applied fracture mechanics

246Applied Fracture Mechanics3.3. Pit to small crack transitionPit to short crack transition involves the determination of the critical pit size for thetransition from pit to fatigue crack. In this stage, the microstructure and the state of stressesaround the corrosion pit play a very important role. The conditions for the pit to cracktransition are generally defined phenomenologically by using a Linear Elastic FractureMechanics (LEFM) approach. A major concern is that the threshold stress intensity factorrange defined for corrosion fatigue cracking is determined considering long cracks and cannot be applied to microscopically small cracks (MSC) initiated from pits.Two criteria are used to describe the transition processes:the stress intensity factor associated with the equivalent surface crack growth at whichthe corrosion pit reaches the threshold stress intensity factor for the fatigue crackgrowth,( K) ( K) ,(11)pitcrackCorrosion fatigue crack growth rate also exceeds the pit growth rate,da da dtcrackdtpit(12)Akid et. al. [57] have shown that in some cases pit development overcomes the nucleationstage of the corrosion fatigue cracking process. However, it appears that transition from pitto short fatigue crack occurs at some critical size associated with microstructure, appliedstress level, mode of load and stress state around the pit. In order to describe the stress stateand the stress intensity conditions around the pit, several models, based in pitting corrosionfatigue mechanisms have been proposed [58-61]. They have been assumed that corrosionpits behave as sites for fatigue crack initiation through a purely mechanical micro-notcheffect in smooth surfaces. Real components are normally undergoing complex stressconditions which induce mixed loading modes; where it has been reported that thethreshold stress intensity factor for short fatigue cracks in structures subjected to mixedloading modes is lower than for structures subject to simple loading mode conditions [62].The general aim of this section is dedicated to determine the influence of mixed stressconditions on the initiation of fatigue cracks from corrosion pits. Here we present the resultsobtained from the analysis of the stress state and the threshold stress intensity aroundhemispherical corrosion pits developed on austenitic stainless steel samples immersed innatural seawater. The concept of pits as surface semi-elliptic cracks is introducedthroughout a linear elastic fracture mechanics analysis. Pit depth to major mouth axes ratio(a/c) and the effect of stress orientation were analyzed in terms of variations of the stressstate and mixed load modes. Also the pit to short crack transition and short crack growth isreported through sensitive electrochemical techniques.From the SEM analysis conducted on the metallic specimens after 15 days of immersion innatural seawater, semi-elliptic corrosion pits with different depth /major mouth axes (a/c