ISSN 1392-1320 MATERIALS SCIENCE (MEDŽIAGOTYRA). Vol. 10, No. 1. 2004 Experimental Analysis of Mechanical Behaviour and Damage Development Mechanisms of PVC Foams in Static Tests I. Gimenez 1 , M.-K. Farooq 2 , A. El Mahi 2∗ , A. Kondratas 3 , M. Assarar 2 1 Technology Transfer Center of Le Mans, 19-21 rue Thalès de Milet, 72000 Le Mans, France 2 Institute of Acoustics and Mechanics, University of Le Mans, Avenue Olivier Messiaen, 72085 Le Mans cedex 9, France 3 International Studies Centre, Kaunas University of Technology, Donelaičio 73, LT-3006 Kaunas, Lithuania Received 23 October 2003; accepted 16 December 2003 Cellular foams are being used increasingly as core materials in conjunction with high strength skins, to produce strong, stiff, and light weight sandwich structures for aerospace, marine and transport industry. Due to their higher impact resistance and energy absorbing capability, cellular foams are being extensively used in automobile applications. This paper presents the results of experimental investigation of foam density effect on mechanical behaviour and damage development that induce the rupture of PVC foams in static tests. The experimental investigation was conducted using compression, flexural, indentation and shear tests for the foams of four densities. The damage mechanisms and properties of different foam densities were evaluated in monotonic loading tests and were compared. The obtained results show the better performances of the foam with higher density. The strength of foam material increases with the increase of its density. The complete damage of the test specimens is caused by deterioration of the PVC cells. The mode of damage depends on the density of material and the type of loading. Keywords: foam, compression, indentation, shear, flexural. 1. INTRODUCTION * In the context of sandwich composite materials, the material consists of high modulus reinforcing fibres embedded in low modulus polymeric matrix bonded to low density core material laminate. Laminate and core materials are non-homogeneous and anisotropic, therefore properties will vary throughout the entire structure. Mechanical properties of sandwich composite depend on the properties of constituent laminate and core materials. Composite face sheets fail as a result of an interaction among matrix cracks, fibre fracture, delamination, etc. Mechanical properties of laminates strongly depend upon the quantity, orientation of the fibre and type of the matrix. In the core it depends upon the thickness, density and cell structure of the core. Therefore, it is necessary to understand the behaviour of constituents of sandwich in details [1 – 5]. Foam cores in sandwich components can be used as a cost-reducing production aid and for structural applications. Foamed plastics also referred to as cellular or expanded plastics have a higher flexural modulus. They achieve a higher load-bearing capacity per unit in weight, as well as higher energy storage and energy dissipation capacities. Examples of commonly produced foamed plastics are polyurethane, PVC, polystyrene, polypropy- lene, epoxy, phenol-formaldehyde, cellulose acetate, silicone, etc. It is virtually possible to produce every thermoplastic and thermoset polymer in a cellular form. Foamed plastics can be classified according to the nature of the cells into closed-cell and open-cell types. Each individual cell in a closed-cell type of foam, more or less spherical in shape, is completely enclosed by a wall of plastic, while in an open-cell type of foam the individual cells are interconnected as in a sponge. Free expansion during cell formation usually produces open-cell foams. Closed-cell foams are produced in processes, where some pressure is maintained during the cell formation stage. Close cell foams can absorb more energy than open cell foams because of entrapped gas within the cell, which acts as medium of energy absorption. During compression loading, the entrapped gas inside the cell can compress as either isothermally or adibatically depending on compression rate. Foamed plastics are produced in a wide range of densities. The shape, size and distribution of cells can be regular or highly inhomogeneous, depending on the particular material and adopted foaming process. Accordingly, polymer foams may be homogeneous, with a uniform cellular morphology throughout, or they may be structurally anisotropic. Foaming of plastics can be achieved in several ways. One possibility is to create gases inside the mass of the polymer. Once the polymer has been expanded, the cellular structure must be stabilised rapidly. The expansion is carried out above the melting point, and the foam is then immediately cooled below the melting point (such a process is referred to as physical stabilisation) if the polymer is thermoplastic. Otherwise, chemical stabilisation can be performed. Gas can be whipped into solution of the plastic as low temperature boiling liquid or incorporated in the plastic mix and then volatilised by heat. Carbon dioxide gas can be produced within the plastic mass by chemical reaction, or other gases (e.g., nitrogen, air) can be dissolved in the plastic melt under the pressure and then allowed to expand by reducing the pressure as the melt is extruded. The typical process of uniaxial behaviour of cellular materials is described in [6 – 7]. Typical stress strain * Corresponding author. Tel.: + 33 2 43 833456; fax: + 33 2 43 833149. E-mail address: Abderrahim.Elmahi@univ-lemans.fr. (A. El Mahi) 34