Mechanical property and its influence on the critical current of Ag/Bi2223 tapes

The stress-elongation (R-A) curve has been experimentally characterized as follows. The initial quasi-elastic region (stage I) was very narrow where the pure Ag component had already yielded due to the thermal contraction. In the following stage II, the slope was reduced owing to the yielding of the Ag alloy. Stage III was defined as the macroscopic multiple fracture taking place in the oxide layer. The R-A behaviour at 77 K has been theoretically simulated based on the rule of mixture. The oxide layer was exerted by a large compressive residual stress due to the thermal contraction. Applying tensile elongation, its compressive stress reduced and became zero at a specific elongation A(rl), after which the sign of stress changed to tensile and its quantity increased. When the tensile stress of the oxide layer reached its fracture strength, macroscopic yielding was expected. Such a result obtained from the simulation can describe well the experimental R-A curve as well as the following elongation dependence of critical current. The critical current decreased linearly with increasing tensile elongation from the beginning of stage I and its gradual decrease followed in stages II and II'. This elongation dependence of critical current was reversible in stages I and II, but irreversible in substage II'. In stage III corresponding to the plastic deformation region, the critical current was reduced largely due to the macroscopic crack formation.