Recent progress on the modelling of the minimum creep strain rate and the creep cavitation damage and fracture

Abstract

Modelling of the long-term creep damage and lifetime is very challenging, because the minimum creep strain rate, the lifetime, and creep strain at failure are strongly stress level dependent.With the aim for developing a set of creep damage constitutive equations for high Cr steel suitable for long-term service, research work was carried out in the modelling of the minimum creep strain rate with a wide stress level, the modelling of creep cavitation damage and rupture, and the coupling between creep cavitation damage and creep deformation.The stress driven cavitation concept was taken in the development of creep cavity nucleation model and cavity growth model. These two models were calibrated together using cavity histogram first, then a simple analytical equation was derived to model creep cavitation damage via the cavitated area fraction along grain boundary. The creep cavitation damage model offered the theoretical background and supports for the stressbased extrapolation method and time-based extrapolation method. Two case study were carried to explore the opportunities.This paper reports a summary of progresses achieved in the modelling of the minimum creep strain rate over a wide stress, the modelling of cavitation damage and fracture, and the lifetime extrapolation methods.This paper contributes to the generic methodology as well as the specific knowledge to the topic area.

The Development of the Cavitated Volume Fraction as a Creep Damage Indicator for Creep Remnant Life Prediction

“…The research presented is one of the initial explorations of the practical implementation and applicability of the cavitated volume fraction (݂ ௩ ) as a creep damage indicative parameter. It further expands the application of the original idea [15] of using accurate and comprehensive cavitation data to develop the cavitation model and rupture model [10,13] and contributes to the creep damage lifetime prediction methodology [12]. This approach is based on creep cavitation data with strong scientific bases.…”

“…Creep deformation and fracture time predictions have mainly been characterised using the continuum damage mechanics (CDM) [3][4][5][6]; empirical models that depend on parameterizing the creep curve [7][8][9] and direct cavitation damage models [10][11][12][13]. It is well known that an accurate description of the creep deformation and lifetime is still a contemporary problem in material science and in structural integrity research.…”

Accurate determination of creep cavity nucleation rate and growth rate with considering the partial coalescence prior to rupture and its impact on creep lifetime prediction

“…This would suggest the need for incorporating partial coalescence effects into the model to enhance its predictive accuracy [26] Background on the current cavitation model (Riedel's Cavitation Model) Cavity nucleation rate has been traditionally associated with the creep strain rate (phenomenological approach) but Qiang has pointed out its inaccuracy in the application of such approach originally; [27] then in [28]. This had motivated research to pursue the stress driven cavity nucleation research; though this is not the mainstream of research activities reported in literature, but it is the right direction to take; It is also expected that the stress driven approach due to its mechanism based, rather than the phenomenological approach, would give better chance to extrapolate as proposed and evidenced by Xu [29].…”