Unalloyed pyrolytic carbon for implanted mechanical heart valves.

摘要

Materials for implanted heart valves face challenges unmatched for most mechanical applications. The valves must function without interruption for lifetimes in the order of 10(9) cycles in a corrosive, hostile environment. A particular carbon has been widely used for the past three decades. The mechanical behavior of this isotropic pyrolytic carbon (PyC), with and without silicon alloying, was studied in this research program. Several questions were addressed: the fatigue strength at lifetimes of 10(9) cycles, the threshold for crack growth, the validity of fracture mechanics for PyC, and the statistics of ultra-high survival under cyclic stress. It was found that cyclic stressing within the scatter band of the static strength did not initiate cracks. Furthermore, artificially induced cracks did not grow under cyclic stressing below a threshold stress intensity factor. This threshold stress intensity factor was valid over a wide range of crack lengths. No failures occurred when batches of 29 specimens were tested above the service stress at 6 x 10(8) cycles. Cyclic stressing did not reduce the static strength. The service stress is a small fraction of the fracture strength; thus, a very high probability of survival can be ensured by a proof test of the assembly.