Analysis of crack tip sliding displacement in anisotropic elastic media and its application to stage I fatigue crack gro
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I.
INTRODUCTION
THE use of single-crystal and directionally solidified Ni-base superalloys for turbine blade and vane applications has enabled significantly increased turbine inlet temperatures, u-6] This has been due, in part, to the improved creep rupture properties of these materials, which lack the weakening grain boundaries of polycrystalline castings. Because superior fatigue strength is also required for turbine blade and other applications, high- and low-cycle fatigue fracture behaviors have been also studied. 0-6] Summarizing the fatigue fracture characteristics in single-crystal and directionally solidified Ni-base superalloys, the crystallographic slip planes play an important role, and they exhibit the typical features of stage I fatigue fracture; the fatigue fracture occurs on {111} planes at lower and intermediate temperatures. 0-6] At higher temperature, on the other hand, it occurs on {100} planes, t3,s'6] It is also reported that the magnitude of stress intensity factor also affects the crystallographic fracture plane. [6] In order to investigate the fatigue fracture in single-crystal and directionally solidified Ni-base superalloys, it is inevitable to evaluate the fracture mechanics parameters, such as crack tip sliding displacement (CTSD), taking into consideration elastic anisotropy, because their elastic and mechanical properties are highly anisotropic. [41 On the other hand, in recent years, it has been apparent that small fatigue crack growth behavior in polycrystalline engineering materials may not conform to that measured conventionally with long crack, where cracks are small, or they approach the size of the microstructurally important features such as grain boundaries. [7-131 In fact, it is well known that small fatigue crack
M. OKAZAKI, Associate Professor, is with the Department of Mechanical Engineering, Nagaoka University of Technology, Tomioka, Nagaoka 940-21, Japan. Manuscript submitted January 19, 1990. METALLURGICAL TRANSACTIONS A
growth is notably affected by grain boundaries. With respect to such a small fatigue crack growth, Tanaka and Akiniwa have more recently modeled the plastic slipband ahead of small crack as follows; that is, one is a slipband not reaching a grain boundary, and the other is a slipband blocked by a neighboring grain boundary. tl~ Based on the model, they theoretically discussed the small crack growth near the grain boundary by applying isotropic elastic fracture mechanics, t9'~~ However, kinetic data on intrinsic stage I fatigue crack growth are s c a r c e , [13! and micromechanisms for crack growth have not been well elucidated. Since the crystallographic slip plane plays an important role in stage I fatigue crack growth, it is evident that the investigation from the viewpoint of anisotropic fracture mechanics is earnestly needed. Such research can offer significant information applicable not only to single-crystal materials but also to polycrystalline materials in general. In this work, the CTSD is analyzed for the case in which the crack lying on
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