NUMERICAL ESTIMATES OF COMPOSITE SANDWICH RESPONSE UNDER QUASI STATIC INDENTATION

NUMERICAL ESTIMATES OF COMPOSITE SANDWICH RESPONSE UNDER QUASI STATIC INDENTATION Alberto Zinno 1 , Charles E. Bakis 2 and Andrea Prota 1 1 Department of Structural Engineering, University of Naples “Federico II” Via Claudio 21 - 80125 Naples, Italy. [email protected] [email protected] 2 Department of Engineering Science and Mechanics, Pennsylvania State University 212 Earth and Engineering Science Building – University Park PA 16802, United States. [email protected] Abstract The present work investigates the numerical modeling of sandwich structures with GFRP or CFRP face sheets and Nomex core under quasi static indentation. The study deals with a 2D configuration, where a sandwich beam is indented by a rigid cylinder across the whole width of the beam. The ABAQUS finite element package is used to model the indentation response of the beams. Geometrical non-linearities are including the contact problem are taken into accouny. The purpose of the numerical modelling is to develop reliable 2D simulations of the non-linear response for use in future investigations of failure modes caused by indentation. In order to verify the finite element model, an analytical approach is developed. The analytical model is based on a higher-order sandwich beam theory that includes the variational principles to include the transverse flexibility of the core. 1. Introduction Sandwich composite structures consist of two thin, stiff and strong fiber reinforced composite face sheets (skins) separated by a thick layer of low density material (core) which may be much less stiff and strong. The role of the face sheets, due to the higher elastic properties, is to resist to bending and in-plane loads, while the transverse shear loads are sustained by the core. The bending stiffness of this arrangement is very much greater than that of a single solid plate of same total weight made of the same materials as the faces. For this reason composite sandwich structures are widely used in high- performance applications where weight must be kept to a minimum, for example aerospace structures, high-speed marine craft and trains or racing cars. However the nature of core materials gives to these structures a low transverse stiffness, causing local bending under concentrated loads. As a result, sandwich structures are susceptible to local damages due to handling, interaction with attached structures or transverse impact. Usually, the local failure starts in the core and results in core crushing, face– core debonding and (or) residual dent formation and, therefore, substantial reduction of the structural strength. Fundamental analyses of sandwich beams such as that presented by Allen [1] and Plantema [2], assume that the core is incompressible in the out-of-plane direction. These models further assume that the skins have only bending rigidity while the core has only shear rigidity. This approach is appropriate for sandwich structures with incompressible cores. To model the local effects at the load points for non-metallic honeycomb sandwich panels with low transverse stiffness, deformation of the core in the vicinity of the applied loads must be considered. Accordingly the elastic foundation model has been applied to this problem [3,4]. However foundation models neglect interactions between the top and bottom skins. The non-planar deformed cross-section of the sandwich beams, observed in experiments, suggests a model that allows non- linear variations of in-plane and vertical displacements through the thickness of the core. Frostig and Baruch [5-7] used variational principles to develop a higher-order

NUMERICAL ESTIMATES OF COMPOSITE SANDWICH RESPONSE UNDER QUASI STATIC INDENTATION

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