# Mechanical Properties of Sandwich Composites Reinforced by Nanoclas: An Overview
## 3 Sandwich Composites Incorporating Nanoclays into the Skin
Moreover, from the low velocity impact tests carried out with an energy of 10 J, these authors observed damage areas 24% lower and 2.2 times lower, respectively, for jute sandwic and PS-foam sandwic reinforced with 4 wt% of nanoclays. Consequently, the post impact compression strength and modulus were 44% and 87% higher for jute sandwic and 38% and 41% higher for PS-foam sandwic, respectively. More recently, in 2018, [PERSON] et al. [38] carried out low velocity impact tests (using a weight-drop impact machine) on sandwich composites similar to those described in ref. [32] and [34]. The impact energy used in this study was 30 J and, as reported in [33], only neat sandwich composites and sandwich composites reinforced by 4 wt% of nanoclays were analyzed. It was possible to conclude that all parameters obtained from the impact tests are very similar between both sandwic. For example, the peak load was only 2.2% higher than the value obtained for the neat sandwich, while the displacement and absorbed energy were 4.2% and 2.5% lower, respectively. On the other hand, considering the damage degree, which is defined as the ratio between the energy absorbed and the energy of impact (less than one up to incursion and equal to one when it occurs), this parameter reached the minimum value (0.24) for nano-enhanced sandwic. These results clearly evidence the benefits obtained with nano-enhanced resins. Although the damage caused by the impact load is not visible in both sandwic, authors evaluated the residual bending properties using four-point bending tests. For comparability, they evaluated the pre- and post-impact flexural properties. While the non-impacted samples showed values of strength and stiffness around 7.2% and 66.3% higher than those obtained for neat sandwich composites, after impact they are about 3.5% and 80%, respectively. This means that, after impact, the strength and stiffness decreased by 13.8% and 12.2%, respectively, for neat sandwic, while these values are 16.7% and 4.9% for nano-reinforced sandwic. Finally, in 2019, [PERSON] et al. [39] gave continuity to another study, where the effect of modified nanoclays on low velocity impact damage area and damage tolerance capability was analyzed using sandwich laminates similar to those described in ref. [34]. For this purpose, low velocity impact tests were performed with impact energies of 10 J and 20 J and compression after impact (CAI) tests were carried out in order to evaluate the residual compressive strength and residual compressive stiffness of the sandwich laminates. The damage areas were estimated and a decrease of 19.5% and 13.5% was observed for the energies of 10 J and 20 J, respectively, when 6 wt% of nanoclays were added to the resin. Matrix cracking is the main damage mechanism observed in laminates involving only jute fibers (for 10 J the damage is not significantly visible on the bottom face of the laminate, but for 20 J it is visible on both sides due to fiber breakage and cracks in the matrix), while in hybrid sandwich laminates, delamination is the predominant mode of failure with the beginning between the jute core and the fiber glass skin interface. It was also observed that the residual compressive strength is very low for laminates involving only jute fibers and the damage tolerance capability improved with the incorporation of glass fibers and nanoclays reinforcement in the hybrid sandwich laminates. For example, the residual compressive strength for hybrid sandwic with 6 wt% of nanoclays increased about 3.5 times over the laminates involving only jute fibers.