A mechamcal model of fatigue crack propaganon was studied. It is hypothestzed that propagation is caused by cumulative damage due to strata cycling of the material at a crack tip. With Miner's cumulative damage law and Manson and CofI'm's strain cycle fatigue law, crack propagation rate of a material can be calculated. The calculated values are compared with data on 7075-T6 Al, 2024-T4 A1, AM 350 stainless steel, and 18 Ni maraging steel. The calculated and experimental values correlate reasonably well. The results indicate that fatigue crack propagation resistance of a material as related to its capability to sustain strain cycling, ~.e. its cyclic ductility.

Based on the elastic interaction between a solute atom and a tensile crack-tip stress field, a mechanism of stress-corrosion cracking was proposed and analyzed. This elastic interaction provides a potential for solute atoms to migrate toward the tip of a crack. The elastic interaction and the equilibrium concentration of solute atoms near a crack tip were calculated. The solute atom concentration increases rapidly toward the crack tip if the solute atom is interstitial or if it relaxes the crack-tip stress field. The high concentration of solute atoms at the tip of a crack will enhance the reaction between solute and solvent atoms. The weak fracture strength of the reaction product may cause crack growth. Two crack growth models were analyzed: One is based on the assumption of the “homogeneity” of the fracture and deformation properties of a material, and the other takes a structural size of a material into consideration. The proposed models are compared with available data on magnesium-aluminum alloy, 4340 steel, and soda-lime glass.

A fatigue crack is often initiated by a localized cyclic plastic deformation in a crystal where the active slip plane coincides with the plane of maximum shear stress. Once a crack is initiated, the crack will propagate on the maximum shear plane for a while and, in the majority of the cases, will eventually change to the plane of the applied tensile stress.The "shear" and "tensile" modes of fatigue crack propagation are termed stage I and stage I1 fatigue crack growth. They are also known as mode I1 and mode I fatigue crack growth. However, the mechanism of the tensile mode fatigue crack propagation is shear in nature.Considerable progress has been made recently in the understanding of mode I1 fatigue crack growth. This paper reviews the various test methods and related data analyses.The combined mode I and mode I1 elastic crack tip stress field is reviewed. The development and the design of the compact shear specimen are described and the results of fatigue crack growth tests using the compact shear specimens are reviewed. The fatigue crack growth tests and the results of inclined cracks in tensile panels, center cracks in plates under biaxial loading, cracked beam specimens with combined bending and shear loading, center cracked panels and the double edge cracked plates under cyclic shear loading are reviewed and analyzed in detail.

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The Griffith energy criterion is equivalent to a critical stress and strain environment criterion for brittle fractures. For quasi-brittle fracture, i.e. the case where small amounts of plastic deformation at the crack tip precedes fracture, the energy criterion is still equivalent to stress and strain environment criterion. Based on the concept of a critical stress and strain eavironrnent, it is proposed as an engineering criterion for ductile fracture, that at fracture the size of the highly strained region at crack tip is constant.