Abstract
It is known that the fatigue process begins with the plastic deformation of the surface layers of the metal fittings. Moreover, the displacement of dislocations under conditions of re-alternating loads is observed at loads below the elastic limit of the metal. The rate of local plastic deformation during cyclic deformation is several orders of magnitude higher than the rate of deformation under static loading. Dislocation slip begins in grains with a favorable orientation near stress concentrators. As the number of cycles in the surface layers increases, the density of dislocations and the number of vacancies increases. When the base number of NR cycles is reached, a surface reinforced layer of metal with a large number of germinal cracks is formed, the size of which does not reach a critical value. Increasing the number of cycles cannot cause further development of fracture in such a layer. Only when the stresses exceed the endurance limit of the crack reach a critical length, after which the process of their discharge into the main crack begins with the spread of the latter. The results of experimental studies indicate a strong effect of diffusion hydrogen on static and cyclic parameters of crack resistance. It was found that with increasing flooding, especially when the hydrogen content exceeds 5 cm3/100g, both static strength and long-term strength (fatigue) decrease sharply. Moreover, for these areas of hydrogen solution in reinforcing steel is characterized by a viscous nature of fracture, while for heavily flooded reinforcement (from 5 to 12 cm3/100g is characterized by brittle fracture by the mechanism of microfission in the hardened (martensite or troostite structure). allowed to determine the optimal hydrogen content in reinforcing steel (3…5 cm3 /100g), the excess of which will reduce the crack resistance of reinforcement during long-term operation, especially in corrosive environments. The results of the research confirm the above data. bainite structure y sharply reduces the crack resistance of reinforcing steel, which makes it impossible to use in the manufacture of reinforcement involved in reinforced concrete structures designed for long-term operation (more than 50…60 years). Thus, the obtained diagram can be recommended to designers of reinforced concrete structures for hydraulic purposes, as it greatly facilitates the reasonable choice of reinforcement in the development of reinforced concrete structures for responsible and long-term use.
Highlights
It is known [1 – 5] that the resistance of the metal to fatigue failure is characterized by the limit of endurance, ie it is the highest stress that can withstand the metal without failure at any number of cycles
The endurance limit is most often determined in tests with alternating symmetric cycle (R=-1), and the endurance limit is denoted by σ-1
The endurance limit is limited to 107 cycles
Summary
It is known [1 – 5] that the resistance of the metal to fatigue failure is characterized by the limit of endurance (fatigue), ie it is the highest stress that can withstand the metal without failure at any number of cycles. On fatigue curves it is possible to find such important indicator, as durability at fatigue under which accept number of cycles of loading which maintains steel at destruction at a certain pressure. It is known from the literature [1] that the endurance limit in metal correlates well with the mechanical properties of metals. The value of σ-1 is on average (0.4...0.6) σB – for carbon and alloy steels; (0.3....5) σB – for bronze and brass. This characteristic can be compared with Brinell hardness:
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