Two-dimensional peridynamic simulation of the effect of defects on the mechanical behavior of Bi₂Sr₂CaCu₂O_x round wires

Ag/AgX sheathed Bi₂Sr₂CaCu₂O_x (Bi2212) is the only superconducting round wire (RW) with high critical current density (J_c) at high magnetic (>25 T) and is thus a strong candidate for high field magnets for nuclear magnetic resonance and high energy physics. A significant remaining challenge, however, is the relatively poor electromechanical behavior of Bi2212 RW, yet there is little understanding of the relationships between the internal Bi2212 microstructure and the mechanical behavior. This is in part due to the complex microstructures within the Bi2212 filaments and the uncertain role of interfilamentary bridges. Here, two-dimensional peridynamic simulations are used to study the stress distribution of the Bi2212 RWs under an axial tensile load. The simulations use scanning electron micrographs obtained from high J_c wires as a starting point to study the impact of various defects on the distribution of stress concentration within the Bi2212 microstructure and Ag. The flexibility of the peridynamic approach allows various defects, including those captured from SEM micrographs and artificially created defects, to be inserted into the microstructure for systematic study. Furthermore, this approach allows the mechanical properties of the defects to be varied, so the effects of porosity and both soft and hard secondary phases are evaluated. The results show significant stress concentration around defects, interfilamentary bridges and the rough Bi2212/Ag interface. In general, the stress concentration resulting from porosity is greater than that of solid-phase inclusions. A clear role of the defect geometry is observed. Results indicate that crack growth is likely to initiate at the Ag/Bi2212 interface or at voids, but that voids may also arrest crack growth in certain circumstances. These results are consistent with experimental studies of Bi2212 electromechanical behavior and magneto-optical imaging of crack growth. (paper)