Like crack initiation, the extent o

Like crack initiation, the extent of crack propagation increases with increasing |dT/d(fS1/2)| near (fS)1/2 = 1. First, as already mentioned, the liquid channel along the grain boundary is longer with a higher |dT/d(fS1/2)| near (fS)1/2 = 1, which may act as a sharper crack to promote crack propagation. Second, suppose a crack already exists near (fS)1/2 = 1. Let the bottom surface of X at z inFig. 4b be the tip of the crack. Increasing |dT/d(fS1/2)| near (fS)1/2 = 1 decreases the RHS of Eq. (10) to make it easier to exceed and thus cause a void to form in X, that is, easier for the crack to propagate into X. The crack length, which increases with increasing extent of crack propagation, is often used as the index for the crack susceptibility in crack-susceptibility tests. Thus, the crack susceptibility can be expected to increase with increasing |dT/d(fS1/2)| near (fS)1/2 = 1.