Axial tensile stress-strain characterization of ITER model coil type Nb_3Sn strands in TARSIS

摘要

For a few years there has been an increasing effort to study the impact of (bending) strain on the transport properties of superconducting wires. As the stress distribution, originated by differences in the thermal expansion and electromagnetic load, is the driving factor for the final strains, the axial and transverse stiffness of the strand play a crucial role in the final performance. Since the strain state of the Nb_3Sn filaments in strands determines the transport properties, basic experimental stress-strain data are required at the strand level for accurate modelling and analysis and eventually for optimizing cable and magnet design. We performed axial tensile stress-strain measurements on several types of Nb_3Sn strands used for the manufacture of the International Experimental Thermonuclear Reactor (ITER) central solenoid and toroidal field model coils and a powder-in-tube processed wire. In total 48 wire samples were tested at boiling helium, boiling nitrogen and at room temperature. We present the computation of the stress-strain characteristic with a straightforward 1D model using an independent materials database, obtaining a good agreement with the experimental results. The details from the take-off origin of the measured stress-strain curves are discussed and the data are evaluated with respect to some commonly used functions for fitting stress-strain curves. The measurements are performed in the new setup TARSIS (test arrangement for strain influence on strands). A double extensometer connected to the sample enables us to determine the strain level whereas a load cell is used to monitor the stress level. For higher levels of applied stress (100 MPa), we found typically a higher strain for bronze route wires compared to a powder-in-tube and internal tin type of strand. Stress-strain results are essential to assess more accurately the impact of thermal and electromagnetic induced stress on the strain state of the Nb_3Sn filaments for wires from various manufacturing processes.