In this study, a structural-deformation analysis of the process of crack development is performed, on the basis of which an engineering methodology is developed for assessing the endurance limit and resource of large-sized structures. A simple analytical dependence (correlation аналитическая зависимость) was obtained, which allows one to determine the critical size of macrocracks for ferritic-pearlitic steels without using the well-known Griffith formula. The results of calculating the cracks critical lengths of various steels depending on their yield strength are presented. The analytical dependence of the calculation of the fatigue limit for the most dangerous symmetric loading cycle according to the standard set of mechanical characteristics of ferrite-pearlite steel is presented. The obtained results make it possible to calculate the endurance limit of structural elements of marine equipment and other structures subject to cyclic loads

Introduction. Welded joints in large-sized metal structures (e.g., in the structures of ship hulls) subject to low-cycle fatigue are considered. The characteristic appearance of soft interlayers, which are significantly plastically deformed under working loads, was noted. Deformation of the metal structure with damage, especially in the form of cracks, reduces the strength and reliability of structural elements and joints. Pre-deformation negatively affects plasticity; therefore, much depends on the residual plasticity of the cracking material. At the same time, with a decrease in residual plasticity, such an important reliability indicator as the resistance of the material to crack propagation — the fracture toughness – decreases. The paper is devoted to the development of a model that includes analytical dependences for assessing the crack resistance of metal structures and their welded joints with soft interlayers according to the crack resistance limit for all crack sizes.Materials and Methods. The theory and methods of linear mechanics of materials destruction, structural-mechanical approach are used. The calculation results were analyzed and compared to the experimental data and other analytical solutions. The numerical experiment was performed for the ferrite-perlite steel grades of 10, 50, 22K, St3sp, etc., widely used in industry, as well as for alloy steels hardened to medium and high strength of 30KhGSA, 37KhN3A, etc. Results. Analytical dependences are obtained for calculating the relative crack resistance limit according to three main known mechanical characteristics of the state of the material of the soft interlayer of the welded joint.Discussion and Conclusions. The results obtained can be used to assess the crack resistance of pre-deformed structural elements and welded joints (including those with soft interlayers) operating under a transverse load. The results of experimental data and analytical calculations are shown in dimensionless form, which enables to obtain invariant results with respect to the fracture toughness limit.

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In the constructions and weld seals microdefects while repeated stress lead to metal structure damage, low-cycle and high-cycle fatigue, and cracks dissemination. The calculation method of operating life estimate presupposes a microcrack development hypothetical speed application. This speed in many cases is unknown. Initial speed determination suggested in this work is based on endurance limit values design analysis when considering a combined problem for the first two sections of fatigue failure kinetic diagram.