Direct visualization of the molecular damage precursors to crack nucleation*

Loading a solid with a subcritical stress can lead to sudden fracture after a long period of seemingly quiescent stability. Contrasting hypotheses exist to explain delayed brittle fracture, yet to date, unambiguous experimental proof of the mechanisms underlying this unpredictable mode of failure remains absent. To elucidate the damage processes that precipitate crack nucleation, we use quantitative micromechanical mapping at the site of crack nucleation within an elastomer by means of Laser Speckle Imaging. This allows us to visualize the damage processes in the moments prior to crack nucleation. We find that the local rigidity of the rubber gradually degrades and becomes zero at the exact moment when a macroscopic crack becomes visible. This pre-fracture damage zone grows in space and time along a self-catalytic pathway, exhibiting signs of a critical transition. Our results paint a microscopic picture of the elusive origins of delayed fracture through damage accumulation.