title: Modelling of loading, stress relaxation and stress recovery in a shape memory polymer.
abstract: A multi-element constitutive model for a lactide-based shape memory polymer has been developed that represents loading to large tensile deformations, stress relaxation and stress recovery at 60, 65 and 70°C. The model consists of parallel Maxwell arms each comprising neo-Hookean and Eyring elements. Guiu-Pratt analysis of the stress relaxation curves yields Eyring parameters. When these parameters are used to define the Eyring process in a single Maxwell arm, the resulting model yields at too low a stress, but gives good predictions for longer times. Stress dip tests show a very stiff response on unloading by a small strain decrement. This would create an unrealistically high stress on loading to large strain if it were modelled by an elastic element. Instead it is modelled by an Eyring process operating via a flow rule that introduces strain hardening after yield. When this process is incorporated into a second parallel Maxwell arm, there results a model that fully represents both stress relaxation and stress dip tests at 60°C. At higher temperatures a third arm is required for valid predictions.
authors.affiliation: IRC in Polymer Science and Technology, School of Engineering and Informatics, University of Bradford, Bradford BD7 1DP, UK. Electronic address: j.sweeney@bradford.ac.uk.
authors.forename: J
authors.initials: J
authors.lastname: Sweeney
authors.affiliation: IRC in Polymer Science and Technology, School of Physics & Astronomy, The University of Leeds, Leeds LS2 9JT, UK.
authors.forename: M
authors.initials: M
authors.lastname: Bonner
authors.affiliation: IRC in Polymer Science and Technology, School of Physics & Astronomy, The University of Leeds, Leeds LS2 9JT, UK.
authors.forename: I M
authors.initials: IM
authors.lastname: Ward
mesh_terms: D004548:Elasticity; D008422:Materials Testing; D008962:Models, Theoretical; D011098:Polyglactin 910; D013314:Stress, Mechanical; D013696:Temperature; D013718:Tensile Strength; D016474:Weight-Bearing
chemical_list: D011098:Polyglactin 910
keywords: Constitutive modelling; Mechanical properties; Modelling; Shape memory