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.
In this work the effects of specimen size and type on creep crack growth rates in stainless steel are examined. Experiments have been carried out on high constraint compact tension specimens (CT) and low constraint centre cracked panels (CCP) of ex-service 316H stainless steel. All testing was carried out at 550°C. Constraint effects have been observed in the data, with the large CT specimens having the fastest crack growth rate and the small CCP specimens the slowest. These trends are consistent with those that would be predicted from two parameter (C*–Q) theories. However, it is found that a constraint dependent creep crack growth model based on ductility exhaustion overpredicts the constraint dependence of the crack growth data.
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