PSI - Issue 23

A.A. Shanyavskiy et al. / Procedia Structural Integrity 23 (2019) 63–68 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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“fatigue limit” there is no idea of the ratio of σ – 1 / σ 0.2 . To make this conclusion, the results of fatigue tests achieved on various aviation steels and alloys were analyzed by Shanyavskiy and Soldatenkov (2019). The vast majority of alloys based on Fe, Al, Mg, Ti, Cu, Ni is appeared to meet the inequality: σ – 1 /σ 0.2 < 1 (Fig. 2). It means that for the majority of alloys, a mesoscale level corresponding to HCF regime exists in structures. As the ratio of σ – 1 /σ 0.2 increases, the mesoscale level corresponds to a smaller stress interval. Finally, a small number of alloys that are not used in the load-bearing structural elements may experience residual strain, meeting the inequality: σ – 1 /σ 0.2 ≥ 1.

Fig. 2. The ratio σ – 1 /σ 0.2 dependences on (a) σ 0.2 and (b) σ 0.2 /σ U for more than 250 aviation materials based on Fe, Al, Mg, Ti, Cu, and Ni.

By quantitative analysis, it was found that fatigue limit dependences on both yield stress and ultimate strength can be approximated by power-law relations in the following forms: σ – 1 = A ·(σ 0.2 ) α or σ – 1 = B ·( σ U ) β . The following values of factors for dependences applicable to aviation materials under consideration were obtained: for Cu-based alloys, A = 7.88, α = 0.51 and B = 1.44, β = 0.75; for Fe-based alloys, A = 6.52, α = 0.63; for Al-based alloys, B = 0.66, β = 0.86. For materials based on Mg, Ti and Ni, a large scatter is observed on fatigue limit dependence on both yield stress and ultimate strength. That is why functional relations between the considered characteristics of these materials weren’t obtained . Within the new understanding of the metal evolution as a synergetic system, the following situation with the transition from VHCF to HCF, and, further, to LCF was considered (Fig. 3). The presented diagram takes into consideration the differences in the metal behavior under cyclic loading on three scale levels, and metal behavior evolution with increasing stress, in accordance with the dependences shown in Fig. 2.

Fig. 3. Fatigue diagrams of metals with the widths of bifurcation regions, Δ q wi , plotted in accordance with the paradigm of decreasing stress level for σ – 1 /σ 0.2 = 1, and for the general case, when σ – 1 /σ 0.2 < 1.