Fatigue crack growth assessment for life prolongation of thick section components

For much of the allowable range of design, stress intensity-life based fatigue assessment, according to present ASME-Code (Sections III and VIII Div 2) life estimates, are expected to be conservative for non corrosive applications - notably for constant amplitude design cycles above 500. The conservatism stems from the design curve basis of testing small specimens, for which the crack growth portion of fatigue life may be small. For initiation lives less than 500 cycles, it has been pointed out that the design curves may be nonconservative because they exclude constraint or strain state effects on material deformation capacity. In this paper, fatigue crack growth from an initial flaw, 0.1 inch in depth, was modeled using the Paris law, and ΔK modified to include stress ratio, R, dependency. The initial crack size used for fatigue life estimation was based on the sensitivity of the nondestructive examination (NDE) detection methods. The total calculated cyclic endurance was found to be higher than the original ASME design life permitting life extension to be based on a total life approach. However, a suitable recurrent inspection schedule must be established for its safe management. A case study is presented to demonstrate the potential of life extension, inclusive of both crack initiation, and crack propagation based on fatigue crack growth analysis.