Mechanics of Time-Dependent Materials 7: 269–290, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands. 269 Nonlinear Viscoelastic, Viscoplastic Characterization of Unidirectional GF/EP Composite MODRIS MEGNIS ⋆ and JANIS VARNA ⋆⋆ Division of Polymer Engineering,Lulea University of Technology, S-971 87 Lulea, Sweden; E-mail: {modris.megnis, janis.varna}@mb.luth.se (Received 28 January 2002; accepted in revised form 16 December 2002) Abstract. Viscoplastic strains of unidirectional continuous fiber composite (HEXCEL GF/EP prepreg system) are studied experimentally and theoretically. Creep and strain recovery tests are used. Schapery’s nonlinear viscoelastic viscoplastic constitutive equations are used and generalized to describe inelastic behavior of unidirectional composite under isothermal creep and strain recovery conditions. The methodology to quantify the viscoplastic strains with respect to applied stress is proposed. Viscoplastic strains of composite are described by plastic shear strain in material sym- metry axis. Assumptions has been used and validated that the function describing the stress and time dependence of viscoplastic strain can be presented as a product of two, time and correspondingly stress dependent, master curves. Key words: composites, creep, nonlinear, viscoplasticity 1. Introduction Continuous fiber unidirectional polymeric composite materials exhibit inelastic be- havior. The main reason for this is micro-damage accumulated in layers, inelastic stress-strain response of a unidirectional composite in shear and matrix viscoelasti- city. In the present work we will focus on inelasticity caused by shear effects. Description of the micro-damage induced effects is left for a further study. The inelasticity is more pronounced if the loading axis does not coincide with the fiber orientation, see Varna et al. (1997, 1999). Since unidirectional layers are building elements in multi-directional laminates used in practical applications, the inelastic description of one single layer is a necessary step towards inelastic analysis of laminates. Experiments indicate (Varna et al., 1997, 1999) that there are more deformation mechanisms acting than just elasticity and viscoelasticity. For example, consider- ing off-axis or angle-ply laminate subjected to loading ramp containing loading and ⋆ Current address: Materials Research Department, Risø National Laboratory, DK-4000 Roskilde, Denmark, E-mail: modris.megnis@risoe.dk. ⋆⋆ Author for correspondence.