13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- stepped or benched, perhaps irregular shaped broad or macro-pits. Ernst [7, 8] carried out a semi-quantitative model to explain lacy pit cover formation and pit growth, representing the shape of a pit and pits within pits grown from the edge of a 50 μm 304 SS foil in 1 M NaCl at 15℃ and 600 mV. An SEM microfractograph of a typical nucleating corrosion pit on the fracture surface of a specimen that had been precorroded for 384 h was given by Dolley [18]. And elliptical pits developed from the artificial pit were given in [19]. 3. Effect of the corrosion pit on fatigue lives (crack initiation) 3.1 Effect of the corrosion pit on crack initiation and propagation. Corrosion pits acted as pre-existing flaws in the material to nucleate fatigue cracks. Burstein [20] and Li Lei [21] indicated that the evolution of corrosion pit followed three stages: nucleation, metastable growth and stable growth. The pit size observed on the fracture surface is considered to give the critical pit size that depends on the cyclic stress amplitude at which the transition occurred [10]. A comprehensive seven-stage model is developed in [22, 23] for pitting corrosion fatigue damage process, including pitting nucleation, pit growth, transition from pit growth to short crack, short crack growth, transition from short crack to long crack, long crack growth, and fracture. Bastidas-Arteaga [24] assessed the total corrosion-fatigue life as the sum of three critical stages: corrosion initiation and pit nucleation; pit-to-crack transition, and crack growth. Turnbull [14, 15] noted that fundamental steps in the overall process of crack development include pit initiation, pit growth, the transition from a pit to a crack, short crack growth and long crack growth, and suggested that cracks do not necessarily initiate from the bottom of the pits, for the reason that there were many cracks with a depth smaller than that of the corresponding pit. While Ebara [25] found that the crack initiated at the bottom of corrosion pit where stress concentration is large and is presumably electrochemically active, and indicated that corrosion fatigue cracks essentially nucleated and grew from one or two large pits at the circular hole surface near the area of maximal stresses [26]. It is indicated that the largest pits did not nucleate cracks, which is due to the result of a combination of the ‘bluntness’ of the larger pits, and they were not located at the root of the notch, where the stress concentration is highest [27]. As regards to the SUS 630 specimen, the fracture surface showed that the fatigue crack propagation displayed high non-linear in route [28]. The initiation and growth of corrosion pit, crack initiation from corrosion pit and the crack propagation appearance can be vividly identified in [25]. Based on the modeling results of Bastidas-Arteaga [24], G. S. Chen [26] and Medved [27], two criteria are proposed to describe the transition from pit growth to fatigue crack growth: (1) the stress intensity factor of the equivalent surface crack has to reach the threshold stress intensity factor, ΔKth, for fatigue crack growth, assuming that a corrosion pit may be modeled by an equivalent semi-elliptical surface crack; (2) the time-based corrosion fatigue crack growth rate also exceeds the pit growth rate. The results of Sriraman and Pidaparti [1, 12] indicated crack initiation from pit sites can be extremely fast at high stress levels and can occur even from relatively small pits. And Kondo [10] pointed that at higher stress levels, transition occurred at fairly small pit sizes. On the other hand, at lower stress levels, transition occurred at larger pits. Medved [27] arrived at the conclusion that pits were deeper than wide with aspect ratios up to 4, many of which nucleated fatigue cracks were not
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