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

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.