Dynamics, Durability of Instruments and Equipment 
DOI: http://www.dx.doi.org/10.24866/2227-6858/2020-2-2

Figura K.

KONSTANTIN FIGURA, Candidate of Engineering Sciences, Associate Professor, ORCID: 0000-0002-0777-0089, ResearcherID: G-9221-2012, ScopusID: 56433299000, e-mail: figura_85@mail.ru
Department of Hoisting, Transport and Road Building Machinery
Bratsk State University
Bratsk, Russia

Influence of asymmetry of the loading cycle on the fatigue strength
of pipelines under corrosion damage

Abstract: The article is devoted to the fatigue characteristics of pipelines subject to corrosion damage: a pipeline operating under asymmetric load with internal pressure by the working fluid is considered. The analysis of publications related to the durability of pipelines subject to corrosion damage and variable loads is carried out; and the most relevant area of research is highlighted – assessment of changes in the effective section of the pipeline due to corrosion and ways to account for changes of stress state of the pipe walls. However, in these publications no changes in the asymmetry of the loading cycle in the process of movement of the working fluid through the pipeline are properly considered. Such oscillations of the loading cycle can be caused by hydraulic shocks, by pulse nature of the fluid motion, or by other reasons. Therefore, this article attempts to identify the degree of influence of the asymmetry of the loading cycle on the fatigue life of the pipeline. The proposed study is based on standard methods for determination of the fatigue characteristics of structures described in the respective state standards, as well as in the most common standards for strength calculation. The applied approach does not consider the process of crack formation in an explicit form, but is more universal for the purpose of engineering calculations. Based on standard methods and improved by the authors of this article, this specific technique allows the use of standard software packages to determine the fatigue characteristics of structures operating under complex loading conditions. The simulation was performed with the ANSYS software package. The respective distributions of durability of the structure, as well as the safety factor for stress amplitudes, are obtained. Dependence of the fatigue life on the asymmetry coefficient of the loading cycle is also built and displayed. It is shown that a pulse change of pressure in the pipeline can lead to a significant decrease of the fatigue life of this type of structures. The results are consistent with the known data linking non-stationary modes of movement of the working fluid with the respective decrease of pipeline’s resource. Also, since there is no theoretical and experimental data in the regulatory and scientific literature linking asymmetry of the loading cycle by the in-pipe pressure with the fatigue strength of the pipeline metal, the technique used by the authors of the article can be used to estimate the resource of pipelines at the design and operation stages.
Keywords: fatigue life, cyclic loading, cycle asymmetry, corrosion damage, pipeline.

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