Seminars and thesis defenses



January 24, 2020


Euler seminar room



Predicting the deformation response of polymeric glasses via shear transformation zone (STZ) dynamics

By Frederik Van Loock (iMMC / IMAP)

The observed deformation behaviour of glassy polymers is actually notoriously complex. In the absence of failure, the measured uniaxial stress-strain curve of a polymeric glass deformed at a temperature below the glass transition temperature typically has an initial linear, (visco-)elastic part, followed by yield, strain softening, plastic flow, and strain hardening. In addition, the deformation response is sensitive to temperature and rate of deformation, physical ageing, and processing and/or deformation history. A vast amount of sophisticated, three-dimensional (visco)elastic-(visco)plastic models are available to simulate this large deformation response. They generally give good fits to measured uniaxial stress-strain curves. However, they are predominantly phenomenological, give limited insight into the micromechanical nature of deformation (and failure), and require the calibration of a large number of fitting parameters. Molecular dynamics (MD) simulations have also been used to shed light on the role of molecular deformation mechanisms driving the deformation of glassy polymers. These computations confirm that the inelastic deformation occurs through thermally activated molecular rearrangements and conformational changes of a collection of polymer chains. However, MD simulations are restricted to short time and length scales, limiting their use when attempting to predict the response of glassy polymers in bulk or when confined between stiff fibres in fibre-reinforced polymers.
The use of a mesoscale numerical model based on the activation of shear transformation zones (STZs) offers a practical avenue to bridge the state-of-the-art continuum and atomistic simulation approaches. It may thereby help to improve the understanding of the interactions between discrete and elementary distortion mechanisms (and their collective organisation) during plastic deformation of a polymeric glass. The STZ framework was originally developed by Argon to simulate the deformation of metallic glasses through thermally activated shear transformations around free volume regions present in the glass. We have used the implementation of Homer and Schuh to develop a mesoscale finite element model for polymeric glasses based on random, heterogeneous activations of STZs. The model assumes that plastic deformation is governed by both conformational changes of molecular clusters (STZs) and the elastic interaction of these local events with the surrounding matrix. The mesoscale STZ framework only requires the calibration of 7 parameters (of which 5 can be directly measured) and successfully predicts the large deformation response of glassy polymers, including post-yield softening, non-linear unloading behaviour and the Bauschinger effect, by accounting for the inherently heterogeneous distribution of stress (and strain) within the deformed glass.