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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Nomenclature A

Austenite phase

M Martensite phase TMT thermoelastic martensite transformation TIM Thermal induced martensite SIM Stress induced martensite TTs transformation temperatures of the SMA T Min Minimum operation temperature T Max Maximum operation temperature d SMA wire diameter  s shear strain at the metal-polymer interface M s Martensite start TT M f Martensite finish TT A s Austenite start finish TT A f Austenite finish TT  rec shape memory strain recovery  rec shape memory stress recovery  ow one-way shape memory strain recovery  tw two-way shape memory strain recovery E mt Young’s modulus of twinned martensite E mdt Young’s modulus of detwinned martensite  det Young’s modulus of detwinned martensite  Y yield strength T G Glass transition temperature T C Curing temperature  f strain to failure

1. Introduction Shape Memory Alloys (SMAs) belong to a unique class of material that are able to restore their shape even after experiencing very large deformations (Concilio et al. 2021). Among the different alloy systems, the binary nickel titanium one (NiTi) got the highest commercial success, thanks to their excellent mechanical and functional features, coupled with good corrosion resistance and biocompatibility (Duerig et al. 1999). Shape recovery properties in SMAs are directly linked to the so-called thermo-elastic martensitic transformation (TMT), that is a reversible solid state phase transition between a parent phase, the body centered cubic austenite (A), and a product one, the monoclinic martensite phase (M). TMT can be obtained either by temperature variation (TIM, thermally induced martensite), through the so-called transformation temperatures (TTs), or by mechanical stress (SIM, stress induced martensite), through the transformation stresses. Shape memory effect (SME) represents the shape recovery property of SMAs upon heating, due to TIM, and can occur under antagonistic stresses, resulting the development of a mechanical work, that can be exploited in actuation systems. Due to their high recovery stress (up to 800 MPa) and strain (up to 4%), NiTi SMAs exhibit very high energy density if compared with other actuation technologies. However, the industrial use of NiTi is currently limited to a few smart and/or active parts in nice and high demanding applications in different fields, ranging from robotics (Furuya and Shimada 1991, Lange et al. 2015, Lee et al. 2019), automotive (Mohd et al. 2014a, Stoeckel 1990, Mohd et al. 2014b), aerospace (Bil et al. 2013, Hartl and Lagoudas 2007, Ferede et al. 2021), biomedical (Morgan 2004, Petrini and Migliavacca 2011), civil engineering (Sgambitterra et al. 2016, Torra et al 2017) etc. In fact, the widespread use of such materials in large scale industrial application is mainly hindered by both commercial and technical issues as the high material cost, the very difficult processability by conventional manufacturing processes and to lack of engineering knowledge and design tools on material behavior and reliability, especially