Issue 29

M. Romano et alii, Frattura ed Integrità Strutturale, 29 (2014) 385-398; DOI: 10.3221/IGF-ESIS.29.34 386 I NTRODUCTION ibre reinforced plastics are increasingly applied in the automotive sector as well as for personal impact protection issues. Thereby especially the crash behavior and the failure mechanisms have to be researched properly in order to ensure safety for car passengers and facilitate recovery in the crash case. Fibre reinforced plastics consisting of only one kind of reinforcement fibres under high-velocity impact loads mostly fail in a brittle failure mode causing sharp- edged fracture surfaces and splitters. In order to use the outstanding properties of each single kind of fibre and to avoid the afore-mentioned brittle failure modes to an optimum, the idea of using hybrid stacking sequences is adequate. Under these conditions fibre reinforced plastics with hybrid stacking sequences can be used as load-bearing structures and at the same time as safety structure for passengers in automotive or aerospace applications. Moreover, with the hybrid stacked composites lightweight concepts can efficiently be realized regarding energy saving issues. R ESEARCH ENVIRONMENT he research environment is the material behavior of fibre reinforced plastics under transversal loads in the high- velocity range. Therefore a brief literature review is presented. This leads to several conclusions and to the pursued mechanical principle described thereafter. Literature review Energy dissipation mechanisms in fibre reinforced plastics are dominated by inter- and intralaminar failure modes as well as adhesive failure in the interface between fibre and matrix. Whereas interlaminar failure means the delamination of sequent plies, intralaminar failure means fracture of fibre and matrix. Only a small amount of energy is dissipated by friction effects between fibre and matrix and plastic deformation in the polymeric matrix system. Different material parameters affect the failure modes. These are amongst others fibre orientation, fibre volume content, layup, geometry of the specimen and of the impactor as well. Maier 1990 [7] investigated the crash-behavior of tubular specimens with different cross-sections by experiments and finite-element-analyses. The specimens were made of glass fibre reinforced vinyl ester, poly ester and epoxy matrix system. The specimens with the epoxy matrix systems exhibited the highest energy dissipation capacity. Additionally the influence of the processing and the geometry on the energy dissipation capacity has been investigated. Morita et al. 1997 [9] characterized the damage tolerance of fibre reinforced plastics under high-velocity impact loads with a varied degree of anisotropy. Thereby the degree of anisotropy means the change of fibre orientation for sequent layers in the layup. The results showed an enhanced energy dissipation capacity with an increased degree of anisotropy for sequent layers in the layup. Simultaneously the damage area in terms of interlaminar delaminations increased. Holmquist and Johnson 2002 and 2005 [5, 4] investigated the reaction of silicone carbide to transversal impact loads in the high-velocity range. The investigated material is a ceramic. The originally high energy dissipation capacity could even be enhanced by creating predefined residual stresses, suited to the specific material properties. Muhi, Najim and Moura. 2009 [11] considered the effect of hybridization of glass fibre reinforced plastics with single layers of aramid fabrics under transversal high-velocity impact. Therefore five kinds of different layups have been investigated experimentally. These are a monolithic layup consisting of only four layers of glass fabrics and in comparison four hybrid layups where at different positions in the layup one layer of glass fabric is substituted by one layer of aramid fabric (Kevlar 29) at a time. Additionally three different geometries of the impactor have been used. As a completion analytical approaches following Morye et al. 2000 [10] have been carried out. The hybridization in terms of substituting one layer of glass fabric with one layer of aramid fabric yields enhanced penetration resistance as well as sensitivity to the position of the hybrid substitute layer in the construction of the layup. In detail the hybridized layup where the opposite impact side has been substituted exhibits the highest energy dissipation properties. Fadhel 2011 [3] carried out finite-element-analyses for pure polycarbonate specimens under high-velocity impact-loads varying their thickness. Additionally two different geometries of the impactor have been investigated. Even though the work is focused on pure polymeric materials the essential conclusion of the simulations is that an enhanced elastic sag could be identified as the main reason for an increasing dissipated energy. Melo and Villena 2012 [8] investigated the influence of the fibre volume content on the energy dissipation capacity of fibre reinforced plastics. For glass fibre reinforced plastics with epoxy, polyester and vynilester matrix systems higher F T