PSI - Issue 50

I.G. Emel’yanov et al. / Procedia Structural Integrity 50 (2023) 57–64 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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For many metal alloys, material degradation is expressed in a drop in cycling strength, which is clearly shown in Fig. 2, when the tensile strength in the process of cyclic operating time intersects the Weller curve (curve W ).

Fig. 2. Intersection of tensile strength with the Weller curve

In Fig.2, S B0 is the tensile strength of the sample material during static tests, S B is the tensile strength of the sample material after cyclic operating time, σ M is the maximum stress of the cycle , n is the number of cycles. The proposed method for calculating material fatigue can be represented as the Korten-Dolan formula, Emelianov and Mironov (2012), k n m S B M n S BO     , ) ( , (4) where m is the fatigue curve index for the material under study, determined from the experiment (for most structural steels, it is close to 2), k σ is the kinetic coefficient, which is determined from the failure condition (Fig. 2) S B M N M    , ) ( , (5) where σ M is cycle stress, N is the number of cycles worked out, which can be found from the Weller curve for the material under study. Considering relations (4) and (5), one can obtain an equation for determining the current value of the ultimate strength S B depending on the number of cycles worked out where σ M is the current stress of the cycle, S B0 is the ultimate strength of the material at the initial moment of time ( n =0), N is the number of cycles to failure at a certain value of σ M , n is the number of loading cycles worked out at the current moment of time, m is the kinetic coefficient determined experimentally. Thus, according to formula (6), it is possible to determine the ultimate strength of the material depending on its operating time at a certain stress value σ M . The proposed technique makes it possible to take into account the fatigue of materials at different levels of operating stresses. The transition from one stress level to another is carried out from the condition of equality of the ultimate strength of the material under different loading history ( 2 , 2 ) ( 1 , 1 ) S B M n S B M n    (7) n m N m S B M   0 S B M n S B  ; ) (   0 , (6)

The calculation technique was described in more detail for the case of quasi-random loading in the works, Mironov (2017), Ogorelkov (2018). Considering the change in the ultimate strength during operation makes it