VAMAS round robin test on bending strain effect measurement of Bi-2223 tapes

In the framework of Versailles project on advanced materials and standards (VAMAS) a round robin test (RRT) has been implemented to determine a standard test method for the bending strain effect measurement by using three kinds of Ag alloy sheathed Bi-2223 multifilamentary tapes and with the participation of 12 laboratories worldwide. In the RRT, specially designed bending devices having curvatures corresponding to 0%, 0.2%, 0.4%, 0.6%, 0.8% and 1.0% bending strains were prepared and delivered to the participants. Bending of a specimen was done at room temperature and the critical current of the specimen was measured at 77 K and zero magnetic field. The bending and subsequent critical current measurement were started from 0% strain, and repeated on a single specimen up to 1.0%. The critical current vs. bending strain curves obtained were compared among laboratories whereby the critical current was normalized to that for the 0% strain state. The coefficient of variation (COV) (= the average divided by the standard deviation) was very small for the bending strain of 0.2%, increased at 0.4% strain, and remained at the same level up to 1.0% strain. The COV was, however, kept within 6.2% for all strains. Results were further compared in terms of bending strain limit and the COV in bending strain limit among participants was found to be 13.9%, rather large. Possible error sources are discussed. (C) 2002 Elsevier Science B.V. All rights reserved.