A Monte Carlo–shear lag simulation of tensile fracture behaviour of Bi2223 filament

Abstract

The damage evolution in Bi2223 filaments and its influence on critical current was described by a Monte Carlo-shear lag simulation method. The experimentally observed zigzag crack propagation across aligned Bi2223 grains under tensile strain was effectively modelled by including transverse and longitudinal failure modes for individual grains. From the simulated stress-strain curve, the survival parameter (slope of the stress-strain curve normalized with respect to the original Young's modulus) was estimated with increasing applied strain. With this parameter combined with the strain sensitivity of the critical current, the measured change of critical current of the composite tape with applied strain could be described well.

Residual and fracture strains of Bi2223 filaments and their relation to critical current under applied bending and tensile strains in Bi2223∕Ag∕Ag alloy composite superconductor

“…4,9,10,13,15,20,21,27 The present sample had been cooled down to 77 K ͑1͒ and then heated to room temperature RT͑2͒, at which the residual strain of Bi2223 was measured, as has been shown in Fig. 2.…”

“…1-27 For description of damage behavior of Bi2223 filaments that transport superconducting current, two important parameters have been mentioned. 4,13,14,16,19,21,27 One is the intrinsic fracture strain Bi,f and another is the residual strain Bi,r . If the Bi2223 filaments are tested alone, they fracture at the intrinsic fracture strain Bi,f .…”

“…The residual strain of Bi2223 filaments is compressive in the current-transportation direction since the coefficient of thermal expansion of Bi2223 is lower than that of Ag and Ag alloy. 4,9,10,13,15,20,21,27 Thus, the fracture of the Bi2223 filaments embedded in the composite tape is retarded by − Bi,r under applied tensile strain. As the fracture strain Bi,f of the Bi2223 filaments is not necessary high ͑around 0.1%͒, 4,9,11,13,18,19,21,22,27 the residual strain Bi,r contributes largely to the strain tolerance of critical current of the composite tape.…”

“…The applied tensile strain at which damage of Bi2223 filaments takes place in the composite tape, corresponding to Bi,f -Bi,r , was estimated from the load-strain curve at room temperature. In this approach, the feature that the load carrying capacity of the composite tape with fractured Bi2223 filaments is reduced in comparison with that without fractured ones 11,13,21 was used. Combining the estimated Bi,f -Bi,r value with the Bi,r value estimated in ͑1͒, we obtained Bi,f value, as will be shown in Sec.…”

“…At the end of region III, they fracture, due to which the load carrying capacity of the composite is reduced in comparison with that of the composite without fracture of the filaments. 11,13,21,27,29 Figure 5͑b͒ shows enlarged stressstrain curve of the part surrounded by the rectangle in ͑a͒. The open circle in Fig.…”