Investigation of cracks in original material of Cleuson-Dixence shaft

Abstract

After the big accident in the Cleuson-Dixence Hydropower Plant in December, 2000, several investigations were performed. This contribution summarises the efforts undertaken to clarify the mechanism of the catastrophic failure by the investigation of original material taken from the shaft near the zone of the accident. By extensive non-destructive testing, four plates were removed by flame cutting and tested in the laboratory again. From these results some specimens were taken for further investigations using light microscope and scanning electron microscope. It was found that the investigated cracks are caused by the hydrogen induced cold cracking mechanism after welding, which is also called delayed cracking. By applying a proper pre- and post-weld heat treatment, these cracks can be avoided and by a diligent use of non-destructive testing methods, the required quality of the welding can be assured.