Issue 48

O. A. Mocian et alii, Frattura ed Integrità Strutturale, 48 (2019) 230-241; DOI: 10.3221/IGF-ESIS.48.24 234 Energy absorption Fig. 2 presents typical energy variation curves as a function of time for all tested sandwich panels. Results are presented comparatively in terms of core type and impact velocity, for each type of facesheet. a) b) c) d) Figure 2 : Variation of energy in time for different facesheet types: a) composite type A; b) composite type B; c) composite type C; d) aluminum. Three important particularities can be identified in the energy-time variation curve: the impact energy corresponding to the maximum value on the curve, the absorbed energy corresponding to the constant value on the curve after the decrease of energy and the elastic recovered energy represented by the difference between the impact and absorbed energy. If the curve shows an increase of the energy in time it results that the panel has been perforated (Figs. 2 a, b, c at 4 m/s and PUR foam core, as an example). In all cases the absorbed energy increases with increasing impact energy. For sandwich panels with aluminum facesheets there is always a recovery of energy regardless the impact velocity. Instead, for sandwich panels with composite facesheets, a recovery of energy is achieved only for the smallest impact velocity of 3 m/s. Not only the impact velocity influences the absorbed energy but also the facesheet and core type (see Fig. 3). When using PUR foam as core, for the impact velocity of 3 m/s, the sandwich panels with aluminum facesheets absorb approximately 13 % less impact energy than those with composite facesheets. On the contrary, sandwich panels with PS core absorb almost the same amount of energy, regardless the facesheet type, due to its elastic behavior. At 3.5 m/s all panels with PUR foam core (Fig. 3a) absorb almost the same amount of impact energy. For panels with PS foam core (Fig. 3b), the highest amount of energy is absorbed by those with composite facesheets type C, approximately 25% more than those with composite facesheets type A, which absorb the smallest amount of energy. For the highest impact velocity, sandwich panels with aluminum facesheets absorb the same amount of energy, regardless the foam core type. Composite sandwich panels absorb considerably less energy, approximately 14 % less, since they are completely perforated at this impact velocity, except the case of sandwich panel with composite facesheets type C and PS foam core. The energy absorption can also be quantified by several parameters, such as: normalized absorbed energy (NAE - ratio between absorbed energy and initial impact energy), specific energy absorption (SEA - ratio between absorbed energy and mass) and crush force efficiency (CFE - ratio between average crushing force and peak force), [36]. These parameters are comprised in Tab. 3 and graphically illustrated in Fig. 4. 0 30 60 90 120 0 5 10 15 20 25 30 35 Energy [J] Time [ms] SPA_PUR_3 SPA_PS_3 SPA_PUR_3.5 SPA_PS_3.5 SPA_PUR_4 SPA_PS_4 0 30 60 90 120 0 5 10 15 20 25 30 35 Energy [J] Time [ms] SPB_PUR_3 SPB_PS_3 SPB_PUR_3.5 SPB_PS_3.5 SPB_PUR_4 SPB_PS_4 0 30 60 90 120 0 5 10 15 20 25 30 35 Energy [J] Time [ms] SPC_PUR_3 SPC_PS_3 SPC_PUR_3.5 SPC_PS_3.5 SPC_PUR_4 SPC_PS_4 0 30 60 90 120 0 5 10 15 20 Energy [J] Time [ms] SPAl_PUR_3 SPAl_PS_3 SPAl_PUR_3.5 SPAl_PS_3.5 SPAl_PUR_4 SPAl_PS_4