Abstract
This paper reports (1) the latest development and application of a modified hyperbolic sine law for minimum creep strain rate and stress for both low Cr and high Cr steels, and (2) the development of a creep fracture criterion based on cavity area fraction along grain boundaries for high Cr steel. This work is part of the fundamental development of creep damage constitutive equations which were identified through a critical literature review. In the former the application of the new law results in an improved fitting; in the latter, a new creep fracture criterion based on cavity area fraction along grain boundaries was derived and quantitatively calibrated using the latest detailed cavity nucleation and growth kinetics models for high Cr steel. Furthermore, this paper revealed the trend of nucleation rate coefficient with stress, and the trend of creep life time coefficient with stress, which provide reliable and universal prediction capabilities. This paper contributes to the specific knowledge on the minimum creep strain rate and stress function, the development of a scientifically sound and novel creep rupture criterion based on the cavity area fraction along grain boundaries for high Cr steel, and the provision of creep damage/life prediction tools.
Highlights
Stress breakdownIn the development of high temperature materials for long term use, the accelerated creep test may be used it may not be reliable in predicting long time service
This paper contributes to the specific knowledge on the minimum creep strain rate and stress function, the development of a scientifically sound and novel creep rupture criterion based on the cavity area fraction along grain boundaries for high Cr steel, and the provision of creep damage/life prediction tools
This paper aims to overcome the inaccuracies involved in the process of modelling minimum creep strain rate and stress, and in turn develop and apply a more scientifically sound creep fracture criterion
Summary
Stress breakdownIn the development of high temperature materials for long term use, the accelerated creep test (short-term) may be used it may not be reliable in predicting long time service. The stress breakdown phenomenon for high Cr steel received positive reports in past literature [1] and was praised by other research papers and reviews. It can be summarized as (1) there is a change of mechanisms in creep deformation, fracture, and the coupling between creep deformation and creep damage; and (2) it is understood that the nature of the cavity nucleation, growth and coalesce changes with the stress level, resulting in a change from a ductile to brittle fracture. The relevant equations are listed below for further use: Dyson:
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