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На основі запропонованої структурної концепції порогових розмахів коефіцієнта інтенсивності напружень (КІН) розроблено модель, що описує кінетику росту фізично малої (ФМТ) і довгої (ДТ) втомних тріщин. Модель дозволяє розраховувати швидкість росту ФМТ і ДТ під час одновісного регулярного циклічного навантаження за даними про статичні характеристики механічних властивостей та про мікроструктуру вихідного матеріалу. Модель апробовано на результатах випробувань на тріщиностійкість за циклічного симетричного плоского згину зразків із титанового сплаву ВТЗ-1 у різних структурних станах. Отримано задовільне співпадання розрахованих і експериментальних кінетичних діаграм утомного руйнування (КДУР).
A structural concept of fatigue thresholds (threshold stress intensity factor ranges) of the material is proposed. The concept establishes the relationships between the internal fatigue threshold , the structural fatigue threshold, that is, the fatigue threshold for microstructurally short cracks (MSC) , the effective fatigue threshold , the fatigue threshold for physically small cracks (PSC) , and the fatigue threshold for long cracks (LC) , and makes it possible to calculate these quantities from the known elastic and microstructural characteristics of the starting material. Based on the proposed concept, a model has been developed for calculating the PSC and LC growth rate at a regular cyclic symmetrical uniaxial load under high–cycle fatigue (HCF) and low–cycle fatigue (LCF) conditions. As a criterion of transition from PSC to LC, we consider the depth of PSC, at which the reversible plastic zone size at the crack tip will exceed the grain size d. Under HCF conditions , that is, at the load amplitude below the limit of proportionality , the PSC growth should be divided into two areas of growth: the first area is from d to , when the crack is propagated along the plane of the most favorable displacement of individual grains, and the second one is from до when the crack changes the growth mechanism and is propagated in the plane perpendicular to the tensile load direction. A criterion for this change in the growth mechanisms is the attainment by of the level . Under LCF conditions, i.e., at the load amplitude higher than , a fatigue crack is propagated, from the instant of its initiation to that of the fracture, in the plane perpendicular to the applied tensile load direction. The formulas are presented for calculating and as a function of the level of the applied load amplitude. For each of the areas of PSC and LC growth, the equations are represented as a power law–dependence of the crack growth rate on the stress intensity factor range . In this case, is expressed in a traditional form of linear elastic fracture mechanics – in terms of stress and crack length. The equation parameters are expressed in terms of the characteristics of elasticity: modulus of elasticity Е, Poisson’s ratio , limit of proportionality , and the characteristics of microstructure: grain size d, the Taylor factor М, the magnitude of the Burgers vector b and the distance between adjacent slip planes in the crystal lattice h. The proposed model was tested using the results of fracture toughness tests performed for specimens of titanium VT3–1 alloy in different microstructural states under cyclic symmetric plane bending. Good agreement between the calculated and experimentally determined and the kinetic fatigue crack growth curves (KFCGC) is obtained. |
