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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integration. Figure 7.b shows that thermal activation cycles do not significantly affect the mechanical strength of the SMA-polymer samples. In fact, similar values of the maximum stress were recorded in the two types of samples. However, tests carried out under constant thermal activation of the SMA wire show a completely different behavior, as illustrated in Fig. 7.c. In fact, the maximum load is significantly lower than the previous cases, that is with maximum normal stresses around 300 MPa. This is attributed to the combination of mechanical and shape recovery stresses, resulting from thermal activation of the SMA wire, coupled with a reduction of polymer strength when increasing the temperature in the near T G region. This seems to be confirmed by the higher ductility observed in these samples, that is with a large stick-slip plateau beyond the maximum stress. 4. Conclusions This work reports results of preliminary investigations of an R&D project on the development of shape memory alloy -based composites with shape morphing capabilities for active aerodynamics of motorized vehicles. The work addresses a major issue for SMA-polymer integration, that is the limited interfacial strength between metals and polymers. In particular, pull-out tests of SMA-polymer samples were carried out under different thermo-mechanical loading conditions and the results can be summarized as follow: • As manufactured SMA-polymer samples show a significantly high adhesion strength, that is corresponding to a normal stress in the wire of around 900 MPa. This value is significantly higher that the critical detwinning stress of the SMA and is comparable with maximum recoverable stress in the SMA during thermal activation; • The load capacity of the SMA-polymer samples seems to be mainly unaffected by a few thermal activation cycles (up to 1000). In fact, maximum stress obtained from pull-out tests is very similar to that of as manufactured samples; • A marked drop of the adhesion strength is observed when applying both mechanical and thermal stresses, that is from pull-out tests during constant thermal activation. This is attributed to the combination of mechanical and shape recovery stresses, coupled with a reduction of polymer strength the near T G region. Based on these preliminary results further studies will be carried out with the aim of improving the metal-polymer interface strength. This can be obtained by different approaches, that is either by surface modifications by chemical/mechanical treatments, or by improving the chemical compatibility between polymer and SMA. Acknowledgements This work was carried out within the R&D industrial project ARIA (Active Responsive Intelligent Aerodynamics), funded by the Italian Minist ry of Research with the “PON Research and Innovation” framework 2014-2020. In addition, this works is part of the research program proposed by the Department of Mechanical, Energy and Management Engineering for the R&D project “Attraction and International Mobility”. Barrett, R., Gross, R.S., 1996. Super-active shape-memory alloy composites, Smart Materials and Structures 5, 255-260. Bil, C., Massey, K., Abdullah, E.J., 2013. Wing morphing controlwith shape memory alloy actuators. Journal of Intelligent Material Systemsand Structures, 24(7), 879 – 898. Concilio, A., Antonucci, V., Auricchio, F., Lecce, L., Sacco,E., 2021. Shape Memory Alloy Engineering, 2nd Edition, Butterworth-Heinemann – Elsevier. Di Cocco, V., Iacoviello, F., Maletta, C., Natali, S., 2014. Cyclic microstructural transitions and fracture micromechanisms in a near equiatomic NiTi alloy. International Journal of Fatigue 58, 136-143. Duerig, T., Pelton, A., Stockel, D., 1999. An overview of nitinol medical applications. Materials Science and Engineering A 273 – 275, 149 – 160. Ferede, E., Karakalas, A., Gandhi, F., Lagoudas, D.C., 2021. Numerical investigation of autonomous camber morphing of a helicopter rotor blade using shape memory alloys. In 77th Annual Vertical Flight Society Forum and Technology Display 2021. References