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Topological defects govern crack front motion and facet formation on broken surfaces


Cracks develop intricate patterns on the surfaces that they create. As faceted1,2 fracture surfaces are commonly formed by slow tensile cracks in both crystalline and amorphous materials3,4,5, facet formation and structure cannot reflect microscopic order. Although fracture mechanics predict that slow crack fronts should be straight and form mirror-like surfaces6,7,8,9, facet-forming fronts propagate simultaneously within different planes separated by steps. Here we show that these steps are topological defects of crack fronts and that crack front separation into disconnected overlapping segments provides the condition for step stability. Real-time imaging of propagating crack fronts combined with surface measurements shows that crack dynamics are governed by localized steps that drift at a constant angle to the local front propagation direction while their increased dissipation couples to long-ranged elasticity to determine front shapes. We study how three-dimensional topology couples to two-dimensional fracture dynamics to provide a fundamental picture of how patterned surfaces are generated.

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Figure 1: Fracture surface patterns in brittle polyacrylamide gels.
Figure 2: Image sequences of facet-forming crack fronts and the resulting surface patterns.
Figure 3: Step-lines propagate at a constant angle to the local front normal in the crack frame.
Figure 4: Local dissipation at a step determines the long-range shape of the front.


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J.F. and I.K. acknowledge the support of the Israel Science Foundation (grant no.1523/15), as well as the US-Israel Bi-national Science Foundation (grant no. 2016950). I.K. thanks I. Svetlizky and E. Katzav for fruitful discussions about step stability. I.K. is grateful to P. M. Chaikin for an enlightening conversation on the complexity of fracture surfaces.

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Authors and Affiliations



I.K. and G.C. designed the gel preparation method and fracture experiments. I.K. synthesized the gel samples, performed the fracture experiments and surface profilometry, and analysed data. J.F. conceived the 3D crack front imaging, and initiated and supervised the research. The manuscript was written by all authors.

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Correspondence to Jay Fineberg.

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The authors declare no competing financial interests.

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Kolvin, I., Cohen, G. & Fineberg, J. Topological defects govern crack front motion and facet formation on broken surfaces. Nat. Mater. 17, 140–144 (2018).

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