Toughening mechanism of macroscale heterogeneous soft materials: a systematic study from the perspective of energy release rate

Abstract

Due to their enhanced fracture and fatigue resistance, macroscale heterogeneous soft materials consisting of alternating hard/soft phases have become increasingly prevalent in a wide range of applications. Despite their widespread use and apparent advantages, a comprehensive and precise understanding of the toughening mechanisms behind them remains elusive. Here, we systematically study the fracture mechanics of hyperelastic bi-material sheets (i.e., the basic building block of macroscale heterogeneous soft materials) through a combination of experiments, numerical calculation, and analytical analysis. First, employing heterogeneous PAAm hydrogels as a model system, we experimentally observe the hindering effect of the bi-material interface on the crack propagation from the soft phase into the hard phase, and identify the toughening rule of macroscale heterogeneous soft materials – the increase in the shear modulus contrast between the soft and hard phases leads to a substantial enhancement in the stretch at break of pre-cut heterogeneous soft materials. Second, through finite element calculations, we uncover the toughening mechanism of macroscale heterogeneous soft materials from the perspective of energy release rate: when the soft-phase crack propagates close to the soft-hard interface, the energy release rate rapidly plummets, and the reduction in the energy release rate is more obvious with the increase of the shear modulus contrast between the soft and hard phases, which plays a pivotal role in toughening heterogeneous soft materials. Lastly, an analytical fracture theory from the perspective of crack-tip deformation is derived for macroscale heterogeneous soft materials. By comparing the measured/calculated stretches at break from experiments, finite element calculation, and the analytical model, we reveal the precision, advantage and depth of understanding the toughening mechanism of macroscale heterogeneous soft materials from the perspective of energy release rate. The findings are applicable to a wide variety of hyperelastic soft materials, including biological materials, hydrogels and elastomers, offering valuable insights into the design of heterogeneous soft materials with superior mechanical properties.