# Characterization of Polycarbonate and Glass-Filled Polycarbonate Using Multi-Relaxation Test—Role of Glass Fiber on Viscous Behavior of Matrix in Fiber Composites

## TL;DR

This study examines how short glass fibers affect the viscous behavior of polycarbonate in composites using a multi-relaxation test and a spring-dashpot model.

## Contribution

A novel method using a three-branch model and genetic algorithm is introduced to separate viscous stress in fiber composites.

## Key findings

- Short glass fibers increase the short-term viscous stress in polycarbonate composites.
- The long-term viscous stress is minimally affected by the presence of short glass fibers.
- A genetic algorithm enabled accurate fitting of relaxation data using a three-branch model.

## Abstract

The work presented here describes an approach that separates the viscous stress from the quasi-static counterpart for polycarbonate (PC) and its short glass fiber composite (GF-PC), with the aim to characterize the influence of short glass fiber on the viscous behavior of PC as the matrix of GF-PC. A multi-relaxation (MR) test was used for the mechanical testing and a three-branch spring–dashpot model for the data analysis, using a genetic algorithm to establish 100 sets of fitting parameter values that enabled the three-branch model to regenerate the measured stress decay during relaxation. Using the spring modulus Kv,s of the short-term branch in the three-branch model, two groups for these fitting parameter values were established as a function of specimen displacement (named stroke) of GF-PC, one of which shows a trend that is similar to the trend of the corresponding fitting parameters for the pure PC, and thus is believed to reflect the influence of glass fiber on the PC matrix of GF-PC. The study concludes that the short glass fiber increases the short-term viscous stress, but its role on the long-term viscous stress is marginal.