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I. INTRODUCTION

FATIGUE in engineering structures involves crack-initiation and growth processes. Current life-prediction approaches for military gas turbines address both crack initiation and propagation lives. Typically, a gas-turbine component is designed to have (1) a minimum low-cycle fatigue (LCF) crack-initiation life exceeding the total specified service life and (2) a crack-propagation life from an assumed initial inspection that is twice the number of service cycles between inspections.[3] According to Cowles,[3] the current life-prediction systems are expensive to establish and substantiate, because large experimental databases are required. The current life-prediction systems may be improved by development of better crack-initiation-life models, probabilistic treatment of the variability of crack-initiation life, and enhanced local notch analysis capability.[3] One of the limitations of current crack-initiation-life models is that they are incapable of predicting the crack size at fatigue-crack initiation. Consequently, an initial crack size must be assumed in the prediction of crack-growth life. Recent advances in materials processing have facilitated the design and manufacture of components with tailored microstructures at desired locations. For example, a coarsegrained microstructure may be placed in regions where creep resistance is desired, while a fine-grained microstructure may be introduced in regions where fatigue-crack-initiation resistance is needed. This design approach requires precise control of the microstructure during processing as well as detailed knowledge of the microstructure/property relationship, so that optimization of microstructures to achieve a balance of material properties can be executed in an efficient and effective manner. In particular, the time to material development can be significantly reduced if computer-based methods can be implemented to reduce the amount of iterations required to develop the desired microstructures and properties for the intended applications. For computer-based design of damage-tolerant materials and structures, microstructure-based models of fatigue-crack initiation and growth are required. These models would also be essential KWAI S. CHAN, Institute Scientist, is with the Southwest Research Institute, San Antonio, TX 78238. Contact e-mail: [email protected] Manuscript submitted November 9, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A

for the development of probabilistic life-prediction methods, since the tail ends of the defect distribution, which are often related to the microstructural-size distribution, mostly influence fatigue life. Prior reviews established the important roles of microstructure in fatigue-crack initiation and growth in metals. The need to introduce a microstructural-size parameter in a fatigue-crack-growth model was identified in a review article by Bailon and Antolovich,[4] which compared fatigue-crackgrowth models against experimental data available at that time. A microstructure-based fatigue-crack-growth model for la