Zhongyu Lu

Damage modelling: the current state and the latest progress on the development of creep damage constitutive equations for high Cr steels

Abstract This paper reviews the fundamentals of the development of creep damage constitutive equations for high Cr steels including (1) a concise summary of the characteristics of creep deformation and creep damage evolution and their dependence on the stress level and the importance of cavitation for the final fracture; (2) a critical review of the state of art of creep damage equation for high Cr steels; (3) some discussion and comments on the various approaches; (4) consideration and suggestion for future work. It emphasises the need for better understanding the nucleation, cavity growth and coalesces and the theory for coupling method between creep cavity damage and brittle fracture and generalisation.

On the development of creep damage constitutive equations: a modified hyperbolic sine law for minimum creep strain rate and stress and creep fracture criteria based on cavity area fraction along grain boundaries

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.

The interpretation of experimental observation date for the development of mechanisms based creep damage constitutive equations for high chromium steel

It is very important to design a safe factor or estimating the remain lifetime for electric power plant components of steam pipes which mostly are manufactured from high chromium steels and work at high temperature and low stress level. The authors will develop the mechanisms based on creep damage constitutive equations for high chromium steel under lows stress in initial stage: (1) Creep cavities were mostly formed attaching with the precipitation of Laves phase or on grain boundary. The Laves phase should play an active role in the nucleation of creep cavities and suggest to explore the function between cavity nucleation and the evolution of Laves phase; (2) The dominant cavity nucleation damage mechanism controls; (3) Brittle intergranluar is fracture model; (4) High density number of cavity of crept test high chromium steel at high temperature under low stress could be as fracture criterion.