Time and temperature effects on Poisson’s ratio （时间和温度对泊松比的影响）.pdf

eXPRESS Polymer Letters Vol.5, No.8 (2011) 685–697 Available online at DOI: 10.3144/expresspolymlett.2011.67 Time and temperature effects on Poisson’s ratio of poly(butylene terephthalate) 1* 2 S. Pandini , A. Pegoretti 1Department of Mechanical and Industrial Engineering and INSTM Research Unit, University of Brescia, via Branze 38, 25133, Brescia, Italy 2Department of Materials Engineering and Industrial Technologies and INSTM Research Unit, University of Trento, via Mesiano 77, 38123, Trento, Italy Received 26 November 2010; accepted in revised form 2 February 2011 Abstract. The viscoelastic nature of the Poisson’s ratio of a semicrystalline poly (butylene terephthalate) is highlighted by investigating its dependence on time, temperature and strain rate, under two types of loading conditions: i) constant defor- mation rate tests, in which the transverse strain is measured in tensile ramps at various temperatures and at two strain rates; and ii) constant deformation tests, in which, under a constant axial deformation, the transverse strain is measured as a func- tion of time in isothermal experiments performed at various temperatures. In both testing configurations, axial and trans- verse deformations are measured by means of a biaxial contact extensometer, and a correction procedure is adopted in order to compensate the lateral penetration of the extensometer knives. Poisson’s ratio displays the typical features of a retarda- tion function, increasing with time and temperature, and decreasing with strain rate. This behaviour has been compared to that of simultaneously measured relaxation modulus. Keywords: material testing, mechanical properties, viscoelasticity, Poisson’s ratio 1. Introduction Poisson’s ratio of polymeric materials can thus be In the infinitesimal deformation of an idealized thought in the framework of l