13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- New Trends in the Fracture of Lightweight Structures Efstathios E. Theotokoglou1,*, Gavriil E. Theotokoglou1 1 School of Applied Mathematical and Physical Sciences Department of Mechanics Laboratory of Testing and Materials National Technical University of Athens Zographou Campus, Theocaris Bld GR-0157 73, Athens, Greece * Corresponding author: stathis@central.ntua.gr Abstract Analysis of fractured lightweight structures has been performed in this paper. In particular a sandwich beam under three point bending containing a crack in the core material very close to the upper skin interface and parallel to the longitudinal beam axis is investigated. A numerical study of the fractured sandwich beam is developed calculating the stress intensity factors in order to investigate the fracture very close to upper skin interface under mixed-mode loading conditions. In addition a cohesive damage model is presented to simulate crack propagation and kinking into the core under mixed-mode loading conditions. The crack considered, is analyzed with static non-linear two- dimensional finite element analyses. Keywords Lightweight structure, Crack Propagation, Stress Intensity Factor, Cohesive Model 1. Introduction Lightweight composite structures are widely used in aerospace, marine and other modern industrial applications. Sandwich structures used in these applications consist of a lightweight foam core bonded to thin laminas to achieve high values of specific strength and stiffness. One of the main advantages of sandwich structures is their ability to provide increased flexural rigidity without an increase of the structural weight. The core is usually made of PVC, wood, a honeycomb material or lately carbon foam bonded to tough carbon fiber reinforced polyetheretherketone (PEEK) skins. In sandwich structures the foam is typically the weakest part and is the first to fail under static or cyclic loading because it transfers the applied loads as shear stresses. In addition a very critical problem in sandwich structures is the debonding problem between the face and core materials [1-5]. Unstable cracking propagation and kinking in core materials represents one of the weakest failure modes in sandwich composites. The fracture behavior in lightweight composites has been directed toward the understanding of crack propagation, and at the same time toward improving the durability of composites against fracture. A crack flaw may be introduced during processing or subsequent service conditions. It may result from low velocity impact, from eccentricities in the structural load path, or from discontinuities in structures, which induce a significant out-of-plane stress. In our paper a composite beam under three point bending and/or asymmetric three point bending is studied. In the core material of the beam an initially small crack is considered. In this study a computational analysis is developed based on an experimental investigation in composite beams under flexural loading [2]. From the numerical simulations stress intensity factors are calculated using the Finite element method and combined with crack propagation criteria predict the crack kinking into the core under flexural loading. On the other hand cohesive crack models [6, 7] are widely used to simulate crack growth and kinking phenomena. In order to develop numerical methodologies to simulate crack propagation in composite structures, cohesive damage models have attracted much interest [8-13] due to their well established advantages compared to the stress based
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