Comparative assessment of several creep damage models for use in life prediction

The accurate prediction of creep life is of great importance for the structural integrity of high temperature components, such as those used in power generation plant, if safe, efficient and economically responsible operation is to be achieved. Continuum damage mechanics can be used in conjunction with finite element analysis to provide a fundamental step in modelling creep failure. Material constants used in these models are often derived from accelerated creep rupture tests, performed using higher stresses or temperatures than would normally be experienced by real components. In this paper, a comparative assessment of extrapolated failure times for several creep damage models has been under taken for uniaxial, notched bar, closed end straight pipe section and idealised pipe bend geometries (note the pipe geometry was typical of that used in power generation plant) using finite element software. Material constants for each model were determined using creep and creep rupture tests performed on P91 materials under a uniaxial condition and using notched bar specimens to obtain the material properties related to the multiaxial stress state. In all cases, a hyperbolic sine function creep law was found to consistently give reduced failure times, compared to power law based models (e.g. Liu–Murakami and Kachanov-Robotnov), when stresses below those used in the creep tests were considered.