The applied strain energy during the loading process is plotted as a function of L. Matching the colour code in panel b, there is little variation in L during the initial loading process. Until reaching the critical value Gc, 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 L and n = 50 samples of Gc. Sample number for Gc is provided in the data repository 48. Data are presented as mean values s.d.
