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

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2.2. Sandwich plates configuration and testing conditions

The sandwich panels had skins made of aluminum Al 6082-T6 of 1.5 mm thickness glued with Araldite AW106 (Huntsman) to two types of core: polyurethane Necuron 100 of density 100 kg/m 3 and commercial extruded polystyrene of density 30 kg/m 3 having thicknesses of 12 mm, respectively 19 mm. The polyurethane core sandwich was abbreviates as PS_PUR and the polystyrene core sandwich as PS_PE. Although the thickness of the polystyrene core of the PS_PE sandwich is greater than the polyurethane core of the PS_PUR sandwich the average mass of the PS_PE panel is about 186 grams compared to about 199 grams for the PS_PUR panel. So, from the point of view of a lightweight panel, the polystyrene sandwich core stands in fact better. The mass of the energy carrier was always kept constant as 13.15 kg and the velocity of impact was increased from 0.77 m/s up to 3.5 m/s (height of impact from 30 mm up to 625 mm), in some cases reaching 4.5 m/s for both panels as being the maximum speed considered in these tests. Therefore the energy of impact started from 3.90 J and increased to 80.54 J (3.5 m/s), or to a maximum of 133.14 J (4.5 m/s). The two types of sandwich panels PS_PE and PS_PUR were tested in the same conditions at initial impact speeds of 1.5, 2.5, and 3.5 m/s. In Fig. 2 is shown for comparison the response of the two panels with different cores and clearly their behavior is different. The initial force at impact is always greater for the PS_PUR sandwich than for the PS_PE sandwich showing the more brittle behavior of the polyurethane foam. In the case of a linear elastic impact event the impact force curve variation in time should be symmetric for loading and unloading. This does not happen even for the lower speed of impact of 1.5 m/s showing that unloading is accompanied by additional phenomena. In all, observing the curves, the polystyrene core gives a more elastic response to the sandwich panel for all three speeds of testing presented in Fig. 2 (a). For PS_PE the time of impact is about the same (14-15 ms) regardless the speed of impact. The PS_PUR sandwich has a more fragile behavior, the impact events take less in time, from about 9 ms at 1.5 m/s to 13 ms at 3.5 m/s. At 3.5 m/s, as seen in Fig. 2 (b) the force drops suddenly from 11790 N to 4450 N due to the severe damage of the aluminum top face of the sandwich which is essentially penetrated by the striker as to be seen also in Fig. 3 (b). The brittle behavior of the polyurethane foam influences this drop of force and the puncture of the skin is followed by some vibrations of the striker. At the same energy and speed of impact in the PS_PE sandwich the maximum force is initially about 6000 N, then drops slightly at the contact with the top aluminum skin, then increases to close to 7000 N due to the elastic recovery of the polystyrene foam; meanwhile some slight vibrations of the striker are noticed and the force drops. At 2.5 m/s again the elastic response of the polystyrene core plays its role, the force increases back after the initial contact, but to a smaller value than the initial one of 5000 N, and afterwards drops. 3. Response of sandwich panels at low-velocity impact testing

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(a) (b) Fig. 2. (a) Response of polystyrene sandwich PS_PE; (b) Response of polyurethane sandwich PS_PUR.

A more clear observation of the two types of impacted sandwich panels is presented in Fig. 3 (a) for 2.5 m/s,