1 Study of Non-linear Stress-Strain Curves of Locally Available Low Density Polyurethane Foams Proceedings of the Technical Sessions, 27 (2011) 1-8 Institute of Physics – Sri Lanka Study of Non-linear Stress-Strain Curves of Locally Available Low Density Polyurethane Foams I. D. Ishan Pradeep Iddamalgoda and Ramal V. Coorey Department of Physics, University of Colombo, Colombo 3 ABSTRACT This paper describes the design and construction of two experimental setups to study the non-linear stress-strain behaviour of locally available polyurethane (PU) foams of low densities in the range of 12 ─ 23 kgm ─3 . The experimental stress-strain curves were parabolic and they showed an elastic region followed by a short plateau region about the maximum stress (collapse stress) followed by a reduction in stress at higher strains. However, they didn’t exhibit a long plateau and densification (a rapid increase of stress after collapse stress), which is the usual stress-strain behavior of high density PU foams. Experimental stress-strain curves were used to obtain elastic properties such as Young’s modulus, in the range of 7200 − 9900 Nm ─2 , Bulk modulus in the range of 9000 − 11800 Nm ─2 , collapse stress in the range 1150 − 2860 Nm ─2 , and Poisson’s ratios in the range of 0.3 − 0.4. All experiments were performed at constant temperature and humidity conditions. Furthermore, power-law relationships n A ρ ∝ were obtained for each elastic property (A) as a function of foam density (ρ ) where ─ 0.02 < n < 1.4. 1 INTRODUCTION Polyurethane (PU) foams refer to different types of foam consisting of polymers consisting of a chain of organic units connected by urethane links and are formed by the reaction of at least two isocyanate functional groups. PU foams are classified according their range of densities as solid elastomers, microcellular foams/elastomers, high and low density foams. Low density PU foams are classified as flexible, rigid and semi-rigid foams. PU foams are ‘open-cell’ foams where a very little energy is absorbed in the linear elastic region but it gives a long flat plateau that allows large energy absorption at constant load [1, 2]. This property of PU foams has been used in many industrial applications such as designing cushion material types, packing, foam-based systems for damping vibrations etc. [1, 3]. For example stiffness of cushion material is a measure of Young’s modulus. The mechanism of Goods and co-workers have shown that the mechanical properties (A) of cellular solids are related to their density (ρ ) and the relationship is expressed as a power law function, n A k ρ = , where k is a constant and n is the density exponent [4]. Another objective in this research is to find the power law relationship of elastic properties such as Young’s modulus, Bulk modulus, collapse stress, Poisson’s ratio as a function of density of foam materials. Since, foam density is the parameter that can be measured once the foam