PSI - Issue 38

Di Song et al. / Procedia Structural Integrity 38 (2022) 546–553 Di Song and Chao Yu / Structural Integrity Procedia 00 (2021) 000–000

548

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meets the ASME requirements for the medical application (endovascular stents) of NiTi SMAs (i.e., with the wall thickness less than 400μm and the Af between 298 and 303 K), and presents a super-elasticity at the human-body temperature (i.e., 310 K). The specimen length and the gauge length are 60 mm and 20 mm, respectively. All the multiaxial fatigue tests are performed under the stress-controlled cyclic loading conditions by an MTS858-5KN machine, and the test temperature is set as 310 ± 0.5 K using a constant temperature/humidity chamber of SDH4004 (made by Chongqing Inborn Experiment Instrument Co., Ltd, China).

800

 f

AM =610MPa

 s

AM =472MPa

600

200 Axial stress  , MPa 400

 f

MA =65MPa

 s

MA =148MPa

0

0

2

4

6

8

10

12

Axial strain 

Fig. 1 The representative tension-unloading stress-strain curves of NiTi SMAs. The representative tension-unloading stress-strain curve of NiTi shape memory alloy adopted in this paper is shown in Fig. 1. It can be observed that such alloys present obvious martensite transformation and reverse transformation process, which is different from other metals and steels. The recoverable strain in a tension-unloading cycle can reach approximately 8%, and it is called the “superelasticity” of shape memory alloys. Such unique deformation property leads to the deviation by using the classical fatigue models to predict the fatigue lives of NiTi SMAs, and the prediction model for the residual strains accumulation per cycle are also few in the literature. The loading cases of non-proportional multiaxial loadings are given in Fig. 1 and Fig. 2, respectively. It should be noticed that in the prediction of residual strains, two types of non-proportional multiaxial paths (square, butterfly typed) are involved; and in the prediction of fatigue lives, there are five types of paths (square, hourglass-typed, butterfly-typed, rhombic, and octagonal ones) performed in this paper. The multiaxial loading cases in the fatigue tests are with the axial peak stresses of 566, 637, and 683MPa, and their equivalent peak stresses that meet the von Mises criterion are 632, 712, and 766MPa, respectively. (a) (b)

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 3





Fig.2 The prescribed non-proportional multiaxial loading paths used in the residual strain prediction model: (a) square; (b) butterfly-typed.