Fracture mechanics evaluation of reactor vessels using stress intensity factors from enriched finite elements

The paper describes a procedure involving use of refined 3-D finite element method by developing the potential energy equations in terms of element stiffness including the effects of both thermal and mechanical loads. Following the method of computing strain energy release rates, Gsub(I), Gsub(II) and Gsub(III), corresponding stress intensity factors, Ksub(I), Ksub(II) and Ksub(III) are developed. Considerations of the classical near-crack-tip equations are made and certain deficiencies with substitution method eliminated. The method essentially highlights the use of high-order, fracture mechanics enriched and modified isoparametric finite element by utilizing the technique in which the corner node is made to correspond with the crack-tip. The geometry of the basic element consists of a 20-node isoparametric brick element utilizing 60-degrees of freedom system at the element level with modifications. Certain special techniques such as multilevel substructuring and condensation of degrees of freedom are carried out to increase the computational efficiency. (orig.)