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 based on polarized optical microscopy (str-POM), the stress analysis approach to elastomeric fracture uncovers new insights. We show new phenomenology in contrast to the standard description of linear elastic fracture mechanics (LEFM). First, str-POM measurements show emergence of a stress saturation zone whose dimension r_ss is independent of the stress intensity factor K. This elastic zone is plastic zone whose size would scale quadratically with K. The absence of stress divergence allows us to measure tip stress s_tip at the onset of fracture, identified as inherent material strength, i.e., s_tip(F) = s_F(inh). We are able to explain why LEFM applies well to elastomers, i.e., why toughness (either given as critical energy release rate Gc or critical stress intensity factor Kc) is a material constant, and we have identified parameters that determine the magnitude of toughness. Second, the popular Rivlin-Thomas energy balance description of elastomeric fracture in pure shear has acquired a fresh and different interpretation based on str-POM observations, which show that the stress buildup at cut tip explicitly scales with specimen height h0, leading to Gc = wch0 being constant. 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.