Effects of crack aspect ratio on the behavior of small surface cracks in fatigue: Part II. Experiments on a titanium (Ti
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TRODUCTION
THE growth of a small crack occupies a significant part of the total fatigue life of many engineering components and structures. Extensive research[1–10] indicates that small cracks propagate faster than large cracks under equivalent stress intensity factor ranges (DK), and this characteristic is often referred to as ‘‘anomalous behavior.’’ This nature of small cracks is suggested to be due to low crack closure levels due to the lack of a fully developed crack wake,[11,12,13] large plastic zones at the tips cracks,[14,15] and microstructural factors such as grain size, slip behavior, and crystallography.[16,17,18] Additionally, it was suggested[19,20] that conventional methods of analysis of small crack data, employing secant or polynomial data reduction, could also lead to this behavior. However, an important parameter describing the shape of small surface cracks, namely, the aspect ratio, a/c (a is the crack depth and c is the half-surface length), has long been ignored in most of the investigations. The shapes of small cracks were often approximated to that of a semicircular crack (a/c 5 1), although significant deviations from this shape were reported.[16,18] The anomalous behavior of small cracks could also be partly due to such K.S. RAVICHANDRAN, Assistant Professor, is with the Department of Metallurgical Engineering, The University of Utah, Salt Lake City, UT 84112. J.M. LARSEN, Research Group Leader, is with the Materials Directorate, Wright Laboratory (WL/MLLN), Wright Patterson Air Force Base, OH 45433-7817. Manuscript submitted March 20, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
assumptions, in situations of large variations in aspect ratio, especially during the early stages of small-crack growth. This aspect was investigated using theoretical simulations of crack growth in the accompanying Part I of the research.[21] It was found that assumptions of a constant aspect ratio (a/c 5 1), at small crack sizes of the order of a few grain diameters, would incur significant errors in DK calculations, leading to scatter in small-crack growth data. We are interested in verifying the results of the simulation as well as in generating experimental data in order to assess the implications of incorporation of actual variations in a/c on small-crack behavior. Such a study has not been undertaken before, primarily due to the experimental difficulty involved in measuring continuous changes in aspect ratio during fatigue. In addition, shapes of small cracks are often irregular, leading to ambiguity in the definition of crack aspect ratio for DK calculations. Nevertheless, a clear understanding on the effects of aspect ratio on small-crack growth is needed to resolve the issue of anomalous behavior. Toward this objective, an experimental approach previously developed by the authors[22–25] has been used in this study to examine the effects of variations in aspect ratio during small-crack growth. This technique involves determination of continuous variations in aspect ratio using crack compliance and sur
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