Fatigue crack path prediction in UDIMET 720 nickel-based alloy single crystals
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I. INTRODUCTION
NICKEL-BASED superalloys are used in both turbine disk and blade assemblies due to their excellent high-temperature properties. Turbine blades require particularly good creep and creep-fatigue resistance and are often produced in a single-crystal form containing a high volume fraction of strengthening g8 precipitate. Turbine disks, however, experience lower temperatures but higher stresses, have a lower volume fraction of g8 and are typically produced in a polycrystalline form. The early stages of small fatigue-crack formation (the stage I crack regime) take up much of the fatigue life of highly stressed components such as aeroengine turbine disks. The short fatigue-crack growth behavior of many materials, including turbine disk alloys, has received considerable attention.[1–4] It is now well established that such cracks grow principally in stage I (particularly at lower temperatures) at higher crack-growth rates than larger defects tested under the same nominal DK conditions, exhibit crack growth at DK values below the long crack DKth value, and furthermore exhibit considerable scatter in growth rates. Such behavior poses problems in adopting appropriate lifing procedures for such safety-critical components as turbine disks. These apparently anomalous behaviors are generally attributed to a number of factors: (1) the short length of the crack compared with the scale of crack-tip plasticity can render the LEFM (linear elastic fracture mechanics) assumption of similitude invalid, i.e., DK does not adequately characterize the crack-tip stress state for all small defects; (2) shorter cracks may have insufficient crack wake to develop closure levels equivalent to those found for longer defects; and (3) P.A.S. REED, Senior Lecturer, and I. SINCLAIR, Lecturer, are with the Materials Group, School of Engineering Sciences, University of Southampton, Southampton SO17 1BJ, United Kingdom. X.D. WU, formerly Research Assistant, Materials Group, School of Engineering Sciences, University of Southampton, is Research Engineer, British Aerospace, Bristol, United Kingdom BS99 7AR. Manuscript submitted July 29, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
when cracks are of the order of local microstructural elements in scale, growth may be highly sensitive to local microstructural features, unlike the broad averaging effect of a macroscopic crack front. The microstructural sensitivity of the crack growth poses particular problems in quantifying crack-growth behavior; in addition, the mechanism of crack growth (stage I) differs from typical long-crack behavior (stage II), so the intrinsic crack-propagation rate itself may be different. Additional extrinsic factors will also affect the growth rate: growth is dependent on the orientation of a particular grain, the nature of the grain (or other microstructural unit) boundary, and the change in angle as the crack crosses into a neighboring grain with a differing orientation. All these factors may combine to obscure the intrinsic crackgrowth processes that are occur
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