Delamination behaviors of GdBCO CC tapes under different transverse loading conditions

Abstract

In superconducting coil applications particularly in wet wound coils, coated conductor (CC) tapes are subjected to different type of stresses. These include hoop stress acting along the length of the CC tape and the Lorentz force acting perpendicular to the CC tape's surface. Since the latter is commonly associated with delamination problem of multi-layered CC tapes, more understanding and attention on the delamination phenomena induced in the case of coil applications are needed. Difference on the coefficient of thermal expansion (CTE) of each constituent layer of the CC tape, the bobbin, and the impregnating materials is the main causes of delamination in CC tapes when subjected to thermal cycling. The CC tape might also experience cyclic loading due to the energizing scheme (on -off) during operation. In the design of degradation-free superconducting coils, therefore, characterization of the delamination behaviors including mechanism and strength in REBCO CC tapes becomes critical. In this study, transverse tensile tests were conducted under different loading conditions using different size of upper anvils on the GdBCO CC tapes. The mechanical and electromechanical delamination strength behaviors of the CC tapes under transverse tensile loading were examined and a two-parameter Weibull distribution analysis was conducted in statistical aspects. As a result, the CC tape showed similar range of mechanical delamination strength regardless of cross-head speed adopted. On the other hand, cyclic loading might have affected the CC tape in both upper anvil sizes adopted.

Evaluation of delamination characteristics in GdBCO CC tapes under transverse load using anvil test methods for various anvil contact configurations at 77 K

“…During the anvil test, the stress concentrated along the micro-cracks at the slit edges, affecting the delamination behavior of the superconducting layer side. The same results were also obtained when the superconducting layer was placed facing upward toward the upper anvil listed in table 2 [18].…”

“…In our previous reports, we used a transverse tensile testing frame and anvil test methods, devised to perform systematic delamination testing procedures [16][17][18]. We have found that the electromechanical delamination strength, σ el , significantly increased when smaller-sized anvil was used even when it was done experimentally and numerically simulated [12,15].…”

“…On the other hand, using wider anvils resulted to lower delamination strength due to the influence of edge geometry such as burrs [17]. Under cyclic transverse tensile loading, the σ el was affected by the anvil sizes adopted [18] wherein most of the CC samples exhibited abrupt I c degradation behaviors. Impurities in the substrates of CC tapes can impose lower delamination strengths [19].…”

Enhancement of electro-mechanical behaviors in RE–Ba–Cu–O composite superconducting tapes with laser slit edges

“…It has been widely used in recent years to describe the strength properties and failure behaviors of several materials, including metalmatrix and ceramic-matrix composites [35]. Particularly, the Weibull distribution has successfully described the delamination strength of 2G-HTS tapes [36]. In this study, the twoparameter Weibull distribution function was used, which is given as [37,38]:…”

National round robin test (RRT) for delamination strength testing YBCO HTS wires at room temperature (RT) and 77K

“…In these works, effects of the cutting edges of the CCs was found to reduce the measured values of MDS [17][18][19]. Anvil configurations [20,21], loading speeds [22] and thickness of the silver layer [23] on the MDS measurement had little influence. Since the CC delamination is intimately related to the fracture and damage of ceramic, the threeparameter Weibull distribution based on the weakest link model often provide a proper statistics analysis [24,25].…”