Fitness for service evaluation of cracked divider plate bolt locking tabs for nuclear steam generators

This paper is to address the structural integrity issues related to continued service without repair or replacement for cracked locking tabs on divider plates of nuclear steam generators. Significantly high residual stress introduced by cold bending of locking tabs at installation was simulated by elastic-plastic finite element method and considered in the fitness for service evaluation. Significant work hardening resulted from the accumulation of large and inhomogeneous plastic deformation introduced by the in situ bending was quantified and considered. Failure and degradation mechanisms for crack stability and propagation were identified. Linear elastic fracture mechanics (LEFM) theory with crack tip plastic zone correction was adopted to establish critical crack sizes. Life of safe operation of the cracked locking tabs from inspected crack sizes to the critical crack sizes was then evaluated. Initial crack sizes at installation were also established through a highly contrived backward fitting evaluation procedure.