Many structural components are subjected to cyclic loading. In these components, fatigue damage is the prime factor in affecting structural integrity and service life. Fatigue damage is typically divided into three stages: crack initiation, crack propagation and final failure. These three stages are important in determining the fatigue life of structural components. In many cases, the crack initiation behavior can, however, be the dominant event for life prediction analyses and design considerations, such as the applications of S (applied stress) Vs N (fatigue life cycle) curves. If the early stage of crack initiation can be detected and the mechanisms of crack initiation better understood, fatigue failure may be prevented.
In addition, fatigue crack growth rate properties are also important for structural reliability assessment. In particular, fatigue crack propagation rate results can be used to predict the crack growth life of a component. Thus, it is of paramount importance to thoroughly characterize and understand both the crack initiation and propagation behavior for structural integrity evaluation. Many pressure vessels are subjected to complex cyclic loading spectrums ranging from small vibrations to large load excursions. To conduct structural reliability analyses of the pressure vessels, fatigue crack initiation and growth properties are crucial material data input parameters. Moreover, an understanding of the fatigue crack initiation and growth characteristics of the required reactor pressure vessel grade materials is essential to evaluate useful life. The fatigue design curves in the American Society of Mechanical Engineers (ASME) Code are based on simplified approaches developed more than 20 years ago. In particular, the S-N data initially included in Section XI of the ASME Boiler and Pressure Vessel Code relative to pressure retaining materials for vessels utilized in nuclear applications was obtained in the same time period. Thus, the ASME Code needs to be updated to improve the fatigue design methodology. Specifically, fatigue crack initiation characteristics need to be considered in the ASME code for more accurate life prediction analyses of pressure vessel components.
In this review, the fatigue crack initiation and propagation investigation was conducted on pressure vessel steels. The crack initiation and growth characteristics were studied using smooth cylindrical bars. Replication techniques were used to monitor both crack initiation and propagation. In addition, the fatigue fracture modes were investigated using scanning electron microscopy. Finally, fatigue life analyses were performed and compared with the experimental results
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