FATIGUE BEHAVIOR AT ROOM-TEMPERATURE AND ITS INFLUENCE ON SUPERCONDUCTING PROPERTIES AT 4.2 K OF MULTIFILAMENTARY NB3AL COMPOSITE WIRE

Fatigue behaviour at room temperature and its influence on superconduc ting current and upper critical magnetic field at 4.2 k were studied u sing Nb3Al multifilamentary composite wire with a copper ratio of 1.55 and an overall diameter of 0.812 mm in which 241 Nb3Al filaments are embedded. The Nb3Al filament itself was not fractured by the fatigue w hen tested separately. In the composite, the fatigue cracks nucleated in the copper, which grew and caused the fracture of the Nb3Al filamen ts. When the maximum stress in the fatigue test was low, one crack amo ng 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 comp osite 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 tha t for the low and high maximum stress ranges. The mode I fracture toug hness of the present composite wire, estimated from the size of the st rength-determining fatigue crack in combination with the maximum stres s, was around 7 MPa m(1/2).