Modeling creep crack growth in high chromium steels

Components used in power generation plants are continually exposed to high temperatures and risk of failure resulting from net section rupture, creep crack growth or fatigue crack growth. Safe and accurate methods to predict creep crack growth (CCG) are therefore required in order to assess the reliability of such components. With advances in finite element (FE) methods, more complex models can be applied in the study of CCG where simple analytical solutions or approximate methods are no longer applicable. Continuum Damage Mechanics (CDM) offer the possibility to account for the loss of load carrying capabilities in the material due to damage processes, the increase of the local effective stress and consequent increase of the creep rate. The possibility to accurately simulate CCG depends not only on the damage formulation but also on the creep model since stress relaxation, occurring in the near tip region, controls the resulting creep rate and, therefore, crack initiation and growth. In this perspective, primary and tertiary creep regimes, usually neglected in simplified creep models, plays a relevant role and need to be taken into account. In this paper, an advanced multiaxial creep model [1], which incorporates damage effects, has been used to predict CCG in ASTM P91 high chromium steel. The model parameters have been determined based on uniaxial and multiaxial (round notched bar) creep data over a wide range of stress and temperature. Successively, the creep crack growth in standard compact tension was predicted and compared with available experimental data.