Issue 30

N. Mortas et alii, Frattura ed Integrità Strutturale, 30 (2014) 403-408; DOI: 10.3221/IGF-ESIS.30.48 406 load [3, 4]. When the fillers are added better results can be found. The sandwiches manufactured with epoxy resin enhanced by nanoclays presents best performance in terms of elastic recuperation with values about 25.7% higher. However, in real applications, the planks (boards) of balsa wood have finite lengths. In this case there are discontinuities between the cores of the sandwiches, because the boards do not touch. This absence of material, in terms of core, affects the mechanical properties of the sandwich, especially the impact strength. In order to detect the effect of the gap’s dimension, Fig.3 shows the typical load-time, load-displacement and energy-time curves. In this case, the control samples are compared against the specimens containing a gap of 20 mm for sandwiches manufactured with pure resin (Fig. 3). 0 0.5 1 1.5 2 2.5 0 2 4 6 8 10 Load [kN] Time {ms] 0 0.5 1 1.5 2 2.5 0 2 4 6 Load [kN] Displacement [mm] a) b) 0 1 2 3 4 5 6 0 10 20 30 Energy {J] Time [ms] c) Figure 3 : Comparison of the gap’s effect in terms of a) load-time curves; b) load-displacement curves; c) energy-time curves. These curves and the curves shown on Fig. 2 are very similar but with different values. The gap promotes a decrease of the maximum impact loads and elastic recuperation (energy dissipated) but the opposite tendency occurs for displacement and contact time. As consequence of a lower energy dissipated, the damage increases. Tab. 1 shows the average values observed and the benefits promoted by the resin nano-enhanced. If, in terms of control samples, the benefits obtained with resin nano-enhanced were previously analyzed, the effect of the gap follows the same tendency. In both cases, the gap promotes a decreasing of the maximum impact load with values around 29.4%, for the sandwiches manufactured with pure resin, and about 24.3% for the other ones with nanoclays. On the other hand, in terms of displacement, the opposite tendency was observed. In this case, the displacement increases for both conditions, but with higher values for the sandwiches with pure resin. Values around 22.9% were found, while an increasing about 22% was observed for the sandwiches with resin nano-enhanced. This small difference in terms of displacement, however, was sufficient to promote the lowest damage areas when the resin was enhanced by the nanoclays. In fact, the gap with 20 Control samples Control samples Control samples  0 = 20 mm  0 = 20 mm  0 = 20 mm