PSI - Issue 24

Alessandro Pirondi et al. / Procedia Structural Integrity 24 (2019) 455–469 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

459

5

However, tensile stress monotonically increases to a value of 550.69 MPa (Fig. 4) as the wire temperature is brought up to 96.7°C (A f ) and this large change affects the stress distribution as depicted in Fig. 2b and Fig. 3b.

Fig. 3.  22 distribution in 90° fibre-oriented top layer: a) 1

st Cylindrical Shape; b) Actuation stage; c) 2 nd Cylindrical Shape (Gandhi et al. (2018)).

Fig. 4. Plot of average axial stress developed in Nitinol wire during various stages (Gandhi et al. (2018)).

From the FEA results, a conclusion can be drawn out that it is possible to use SMA wires to snap the shape of unsymmetric laminates from one stable configuration to the other. However, two main limitations affect the model developed in this section, namely: i) a simplified mechanical behavior of SMA wires that does not come from a thermodynamically consistent scheme; ii) the number and size of wires is quite large with respect to the size and thickness of the laminate, therefore manufacturing issues must be carefully evaluated; iii) beside manufacturing issues, a large preliminary effort has to be spent in finding by trial-and-error a suitable, but not necessarily optimal, combination of design parameters that makes it possible the SMAC plate snap-through. For these motivations, a more complete and sound thermomechanical behaviour of SMA wires has to be adopted and an optimization procedure is required, which will be described in the next section.