MICROSTRUCTURAL PARAMETERS FOR CREEP DAMAGE QUANTIFICATION

This paper is concerned with the use of mechanistically based parameters that can be used to quantify cavitation creep damage for the purpose of life assessment of high temperature components. They are referred to as "line parameters" in that they relate the number of damaged grain boundaries intersected by or very close to a reference line to the total of grain boundaries intersected by this reference line. The parameters considered in this work are (i) the "A-parameter" based on the concept of constrained cavity growth and the phenomenological damage equations by Kachanov and Rabotnov, (ii) the "rho-parameter" again based on the model of constrained cavity growth but relating to a model of a creeping solid containing microcracks and (iii) a new parameter A*, which is based on the "A-theory" but takes the mechanical interaction of cavitating facets during creep into account. A* performs best in relating creep damage to consumed life fraction. It is more sensitive to the consumed life fraction than "A" and the metallographic data are in better agreement with the predictions of the corresponding damage evolution law than in the case of "rho". The results of a Monte Carlo simulation performed in this study indicate that one of the reasons for scatter in creep rupture lives of engineering steels is the statistical variation in the time required for the formation of highly damaged regions.