Anomalous fracture behavior of soft layered materials

Soft layered material systems are ubiquitous in nature and engineering – from natural biological tissues to engineered devices. However, from the mechanics perspective, why soft layered materials are favored by natural selection and engineering design remains largely unexplored. Here we study the fracture mechanics of heterogeneous soft layered materials. We develop a theoretical framework for analyzing the co-evolving channel cracking and interfacial delamination in soft film/substrate systems, which is then applied on investigating the fracture of bilayer hydrogels – a representative soft layered material. Through both experiment investigation, theoretical analysis and numerical modeling, we find that the heterogeneous soft layered materials exhibit anomalous size-independent fracture behaviors with fracture strains independent of the flaw size and overall structural dimensions, in stark contrast to ordinary homogeneous materials whose stretch at break reduces undesirably with increasing flaw size and structural dimensions. The size-independent fracture behavior leads to notable toughening of soft layered materials. The findings hold for a broad range of hyperelastic soft materials, from biological materials to hydrogels and elastomers, opening potentially new avenues for the development of fracture-resistant soft materials, and motivating new investigations of the development and applications of heterogeneous soft materials.