PSI - Issue 33

Emanuele Sgambitterra et al. / Procedia Structural Integrity 33 (2021) 1073–1081 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Elastic strain range and strain to failure of the polymer (  f ) must be higher than the shape memory strain recovery of the SMA (  rec ).

Fig. 1. Schematic depiction of the thermal hysteresis (strain vs temperature) of the SMA and main thermo-mechanical constraints for SMA polymer integration.

Based on these constraints, the commercial materials reported in Table 1 were selected for SMA-polymer sample preparation in this preliminary study.

Table 1. Selected materials.

Material type

Trade name SmartFlex

Manufacturer Saes Memry

Shape memory alloy

Polymer matrix

Epoxy crystal ng Cores s.r.l.

2.2. SMA thermo-mechanical properties Shape memory alloy wires with diameters ( d ) of 0.3 mm were used in this investigation as they combine significantly high recovery force (F rec around 20 N) with a large surface to volume ratio. This latter parameter is of major concern as it directedly affects the adhesion strength at the metal-polymer interface. Figure 2.a reports the measured stress strain curve of the wire as obtained from an isothermal tensile test carried out in martensitic conditions (T=25°C), together with significant mechanical parameters. The test was carried out after a complete heating/cooling cycle between TTs in order to reset the material pre strain carried out by the manufacturer and to get a martensitic microstructure at room temperature. Figure 2.b reports the differential scanning calorimetry curve of the material together with the measured values of the TTs. However, it’s worth noting that TTs measured from DSC experiments represent the temperature activation ranges of the SMA in stress free condition that can be different from the ones experienced in service, due to the marked thermo-mechanical coupling of SMAs. To this aim, both TTs and shape recovery strain (  rec ) of the material, that is in terms one-way (  ow ) and two-way strain (  tw ), were measured as a function of the martensite pre-strain, as shown in Fig. 3. In particular, strain temperature measurements, shown in Fig. 3.a, were carried out under complete thermal cycles in the range -30÷130 °C, for different values of the martensite pre-strain and under a fixed tensile load of 10.6 N corresponding to an axial stress of 150 MPa.