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 .

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Progress of the fatigue damage with increasing number of stress cycles at room temperature and its influence on the critical current at 4.2 K of Nb3Al multifilamentary composite wire were studied. The main results are summarized as follows. (i) With increasing number of stress cycles, the damage progressed in the order of the following three stages: stage I, formation of cracks in the clad copper; stage II, stable propagation of the cracks in the clad copper into the inner core portion, causing fracture of the Nb3Al filaments; stage III, unstable propagation of the main crack, causing overall fracture of the composite. (ii) The critical current remained nearly constant in stage I but was reduced in stage II, while the residual strength was reduced slightly in stage I and substantially in stage II. The reduction in residual strength and critical current in stage II arose from the fracture of the stress-carryable and superconducting-current-transportable Nb3Al filaments. Stage II occurred in the late stage of fatigue life. (iii) The extent of the damage and therefore the reduction in residual strength and critical current in stage II was dependent on the maximum stress level. At the high maximum stress level, where the damage was mainly cracking in the clad copper and partially a small number of breakages of the filaments, the reduction in residual strength and critical current was minor. With decreasing maximum stress, the damage accumulated more, resulting in larger reduction in residual strength and critical current, while the accumulation process varied with the stress level; at the intermediate stress level, many cracks in the clad copper grew into the core region but, at the low stress level, one of the many cracks in the clad copper grew into the inner core region.

The influences of fatigue damage introduced at room temperature on critical current at 4.2 K and residual strength at room temperature of Ti-Nb superconducting composite wire with a low copper ratio (1.04) were studied. The experimental results were compared with those of Nb3 Al composite. The following differences between the composites were found: the fracture surface of the Ti-Nb filaments in the composite varies from a ductile pattern under static loading to a brittle one under cyclic loading, while the Nb3 Al compound always shows a brittle pattern under both loadings; the fracture strength of the Ti-Nb composite is given by the net stress criterion but that of Nb3 Al by the stress intensity factor criterion; in the Ti-Nb composite the critical current Ic decreases with increasing number of stress cycles simultaneously with the residual strength c ,r , while in the Nb3 Al composite Ic decreases later than c ,r . On the other hand, both composites have the following similarities: the filaments are fractured due to the propagation of the fatigue crack nucleated in the copper; with increasing number of stress cycles, the damage progresses in the order of stage I (formation of cracks in the clad copper), stage II (stable propagation of the fatigue crack into the inner core) and stage III (overall fracture), among which stage II occurs in the late stage beyond 85 to 90% of the fatigue life; at intermediate maximum stress, many large cracks grow into the core portion at different cross sections but not at high and low maximum stresses; accordingly, the critical current and residual strength of the portion apart from the main crack are low for the intermediate maximum stress but not for low and high maximum stresses.