Shape memory hooks and their properties

Shape memory (SM) hooks belong to the category of SM fasteners utilizing the effects of the martensitic phase change transformation. The SM hooks may be incorporated into textiles and function in the same manner as Velcro fasteners do. The main objective of our research was to find the influence of the material properties and geometry parameters of pseudoelastic NiTi micro-hooks on the maximum interlocking force necessary for releasing the NiTi hook connection. Since it is not trivial to predict the maximum interlocking force through modeling (due to the non-linear material characteristics, the large shape changes and the contact problem description) we chose an experimental approach first. We measured the interlocking force using TiNi samples made of NiTi wires of various diameters. The shape setting was carried out giving various heat treatment to our NiTi samples. Several hook shapes were tested with a mechanical force tester. Another purpose of the study was to propose a method for aligning of the micro-hooks into an array and attaching them to a flexible substrate.
We have found an optimal heat treatment resulting in minimum R-phase in the NiTi hooks and stable stress-strain loops. Furthermore, we propose the manufacturing route using elliptic NiTi springs aligned in rows.