Fig. 4. Fatigue behavior of PA gels with different mesoscale phase contrasts.

(A and B) Crack propagation rate Δc/ΔN in steady state as a function of λ_max for sample series PA-2.5-C_MBAA (A) and PA-C_m-0.1 (B). The numbers beside each curve in (A) represent C_MBAA (mol %). The dashed lines are a guide for the eyes. The arrows in (A) and (B) indicate the transition points of slow-to-fast fatigue crack growth. (C and D) Crack propagation rate Δc/ΔN as a function of energy release rate G corresponding to (A) and (B), respectively. Inset shows the initial linear regime in the linear scale. Symbols in (C) are the same as those in (A). (E) Energy release rate at transition points of slow-to-fast fatigue crack growth (G_tran) (left y axis) and the threshold G₀ (right y axis) extracted from (C) and (D) versus I_m for the three series of samples. (F) Correlation between λ_tran for slow-to-fast mode transition in the fatigue test and the maximum stretch ratio λ_affine for affine deformation of the phase structure in the tensile test for the three series of samples. The data of series PA-2.0-0.1(T_dial) are extracted from (25). The error bars are SEs from two to four measurements. Note that in these PA gels, G₀ and λ_tran are characteristic material parameters, while G_tran is geometry dependent (see Fig. 5). The measurements were performed with samples of pure shear geometry (L₀ = 50 mm, H₀ = 10 mm, and c₀ = 10 mm) and nominal strain rate 1 s⁻¹.