Cyclic Strength and Nonlinear Material Fracture Mechanics (by the example of steels)

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It was shown that a material fatigue fracture diagram can be viewed as a locus of points with σ and √ l coordinates' product equal to K₁c/2, and σ and l product -- to G₁c/2, where K₁c and G₁c are non-linear fracture mechanics force and energy criteria. It was established that the average number of interatomic bonds destroyed within one alternate stress ∇1cs cycle is directly proportional to σ that is twice as large as a peak value of σᵃ. It was found that low-cycle fatigue is characterized by σ>σ₀.₂ and σ1cs> 1, high-cycle fatigue -- by σ= σ₀.₂ and ∇1cs = 1, and giga-cycle fatigue -- by σ< σ₀.₂ and ∇1cs < 1. An individual interatomic bond cannot be destroyed part by part but as a single unit. The latter means that in giga-cycle fatigue a single interatomic bond is destroyed within several cycles rather than within a single cycle. The factors F (collapsibility) and R (resistibility) were proposed and mentioned as essential material physical constants. The introduced notion ∇1cs and the established linear nature of ∇1cs relationship allow to: a) clarify the fatigue crack growth physical nature in low-, high- and giga-cycle fracture zones; b) determine the nature of a fatigue fracture diagram disruption; c) plot the fatigue fracture diagram using the results obtained in a single specimen cyclic strength test with a selected value of σ≥ σ₀.₂. For giga-cycle fatigue it is important (with similar purpose in mind) to determine this dependence for σ< σ₀.₂. It is recommended to use G₁c criterion to find the lcr length value which in contrast to K₁c has a clear physical nature.