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I.

INTRODUCTION

MICROSTRUCTURE is now known to affect both the initiation and propagation of fatigue cracks, but work on a variety of metals and alloys has shown that microstructural conditions resisting crack nucleation are not necessarily beneficial to and in some cases are deleterious to fatigue crack propagation resistance) From a material selection viewpoint it therefore is crucial to understand the contributions of microstructural parameters separately to fatigue crack initiation and propagation behavior. This is particularly true for design approaches which typically either assume initiation to govern component life or which assume preexisting flaws, so that fatigue crack growth, especially near threshold, is important to life prediction. Investigations of the effect of metallurgical variables on fatigue crack initiation have identified grain size, yield strength, and stacking fault energy to be of major importance. Grain size reductions have been seen to lengthen the constant-load life of stainless steel,2 brass) '4 other copper alloys,4 low carbon steels, 5 9 aluminum,~O,Hand titanium. ~2-~7 In other research the fatigue lives of copper and aluminum were found to be "almost unaffected by grain size". 3'4 Fatigue limits have been directly correlated in many alloys to yield strength, with higher endurance limits accompanying higher yield strengths) s-24 In agreement with the observation that fatigue strength is directly proportional to the difficulty of dislocation cross-slip, 4'2s it has been observed that higher fatigue resistance can be achieved by homogenizing slip deformation, thus avoiding local concentrations of plastic deformation. In alpha-brass (a difficult cross-slip material) decreasing grain size acted to increase fatigue life while in copper and aluminum (in which cross-slip is easy) fatigue life was insensitive to grain size.4 The aim of the present study was to characterize the influence of microstructure on the fatigue crack initiation of fully pearlitic steels. The independent variation of prior austenite grain size, pearlite colony size, and pearlite interlamellar spacing provided the opportunity to independently G.T. GRAY, III, formerly with Carnegie-Mellon University, is now at the Los Alamos Scientific Laboratory, Los Alamos, NM. A.W. THOMPSON and J. C. WlLL1AMS are, respectively, Professor of Metallurgical Engineering and Materials Science and Dean of Engineering, Carnegie-Mellon University, Pittsburgh, PA 15213. Manuscript submitted November 28, 1983. METALLURGICAL TRANSACTIONS A

vary grain size and yield strength. 26 The role of pearlitic microstructure on the notched and smooth-bar crack initiation is discussed in terms of initiation mechanisms and specimen geometry.

II.

EXPERIMENTAL PROCEDURE

The material used for this investigation was the same hot rolled rail steel used in our earlier work. 27,28 It was made by U.S. Steel and supplied by the Association of American Railroads. From this material, standard tensile (ASTM E8) and fatigue crack initiation specimens were cut from