PSI - Issue 36

Sergiy Bezhenov et al. / Procedia Structural Integrity 36 (2022) 356–361 S. Bezhenov / Structural Integrity Procedia 00 (2021) 000–000

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3. Results and Discussion

The results of fatigue tests were presented in accordance with hypothesis Bezhenov (2008) about the existence of a pole of curves of HCF for materials of the same class with di ff erent technological heredity, according to which the traditional two-parameter equation of the fatigue curve (Wo¨hler’ line) is transformed into an equation where, in addition to the values of number of cycles before failure and amplitude of cycle stresses, the variable is also the slope of the fatigue curve, the value of which is due to the technical condition of the product and is determined by the following ratio 1: Here N P and σ P are the pole coordinates of the HCF curves, which are invariants for a material of a certain class; C HCF – is the HCF resistance constant, which on a logarithmic scale is a measure of the maximum allowable damage to the material in a particular technical condition. During the operation of power plants, the main parameters that will determine the reliability and resource are the fatigue resistance characteristics of the most important elements. Such parameters, according to the HCF model, are the parameter m , which characterizes the rate of accumulation of damage to the structure of the material from fatigue, and the value of the endurance limit, which limits the maximum cyclic loads that can be allowed during operation. The characteristics of resistance to HCF of the studied materials are given in the following Table 2. The strengthening e ff ect is clearly manifested in the increase in the endurance limit σ − 1 and the parameter m , while the limits of strength and proportionality remain unchanged. m = lg( C HCF / N P ) lg σ P (1)

Table 2. Mechanical properties of the samples investigated under the high-cycle fatigue conditions.

parameter m

σ − 1 , MPa

material

N P , cycles

σ P , MPa

N cr , cycles

N

H

N

H

medium carbon steel

100 555

1050 1100 1000

200e + 3 200e + 3 189e + 3 71e + 3

7.5944 7.3321 13.144 8.1228

9.1325 8.1964 25.133 14.444

285 360 530 485

355 405 690 630

low-alloy steel

heat-resistant Ni-based alloy deformable Ti-based alloy

1000 3200

980

According to the HCF model by Bezhenov (2012) the rate of cyclic degradation of the material is given by the value of the parameter m as a measure of the resistance of the local volume to external loads in the HCF conditions. In this case, the intensity of the AE is associated with the intensity of the change in stress-strain condition of the local volume, which does not contradict the known AE models by Liptai et al. (1971); Tetelman and Chow (1972); Dungean (1975). The results of AE control were presented base on AE model of cyclic degradation of materials Bezhenov (2015) in which the dynamic dislocation model of fluidity by Yokobori (1978) and the law of discreteness of the structural-energy theory of destruction of materials by Ivanova and Terent’yev (1975) are combined. The result is a dependence 2, which establishes a relationship between the fatigue resistance characteristic of the material and its AE activity.

lg σ P lg ∆ σ a ·

( j ) ( i ) · m ·

n AE ( i ) = µ

k a ( i ) · k N ( i ) · k σ ( i )

(2)

Here the index i determines the stage of non-localized failure, and the index j is the ordinal number of the energy level of failure with a certain value of the sti ff ness of the stress state of the material µ ( j ) ( j = I – VII , according to the energy spectrum Ivanova and Terent’yev (1975)). It should be noted that in terms of studies of all correction factors, only the coe ffi cient k N at the stage of microyielding had a significant e ff ect, which takes into account the presence of reverse processes of local adjustment of the material structure and depends on the duration of the stage of dynamic stabilization of the structure N ′ .