Use of local approach for predicting fatigue cracks in sleeves subjected to repeated thermal shocks

In pressurized-water nuclear plants, certain parts of the circuits have special zone where, for construction reasons, there is a slot that causes a high concentration of stresses. This is the case in particular of nozzles, protected by sleeves, which are subjected to numerous transients and thermal shocks. Loop tests have been carried out on models to evaluate the risk of the initiation or propagation of cracks in these structures. These appear after a few hundred cycles (500) of 210 deg. C amplitude and attain a depth of the order of one millimetre at 3000 cycles. To analyse how these fatigue cracks begin, an original method based on local approach has been developed in France. A law giving the number of cycles to initiate a crack in terms of the stress σ_θθ has been determined experimentally by numerous rupture mechanics tests on samples. Concordance of calculation and tests has been very satisfactory and has validated this method for thermal shocks on axisymmetrical structures. It is however desirable that an analogous method should be developed and validated for situations in which the plasticity is not confined. (author)