This paper presents a failure assessment approach for the handling of combined thermomechanical loading of nuclear pressure vessels. The proposed approach is based on a failure assessment curve that accounts for the thermal and mechanical stresses in terms of the elastic stress intensity factor with a plastically corrected crack length. The square of this stress intensity factor divided by the effective elastic modulus is added to a J-integral deformation plasticity solution term. The result is valid for the full range of material/structural behavior, from linear elastic to fully plastic. The fracture behavior parameter, the total J-integral, can then be formulated in terms of a failure assessment curve to account for the combined loading states.

The proposed approach was benchmarked using a modified Babcock & Wilcox version of the ADINA computer program. The program generated incremental plasticity finite element crack solutions for a single edge cracked plate and a circumferentially flawed cylinder subjected to thermomechanical loadings. The ADINA finite element results agreed quite well with the proposed failure assessment approach.

The failure assessment approach is illustrated by a sample problem of a postulated accident condition, based on the occurrence of an overcooling transient (thermal loading) combined with an uncontrolled repressurization (mechanical loading) of a typical nuclear reactor vessel.