Issue 29

A. Fortini et alii, Frattura ed Integrità Strutturale, 29 (2014) 74-84; DOI: 10.3221/IGF-ESIS.29.08 77 ρ ΔH ε    A A CR L C T (1) ρ ΔH ε    M M CR L C T (2) where the NiTi density ρ is assumed 6.45 g/cm 3 and the critical temperature T CR is calculated as (A s +A f )/2. To memorise the bent shape, the strip was subjected to a double heat treatment: it was previously strained at room temperature and wounded on a cylindrical jig to reach a circle shape. This fixture was placed into a tube furnace, heated at 450 °C for 25 min and quenched by water-cooling. These specific temperature and time were chosen according to the results of a previous experimental study [15], which demonstrated that this combination allows reaching a 92% of shape recovery. Subsequently, the strip was strained at room temperature applying opposite bending couples acting at the ends and locked into an arc clamp. The strip was thus thermally treated as previous to memorise this bent shape, with an initial curvature radius R 0 equal to 42.32 mm. Training procedure The heat-treated NiTi strip was subjected to the thermomechanical cycling by means of the shape memory cycling method [13]. A specifically designed training sequence was applied to the sample repeating the following steps for 30 cycles: pre-straining at a T< M f (room temperature) to reach the desired cold shape, ii) heating the specimen above A f (140 °C) through a hot air stream flow, iii) cooling below M f (room temperature) by natural convection. Therefore, the strip was strained to a flat shape, applying a uniform bending load at room temperature, and fixed into a supporting structure, with one end held in a clamp. In order to ensure reaching the phase transformation temperature on heating, a K-type thermocouple was placed in the inner radius of the strip and kept in good thermal contact with it by conductive aluminum tape. Heating the strip to high temperature above A f was achieved by hot air stream flow whereas cooling to room temperature, below M f , was realised through natural convection. To study the curvature evolution, sample images were acquired by a digital camera at the end of the heating process, Fig. 2a, as well as after cooling to room temperature, Fig. 2b. Interpolating the axis of the strip with a three-point arc, by means of a CAD software, the hot radius, R H , and the cold radius, R C , were collected. (a) (b) Figure 2 : (a) Hot shape; (b) Cold shape. It is well known that there is a limit to the amount of reversible strain that can be achieved in TWSME [14]. In particular, the two-way recoverable strain is significantly less than the one-way recoverable strain and it is typically in the neighborhood of 2%. In the present study the bending deformation strain was calculated according to Eq. (3): 0 ε 2R   tr t t (3)