New approaches in fatigue fracture mechanics
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NEW APPROACHES IN FATIGUE FRACTURE MECHANICS O. P. Ostash
UDC 539.43
It is shown that the size d* of the process zone is a basic parameter of the material depending on its microstructure, mechanical properties, and testing conditions and specifying various stages of the fatigue fracture of materials and structural elements. The proposed unified model of fatigue fracture based on the parameter d* makes it possible to determine the service life of cyclically loaded structural elements with stress concentrators solely according to the diagrams of fatigue macrocrack growth rate. It is assumed that the size d* of the process zone determines the specific features of the well-known regularities of fatigue fracture, including the characteristic points in the Kitagawa–Takahashi and Smith–Miller diagrams.
The fatigue life of materials (structural elements) is determined as the sum of the period of initiation ( Ni ) and the period of subcritical growth ( N p) of a fatigue macrocrack, where the period Ni includes the time to the initiation of a system of microcracks ( N n), formation of the predominant microcrack, and its transformation into the macrocrack ( N tr ) [1–3]. Due to the efficiency of the methodical approach based on the application of the range of stress intensity factor ( Δ K ) as a fracture parameter in the stage of growth of the macrocrack (Table 1), it became possible to solve the problem of standardization and accumulate a large body of data [1]. As for the period N i , the progress is much less pronounced. This is why the problem of determination of the characteristics of materials and evaluation of the service life of structural elements in the stage of initiation of fatigue macrocrack is one of the most urgent problems of contemporary fatigue fracture mechanics. The basic parameters of this stage of fatigue fracture still require additional analysis mainly to determine the effective (cyclic) stress concentration factor ( Kf ), the local stress range (Δσ*y ) , and the criterion of length of the initial macrocrack ( ai ), i.e., of the transformation of a microcrack into the macrocrack (Table 1). The processes of initiation and growth of the macrocrack are traditionally regarded as different processes. Therefore, they are studied separately. However, the major difference between these processes is connected with the fact that the effect of crack closure is observed in the stage of growth of the macrocrack but is absent in the stage of its initiation [3]. In view of this fact, the indicated processes can be regarded as similar [4, 5]. Significant progress in understanding and description of the initiation and growth of fatigue macrocracks was attained as a result of the introduction of a concept of process zone and a new structural mechanical parameter of materials, i.e., the size d* of the process zone [6–8]. In what follows, we summarize the principal results of investigations carried out in this field and analyze the relationship between the parameter d* and the wellknown phenomena and regularities of fa
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