The crack-tip mechanics and growth rates of small fatigue cracks in ASTROLOY
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I.
INTRODUCTION
SMALL fatigue cracks in ASTROLOY* grow at rates *ASTROLOY is a trademark of General Electric Company, Fairfield, CT.
in excess of those expected from extrapolation of results from large cracks, as shown by a number of investigations.[~ 5i This phenomenon is similar to the growth of small cracks in aluminumI61 and titanium alloystT] and in steels, tSl In many of these alloys, grain size has been found to influence some aspects of fatigue crack initiation and growth; however, small cracks also grow faster in single crystals of ASTROLOY. tgI It has even been shown that crack growth rates of physically short (through the thickness) cracks are higher than large cracksY ~ There has been considerable speculation about the reasons for the unexpectedly high growth rates for small cracks and the effects of grain size, but the mechanisms of fatigue crack growth from a fundamental viewpoint are still under investigation. In Reference 3, some of the many reasons for differences in behavior between small and large crack behavior were explored in detail, and some agreement was found in the articles that (1) fatigue crack closure differences between small and large cracks were important and (2) that closure alone could not provide a satisfactory explanation for the differences in the crack growth rates observed. The objective of the work reported here has been to ex-
D.L. DAVIDSON, Institute Scientist, and S.J. HUDAK, Jr., Department Director, are with the Materials and Structures Division, Southwest Research Institute, San Antonio, TX 78228. Manuscript submitted February 7, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
amine small fatigue cracks in ASTROLOY as they grew, using new tools for observationt11,12] and measurementY 31 From this information, we have sought to develop a more complete understanding of small crack mechanics,t14] examine many of the hypotheses proposed previously for small crack behavior, and devise a method for predicting small crack growth rates under constant amplitude loading for use in engineering damage tolerance assessments where stress and crack size are known. II.
EXPERIMENTAL TECHNIQUE
Naturally initiated fatigue cracks were chosen for study in this work, because no technique exists for artificially introducing small cracks reliably, and cracks initiated at notches are often strongly influenced in unpredictable ways by the stress concentration of the notch. All experimental work, therefore, began with the difficult task of initiating and finding fatigue cracks that were physically very small. Coarse- and fine-grained ASTROLOY were chosen for this study because these materials are currently used in gas turbine engines. The material was furnished by Rolls Royce, plc, Derby, U.K., who cut specimen blanks from forgings similar to those used in turbine disks. Information on composition, thermal and mechanical history used during manufacture, grain size, and tensile and cyclic properties is listed in Table I and given in Reference 14. The material was strengthened by the 7' pha
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