The alloys under study include 316L stainless steel, 6Al4V titanium, and several cobalt alloys, including L605, Elgiloy and MP35N, in both fine wire and bar stock form. Last year we reported that we had observed that the fatigue life of MP35N wire could be changed by two orders of magnitude by prestretching the wire an amount consistent with its as-received cold worked condition. Such cold working is common during the fabrication of interventional cardiology devices and is sometimes performed during surgery in orthopedic procedures. We are completing those experiments by characterizing the hardness and grain structure of the virgin and prestretched wire and attempting to correlate the fatigue behavior with the results of stress-strain tests. However, microhardness measurements indicate that hardening is occurring and manifesting itself in the change in fatigue behavior.
Experiments using noncontact video extensometry and other methodologies are underway.
We have also initiated a full-factorial experiment to evaluate the effects of test frequency and physiological environment on the fatigue behavior of 316L and 6Al4V titanium alloys. These alloys were selected as being the most and least susceptible to corrosion fatigue among the commonly used implant alloys. The results of this project should allow us to develop definitive guidance regarding the experimental protocols for characterizing the fatigue behavior of these implant alloys and devices made from them.