Influence of Molecular Conformation on the Constitutive Response of Polyethylene: A Comparison of HDPE, UHMWPE, and PEX E.N. Brown & R.B. Willms & G.T. Gray III & P.J. Rae & C.M. Cady & K.S. Vecchio & J. Flowers & M.Y. Martinez Received: 15 December 2006 / Accepted: 16 February 2007 / Published online: 22 March 2007 # Society for Experimental Mechanics 2007 Abstract The current work presents the characterization and comparison of the mechanical response of three different industrial forms of polyethylene. Specifically, high-density polyethylene (HDPE), ultra high molecular weight polyethylene (UHMWPE), and cross-linked poly- ethylene (PEX) were tested in compression as a function of temperature (-75 to 100°C) and strain-rate (10 -4 to 2,600 s -1 ). The responses of UHMWPE and PEX are very similar, whereas HDPE exhibits some differences. The HDPE samples display a significantly higher yield stress followed by a flat flow behavior. Conversely UHMWPE and PEX both exhibit significant strain hardening after yield. The temperature and strain-rate dependence are captured by simple linear and logarithmic fits over the full range of conditions investigated. The yield behavior is presented in terms of an empirical mapping function that is extended to analytically solve for the mapping constant. The power-law dependence on strain-rate observed in some polymers is explained using this mapping function. Keywords Polyethylene HDPE UHMWPE PEX . Materials characterization . Temperature . Strain-rate . Split Hopkinson pressure bar SHPB Introduction It is well known that the mechanical response of polymers is strongly affected by strain-rate [1–6]. Increasing the strain-rate leads to higher elastic modulus because the polymer chains have reduced relaxation time [7]. This also results in increased yield stress. While we [8–16] and others (see for example [17–20]) have published extensively on the mechanical behavior of numerous semi-crystalline polymers, these studies have largely focused on character- izing the material in a single form. From study to study, the pedigree (record of the material including variables such as chemical composition, processing, thermal and loading histories, density, crystallinity, molecular weight, etc.) of the material being characterizing ranges from extremely well known to completely unknown. Some of this research has probed the effect of changing polymer morphology, such as changing the percent crystallinity [8] or orientation of the polymer chains [21], while others have made comparisons based on changing composition, such as polytetrafluoro- ethylene versus polychlorotrifluoroethylene [15]. The current work investigates three polymers with the same chemical composition in the monomer repeat unit (C 2 H 4 ) n , but vastly different long-range conformations. We present the characterization and comparison of the large strain compressive behavior of three different industrial forms of polyethylene (PE): high-density polyethylene (HDPE), ultra high molecular weight polyethylene (UHMWPE), and cross-linked polyethylene (PEX). Poly- ethylene is a commodity thermoplastic extensively used in Experimental Mechanics (2007) 47:381–393 DOI 10.1007/s11340-007-9045-9 DO09045; No of Pages E.N. Brown (*, SEM member) : R.B. Willms : G.T. Gray III : P.J. Rae : C.M. Cady Materials Science and Technology Division, Structure/Property Relations, Los Alamos National Laboratory, MS G755, Los Alamos, NM 87545, USA e-mail: en_brown@lanl.gov K.S. Vecchio : J. Flowers Materials Science and Engineering Program, University of California San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0411, USA M.Y. Martinez WT Division, Materials Engineering, Los Alamos National Laboratory, MS P941, Los Alamos, NM 87545, USA