Recent Advances in Studying Fracture Mechanisms and Fracture Toughness of Double-Network Hydrogels

Prof Zishun Liu

City University of Hong Kong (Dongguan)

Abstract: Double-network (DN) hydrogels are renowned for their exceptional fracture toughness, attributed to their unique interpenetrating network structures. However, fully understanding the mechanisms underlying their fracture behavior and toughening processes necessitates advanced computational, theoretical and experimental approaches. This work integrates computational modeling, network mechanics, and experimental validation to elucidate the interplay between microstructure, energy dissipation, and fracture behavior in DN hydrogels. A mesoscopic network mechanics model is developed, incorporating stretch-based criteria to govern polymer chain fracture. This enables the prediction of ductile-to-brittle transitions in the DN hydrogels. Numerical simulations reveal that stretch ratios induce necking and hardening effects, while uniform stress distribution within the first network enhances toughness. Theoretical analysis of fracture toughness is performed using uniaxial tensile and tearing tests, partitioning apparent toughness into intrinsic and dissipative components. An exponential function is introduced to correlate fracture energy with specimen free width, addressing inconsistencies in crack-tip field characterization. Multi-scale modeling bridges molecular dynamics and finite element methods to simulate hyperelasticity, large deformations of DN hydrogels, with results validated against experimental swelling and phase transition behaviors. This study establishes a unified computational-theoretical-experimental framework, linking microstructural interactions-such as network stretch criteria and cross-linking density-to macroscopic toughness properties. The findings contribute to the development of predictive models for DN hydrogel design, offering valuable insights for applications in soft smart fatigue-resistant materials. This work also lays the foundation for next-generation hydrogels with tailored fracture resistance.

Bio: Dr. Zishun Liu is a Chair Professor and Vice-President (Research & Innovation) at City University of Hong Kong (Dongguan) and visiting Chair Professor at Xi’an Jiaotong University (XJTU). Dr. Liu received B.Eng., M.Eng., PhD degrees in Applied Mechanics from XJTU and the National University of Singapore (NUS) respectively. He is also the General Secretary of Int. Association of Applied Mechanics (IAAM) and the Honorary President of Singapore Association of Computational Mechanics. He is IAAM fellow and SACM fellow. His research interests are in the areas of Mechanics of Soft Materials, Computational Solid Mechanics & Biomechanics, Nanomechanics, Vibro-Acoustic. He has published more than 230 SCI research papers. Dr Liu is an active member of various leadership roles in editorial boards and professional communities as follow: Dr. Liu is an Editor-In-Chief of Int. Journal of Applied Mechanics and Int. Journal of Computational Materials Science and Engineering, and Editor of Journal of Mechanics of Material and Structures, Associate Editor of Journal of Applied and Computational Mechanics. He also serves on the editorial boards of Int. Journal of Computational Methods, Int. Journal of Structural Stability and Dynamics, Acta Mechanica Sinica, Acta Mechanica Solida Sinica. Dr Liu is a General Member of the International Association for Computational Mechanics (IACM). As a Chairman, Dr. Liu has organized more than 20 Int. Conferences in the field of applied & computational mechanics.