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(a) Displacement needed to drive crack propagation Δs as a function of contact radius c, when h/a = 0.1. Data are normalized by Δc, i.e. the displacement needed to initiate crack propagation. (b) Force needed to drive crack propagation, Fs as a function of normalized contact radius c/a when h/a = 0.1. The force is normalized by its value when c = a, i.e. Fc. The decrease in Fs as crack advances indicates that crack growth is unstable under force control. (Online version in colour.)
