The Influence of Overheating and Notches on TiNi Shape Memory Alloy Spherical Segments Functionality

The effect of spherical segment form recovery by leap under re-heating has put into practice in shutoff armature and thermoswitches. The main condition of reliability of devices containing spherical segments is guaranteeing of their stable functioning at multiple cycles of “bending in martensite ↔ re-heating up to austenite with an abrupt clap and, if a counter body presents, with an impact” for doing of operative action. That’s why the guaranteed reliability of such elements sort is necessary.

The research methods of the influence of overheating of spherical segments in free and built-in states after some number of cycles are represented in article.  It is shown, in particular, that the aging at temperatures of 100°Ń, 150°Ń during near an hour leads to clap effect intensification. Impact force is visibly increases. If the aging segment is in built-in state we can observe shape memory effect suppression and decrease of the impact force at the same time. In the previous articles we have found out that the edges are the most spherical segment stressed areas. The absence of stress concentrators on edges is, for example, a result of segment treatment by electro-spark machine tool. These concentrators initiate arising and development of cracks. These processes are controllable. It was found that the high stressed area during a bending, mirror-like to the initial form, in martensite and under re-heating is located at some distance from edge and depends on segments radius of curvature. The most stressed area, which is greatly sensitive to fracture, was found out using the specially made notches. Depending on radius of curvature, the interconnection between a crack resistance and notch dimensions is given in this work. We have made the scheme of arising and growth rate of cracks under mechanical cycling, which takes into account the specially made notch dimensions.