## 1 Introduction  
The technical term \"Crashworthiness\" exactly describes the capability of a structure to protect its occupants during an impact event [1, 2]. In aeronautics, the first research work on aircraft crashworthiness dates back to the 1917s and was carried out by [PERSON], an American pilot, engineer and aviation safety pioneer [3, 4]. He was the only survivor of four passengers in a plane crash and was convinced that the reason for his survival was related to the moderate deformations occurred in specific plane areas. This led [PERSON] to study the dynamics of air and road accidents and realised that in some of them it was possible to improve the chances of survival by using \"shock absorber systems\". Later, in 1950, the US Army began a research project based on the analysis of aircraft crashes and passenger injuries, publishing in 1967 the _Aircraft Crash Survival Design Guide_ a guideline for the'safe design' of shock-absorbing structures [5, 6, 7, 8, 9]. In particular, this volume [5, 6, 7, 8, 9] of the _Aircraft Crash Survival Design Guide_ discusses the following topics: [5] Volume I--Design Criteria and Checklists, [6] Volume II--Aircraft Design Crash Impact Conditions and Human Tolerance, [7] Volume III--Aircraft StructuralCash Resistance, [8] Volume IV--Aircraft Seats, Restraints, Litters and Cockpit Cabin Delethalization and, [9] Volume V--Aircraft Postcrash Survival. Further steps in this direction were accomplished in 1972 as a result of cooperation between NASA, FAA and many aerospace industries. This collaboration led to the development of new (at that time) structural concepts, still widely used nowadays, arising from experimental tests assisted by the first numerical analyses in this field. During this period, there were also performed impact tests on small civil aircraft, which provided a great deal of qualitative and quantitative information on the behaviour of the aircraft and of the occupants in case of impact events. In parallel with these developments in aeronautics, passenger safety started to become a key concept in the design of all transportation systems. Indeed, a large number of research studies on the topic of crashworthiness for different types of transport systems can be found in the literature [10; 11; 12; 13; 14]. In the frame of the work presented in [10], the authors investigated the idea of using thin-walled circular foam-filled tubes with stiffened annular rings to increase specific energy absorption (SEA) characteristics and to reduce force peaks in the main critical components of the automobile. Concerning the railway sector, the activity performed in [11; 12], where the authors present a study on the impact resistance of expanding circular tube energy absorbers with cylindrical anti-chamber under eccentric load employable in subways trains, merits mention. Moreover, in [14], an analytical model to predict mean crush force recorded in a shock absorber adoptable for transport applications is provided. Due to the high capacity of metallic materials to dissipate absorbed energy by plastic deformations, the state of the art of shock absorber systems is characterized, mainly, by metallic solutions [15; 16; 17]. On the other hand, however, metallic shock absorber systems usually lead to significant weight increases [17; 18]. For this reason, it is usual to find in both the automotive as well as the aerospace applications, metallic thin-walled mechanisms [10; 19] or honeycomb structures [20]. Furthermore, due to their fragile nature, the use of composite materials in shock-absorbing systems is, generally, not attractive. Indeed, composite materials, when subjected to impact loads, absorb energy by fibre and/or matrix breakage and by delamination formation and growth. Indeed, a small amount of plasticization in matrix takes place, however, the rate of energy dissipated through plasticisation is considerably lower if compared to metallic solutions.