Abstract
The paper presents a precise analysis of the influence of non-proportional loading of specimens on fatigue life during initiation and propagation of fatigue cracks. Simulation of the fatigue life of specimens was based on relations describing propagation rate of the fatigue cracks. The Paris and Forman relations were applied; they were integrated after previous introduction of relationships for the equivalent range of the stress intensity factor ΔKeq and including the phase shift angle ϕ between amplitudes of the bending moment and the torsional moment. Under bending with torsion, range of the equivalent stress intensity factor ΔKeq includes ranges of stress intensity factors for loading modes I and III, i.e. ΔKI and Δ KIII. The performed tests of 10HNAP constructional steel under cyclic bending with torsion allowed us to determine the influence of the phase shift angle ϕ on the fatigue life. It has been proved that increase of the phase shift angle from ϕ= 0° to ϕ= 60° and the ratio of amplitude of the bending moment Mag to amplitude of the torsional moment Mas equal to 1.33, 2 and 4 cause increase of the fatigue life of the tested specimens. The maximum increase of the fatigue life of specimens made of 10HNAP steel was 73% (Mag/Mas= 2, ϕ= 45°).
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