Cyclic Strength and Nonlinear Material Fracture

It has been shown that a material fatigue fracture diagram can be viewed as a locus of points with their σ and √l coordinates’ product equaling to K1c / 2 and their σ and l product – to G1c / 2, where K1c and G1c are respectively the force and energy criteria of non-linear fracture mechanics. The average number of interatomic bonds destroyed within a single alternate stress cycle is directly proportional to σ, which is twice as large as a peak value of σa. The study establishes the characteristics of low-cycle fatigue (σ > σ0.2 and Δ1cs > 1), high-cycle fatigue (σ = σ0.2 and Δ1cs = 1), and giga-cycle fatigue (σ < σ0.2 and Δ1cs < 1). An individual interatomic bond is destroyed as a single unit, implying that in case of giga-cycle fatigue, a bond is destroyed over several cycles rather than within a single cycle. The factors F (collapsibility) and R (resistibility) are suggested as essential material physical constants. The introduced notion Δ1cs and the established linear relationship enhance understanding of fatigue crack growth and enable the plotting of fatigue fracture diagrams based on cyclic strength testing results.