Observation of Crack Formation Caused by Twisting in REBCO Coated Conductors by Using Scanning Hall-Probe Microscopy

In magnet applications using REBa 2 Cu 3 O y (RE = rare earth, Y, Gd, etc., REBCO) coated conductors, micro-cracks caused by mechanical slitting are one of the most worrisome phenomena because they could be the origin of critical current degradation. In this study, we have investigated the crack formation caused by twisting the REBCO tape by using scanning Hall-probe microscopy (SHPM) and scanning electron microscope (SEM). First, twist, the most severe load on the crack, was applied to the 4 mm-wide REBCO tape divided by mechanical slitting, and the variation of critical current with the torsion angles was investigated. Then, magnetization distribution measured by SHPM revealed that these decreases in critical current correspond to crack formation. The micro-cracks introduced initially during the slitting process are not the origin of the crack formation that spread throughout the tape. On the other hand, for the REBCO tape after the irreversible twist test, additional cracks have also been confirmed even though the critical current has not yet decreased.