Predicting the Brittle-to-Ductile Transition in Amorphous Polymers

TL;DR

This paper introduces a simple scalar model to predict the brittle-ductile transition in amorphous polymers based on visco-elasto-plastic behavior, strain rate, and temperature, aligning well with experimental data.

Contribution

It formulates a new scalar model linking BDT to relaxation times and strain rate, providing a predictive framework for various polymers.

Findings

The model predicts BDT as an upper bound on strain rate inversely related to beta-relaxation time.

Good agreement between model predictions and experimental data for three polymers.

The model offers a unified approach to understanding BDT across different polymer chemistries.

Abstract

Brittle-ductile transition (BDT) is an important characteristic of amorphous (and semicrystalline) polymers. For a given strain rate, at temperatures above BDT, the polymers exhibit strain softening followed by yield and strain hardening, while at temperatures below BDT, the same materials exhibit brittle failure at relatively low strains. Surprisingly, today there is no simple model describing BDT as a function of polymer chemistry, sample history, deformation type, and strain rate. Experimental data suggest that BDT is often, though not always, associated with the beta-transition. We formulate a simple scalar model to describe the visco-elasto-plastic shear stress-strain curves as functions of temperature and strain rate. We also show that within this model, there is always an upper bound on the strain rate where the material can have a uniform viscoplastic flow; this upper bound is taken to represent the BDT. We stipulate that this upper bound is inversely proportional to the Johari-Goldstein beta-relaxation time. Using our "general" Sanchez-Lacombe "two-state, two-(time)scale" (SL-TS2) model, we compute the BDT for three polymers (polystyrene, poly(methylmethacrylate), and poly(vinylchloride)) and found a good agreement with experimental data.