Study On Thermomechanical Treatment, Mechanical Properties and Fatigue of Nitinol Superelastic Thin Sheet
M. Mertmann, W. Oswald, Memry GmbH, Weil am Rhein, Germany; R. Steegmueller, Admedes Schuessler GmbH, Pforzheim, Germany; A. Schuessler, ADMEDES SCHUESSLER GmbH, Pforzheim, Germany
Nitinol thin sheet with an ingot composition in the range of standard superelastic material with about 50.8 at-% Ni, balance Ti was used to manufacture a series of samples with different thermomechanical conditions. Production parameters such as cold work, heat treatment temperature and heat treatment time were varied. All finished samples were of the same final thickness of 0.3 mm and were undergoing the same industrial surface finishing process in order to obtain a smooth, defect and oxide-free, shiny surface. Before carrying out the laser cutting the material was characterized by tensile testing, DSC and bend-and-free recovery (BFR) test. Afterwards, miniature dogbone specimens were cut from the as-manufactured sheets in both directions, longitudinal as well as transverse to the rolling direction. These samples were surface finished using a standard deburring and electropolishing process. On a few sheet coupons there were also samples taken in 45° to the rolling direction. All qualified samples were then exposed to fatigue testing in a 3-point bending mode until fracturing occurred at a frequency of 20 Hz in 37°C constant air temperature conditions. The results showed that there is a significant effect on the fatigue performance of the samples of both, the applied thermomechanical treatment as well as the sheet anisotropy. It is also obvious that the achieved strain data is in average lower than the data obtained in comparable studies on tube or wire, which can be attributed to the different test setup (3-point bending in air) as compared to most other studies as well as the larger surface. The obtained results were used to chose an ideal set of process parameters for sheet material, which has specifically long fatigue life as compared to material processed according to the standard manufacturing route.
Summary: A wide range of different thermomechanical treatments is performed on commercially available superelastic Nitinol thin sheet. The material properties are characterized by mechnical and thermal procedures. After laser cutting of dogbone samples a 3-point bending fatigue test is applied.