Prediction of Plane-Strain Creep-Crack Growth Using Continuum Damage Mechanics

The paper examines the possibility of using creep Continuum Damage Mechanics to predict plane strain creep crack growth in compact tension specimens using finite element based numerical methods, coupled with uni-axial and multi-axial stress laboratory creep data, and with mechanisms-based constitutive equations. The predictions of the behaviour of the compact tension specimens made using the single state damage variable equations due to Hayhurst, Dimmer, and Morrison (1984) are shown to be unable to predict that zones of damage do not grow in discrete planar regions as observed in laboratory experiments. To overcome the deficiencies of this approach, new constitutive equations have been developed, which combine the equations of Hayhurst et al. (1984) with those of Cocks and Ashby (1982), to describe the stress-state dependence of the physical mechanisms. The behaviour of the compact tension specimen determined using these new equations predicts lifetimes of the correct order and evolution of damage zones which reflect the growth of discrete cracks as observed experimentally. The paper shows that creep Continuum Damage Mechanics can be used with finite element based numerical procedures to predict creep crack growth, provided that mechanisms-based constitutive equations are used which describe the appropriate stress-state behaviour for each mechanism zone, and that continuity of mechanical properties are achieved across all mechanism boundaries.