Stress amplification and relaxation imaging around cracks in nanocomposite gels using ultrasound elastography.

The quantification and modeling of gel fracture under large strain and dissipative conditions is still an open issue. In this study, a novel method for investigating the mechanical behavior of gels under highly deformed states, specifically in the vicinity of the crack tip, was developed to gain insights into fracture processes. Shear wave elastography, originally developed for the biomedical community, is employed as a powerful tool to quantitatively map the local elasticity of model gels. Here, the local stress is experimentally measured from the shear wave velocity according to nonlinear acoustoelasticity theory. The stress concentration observed at the crack tip in elastic gels is validated using classical finite element methods. Subsequently, the mechanisms of network rearrangements in viscoelastic gels (with silica nanoparticles) are analyzed both spatially and temporally. These gels consist of 90 wt% water and are synthesized with sticky nanoparticles to introduce exchangeable sacrificial bonds that facilitate stress relaxation. The nanoparticles efficiently provide stress relaxation around the crack tip, mitigating a stress singularity. The amplitude of stress relaxation was measured quantitatively and appears to be higher closer to the crack. This paper showcases the feasibility and potential of a new experimental approach that enables non-invasive and dynamic mapping of gel fracture mechanics.