Problems in the derivation process of Griffith's equation on the critical condition of crack propagation are discussed, and new equations are derived according to his energy balance criterion.
One of the typical equation which has similar expression to that of Griffith's is under plane stress condition.σ0=2(Eγs/l)1/2, where, σ0 is critical tensile stress for crack propagation, i.e. static fatigue limit and E is Young's modulus, γs surface free energy and l specimen length.
Assumptions used in the derivation of the above equation are, (1) crack extends in a plane normal to the stress axis, (2) specimen has a uniform cross-sectional area “aΤ”; along the stress axis, and (3) the area of inherent crack “;a”; satisfies a condition a<<aΤ. Under these assumptions, existence of the small crack (a<<aΤ) causes little change in the critical stress σ0.
