ROTARY BENDING FATIGUE OF COILS AND WIRES USED IN CARDIAC LEAD DESIGN

Implantable electrical conductor wires and coils of MP35N and drawn MP 35N tube filled with a silver core (DFT) were tested in rotary bending to evaluate fatigue characteristics and failure mechanisms, Detailed stress analysis enabled evaluation of helical coil fatigue in bending with respect to theoretical stress, The relations between the maximum cyclic stress (S) and the cycles to failure (N) were obtained by intro ducing cyclic stresses on over 600 samples, Microscopic evaluation usi ng scanning electron microscopy was performed to identify failure mech anisms, The results show that the stress analysis presented is a viabl e means for assessing the stress state in a composite coil structure i n rotary loop testing, comparable S-N fatigue data sets for MP35N and DFT conductors are not consistently significantly different, S-N relat ionships for coils and wires of the MP35N and DFT appear to lie on the same curves, and failures of all sample types initiate on the outside surface of the wire, Differences in crack propagation and final fract ure are shown as a function of sample type and cyclic stress, (C) 1998 John Wiley & Sons, Inc.