Issue 29

M. Romano et alii, Frattura ed Integrità Strutturale, 29 (2014) 385-398; DOI: 10.3221/IGF-ESIS.29.34 387 values of the fibre volume content in the specimens yield significant higher energy dissipation properties. Additionally the materials with epoxy matrix system exhibited the highest energy dissipation capacity. Rosenberg and Dekel 2012 [21] basically describe the experimental setup and procedure as well as the evaluation of the experimentally determined results when transversal impact-tests in the high-velocity range are described. Pursued mechanical principle The object in the carried out investigations is the identification of the energy dissipation capacity of fibre reinforced plastics under high-velocity impact loads. Thereby besides monolithic layups selected hybrid layups with and without separating layer are investigated. The aim is to enhance the energy dissipation capacities by hybrid layups based on fabrics of different kinds of fibre reinforcements besides using the outstanding material properties of fibre reinforced plastics as load bearing structures. Therefore two basic ideas are followed. Hybrid stacking sequences As a first approach the use of hybrid stacked layups instead of monolithic layups is relatively simple. Thereby the characteristic properties of carbon and glass fibres, completed by basalt fibres, respectively, can be used through the thickness in by reasonably defined hybrid stacking sequences. Aramid fibres have not been considered, because of their distinct hygroscopicity and resulting in difficulties when being impregnated as described later on. Separating layer as elastic support A second approach is the use of separating layers as a core material. Here, especially relatively thin separating layers with relatively low stiffness are used. In the present application, however, it is not used with the aim of enhancing the moment of inertia as usually done. In contrast the used separating layers with the afore described properties provide an elastic support and a shear plane for the single layers in the layup. Thereby transversal loads can increasingly be transferred to in- plane loads. This effect is reached by additionally providing transversal deflection due to shear effects and therewith enhances the resulting in-plane loads in the material [6]. The reason for this pursued principle is, that fibre reinforced plastics show its outstanding properties under in-plane tensile loads in direction of the reinforcement fibres. The aim is to reduce the effect of simply stamping the material by the impactor but enable the afore described structural mechanic effects similarly to Fadhel 2011 [3]. Limit of the experimental investigations With the afore described basic ideas the initial situation for using the light-weight properties of fibre reinforced plastics as well as enhance their resistance to transversal impact loads in the high-velocity range can be created. These two basically different properties enable the application of fibre reinforced plastics as load-bearing structure and at the same time as protective structure. If thereby for load-bearing structures additional protective structures can be omitted, a holistic light- weight approach can be achieved. Alternative approaches for enhancing the resistance of fibre reinforced plastics under transversal impacts in high-velocity range by the impact resistance of the matrix, i.e. by definably adding small amounts of thermoplastics in the thermoset matrix system is not the aim of the carried out investigations. They are merely focused on the material suited or fibre suited application of different kinds of reinforcement fibres in hybrid stacked layups, respectively. General experimental conditions Knowing that different layer orientations have influence on the energy dissipation capacity [9] all test panels have been built up with a [(0/90) n ] orientation. This was done as a first attempt for characterizing the general material behavior The fibre volume content is a significant parameter for the quality and reproducibility of fibre reinforced plastics [6], [24], [25]. Moreover it has a massive influence on the energy dissipation capacity [8]. In order to experimentally investigate the specific capacities of monolithic and hybrid stacking sequences regarding energy dissipation high-velocity impact tests with an impact velocity of approx. 560 m/s have been carried out. Thereby bearing balls have been used as an impactor. With a mass of approx. 1.12 g the initial kinetic energy before the impact lies in the range of 175 J. The reason therefore is that the spheric bearing balls are insensitive to variations in the angle of incidence. In contrast cylindrical blunt-ended or conical projectiles would cause distinct stress peaks in the specimen. This is due to variations of the angle of incidence caused by statistic deviations. Analog observations have been made e. g. by Muhi, Najim and Moura. 2009 [11]. It is additionally observed that the selected spheric geometry provides reasonable values for the energy dissipation properties [11].