Study of the Influence of Inclusions on the Behavior of NiTi Shape-Memory Alloys in Thermal Cycling by Means of Finite E
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INDUSTRIAL applications of shape memory alloys are increasingly penetrating markets. The continuous progress in material manufacturing during the last decade allows provision of more and more reliable materials; Ti-rich Ni-Ti alloys, in particular, are used to develop compact and light actuators. Moreover, thanks to the introduction of thin Ni-Ti wires in applications where miniaturization needs are crucial, extremely low response times can be achieved.[1] The technical and economical success of SMA actuators strongly depends on the correct material exploitation, in particular, when extremely high stability and fatigue life are required; in this case, design shall be developed by skilled engineers. A further improvement of stability (functional fatigue) and fatigue resistance could make design easier and promote a faster spreading of this technology. This objective drives research activities devoted to better understanding the fatigue effect on the material and to developing improved alloys or material processing. In particular, activities aiming at reducing the inclusion contents in Nitinol are ongoing.[2] Despite the fact that this effort could be fully justified by standard notions about fatigue predicting the detrimental effect of inclusions on fatigue life (e.g., Reference 3), the real consequence of inclusions is still controversial: past publications claim no difference exists among materials MARCOFABRIZIO URBANO, Head of Technology and Engineering, is with the SAES Getters S.p.A, Lainate, Italy. Contact e-mail: [email protected] Manuscript submitted March 25, 2011. Article published online October 19, 2011 2912—VOLUME 43A, AUGUST 2012
with different density of carbon inclusions for superelastic Nitinol.[4] In another work,[5] through fracture mechanics considerations, it is proposed that inclusions with dimensions smaller than 50 lm may not influence the fatigue properties. However, the fracture mechanics approach, as admitted by the authors, is complementary and not substitutive of traditional life-prediction methodologies, which could lead to different conclusions. Indeed, experimental works demonstrating the correlation or independency of inclusion density and dimension vs fatigue life on a solid statistical basis are still missing. Theoretical works, on the contrary, indicate hard particles such as inclusions as crack starters; by utilizing finite element method (FEM), the stress/strain field evolution during rotary bending was studied.[6] According to the authors, the existence of inclusions changes the stress distribution in the cross section and the stresses greatly increase. When the inclusion is at the matrix surface, the maximum stress is greatly increased and may induce the fatigue crack to form. The effect of inclusions in superelastic fatigue is thus still poorly understood. When actuation materials are considered, the situation is even worse. Scarce literature is available in general on the subject of thermal cycling fatigue. Moreover, to our knowledge, the role of inclusions has never
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