## 1 Introduction  
The term \"hybrid\", from Latin \"hybrida\", has been used, for long time, to describe the offspring of two plants or animals of different species or varieties. Generally, it refers to a thing made by combining at least two different elements. The hybrid concept, applied to materials, refers to when the combination of these different elements allows for the production of some new and unexpected properties and functions. In the case of composites, the term hybrid refers to the composite material which is composed of either one fiber material along with one or more matrix materials, or one matrix material along with one or more fiber materials [1]. Most of the proposed cases for the aerospace sector refer to the use of different fibers (organic, carbon, natural or glass fibers) in the same organic matrix. In typical micro-composite materials, the mixture of matrix and fiber results in averaging the properties of both, whereas new properties have to be achieved by hybrid composites [2]. For example, vehicles and packaging products have proved that natural fibers are a viable alternative to synthetic fibers. In fact, hybridization offers additional advantages since the mixing of cheaper, low-quality fibers with more expensive fibers of higher quality can improve the properties of a composite without significantly affecting the cost [3]. Among the improvements which resulted from hybridization are those affecting the damping or the fatigue behavior. All these aspects make hybrid composites very attractive for high-added value markets with severe technical constraints, such as the automotive and transportation, marine, aerospace, wind energy, and sports markets [1]. The material class of hybrid composites is very large, and research studies are uncountable. Approximately 40,000 manuscripts refer to \"hybrid composite materials\" in the main relevant fields (title, abstract, keywords); more than 2000 already in the current year (Scopus font). Nevertheless, research trends are very clear, and some important achievements have been reached recently. In the last year of research, by restricting the analysis to structural composites, the most common form of hybridization refers to the contemporary use of carbon and glass fibers. These fibers are generally inserted in an epoxy matrix, and conventional manufacturing procedures are adopted [4]. In terms of the expected improvement from hybridization, most of these studies discuss positive effects for the dynamic behavior of hybrid materials, improving qualities such as fatigue [4,5], impact [6,7], and damping, with and without aging [8,9]. However, they continue to propose the typical composite hybridization by combing carbon and glass fibers, mainly into an epoxy matrix. In fact, carbon fiber reinforced (CFR) and glass fiber (GF) composites are the most used composite materials for transportation, but they are rarely combined in the same product because of the very different static performances of glass and carbon fibers. In some cases, instead of damping properties, which are measured under cycling, effects on toughness are investigated, typically extracted from quasi-static tests [10]. Results confirm that the hybrid composite shows lower stiffness and strength than traditional CF laminates, but it has the advantage of an increased damping behavior. Hybridizing CFR laminate with GF lamina is also found to be an efficient way to improve delamination tolerance without reducing laminate resistance to flexural and impact loads [11]. Toughness improvements are also found by combining CFs with other fiber types, such as polyarylate and polybenzobisoxazole [12], Kevlar [13,14], aramid [15], liquid-crystal polymers [16], basalt [17], and carbon nanotubes [18, 19, 20]. The interaction mechanisms seem to be the same as the common CF-GF hybrid composites, but different extents of damping properties may be reached by tuning the related compliance.