numero25

J.T.P de Castro et alii, Frattura ed Integrità Strutturale, 25 (2013) 79-86; DOI: 10.3221/IGF-ESIS.25.12 85 industry, where very light structures must operate in saline environments. But environmentally assisted cracking problems have been treated so far by simplistic structural integrity assessment procedures based on an overly conservative policy of totally avoiding material-environment pairs susceptible to EAC conditions. When such conditions are unavoidable during the service life of the structural component in question, the standard solution is just to choose a nobler material to build it, one that is resistant or immune to crack initiation and propagation by EAC in the operational environment. Alternatively, the solution may be to recover the structural component surface with a suitable properly adherent and scratch resistant EAC-resistant coating. But in many cases there are no such coatings available in the market. Such inflexible design criteria may be safe, but they can also be too conservative if the material is summarily disqualified when it may suffer EAC in the service environment, without considering any stress analysis issues. Such decisions may cause severe cost penalties. Indeed, even though EAC conditions may still be difficult to define, due to the number of metallurgical, chemical, and in particular mechanical variables that affect them, structural integrity assessment procedures should always be used on the design stage to define a maximum tolerable flaw size. In fact no crack can grow unless driven by a tensile stress, caused by the superposition of applied loads, residual stresses induced by previous loads or overloads, and maintenance or manufacturing procedures. Thus EAC cracks cannot be properly evaluated neglecting the stress and strain fields that may drive them. But such uneconomical decisions can be avoided, since we already know how different the behavior of deep and shallow fatigue cracks is, and how it can be treated in structural design. Therefore, it is now possible to propose an extension of the proved criteria for accepting shallow fatigue cracks to environmentally assisted cracking problems, assuming they also present a notch sensitivity behavior that can be mechanically described. If cracks behave well under EAC conditions, then a Kitagawa-like diagram can be used to quantify tolerable stresses, using the material EAC resistances to define a “short crack characteristic size under EAC conditions” by :   2 0 (1/ ) IEAC EAC a K S      (19) In this way all corrosion features are assumed to be properly described by the resistance to crack propagation K IEAC and by the resistance to crack initiation S EAC under fixed stress conditions in the analyzed material-environment pair. In other words, this model supposes that the mechanical parameters that govern the environmentally assisted cracking problem behave analogously to the equivalent parameters  K th (R) and  S L (R) that control the fatigue problem, see Fig. 5. Figure 5 : A Kitagawa-Takahashi-like plot proposed to describe the environmentally assisted cracking behavior of short and deep flaws for structural design purposes. Consequently, if cracks loaded under EAC conditions behave mechanically as they should, i.e. if their driving force is indeed the stress intensity factor applied on them; and if the chemical effects that influence their behavior are completely described by the material resistance to crack initiation from smooth surfaces quantified by S EAC , and by its resistance to crack propagation measured by K IEAC ; then it can be expected that EAC cracks may depart from sharp notches and then stop, due to the stress gradient ahead of the notch tips, eventually becoming non-propagating cracks, as it occurs in the fatigue case. Hence, if the size of non-propagating short cracks can be calculated using the same procedures useful for fatigue case, then the resistance to that kind of defect can be properly quantified using an EAC notch sensitivity factor in structural integrity assessments. Therefore, a criterion for the maximum tolerable crack size under EAC conditions can be proposed as: 1 2 max 0 (1 ) (1 ) ( ) IEAC a K a a a g a w              (20)