Fatigue behaviour at room temperature and its influence on superconducting properties at 4.2 K of multifilamentary composite wire

Abstract

Fatigue behaviour at room temperature and its influence on superconducting current and upper critical magnetic field at 4.2 k were studied using multifilamentary composite wire with a copper ratio of 1.55 and an overall diameter of 0.812 mm in which 241 filaments are embedded. The filament itself was not fractured by the fatigue when tested separately. In the composite, the fatigue cracks nucleated in the copper, which grew and caused the fracture of the filaments. When the maximum stress in the fatigue test was low, one crack among many grew preferentially; when the maximum stress was intermediate, many cracks could grow at different cross-sections before the overall fracture of the composite; when the maximum stress was high, the composite was fractured before large fatigue crack growth. Because of such a maximum stress dependence of the growth of the fatigue damage, the critical current measured using the segments of the fatigue-fractured specimens for the intermediate maximum stress range was lower than that for the low and high maximum stress ranges. The mode I fracture toughness of the present composite wire, estimated from the size of the strength-determining fatigue crack in combination with the maximum stress, was around .

A probe for investigating the effects of temperature, strain, and magnetic field on transport critical currents in superconducting wires and tapes

“…These results also demonstrate that the probe is suitable for studying the influence of fatigue on the transport properties of superconducting wires and tapes. Such studies are complementary to other investigations which address multiple cyclic loading ͑up to 10 7 cycles͒ at room temperature, prior to measurements at 4.2 K. [31][32][33]…”

“…This property offers the additional possibility of using the probe to examine the effect of fatigue on the transport properties of wires and tapes. [31][32][33] These general considerations convinced us to choose CuBe alloy as the material for the spring. The spring can be machined first and subsequently precipitate hardened but particular precautions have to be taken during the heat treatment in order to maintain the shape of the spring.…”

Progress of fatigue damages and its influence on the critical current of multifilamentary Nb3Al superconducting composite

“…As mechanical and electromagnetic stresses acting during fabrication and/or service of superconducting composite wires affect the superconductivity, it is necessary to clarify the mechanical behaviour and its relation to the superconductivity. Recently the present authors [1,2] have studied the static and fatigue behaviours at room temperature and their influences on critical current at 4.2 K of an Nb 3 Al composite wire which has been developed for wide engineering application owing to the high critical current densities at high magnetic field and high strain endurance and low strain sensitivity of the critical current [3][4][5][6][7][8][9]. Concerning the static behaviour at room temperature, it was found that (a) the scatter of tensile strength of the multifilamentary composite is very small in spite of the large scatter of strength of the Nb 3 Al filaments and (b) the strength of the composite wire has a very slight dependence on length, although the strength of the Nb 3 Al filaments decreases markedly with increasing length.…”

“…Concerning the fatigue behaviour at room temperature and its influence on critical current at 4.2 K, the following features have been observed [2]. (d) The Nb 3 Al filament itself is not fractured by fatigue when it is not embedded in copper, but is fractured when it is embedded owing to the propagation of the fatigue crack nucleated in the copper.…”

“…In this procedure, the extent of the damage other than the main crack in the fatigue-fractured specimens can be detected. This procedure has been used in our former work [2]. (b) The specimen with a gauge length of 20 mm is fatigued for a given number of stress cycles.…”