A New Fatigue Crack Growth Mechanism of High-Strength Steels

In this study, the fatigue crack damage evolution procedure of AISI 4340 high-strength steel was systematically studied in quasi-in-situ via three-dimensional X-ray technology and scanning electron microscopy. It is found that the surface roughness and uniformity of crack growth in high-toughness steel are higher than those in low-toughness steel. The crack bifurcation and crack closure effect would both prolong the effective crack length; accordingly, the effective fatigue crack growth (FCG) rate decreases due to above crack growth characteristics in high-toughness steel. Based on the experimental results and theoretical analysis, a new FCG mechanism is proposed, i.e. the blunting and re-sharpening to micro-scale void coalescence (BRS-MVC) mixed mechanism. Few micro-scale voids initiated near the crack tip would be surrounded by the blunting crack tip at low Δ K zone, which is controlled by the BRS mechanism. More micro-scale void clusters would occur around the crack tip at high Δ K zone, which may be attributed to the MVC mechanism. The new FCG mechanism provides a unique perspective for the FCG behavior of high-strength steels, which could promote the fatigue life prediction of metallic materials.