Issue 48

O. A. Mocian et alii, Frattura ed Integrità Strutturale, 48 (2019) 230-241; DOI: 10.3221/IGF-ESIS.48.24 235 a) b) Figure 3 : Comparison of absorbed impact energy as a function of facesheet type: a) PUR foam core; b) PS foam core. Sandwich panel NAE [%] SEA [J/kg] CFE [-] Sandwich panel NAE [%] SEA [J/kg] CFE [-] PUR PS PUR PS PUR PS PUR PS PUR PS PUR PS SPA_3 97.96 97.81 357.97 423.7 0.47 0.54 SPC_3 98.78 98.45 346.68 336.82 0.43 0.50 SPA_3.5 94.39 82.62 465.46 483.73 0.54 0.58 SPC_3.5 95.15 99.42 468.99 504.58 0.59 0.67 SPA_4 80.63 76.33 526.05 556.13 0.56 0.62 SPC_4 82.00 88.07 500.52 531.55 0.59 0.60 SPB_3 97.65 96.63 329.40 339.73 0.43 0.58 SPAl_3 85.94 98.06 257.23 315.62 0.56 0.60 SPB_3.5 98.37 97.27 492.25 490.85 0.62 0.63 SPAl_3.5 97.41 95.13 393.54 414.44 0.36 0.60 SPB_4 80.81 74.28 499.72 473.67 0.55 0.61 SPAl_4 94.30 95.32 514.49 545.86 0.49 0.59 Table 3 : Energy absorption quantifying parameters. For sandwich panels with composite facesheets, irrespective of core type, the normalized absorbed energy decreases with increasing impact velocity, respectively with increasing impact energy (see Fig. 4). This happens because the impact energy exceeds the saturation energy, which represents the maximum impact energy that can be absorbed by a structure until it completely loses its load carrying capacity. As it will be further seen in this study, most of the sandwich panels with composite facesheets are perforated at higher impact velocities, meaning that they absorbed only a part of the total impact energy. The rest remained in the impactor as residual velocity. In the case of sandwich panels with aluminum facesheets, at higher impact velocities, NAE slightly increases for panels with PUR foam core and decreases for panels with PS foam core. Due to its elastic behavior the PS foam core manages to better absorb the impact energy at lower impact velocities than the PUR core. The higher values of SEA for sandwich panels with composite facesheets indicate that these panels absorb more energy per unit mass than those with aluminum facesheets. This is because the average weight of sandwich panels with composite facesheets is approximately 15 % less than of those with aluminum facesheets. Also, sandwich panels with composite facesheets absorb a greater amount of energy than those with aluminum facesheets, especially for the lowest impact velocity, due to the different and complex energy absorption mechanisms: matrix cracking, debonding, fiber rupture, core crushing and delaminations. This type of failure determines a small value for the CFE parameter. For a structure to have good energy absorption capabilities, both SEA and CFE parameters should have high values. From Fig. 4 b) and c) although aluminum sandwich panels with both PUR and PS foam core have the highest values for CFE at 3 m/s, they have the smallest value for SEA. Consequently, even though they fail in a progressive mode they do not manage to absorb enough impact energy per unit mass. At this impact velocity, the sandwich panel with composite facesheets type A and PS foam core proves to be the most efficient, with SEA of almost 425 J/kg and CFE as 0.54. Though, as it will be later seen in this study, this panel is also the most damaged one at 3 m/s. The CFE parameter for aluminum sandwich panels with PUR foam core considerably reduces its value at higher impact velocities. For example, at 3.5 m/s reaches a value of 0.36, which is 35 % less than for the lowest impact velocity of 3 m/s. Instead, the value of the CFE parameter for sandwich panels with aluminum facesheets and PS foam core remains almost the same, around 0.60, irrespective of impact velocity. For impact velocities of 3.5 m/s and 4 40 60 80 100 120 3 3.5 4 Absorbed energy [J] Impact velocity [m/s] SPAl_PUR SPA_PUR SPB_PUR SPC_PUR 40 60 80 100 120 3 3.5 4 Absorbed energy [J] Impact velocity [m/s] SPAl_PS SPA_PS SPB_PS SPC_PS