Crack-Blocking vs. Crack-Neutralizing: Fracture Mechanics of Soft Polymer Networks

This talk presents our recent efforts to elucidate the nonlinear mechanics and fracture behaviors of soft polymer networks, focusing on two fundametally distinct defect-response mechanisms: “crack-blocking” in strain-induced crystallizing (SIC) elastomers and “crack-neutralizing” in liquid crystal elastomers (LCEs). By integrating digital image correlation, high-speed thermography, custom-made biaxial tensile testing, and micro-beam scanning WAXS, we quantify the coupled fields of strain, stress, and crystallinity around cracks and circular defects in natural rubber (NR). In SIC elastomers, the development of a highly localized crystallization zone near a crack tip acts as an efficient crack-blocking mechanism, supressessing crack growth through localized hardening. This approach allows us to quantify the correlations among these fields in the near-tip region.[1,2] Extending this methodology to nano-filler-reinforced NR reveals how filler introduction modulates SIC and enhances the crack-blocking performance.[3] Moreover, stretching NR sheets with circular defects yields, within a single experiment, a spectrum of deformation states—from uniaxial to biaxial—enabling us to derive a crystallinity relationship expressed as a function of two orthogonal strains.[4] Notably, the resulting trend contrasts with expectations from classica SIC theory.

We further investigate LCEs, whose in-plane liquid-like elasticity—arising from the coupling of LC alignment and rubber elasticity—becomes evident under independent biaxial stretching.[5] When stetched normal to an initial notch, LCEs show a strikigly different fracture response.[6] At small stretches, strain localizes sharply near the crack tip. Once this localized strain reaches a characteristic saturation level, further stretching increases only the far-field strain. The material ultimately develops an almost uniform strain field, effectively neutralizing the mechanical influence of the defect. This crack-neutralizing behavior markedly delays macroscopic fracture and represents a distinct fracture mechanism rooted in liquid-like elastic response.