Remaining Creep Life Assessment Techniques Based on Creep Cavitation Modeling
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The importance of creep cavitation and its role in failure of critical components has been studied in the past. However, there has been the absence of a quantitative creep cavitation model that can effectively correlate creep cavities with the remaining life of the components. Efforts have been made in the past, but until now, the methods already developed have been able to provide only qualitative guidelines, for two primary reasons: first, due to the lack of understanding, especially about the creep cavity nucleation, which is the least understood phenomenon to date; and second, due to the lack of a justifiable model that can correlate the surface cavities observed in the replica taken from in-service components to the creep cavitation occurring in the bulk, which, obviously, is more representative of the creep life of the component. In the model that I discuss here, the creep cavitation has been beautifully correlated to the remaining life using mathematical expression based on statistical probability distribution.[1]
A remarkable achievement of the model is that it correlates the A parameter, i.e., number fraction of cavitated grain boundaries observed on the replica, with the continuum damage variable D, which is the area fraction of cavities present on the grain boundary facets. The model makes use of the Kachanov–Rabotnov creep damage theory and involves minimum mathematical calculations. The methodology of calculation is quite economical, as the only experimental input required is the value of Acr (which is the critical value of the A parameter at failure that can be easily determined by performing a hot tensile testing), as compared to creep testing and strain rate monitoring, which require expensive setup of a creep machine and diligent software to incessantly monitor strain rate, a process that is presently very expensive. Creep cavity nucleation has been one of the least understood phenomena to date. The creep cavity nucleation rate affects the remaining life of a component. There are some rate laws that are specific to components and the grades of steel such as the relationship given in Eq. [1] for the type IV cracking in weldments: tr ¼ Brn expðQ=RTÞ½3
½1
where B is a constant depending on the grade of steel, Q is the activation energy specific to the grade of steel, R is the universal gas constant, and T is the operation temperature in Kelvin. The extensive research work and literature survey have revealed the following facts. (1) The presence of any non metallic inclusion; simply put, any region that can act as a stress concentrator acts as initiators of creep cavities (Figure 1). (2) In the absence of inclusions, the creep cavities generally nucleate at grain boundary triple points (Figure 2). (3) The creep cavities generally grow and get linked to each other in the direction perpendicular to the stress axis (Figure 3). (4) The number of cavities per unit grain boundary area linearly increases with creep exposed time. (5) As the amount of applied stress increases, the rate of creep cavitation increase
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