Growth of small fatigue cracks in PH 13-8 Mo stainless steel
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I. INTRODUCTION
INTEREST in the fatigue crack propagation of small cracks developed approximately 20 years ago when it was found that these cracks propagated at greater rates and threshold levels below those determined by long crack experiments and prediction tools. In the last 10 years, much research has been dedicated to the evaluation of progressively smaller cracks (down to the order of the grain size) culminating in the development of stochastic and deterministic models to characterize the anomalous small crack effect. Much of this attention has been focused on small crack growth under low cycle fatigue, whereas research in the area of high cycle fatigue is comparatively limited. This is most probably due to the tedious and time-consuming nature of monitoring small crack growth under high cycle fatigue. It is an unfortunate consequence in light of the fact that most of the fatigue life of structures subjected to high cycle fatigue is spent in the small crack regime. Microstructurally small cracks, as the name suggests, are subject to strong influence of the material microstructure. Variables such as grain size and orientation, distribution of second phases and inclusions, and heterogeneity of local cyclic slip processes can be expected to play a large role in the crack propagation. The initial stage of crack growth is often crystallographic and is commonly termed stage I growth. It is shear dominated as the microcracks are roughly aligned with the orientation of the maximum shear planes. Eventually, the crack becomes insensitive to the microstructural features and transitions to stage II growth. In stage II of crack growth, propagation proceeds in a direction normal to the maximum principal stress range. Fatigue resistance of a material can be related to the difficulty it provides for a stage I shear crack becoming a stage II tensile crack.[1] In A.M. PATEL, formerly Graduate Research Assistant, The George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, is Consultant with Arthur D. Little, Inc., Cambridge, MA 021402390. R.W. NEU, Assistant Professor, and J.A. PAPE, Graduate Research Assistant, are with The George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0405. Manuscript submitted May 5, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
low amplitude, high cycle fatigue, the transition from stage I to stage II is often rapid. The transition is typically observed for crack lengths of 1 to 4 grain diameters.[2] Microstructurally small cracks have been shown to exhibit an oscillatory crack growth rate with respect to crack length. The acceleration and deceleration are associated with interactions with microstructural barriers. If the crack driving force is not sufficient to overcome the largest barrier, a fatigue limit will be realized. If the driving force is sufficient and the crack lengthens, the oscillatory behavior will diminish with each successive barrier and eventually the crack growth can be well characterized with con
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