This paper presents a numerical study of creep crack growth in a fracture mechanics specimen. The material properties used are representative of a carbon-manganese steel at 360 o C and the constitutive behaviour of the steel is described by a power law creep model. A damage-based approach is used to predict the crack propagation rate in a compact tension specimen and the data are correlated against an independently determined C* parameter. Elastic-creep and elastic-plastic-creep analyses are performed using two different crack growth criteria to predict crack extension under plane stress and plane strain conditions. The plane strain crack growth rate predicted from the numerical analysis is found to be less conservative than the plane strain upper bound of an existing ductility exhaustion model, for values of C* within the limits of the present creep crack growth testing standards. At low values of C* the predicted plane stress and plane strain crack growth rates differ by a factor between 5 and 30 depending on the creep ductility of the material. However, at higher loads and C* values, the plane strain crack growth rates, predicted using an elastic-plastic-creep material response, approach those for plane stress. These results are consistent with experimental data for the material and suggest that purely elastic-creep modelling is unrealistic for the carbon-manganese steel as plastic strains are significant at relevant loading levels.

This paper presents a numerical study of creep crack growth in a fracture mechanics specimen. The material properties used are representative of a carbon-manganese steel at 360 o C and the constitutive behaviour of the steel is described by a power law creep model. A damage-based approach is used to predict the crack propagation rate in a compact tension specimen. Elastic-creep and elastic-plastic-creep analyses are performed using two different crack growth criteria to predict crack extension under plane stress and plane strain conditions. The plane strain crack growth rate predicted from the numerical analysis is found to be lower than that predicted from ductility exhaustion plane strain model (known as the NSW model), which uses the creep fracture mechanics parameter C* and the development of creep damage directly ahead of the crack tip to predict creep crack growth rates under plane strain/plane stress conditions. A modified NSW model (NSW-MOD) is presented in which the effect of the damage angle at the crack tip is considered in order to predict this difference. In the model it is assumed that fracture occurs first at the value of the crack tip angle, at which the creep strain, reaches its maximum value. It is found that the new NSW-MOD gives a better prediction of the plane strain upper-bound of the experimental data.

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Finite element predictions of creep rupture in notched specimens are presented in this work. A damage mechanics model linked to the creep strain rate and stress triaxiality has been adopted in order to predict creep life under multiaxial stress conditions and the predicted creep failure strain and time to rupture have been compared with experimental data for a C-Mn steel tested at 360 o C. Finite element analyses have been conducted for primary-secondary (PS) and primary-secondary-tertiary (PST) creep laws. As expected a PST analysis gives more conservative predictions than a PS analysis. An additional term was included in the model to allow for an increase in hydrostatic strain due to creep damage. Under certain conditions incorporating this 'elastic damage' term can lead to an increase in the predicted failure time (i.e. it is less conservative). A further enhancement to the model was to include the effect of crack growth through the use of a nodal release technique. It was found that the predictions obtained using the nodal release technique were very similar to those from the PST creep model with elastic damage. Furthermore, it was found that the inclusion of plasticity (i.e. rate independent inelastic strains) may decrease the conservatism in the prediction (an increase in the predicted life). The sensitivity of the results to the value of the uniaxial creep failure strain and the stress triaxiality model used in the definition of damage were examined and it was found that both these factors strongly affected the predicted rupture time. Mesh size effects were also examined and the finite element predictions were seen to be quite mesh sensitive with a finer mesh giving more conservative predictions.

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