13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Damage Identification and Influence on Mechanical Properties of Closed Cell Rigid Foams Liviu Marsavina1,*, Dan M. Constantinescu2, Emanoil Linul1, Tudor Voiconi1, Dragos A. Apostol2, Tomasz Sadowski3 1 Department Mechanics and Strength of Materials, Politehnica University of Timisoara, Timisoara 300222, Romania 2 Department of Strength of Materials, University POLITEHNICA of Bucharest, Bucharest 060042, Romania 3 Solid Mechanics Department, Lublin University of Technology, Lublin 20-618, Poland * Corresponding author: msvina@mec.upt.ro Abstract The effect of damage on mechanical properties of closed-cell foams is numerically investigated. Representative 3D cell structures are considered for the finite element analysis. In the first instance the mechanical properties were obtained on un-damaged structures. The obtained numerical values for Young’s modulus and Poisson’s are compared with experimental results. Two types of experimental methods were used to determine the mechanical properties of rigid polyurethane foams: Static compression tests providing stress – strain curves and mechanical properties as Young’s modulus, yield stress, plateau stress, densification and Digital Image Correlation used for identification of the damage mechanism, observation of bands of deformation, Young’s modulus and Poisson’s ratio. The influence of density on mechanical properties is also investigated. Two types of damages/defects were defined randomly in finite element analyses by considering missing cell walls and filled cells. The results show that the missing cell walls create a decrease of Young’s modulus, while the filled cells produce the increase of Young’s modulus. Keywords Rigid foams, Damage, Mechanical properties, Numerical analysis, Micromechanical models 1. Introduction Polyurethane foam materials are widely used as cores in sandwich structures. The main characteristics of PUR foams are lightweight, high porosity and good energy absorption capacity, [1,2]. Foam materials crush in compression, while in tension they fail by propagation of a single crack, [3]. Many attempts were performed in order to determine experimentally the compression properties of plastic foams [4-13]. On the other hand, analytical micro-mechanical models based on deformation and fracture of single cell were developed to predict the mechanical properties of foams in compression [14,15]. In recent years the finite element analysis (FEA) micro-mechanical models were used in order to study the effect of damage of cellular structure on the compression properties [16,17]. Usually these models were constructed using 2D beams. This paper presents the results of mechanical properties for three polyurethane foams under compression loading investigating the effect of density, loading direction and loading rate. The Digital Image Correlation (DIC) technique is employed during compression tests in order to identify the damage mechanisms of foams. Afterwards, using a 3D representative volume of the foam, the effect of damage (missing cells and filled cells) was studied. 2. Experimental program In order to identify the mechanical properties of the rigid polyurethane (PUR) foams an extensive experimental program was performed. Closed cell rigid PUR foams with three different densities (100, 160 and 301 kg/m3) were considered for the experimental program, Fig. 1.
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