Resolving stress state at crack tip to elucidate nature of elastomeric fracture

Abstract

Based on spatial–temporal resolved measurements of the stress field at crack tip using polarized optical microscopy (str-POM), we develop an stress analysis approach to elastomeric fracture. Specifically, str-POM measurements reveal emerging phenomenology in three distinct ways. First, there emerges a stress saturation zone (SSZ) whose dimension rss is independent of the stress intensity factor K. This SSZ arises from the fact that the precut suffers natural blunting during cut making. Because of the finite radius of the cut tip, the tip stress σtip is well defined and experimentally accessible, i.e., can be determined within the spatial resolution of str-POM. At the onset of fracture, the tip stress is interpreted to reach inherent material strength σF(inh), i.e., σtip(F)=σF(inh). Second, elastomeric fracture in pure shear is shown by the str-POM observations to involve the same physics: Fracture occurs when the tip stress approaches the inherent strength. Rivlin–Thomas expression for toughness Gc=wch0 follows because the stress buildup at the cut tip explicitly scales with specimen height h0, i.e., Kps=σh0, as expected. Third, the str-POM observations reveal how elastomeric fracture occurs at a common Kc independent of specimen thickness. At a given load there is weaker stress buildup for a thicker specimen due to greater stress saturation at cut tip, and fracture is observed to occur at lower tip stress for a thicker specimen.