PSI - Issue 28

Costanzo Bellini et al. / Procedia Structural Integrity 28 (2020) 2283–2290 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Fig. 8. Influence of cycling on C parameter (Di Cocco and Natali (2018)).

It is possible to describe the parameter C as a function of the number of cycles by using a power formulation: ܥ ൌ ܭ ߝ ௃ (3) where, for the investigated SMA, K=1 and J=-0.046. The parameter D represents the hysteresis introduced by the load condition. For the investigated NiTi alloy, its value is equal to 700 for load condition and 70 for unload condition. 4. Conclusion In this work, a thermodynamic model has been used to evaluate the relation able to predict the quantity of austenite and martensite in the first 100 cycles of loading-unloading tests carried out in a NiTi Shape Memory Alloy (SMA). The analysis of the diffraction spectra allows evaluating the crystalline cells of austenite and martensite, and to calculate the cell parameters. It is noted that cell parameters of martensite exceed the dimension of austenite in the same condition of a non-transformed structure. It means that the excess of dimension could be the cause of the decrease of the stress-strain slope during the austenite-martensite transformation. This aspect must be taken into consideration for the formulation of mechanical models which, starting from the real quantity of austenite and martensite, are able to predict the mechanical behaviour of the SMA. It is worth recommending that, during the structure transformation, a portion of the energy is used to change the microstructure, while the rest is used for the mechanical behaviour. It means that not all the phases participate in mechanical resistance, but some of them are used in atom lattice position changing. References Bellini, C., Natali, S., 2019. A constitutive model to predict the pseudo-elastic stress-strain behaviour of SMA. Procedia Structural Integrity 18, 858–865. Carpinteri, A., Vantadori, S., Di Cocco, V., Iacoviello, F., Natali, S., 2018. Fatigue Analysis of a near-equiatomic pseudo-elastic NiTi SMA. Theoretical and Applied Fracture Mechanics 94, 110–119.