Fracture stresses in soda‐lime glass, a glass‐ceramic, and an alumina ceramic tested in biaxial flexure correlated with the crack lengths prior to branching. The resultant branching constant for the glass agreed closely with the mirror‐mist boundary constant, and evidence indicated agreement between the branching constants and the inner‐mirror boundary constants for the polycrystalline materials. The ratio of the branching constant to the critical stress intensity factor was much smaller for the polycrystalline materials than for the glass. The crack‐branching measurements also indicated the presence of a compressive residual stress as Sociated with the ground sufaces of the