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using the equation σ =K/(Y √π a) ; a=1/π (K/Yσ)^2. The critical stress required for initiating crack propagation is σ, plain strain fracture toughness is K, surface length of the crack is a, and dimensionless parameter us Y.Substituting the given parameters to the equation. 82.4 MPa √m for K, 345 MPa for σ, and 1 for Y in the equation of surface length of the crack.a = 1/π (K / Yσ )^2 = 1/π (82.4 / 1*345 )^2 = 0.01815 m=18.15mm...