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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crystal, never changing it. It can modify its position into the lattice changing just the system. For instance, in many NiTi SMAs the initial structure is cubic (often named as austenite) which can modify in monoclinic (named as martensite) under the effect of external loads. Neither austenite nor martensite is the same as the equivalent structures of iron alloy, but it is only a convention. There are many SMAs, as the copper-based or the ferrous based SMAs, but the most used are the equiatomic nickel titanium SMAs. The NiTi SMAs are characterized by good mechanical and corrosion resistance due to the characteristics of titanium and nickel, and they can be used to produce many components that work in many different environments, even aggressive ones. In order to predict the behaviour of components made of SMAs, many approaches are available in the scientific literature, taking into account also the microstructure changing. However, there are not many models based on the correct identification of the mechanical characteristics starting from the composition of the alloy phases, and the result is that often the geometric transferability is not guaranteed. For instance, there are many models based on the assumptions that the SMAs show a stage of the stress-strain curve where there is not any increment of stress during the deformation (plateau stage - Fig. 1), with the assumption that in this part of the curve the transformation from austenite to martensite, or vice versa, is linear.

Fig. 1. Stress-strain behaviour of a SMA (Carpinteri et al. (2018)).

The latter assumption implies that at the middle point of the plateau the microstructure is constituted by 50% of austenite and 50% of martensite. This is wrong as demonstrated in a recent work of Carpinteri et al (2018), where the measure of the quantity of austenite and martensite in the middle of the plateau is far from 50% both in case of loading and unloading applications. It is worth noting that the effect of hysteresis leads to a delay of transformation, implying a great quantity of transforming phase compared to the transformed one. Approaches using the simplified linear relationship between stress and strain are able to predict the behaviour of SMAs in simple geometry components with uniaxial loads, as shown by Di Cocco et al. (2018). In the work of Maletta (2012) and Maletta and Furgiuele (2011), there are models based on the linear elastic fracture mechanics that are able to predict much more complex cases, especially in case of small scale yielding where the crack tip stress field is calculated, extending the old approaches where the stress transformation takes place at constant stress.