# Impact Response of Composite Sandwich Cylindrical Shells
###### Abstract  
Nowadays, due to the complexity and design of many advanced structures, cylindrical shells are starting to have numerous applications. Therefore, the main goal of this work is to study the effect of thickness and the benefits of a carbon composite sandwich cylindrical shell incorporating a cork core, compared to a conventional carbon composite cylindrical shell, in terms of the static and impact performances. For this purpose, static and impact tests were carried out with the samples freely supported on curved edges, while straight edges were bi-supported. A significant effect of the thickness on static properties and impact performance was observed. Compared to thinner shells, the failure load on the static tests increased by 237.9% and stiffness by 217.2% for thicker shells, while the restored energy obtained from the impact tests abruptly increased due to the collapse that occurred for the thinner ones. Regarding the sandwich shells, the incorporation of a cork core proved to be beneficial because it promoted an increase in the restored energy of around 44.8% relative to the conventional composite shell. Finally, when a carbon skin is replaced by a Kevlar one (hybridization effect), an improvement in the restored energy of about 20.8% was found. Therefore, it is possible to conclude that numerous industrial applications can benefit from cylindrical sandwiccing incorporating cork, and their hybridization with Kevlar fibres should be especially considered when they are subject to impact loads. This optimized lay-up is suggested because Kevlar fibres fail through a series of small fibril failures, while carbon fibres exhibit a brittle collapse. composite sandwich cylindrical shells; low-velocity impact; experimental testing +
Footnote †: journal: Applied Sciences  
17 October 2021  
12 November 2021  
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**1. Introduction**  
A high specific strength and stiffness, good static and dynamic properties, good corrosion resistance, adjustable properties, competitive cost, and fast manufacturing are factors that make composite materials very attractive in diverse fields of engineering. However, the weakness of these materials lies in their compression and transverse properties, which may even restrict their application. For example, their high sensitivity to low-velocity impacts, which can often occur in service or during maintenance activities, should be considered in the design phase because the resulting damage is difficult to detect [1, 2] and, simultaneously, responsible for significant reductions in residual mechanical properties [3, 4, 5, 6]. Regarding low-velocity impact events on through-thickness direction, the literature presents comprehensive knowledge at the level of damage mechanisms [7, 8, 9, 10], residual compression after impact [11], multi-impacts [12] and environmental effects [13, 14, 15]. On the other hand, it is also possible to find strategies to improve the impact performance, for example, using hybridisation [16, 17, 18], nano-enhanced resins [19, 20, 21, 22, 23] or the inclusion of natural materials such as cork. Cork is a natural and sustainable material, which presents excellent fire, smoke and toxicity properties associated with a high specific strength and stiffness, excellent damping characteristics, low thermal conductivity, near-zero Poisson coefficient, high damage tolerance to impact loads and resistance to wear [24, 25, 26]. At the level of benefits found in terms of impact, for example, [PERSON] et al. [27] report improvements in terms of impact strength andglass transition temperature when a polyester resin is filled with cork powder. Another study developed by the same authors shows benefits in terms of the elastic recuperation and damage area when an epoxy resin is filled with cork powder [21]. Simultaneously, the cork increased the penetration threshold and residual tensile strength [21]. [PERSON] et al. [28] and [PERSON] et al.