For various materials, the normalized critical strain energy release rate ( G c − Γ pz )/ Γ * increases in proportion with the crack tip complexity, measured by the normalized geodesic crack length L . The square and the inverted triangle for gel 1 represent sample thicknesses of 100 μm and 380 μm, respectively. The linear proportionality is explained by the energy partition depicted schematically in the inset. The strain energy release rate is partitioned between bond scission at the crack tip, Γ *, which scales with L , and a constant process zone fracture energy Γ pz . The evaluation of Γ * and Γ pz is described in Methods . Error bars are defined from n = 31 segmentations for L . The sample number for G c is provided in the data repository 48 . Better segmentation achieved with the embedded fluorophore in gels 2–4 and the PDMS generates error bars comparable to the symbol size for L . Data are presented as mean values ± s.d. Due to the large CTOD, a tile acquisition was required for gels 2–4 and the PDMS sample; thus, no error bars are reported for these data.
