numero25

G. Qian et alii, Frattura ed Integrità Strutturale, 25 (2013) 7-14; DOI: 10.3221/IGF-ESIS.25.02 13 Figure 7: (a) Predicted results of fatigue life for surface and subsurface crack initiation in air, (b) predicted S-N curves for surface and subsurface crack initiation in 3.5% NaCl solution. C ONCLUSIONS ased on this study, the following conclusions are drawn: (1) During the crack propagation process for specimens tested in air, fracture surface displays three regions with different crack propagation mechanisms. The formation of different morphologies in these regions is attributed to different crack driving forces and different extents of crack tip constraint ahead of crack tip. (2) The K I value of fisheye crack is close to the corresponding Δ K th . And there is an early crack steady growth zone named region A with constant K I value between Δ K th and fracture toughness. (3) The fatigue strength for specimens tested in water and in 3.5% NaCl aqueous solution are significantly decreased compared to that tested in air. The fractography characteristics for specimens tested in aqueous solution are multiple crack origination and intergranular mode with widespread secondary cracks in crack steady propagation period. (4) A model is proposed to study the relationship between fatigue life, applied stress and material property in VHCF in different environmental medias. This model predicts that fatigue life decreases with the increase of applied loading and inclusion size, whereas it increases with the increase of material yield stress. In 3.5% NaCl solution, the fatigue life decreases significantly and surface crack initiation occurred even in VHCF regime. The model prediction is in good agreement with experimental observations. A CKNOWLEDGEMENTS his work was funded by the National Natural Science Foundation of China (Nos. 11172304, 11021262 and 11202210) and the National Basic Research Program of China (2012CB937500). R EFERENCES [1] Stanzl, S., Tschegg, E., Mayer, H. Lifetime measurements for random loading in the very high cycle fatigue range, Int. J. Fatigue, 8 (1986)195-200. [2] Murakami, Y., Yokoyama, N., Nagata, J. Mechanism of fatigue failure in ultralong life regime, Fatigue Fract. Eng. Mater. Struct., 25 (2002) 735-746. [3] Bathias, C., Paris, P., Gigacycle Fatigue in Mechanical Practice, Marcel Dekker, New York, (2005). B T 0.1 1 10 100 0 2 4 6 8 (b) A B  =0.5   / k n s and n i In 3.5% NaCl  =0.1, n i  =0.4, n i  =0.8, n i surface, n s C 0.1 1 10 100 0 2 4 6 8  =   / k (a) In air  =0.1, n i  =0.4, n i  =0.8, n i surface, n s n s and n i A B C