BS in Materials Engineering
Materials Engineering Department
To characterize the decrease in ductility caused by an induced stress concentration as a function of various levels of cold work and heat treatment, Nitinol medical guide wires with a .004″ diameter were tested using both a standard, straight tensile test and a modified, bent tensile test. The ratio of the two break loads (bent:straight) for identically processed samples was used as a proxy for measuring the ductility of the guide wires: the higher the ratio, the more ductile the sample. The average break load ratio as a function of increased cold work, decreased from 1.27 to 0.91 for the heat treated samples, and decreased from 1.28 to 1.13 for the non-heat treated samples. The heat treated wires had lower break load ratios - a difference of 0.105 on average - compared to the similarly cold worked, non-heat treated wires. The magnitude of the decrease in break load ratio between heat treated and non-heat treated wires is dependent on the level of cold work; there was a difference of 0.01 for the lowest level of cold work, and a difference of 0.22 for the highest level of cold work. The decrease in break load - and ductility by proxy - from increased cold work and heat exposure is caused by the suppression of the austenite-martensite phase transformation as a result of these processes.
Available for download on Monday, June 13, 2022