Fig. 4: Subcritical crack front evolution.
From: Complexity of crack front geometry enhances toughness of brittle solids
a, The crack front (marked by dots) is not static, but develops as the applied strain increases from the upper to the lower panel. b, The coloured curves trace the crack tip at incremental applied strains 1–6 corresponding to the colour code in a and c. Curves are registered as described in Methods. c, The applied strain energy during the loading process is plotted as a function of \(\tilde{{{{\mathcal{L}}}}}\). Matching the colour code in panel b, there is little variation in \(\tilde{{{{\mathcal{L}}}}}\) during the initial loading process. Until reaching the critical value Gc, \(\tilde{{{{\mathcal{L}}}}}\) increases to keep the globally critical state. A completely independent sample is shown in yellow points, which reaches the same trend. The critical strain energy boundary at which all cracks are observed to propagate seems to be a universal bound, as it is not exceeded during the loading process in the intermediate or the final applied strain increments. Error bars are defined from n = 31 segmentations for \(\tilde{{{{\mathcal{L}}}}}\) and n = 50 samples of Gc. Sample number for Gc is provided in the data repository48. Data are presented as mean values ± s.d.
