Patterns on broken surfaces are well-known from everyday experience, butsurprisingly, how and why they form are very much open questions. Well-definedfacets are commonly observed1-4 along fracture surfaces which are created byslow tensile cracks. As facets appear in amorphous materials5-7, theirformation does not reflect microscopic order. Fracture mechanics, however,predict that slow crack fronts should be straight, creating mirror-likesurfaces8-13. In contrast, facet-forming fronts propagate simultaneously withindifferent planes separated by steps. It is therefore unclear why steps arestable, what determines their path and how they couple to crack front dynamics.Here we show, by integrating real-time imaging of propagating crack fronts withsurface measurements, that steps are topological defects of crack fronts; crackfront separation into discontinuous overlapping segments provides the conditionfor step stability. Steps drift at a constant angle to the local frontpropagation direction and the increased local dissipation due to step formationcouples to the long-range deformation of the surrounding crack fronts. Slowcrack front dynamics are enslaved to changes in step heights and positions.These observations show how 3D topology couples to 2D fracture dynamics toprovide a fundamental picture of how patterned surfaces are generated.
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