Effect of ageing on critical cut length and morphology of fracture surface in tensile rupture of natural rubber

Abstract

Variation of tensile strength with flaw sizes has been studied both for unaged and aged natural rubber (NR) gum vulcanizates (aged up to 150° C). A precut of varying lengths is given at the centre of the tensile specimens. The morphology of the fracture surfaces has also been reported. A critical cut length is observed for NR vulcanizates. There is an increase in the critical cut length (lc) on ageing. The sharp fall of tensile strength at the critical cut length, however, gradually diminishes. On prolonged ageing, no critical cut length is observed. A mathematical model has been made to explain the behaviour of the critical cut length of NR with ageing time/temperature. Scanning electron microscopic studies support the prediction of Thomas, that there is a change in the mechanism of rupture above thelc. Below thelc. it is a cut growth process and fracture is originated from natural flaws/nicks and proceeds towards the precut at the centre. However, for samples with precut greater than thelc, the fracture is mainly a tearing phenomena initiating from the given precut. A quantative correlation between the tensile strength and the distance between crack lines/tear lines has been found.