Post-mortem analysis of ITER CS helium inlets fatigue tested at cryogenic temperature

In the ITER Magnet System, ten thousand tonnes of superconducting Cable In Conduit Conductor (CICC) arecooled down by a forcedflow of supercritical helium, which is supplied from helium inlets. For the ITER CentralSolenoid (CS), consisting of six independent pancake wound modules, the He inlets consist of three overlappingholes covered by an oblong shaped boss, welded to the CS jacket through full penetration, multi-pass TungstenInert Gas (TIG) welding. Since they are located in a region of high cyclic tensile stresses, i.e.first turn at the innerdiameter of the pancake, the CS inlets are one of the most critical structural components. Qualification of thedesign is done by analysis and a comprehensive design optimization has been performed byfinite element (FE)simulations. In order to guarantee the required fatigue life at cryogenic temperature of these component, a post–welding process consisting in ultrasonic shot–peening is required. Based on a qualified weld procedure, sixmock–ups including each two He–inlets on the opposite surfaces have been produced to run a mechanicalfatigue testing program at cryogenic temperature to validate thefindings of the FE simulations. Five were peenedand one not peened. The paper describes the results of a comprehensive post–mortem failure analysis whichincludes non–destructive (penetrant testing, leak testing, X-ray computed tomography) as well as destructiveexaminations (microoptical and hardness tests, scanning electron microscopy). The initiation site, propagation ofthe crack as well as the tensile overload region have been identified and studied. An estimation of the cycles frominitiation to failure based on the width of the fatigue striations was performed. The paper also includes a fullassessment of the welds according to the most stringent acceptance levels of the standards in forc