A quantitative study on using digital photoelasticity for characterising the effect of the stretching speed on the necking phenomenon

Ahmed A Hamza,Taha Z N Sokkar,Mohammed A El-Bakary,Amira A S Azzam

doi:10.1515/polyeng-2020-0163

Abstract

Abstract The digital photoelastic technique is used to characterise the necking behaviour of isotactic polypropylene (iPP) fibres. The effect of stretching rate on necking initiation is studied. The birth of necking is observed using photoelastic patterns of the stretched fibres to understand how the localised difference between the principle stresses grows to form a necking region. Finally, the formation of multiple necking regions is characterised photoelastically. These multiple necks are initiated using the same formation mechanism and conditions as if there is only a single necking region. It was evident that, fast stretching causes faster arrangement of molecular chains and hence decreases the time required for necking initiation. Recommendations are suggested for optimum mechanical processing conditions of iPP fibres to avoid failure by necking. Photoelastic patterns are given for illustration.

Highlights

The mechanical response of polymers, especially polymeric fibres, strongly relies upon temperature, stress and processing time
Fast stretching causes faster arrangement of molecular chains and decreases the time required for necking initiation
Recommendations are suggested for optimum mechanical processing conditions of isotactic polypropylene (iPP) fibres to avoid failure by necking

Summary

Introduction

The mechanical response of polymers, especially polymeric fibres, strongly relies upon temperature, stress and processing time. Controlling these parameters yields a better polymeric structure. Digital stress analysis is finding the in situ stress values at different points along the fibre by comparing the captured photoelastic pattern to precomputed photoelastic orders, i.e. lookup tables. This analysis is a well-known technique for characterising. Photoelasticity evaluates the stress of models of obvious birefringent regions through polarisation optics. Structures of lines appear as a result of birefringence [5]

Objectives

Methods

Results