# Mechanical Properties of Sandwich Composites Reinforced by Nanoclas: An Overview
## 3 Sandwich Composites Incorporating Nanoclays into the Skin
From the dynamic mechanical tests, it was concluded that all properties (storage modulus, loss modulus, loss factor, area under tan \(\delta\) and glass transition temperature) increased with the nanoclays content, similar to that observed with graphite nanoplatelets. However, the last nano-reinforcement promoted higher improvements than those obtained with nanoclays. For example, the loss factor increased around 80% with the addition of 2.5 wt% of nanoclays and about 125% for the same content of graphite nanoplatelets. Regarding the impact tests, the same tendency was observed in terms of energy absorption, reaching improvements around 100% when 2.5 wt% of nanoclays were added [31]. In 2012, [PERSON] [32] investigated the influence of nanoclays content on sandwich composites under flexural and impact loading. Four different combinations of sandwich composites were prepared with fiber glass/nano-modified polyester skins (using 0 wt%, 2 wt%, 4 wt% and 6 wt% of nanoclays) and a polystyrene foam core. Flexural tests under three-point bending were performed to obtain the core shear stress (\(\tau\)) and the skin bending stress (\(\sigma\)) according to the ASTM C 393-94 standard, while the Charpy impact strength was determined according to ASTM D6110 standard. It was observed that the flexural strength and stiffness increased up to 4 wt% of nanoclays, with improvements of 16.3% in terms of flexural strength, while the highest impact strength was obtained with 6 wt% of nanoclays and with improvements of 50% compared to the control samples. In 2013, [PERSON] et al. [12] studied the benefits of nano-enhanced resins on the impact response of sandwich composites produced by fiber glass/epoxy skins and balsa wood core. The epoxy resin was enhanced by nanoclays with special treatment to improve their dispersion and interface adhesion. More details about the treatment and the dispersion/exfoliation on the epoxy matrix can be found in [33]; the nanoclays content employed was 3 wt% of the epoxy resin/hardener mixture. Authors performed impact tests with a drop-weight testing machine and energies from 4 to 36 J were used. After impact tests, the residual bending strength was evaluated by three-point bending tests. It was conclude that sandwiches enhanced by nanoclays have better impact properties than the control samples [12]. For example, the benefits obtained in terms of maximum displacement were about 23.4% lower than those measured on the control samples, whereas the elastic recuperation was about 45.4% higher. Regarding the failure mechanisms, the greatest damage occurred at the highest energies and, while the control samples showed matrix cracking on both facesheets (upper and lower) with significant damage on the balsa core, the nano-enhanced sandwiches exhibited damage only on the upper facesheet. Moreover, as shown in Figure 2, the neat sandwiches showed lower residual flexural strength for all impact energies, because the nanof fillers increase the stiffness of the facesheets and, consequently, the panel overall bending stiffness. In 2014, [PERSON] et al. [34] evaluated the effect of nanoclays modified polyester resin on flexural, impact, hardness and water absorption properties of jute and glass fabric hybrid sandwich laminates. For this purpose, contents of 2 wt%, 4 wt% and 6 wt% of nanoclays were used. It was possible to conclude that both the flexural strength and the flexural modulus increased up to 4 wt% of nano-reinforcements, but no additional increase was observed for higher contents. In this case, the benefits obtained are 90% and 67% higher than the strength and modulus of the neat jute laminate (100 wt% of jute fibers), respectively. Regarding the impact strength, hardness and water absorption, the optimum loading was 6 wt% of nanoclays. For example, the impact strength increased around 5.5 times the value obtained for neat jute laminates (100 wt% of jute fibers), the hardness increased around 27% compared to the value obtained for composite laminates (60 wt% of jute fibers plus 40 wt% of glass fiber).