PSI - Issue 40

A.V. Zinin et al. / Procedia Structural Integrity 40 (2022) 470–476 A.V. Zinin at al. / Structural Integrity Procedia 00 (2022) 000 – 000

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microscopic "dents" on the surface, which are a sign of the high stresses (Laird 1977). Such a structure is called "tracks" and "rubber tire prints" (Smirnova, Zinin 2021). In Fig. 4a, short tracks between parallel terraces are clearly visible on the width of one groove; in Fig. 4b, "rubber tires prints" are recorded.

Fig. 3 The type of fractures of samples after cyclic tests under various modes: a) high-cycle fatigue loading, σ a = 315 MPa, N p = 1.2x10 6 ; b) high-cycle fatigue loading, σ a = 315 MPa, N p = 9 x10 5 after preliminary low-cycle overloads, ε a = 0.5%, N low = 80.

Fig. 4 Fatigue fracture view of a steel 10G2S1 sample subjected to preliminary training at ε a = 0.5%, N low = 80; x2560; a) track traces on striations; b) "rubber tire prints".

3.3. Influence of overloads on lifetime The effect of overload damage on the lifetime of steel samples was evaluated by comparing the original high cycle fatigue curves and the secondary fatigue curves obtained by loading with preliminary "training". The analysis of the results of fatigue tests of two types of steels presented in Fig. 5, a-c results of fatigue tests of samples made of two types of steels with preliminary overloading by elastoplastic cycles show that cyclically strengthening steel 10G2S1 increases its lifetime under "training" by a small number of cycles ( low N = 5) and amplitude of elastoplastic strain a  = 0.2%. However, modes of hard loading with high amplitudes of deformations and longer duration of "training" fatigue curves show the reduction of lifetime and limited fatigue strength due to increased damaging of the material at preliminary cyclic elastoplastic strain. The cyclically softening steel 14Kh2GMR at all conditions of pre-strain shows the effect of increased damaging of the material by overloading, leading to a decrease in lifetime and a lower level of the limited fatigue strength compared to the "untrained" material. Thus, the results of studies of the effect of overloading on the lifetime of the material show that the presence of individual cycles of elastoplastic strain during non-stationary loading, depending on the material type and process parameters, can lead to both a decrease in resistance to fatigue failure and an increase in fatigue characteristics due to additional strengthening of the material.