FATIGUE-CRACK PROPAGATION NEAR FATIGUE THRESHOLD IN VARIOUS STRUCTURAL-STEELS

The investigations have been conducted by measuring fatigue crack prop agation near fatigue threshold in various structural steels differing in chemical composition and strength level. The fatigue crack propagat ion measurements were carried out using the constant-load-amplitude te st in Paris-region, R-constant and K-max-constant method in near fatig ue threshold region. Scanning electron microscopy at fatigue crack fro nt on fracture surface was applied to interpret the influence of crack closure effects on the measured fatigue threshold. Marked fretting ox ide deposits distributed on the fracture surface at threshold level we re observed in a low load ratio resulting from the combined action of plasticity- and oxide-induced crack closure under laboratory atmospher e. Fatigue threshold dependent on the load ratio appeared to be relate d to the extent of the crack closure effect. By considering the relati onship of reversed plastic zone size and grain size the fatigue thresh old in region of crack closure was calculated theoretically. The resul t has shown a good agreement with the experimentally measured values.