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

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as a damage parameter, which represents the damage evolution rate in the prescribed loadings. The higher damage evolution rate corresponds to the shorter fatigue life. Then, the fatigue life can be simply expressed as a linear form, written as: � = ∙ ( ��� (20) − ��� (1)) + (2) where the ��� (20) is the residual strain of the 20 th cycles, and ��� (1) is the residual strain of the first cycle, and are material parameters. By fitting the fatigue lives of three loading cases of square path 566 and 637MPa, and butterfly-typed path 566MPa, the value of the material parameters and can be determined and shown in Table 2. Table 2 Material parameters in the proposed life-prediction model = −108.62 , = 849.89; By applying the proposed life-prediction model on the five types of non-proportional multiaxial fatigue experiments with various peak stresses, the prediction results are shown in Fig. 5. It should be noted that only 3 loading cases of square and butterfly-typed paths are adopted to obtain the value of parameters, and for all the other loading cases of hourglass-typed, rhombic, and octagonal paths, the fatigue lives shown here are all the predicted ones. It can be observed that the predictions for all the five types of multiaxial paths are within twice error bands, which indicates the accuracy of the proposed life-prediction model in the fatigue life region of less than 10 3 cycles.

Fig. 5 Life-prediction results of the proposed model for five types of non-proportional multiaxial loading paths

4. Conclusions New prediction models for residual strains and fatigue lives of NiTi SMAs have been proposed in this paper. In these two models, the effect of the peak stress, transformation extent, and the non-proportional multiaxial degree on the residual strain in the cyclic loadings and the fatigue lives are taken into considered. The predicted residual strains per cycle accord well with the experimental ones, and the fatigue lives for all the loading cases are located within twice error bands for the five types non-proportional multiaxial loading paths with various loading levels, indicating the models proposed are appropriate for predicting the cyclic deformation and fatigue failure in various non-proportional multiaxial loading cases for NiTi SMAs.