# Mechanical Properties of Sandwich Composites Reinforced by Nanoclas: An Overview
## 2 Sandwich Composites Incorporating Nanoclays into the Core  
The presence of nanoclays can modify the characteristics of a sandwich material, but the benefits obtained depend on their incorporation at the core or skin level. When they are added at level of foam core, for example--a material with low density and weak mechanical characteristics--improvements in toughness and impact performance are achieved. When the core is composed of foam materials, they give the composite structure an insulating property, while fiber-reinforced resins induce better impact properties. For example, Figure 1 shows the benefits of nano-enhanced resins on the impact response of sandwich composites involving fiber glass/epoxy skins and balsa wood core. Several authors have studied the benefits obtained from the incorporation of nanoclays into the core of sandwich composites, especially in terms of impact strength. For example, one of the first studies reported in the open literature about the effect of nanoclays in sandwich structures was published in 2005 by [PERSON] et al. [13]. They mixed nanoclays with resin and glass microballoons, in order to create a syntactic foam with three different densities of microballoons (220, 320 and 460 kg/m\({}^{3}\)) and two different quantities of nanoclays (2 wt% and 5 wt%). The results showed that the compressive strength decreased between 10 and 20% when 2 wt% of nanoclays were added, while for 5 wt%, the compressive strength behaved like the control group. The toughness results showed an increase of 80% and 125% for 2 wt% and 5 wt% of nanoclays and a microballoons content of 220 kg/m\({}^{3}\). In the case of medium density foams (320 kg/m\({}^{3}\)), there was an increase of 150% and 200% for 2 and 5% of nanoclays, respectively, while in high density foams (460 kg/m\({}^{3}\)) the increase in toughness was 125%. Authors explained that the addition of nanoclays increased the plasticity of the material and, consequently, the number of fractures produced under compression decreased [13]. In 2008, [PERSON] et al. [14] compared the debond fracture toughness of sandwich structures, considering different types of nanoparticles (nanoclays, carbon nanofibers and multi walled carbon nanotubes) added to the polyurethane foam core. For each type of nanoparticle, they compared three different contents: 0.5 wt%, 1 wt% and 1.5 wt% [14]. The performance of these materials was evaluated  
Figure 1: Energy versus time curves for sandwic with epoxy resin and epoxy filled by nanoclays (impact energy of 20 J, sandwich composite made by fiber glass/epoxy skins and balsa wood core) [12]. using the tilted sandwich debond test--similar to the double cantilever beam test, but performed on an inclined surface--in order to obtain face/core debonding, as opposed to the crack deflection into the core, i.e., to assure the crack propagation trough the interface [14]. From these tests, it was possible to conclude that the best improvement of debond fracture toughness was obtained with 1 wt% of nanoclays, but further increasing nanoparticles resulted in a decrease of debond fracture toughness. Authors explained this behavior by the effects of size and aggregation/agglomeration of nanoparticles on the interfacial/interphase properties [14]. [PERSON] et al. [15], in 2008, analyzed the response to low-velocity impact loading of sandwich composites containing nanoclays. They were mixed with an epoxy resin to cover the carbon fiber outer layer, while the inner layer was composed of foam with nanoclays. Two content of nanoclays were used for the outer layer (1 wt% and 2 wt%), and two other contents (0.5 wt% and 1 wt%) for the core layer. Impact tests were performed using a drop weight testing machine; three different impact energies (15, 30 and 45 J) were considered. The results showed that the samples with nanoclay insertions in the foam core exhibited higher peak loads than those produced with the neat foam core. The damage analysis also showed that, after the impact tests, the damaged area was lower for samples with nanoclays.