandwich PS_PUR.

The response of the PS_PUR sandwich panel at 4 m/s (blue curve in Fig. 5 b) is quite different as the polyurethane core behaves in a more brittle manner. The maximum force of impact is about the same at both speeds of testing, of about 11 kN, that is around 30 % greater than for the PS_PE panel (less than 9 kN), then drops abruptly showing some stable crack propagation in the core, then the force grows back due to the influence of the core, and eventually drops to zero. At 4.5 m/s, as presented in Fig. 5 (b), both skins fail and many vibrations of the striker are registered mainly after the second drop of the force, thus indicating not only another stable crack propagation in the polyurethane core but also the bottom skin failure which was not produced at 4 m/s. As significant amount of damage is generated in the sandwich panel the vibrations of the striker are registered with higher amplitudes than when only top skin was penetrated and core was damaged. The sandwich panel is not perfectly clamped between the specimen support and the clamping ring and at higher energies of impact the panel starts also to oscillate. Of course that the deep drawing of the top skin is also present, even to a greater extent than before. As at 4.5 m/s the damage of the top and bottom skins of PS_PUR is more significant than for 4 m/s, the two force-time curves do not overlap when force decreases to zero as they did before for the PS_PE panels. 4. Conclusions The impact of polystyrene and polyurethane core sandwich panels with aluminum skins is performed and the force time response is analyzed for different impact energies, keeping the impact mass of gravitationally accelerated type constant and varying the initial speeds of impact. Particularities of the impact response for both types of panels are discussed showing that the polystyrene core panels absorb better the impact energy as their core has a more ductile behavior than the polyurethane core. The penetration produced by the striker is also observed as being a good indication of the overall panel response. Local damage propagation events can be noticed and better understood by analyzing the force variations during the impact of the panels. Finite element simulations of these phenomena are now under development as to have a deeper insight of the experimentally obtained results.