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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]

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]

We present results of a numerical simulation of the effect of cyclic creep (ratcheting) with different programs of stress-controlled uniaxial asymmetrical loading of two metallic materials with different cyclic properties. An improvement of the kinematic hardening rule in the cyclic plasticity model on the basis of defining equations of the endochronic theory of plasticity is proposed. It has been shown that the proposed model allows one to describe with satisfactory accuracy the kinetics of the stress-strain state of specimens under low-cycle asymmetrical loading on the basis of the minimum number of basic experiments. [ABSTRACT FROM AUTHOR]

The data on cyclic deformation of materials with different cyclic properties are analyzed. The relationship between the level of additional strain hardening and the mechanical characteristics of a material is established. The proposed models take into account the influence of the form of stressed state on strain hardening and durability under conditions of nonproportional cyclic deformation. The models are constructed on the basis of the analysis of the experimental data on cyclic deformation of 304 stainless steel. The prediction of strain hardening and durability is performed with separate analysis of the influence of shape of the cycle and the form of stressed state. To take into account the influence of shape of the cycle, we use the coefficient of disproportionality of the cycle and the parameter of sensitivity to disproportionality. To take into account the form of stressed state, we use the coefficient of the form of stressed state and the corresponding parameter of sensitivity to the form of stressed state. The results of prediction of both strain hardening and durability of materials on the basis of the proposed phenomenological models reveal their fairly high efficiency. [ABSTRACT FROM AUTHOR]