# Mechanical Properties of Sandwich Composites Reinforced by Nanoclas: An Overview
## 3 Sandwich Composites Incorporating Nanoclays into the Skin  
The incorporation of nanoclays in the skin of a sandwich composite is usually done through the matrix (where the epoxy resins prevail). In this case, they increase the toughness and improve the impact performance of these materials. However, until now, few studies have focused on studying the effect of adding nanoclays on the skins. In 2008, [PERSON] et al. [28] developed a sandwich for stealth radome (radar + dome) applications on aircrafts and warships. These sandwic are composed by PVC foams and skins of E-glass fabric and an epoxy resin reinforced by nanoclays. Four different contents of nanoclays, from 1 wt% to 4 wt%, were used in this study. According to our best knowledge, they were the first researchers to incorporate nanoclays into the skin of a sandwich composite and to characterize its mechanical performance. For this purpose, 3-point bending tests were performed and, regardless of the nanoclays content, the flexural strength showed little variation, because the small amount of nano-fillers promoted a negligible effect on the mechanical properties of the composite. Finally, the water diffusivity decreased with the increase of nanoclays content because the diffusing water path increased by the presence of nanoclays (which have a plate-type shape). [PERSON] et al. [29] produced, in 2009, sandwich panels with closed-cell PS foams (Styrofoam) and epoxy resin/glass fiber skins with different contents of nanoclays (0, 1, 2, 5 and 10 wt%) in order to improve the impact strength. For this purpose, low-velocity impact tests were performed with energies between 25 and 75 J, which are enough to cause damage ranging from a barely visible delamination to perforation. These authors related the panel stiffness with the load-deflection response, assuming that is linear under low velocity impact, and they observed an increase in stiffness with the addition of nanoclays up to 5 wt% (consequently, the panel overall bending rigidity), but larger contents cause a decrease in stiffness. In this case, when the saturation is reached, it promotes agglomerations that are potential crack nucleation points. Involving similar sandwich composite plates, these authorsdeveloped another study in which two sets of impact tests were performed: low velocity and high mass test and high velocity and low mass test [3]. The energies applied ranged from 5 J to 75 J. The results showed that the addition of 5 wt% of nanoclays led to the highest energy absorption, which is in line with the previous work. Simultaneously, depending of the energy level (velocity), two transitions were observed in terms of failure modes: from upper facesheet cracking plus core indentation to upper facesheet tearing plus core crushing, and from upper facesheet tearing plus core crushing to lower facesheet plus core debonding. All results were compiled by the authors in 2010 [30], where it was emphasized that the highest energy absorption was obtained with 5 wt% of nanoclays. A higher content of nanoclays led to immiscible nanostructures, promoting \"hot spots\" for cracks nucleation. According to the authors, epoxy matrices have a saturation limit between 5 wt% and 10 wt% of nanoclays [30]. In the same year, [PERSON] et al. [31] evaluated the impact strength and dynamic mechanical properties of vinyl ester nanocomposites incorporating nanoclays (1.25 wt% and 2.5 wt%) or exfoliated graphite nanoplatelets (1.25 wt% and 2.5 wt%) for use in sandwich skins. Dynamic mechanical tests were performed in single cantilever deformation mode, with fixed-fixed boundary condition and temperatures between \(-\)50 and 150 \({}^{\circ}\)C with an increase of 3 \({}^{\circ}\)C/min, while low-velocity impact tests were performed on a drop-weight impact test machine according to the ASTM D6110-06 standard.