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Deformation of rectilinear elements of Du 200 pipeline made of 20 steel under cyclic loading with internal pressure has been studied experimentally. Tests were carried out on a pneumohydraulic bench under conditions close to plane stress state, room temperature, and cycle asymmetry under soft loading mode (stress control). Circumferential and axial strains were recorded at three points of a diametrical section located in the middle of the element under study. Two tubular elements were tested at different values of maximum and amplitude stresses based on 100 cycles of loading. Deformation curves (cyclic creep) of tubes in a circular direction have been obtained, the character of which is significantly non-uniform in comparison with deformation of laboratory specimens. It is shown that even a slight increase in maximum (by 4%) and amplitude (by 8%) stresses significantly (up to 50%) increases the cyclic creep strains in the circular direction. Such growth of deformations can be explained by a peculiarity of 20 steel, the deformation diagram of which in the elastoplastic transition area contains a yield point, which determines intensive deformation of the material. Fixed plastic deformations in the axial direction are more than an order smaller than circular ones, and with increasing loading cycles, they change insignificantly and coincide for the two studied tubular elements. [ABSTRACT FROM AUTHOR]

The research on determination of sensitivity of metal materials to non-proportional cyclic loading and the accompanying effects is analyzed. The indirect methods for determining the coefficient of non-proportional cyclic hardening of materials based on simple experiments are considered. A comparative analysis of methods for predicting non-proportional cyclic hardening for out-of-phase with a 90°displacement of biaxial cyclic loading with strain control is performed. It is shown that methods that use only the parameters of the stress–strain curve of materials are less effective, since they do not take into account the effect of cyclic loading amplitude on non-proportional hardening, which is quite significant for metals and alloys with face-centered cubic microstructure. Analysis of experimental data obtained at cyclic non-proportional deformation of stainless and carbon steels, aluminum and titanium alloys showed that the coefficient of non-proportional cyclic hardening varies depending on the loading amplitude. For most of the studied materials with face-centered cubic structure, there is a tendency to increase this hardening coefficient with increasing amplitude of cyclic loading and to its stability or slight decrease for metals with volume-centered cubic structure. Based on the behavior of the vast majority of materials, a logarithmic functional dependence is chosen to take into account the effect of the strain amplitude on the non-proportional cyclic hardening. A modification of the key parameter for determining mentioned hardening is proposed, taking into account both the characteristics of the stress–strain curve and the dimensionless value of the cyclic strain amplitude. It is shown that its use allows one to obtain a better correlation of experimental data with the approximation dependence proposed earlier by the author. The results of determining the coefficients of non-proportional cyclic hardening due to the improved approach meet the requirements of engineering use. [ABSTRACT FROM AUTHOR]

An approach to the determination of steady ratcheting strain rate under biaxial loading depending on the loading cycle geometry is proposed. Based on the data analysis, the microstructural investigation results, as well as the similarity detected between the microstructural formations after deformation at the stress- and strain-controlled modes of loading, it is proposed to use the cycle nonproportionality parameter to predict the ratcheting rate. It is shown that the geometric cycle parameters and basic experiments allow one to adequately predict the ratcheting rate for complex cyclic paths. [ABSTRACT FROM AUTHOR]

A version of the endochronic theory of plasticity is developed to describe the stress-strain state under stress-controlled, asymmetric biaxial non-proportional cyclic loading. In order to solve the obtained non-linear system of differential equations, the numerical procedure is proposed and the relevant computational program is developed. The model validity is verified by satisfactory agreement between the calculated and experimental data. [ABSTRACT FROM AUTHOR]

The paper addresses the mechanical behavior of carbon steel 20 in asymmetric loading under the conditions of plane stress state that occurs in a thin-walled specimen in tension-compression and alternating torsion. The asymmetric cyclic loading of specimens was carried out in a stress-controlled mode along three paths with a frequency of 0.1 Hz with a base up to 1200 cycles. Cyclic stress-strain diagrams have been constructed for the specimens under tension-compression, alternating torsion, and proportional loading. The curves of von Mises equivalent ratcheting strains have been plotted. The highest ratcheting rate is shown to take place in the tension-compression loading case. [ABSTRACT FROM AUTHOR]

We review the currently available approaches, applicable codes and standards for assessment of defects in trunk pipelines and discuss advantages and disadvantages of the National pipeline regulations. The paper outlines the main principles of the newly introduced normative document that regulates the assessment of strength and remaining life of pipelines containing service defects. [ABSTRACT FROM AUTHOR]

We propose a procedure for the numerical analysis of the stress-strain state and criteria of its admissible state in the course of repairs of the air crossings of gas mains with regard for possible defects of the pipe metal. Using the “3D PipeMaster” software complex, we compute different versions and present recommendations concerning the optimal location of pipe-laying machines with an aim of realization of the possibility of lifting the pipe to a height required for repair works with guaranteed validity of the prescribed conditions of strength. The values of tractive forces in the pipe-laying machines are determined. The optimal sites of fixing the pipeline in the transverse direction are indicated and the corresponding forces acting in the cables used to prevent the transverse motions of the pipeline are calculated. [ABSTRACT FROM AUTHOR]

Experimental data on low-cycle fatigue of 304 steel and a VT9 titanium alloy in deformation by complex loading histories, which are the sequence of blocks of cycles of different shapes in the space of total strains, cited in the literature, are analyzed to develop adequate models for life calculations. The four damage accumulation rules and the low-cycle fatigue deformation criterion were used as basic approaches. Life prediction models were compared. It is shown that the application of a modified nonlinear damage accumulation rule can improve life prediction results, with better outcomes obtained for the programs involving nonproportional cycles. [ABSTRACT FROM AUTHOR]