Development of a controlled bending application method for investigating Nb3Sn jacketed strands

In the framework of the International Thermonuclear Experimental Reactor (ITER) R&D programme part concerning the Nb 3 Sn cable-in-conduit conductor of the Toroidal Field coils, a dedicated programme of action was launched within the European Fusion Technology Programme, designed to improve the investigation of the impact of bending strain on recently developed industrially advanced Nb 3 Sn strands. In order to ensure that the ITER TF coils experience relevant mechanical conditions, the Nb 3 Sn strands were jacketed inside a 0.2 mm thick stainless steel tube, simulating the mechanical influence of the jacket on the cable strands. In this paper we will describe in detail the four candidate methods that were investigated for imposing a pure controlled bending strain on the jacketed strands. A common feature of these methods is that they study the reaction of the strand on a heat treatment mandrel and transfer it onto a test mandrel with a different diameter. In practice, the imposed bending strain is limited to 0.5%. Two solutions are considered: a reduction or an increase in diameter of the mandrel. Also, two options are possible for the jacket removal at strand ends for connection to the current leads: before or after the heat treatment. The four options are tested to provide an area of investigation that is as large as possible. Specific support together with tool design and manufacture will be presented. A general comparison of all options as regards specified criteria is also carried out in order to precisely define an action process of the bending application method developed. We find finally that the best approach is the method in which the radius is increased and the stainless steel jacket is removed before heat treatment.