Two features of the uniaxial compression of a glassy epoxy resin: the yield stress rate-dependence and the volumetric instability
DICATA, Faculty of Engineering, University of Brescia, Via Branze, 43––25123, Brescia, Italy
Department of Design and Technology, Faculty of Engineering, University of Bergamo, Viale Marconi, 5––24044, Dalmine (BG), Italy
We report the results of uniaxial compressive tests on a DGEBA epoxy resin at room temperature, well below its glass transition. We first focus on the strength, defined as the stress value corresponding to either a maximum or a flattening of the stress-strain curve, which, for this polymer, may be taken to be coincident with the yield stress, as often assumed for many thermosets. Within the strain rate range (1.E-6, 2.E-3 1/s) we confirm the linear trend relating the logarithm of the strain rate to the yield stress, as already been observed by other investigators even for the same epoxy resin; instead, at strain rates below approximately 1.E-6 1/s, we found a negligible rate-dependence, as our data indicate a lowest limit of the yield stress, of about 87 MPa. On the basis of these results, we propose how to extend to the viscoplastic regime of deformation a nonlinear viscoelastic model previously put forward. Secondarily, within the viscoelastic range, at a stress level significantly lower than the yield stress, our measurements show a mild volumetric instability, allowed by the free lateral expansion, not ascribable to any macroscopic structural effect; such a behaviour has never been reported in the literature, to the best of our knowledge.
Author Keywords: Mechanical characterization; Epoxy resins; Strength; Viscoelasticity; Viscoplasticity; Instability