PSI - Issue 16

Volodymyr Iasnii et al. / Procedia Structural Integrity 16 (2019) 67–72 Volodymyr Iasnii, Petro Yasniy / Structural Integrity Procedia 00 (2019) 000 – 000

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With the change of the stress ratio R from 0 to 0.5 the residual strain significantly increases in first and next cycles even at lower stress ranges (Fig.3a). A significant difference in residual strain is explained by the fact, that during the testing with complete unloading ( R = 0), the residual strain was defined at stress σ min = 0, and during the testing with not complete unloading at stress it was defined at σ min = 0.5 σ max. The functional properties of the pseudoelastic SMA can also be characterized by the strain range. The dependences of strain range on the loading cycle number for different stress range values in the first cycle are shown in Fig. 3b. Values of stress range correspond to the half-cycles to failure. a b

Fig. 3. Dependences of the residual strain – a) and strain range – b) on the number of loading cycles. Δσ = 492 MPa (16), 539 MPa (13), 580 MPa (10), 727 MPa (12) at stress ratio R = 0, Δσ = 243 MPa (18), 305 MPa (19) at R = 0.5. A rapid decrease of the strain range was observed for all values of the stress range Δσ during the first ten loading cycles at R = 0, followed by the stabilization area of strain range, or its less or more intensive reduction till fatigue failure of the specimen. With the increase of stress range from 492 MPa to 727 MPa, the strain range in the first cycle increases from 3.7% to 7.7%. The strain range is significantly less at stress ratio R = 0.5 (part unloading) than at the tests with full unloading ( R = 0), though this tendency is observed with the increase of loading cycles number.

Fig. 4. Dependences of the dissipation energy on t he number of cycles. Δσ = 492 MPa (16), 539 MPa (13), 580 MPa (10), 727 MPa (12) at stress ratio R = 0, Δσ = 243 MPa (18), 305 MPa (19) at R = 0.5.