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

In large-sized metal structures, various stress concentrators are often present, which affects the operation of the material. These are intermittent bonds, holes, welded joints, material defects, etc. As a result of overloads under the action of an external cyclic load on structures in the area of stress raisers, the cycle asymmetry, the level of maximum stresses and deformations increase. In this case, the determination of the limit values of the stress cycles can be performed using a diagram of the limit stress amplitudes. The paper presents an engineering method for calculating the limiting stress amplitudes and constructing Hay’s diagrams. It is based on the use of mathematical models of classical linear and structural-mechanical fracture mechanics. Analytically and by calculation, the validity of the method is shown, which consists in determining the endurance limits and limiting stress amplitudes under high-cycle loading in a wide range of variation of the cycle asymmetry coefficient for ferrite-pearlite steels with a yield strength of up to 400 MPa. Thus, a generalized calculation method has been developed for determining the endurance limits for high values of cycle asymmetry and cycle stresses. The error of the method is estimated in the area of low-cycle load. The influence of constant and average load in a cycle on the endurance limit has been investigated both for high-cycle and low-cycle loading. The proposed approach allows to construct Smith and Hay’s fatigue diagrams for the tensile region, taking into account the structural characteristics of the material and the error allowed for engineering calculations.

In metal structures under operating conditions, as well as in emergency situations due to overloads, the presence of stress concentrators, etc., plastic deformations may occur. Such deformation by tensile stresses introduces some damage to the structure of steel, which adversely affects its subsequent operation under cyclic loads. Preliminary relative plastic deformation, ∼ 1-6%, which occurs in structural steels under tension, unambiguously reduces plasticity. It negatively affects the endurance of the material and leads to the early formation of macrocracks. In addition, after unloading, an internal residual stress field at the meso-micro level remains in the structural elements, which also reduces the endurance limit. In causes the decrease of such an important indicator of reliability as fracture toughness, which reflects the resistance of the material to crack propagation. In this regard, designing structures, it seems important to improve the calculation methods for assessing and predicting their fatigue strength, taking into account the listed factors. The article is devoted to the development of a mathematical model that includes dependencies for assessing the fracture toughness of metal structures according to the fracture toughness limit for all sizes of defects in the form of cracks. Analytical dependences have been obtained for calculating the relative limit of crack resistance based on the main mechanical characteristics of the state of the material. The results of the study can be used to assess the crack resistance of structural elements and welded joints pre-deformed by tension.