Mechanical fatigue and cyclic stability of structures made of thin superelastic TiNi-based wire with phase composition controlled by synchrotron radiation

The results of a synchrotron study of the phase composition and cyclic stability of a thin wire and a metal mesh made of a NiTi alloy filament are presented. Fatigue tests of NiTi alloy wire in air and in aggressive media have shown high fatigue resistance. More than 1 million load-unloading cycles related to stress relaxation due to superelasticity have been performed. When the knitwear is deformed by stretching, the effect of superelasticity is suppressed, and the deformation diagram demonstrates nonlinear hyperelastic behavior. This is due to the formation of a complex stress state with inhomogeneous fields of deformations and stresses in the loop material during stretching, which is confirmed by the results of numerical modeling. An analysis of the results of numerical modeling of the representative cell stretching of the studied metal mesh showed the formation of a complex stress state with inhomogeneous zones of deformations and stresses in the loop material. It is established that during uniaxial metal mesh stretching, stress concentrators appear on the contact pads in areas of greatest curvature, and maximum stress values occur in localized zones. Cyclic loading of a metal mesh in air and in a 1% HCl solution showed that the samples of the mesh passed 1.5 million cycles in air without destruction, which is significantly higher than that of NiTi alloy wire. When cycling NiTi alloy wire in a 1% HCl solution, starting from 104 cycles, there is a decrease in the values of the martensite shear stress during forward and reverse martensitic transformations. In cyclic tests in 1% HCl solution, thin wire samples showed less ability to resist fatigue in contrast to air tested samples.The research was carried out within the framework of the state assignment of the Ministry of Science and Higher Education of the Russian Federation under the project No. FVR-2020-0022.The research was performed using the equipment of the Tomsk Regional Common Use Center of the Tomsk State University with the support of a grant from the Ministry of Science and Higher Education of the Russian Federation 075-15-2021-693 (No. 13.RFC.21.0012).