Thermal fatigue crack networks parameters and stability: an experimental study

A thermal fatigue device--called SPLASH--similar to the facility developed by Marsh [Fatigue crack initiation and propagation in stainless steels subjected to thermal cycling, International Conference on Mechanical Behaviour and Nuclear Applications of Stainless Steels at Elevated Temperature, 1981] has been built in CEA/SRMA in 1985. Since then, it was used mostly on austenitic stainless steels to assess the initiation and growth of thermal fatigue crack networks. In 1998, a leak appeared in an auxiliary loop of the primary circuit of a pressurized water nuclear plant in Civaux (France). Thermal fatigue was suspected and studies began on AISI 304 L type austenitic stainless steel. They were eventually compared to results obtained earlier on AISI 316 L(N). First, the initiation conditions were determined and the damage before initiation was qualitatively observed. Then, some crack networks parameters were chosen and quantitatively determined by image analysis. This part of the study was done at the surface, during crack growth, and at the end of the tests, in depth. Finally, the stability of the crack networks obtained by thermal fatigue was tested under isothermal load controlled four point bending fatigue test, and some conclusions were drawn on the mechanisms of propagating crack selection.