applied fracture mechanics

Early Corrosion Fatigue Damage on Stainless Steels Exposed to Tropical Seawater:A Contribution from Sensitive Electrochemical Techniques 255From figure 21 and table 3, it can be seen that for load mode I, when KII/KI =0, the value ofKIth is directly proportional to the aspect ratio. However, this relationship modifies as loadmode II is dominant with respect to load mode I, (KII/KI ~3) at an angle of 75, from whichthe values a/c = 0.47 and a/c = 0.74 seem to have no effect on Keq. It has been reported thatfor short fatigue cracks under conditions of mixed load mode, the value of the Kth can besubstantially smaller than that of long cracks [62]. Thus, it can be assumed that the conditionfor crack growth initiation is reached at Keq values lower than for single mode loading (KI,KII or KIII).Tangential stress has a coupled influence with tension stress for angles from 30 to 55 (loadmodes I and II). For angles lower than 30, tension stress are favoured (load mode I) andbecome the dominant stress.( (b(a)Figure 23. (a) Maximum principal stress distribution at pit contour and (b) small crack emanating froma corrosion pit (Von-Mises: KIsurface=9.56 MPa√m; KIbottom=7.50 MPa√m) , in a hemispherical shaped pitwith a depth=291 m and developed on a pressured thin-walled 350 m tube specimen made of 316LSS; 0.02= 241 MPa, E= 190 GPa and = 0.33. The internal pressure and the relation a/c were kept 110 barand 0.78.The orientation of the stress in an angle relative to the pit-like crack plane generates anormal tension stress at the microscopic plane of the crack that can be even higher than theexternally applied stress. This induces a stress singularity that makes the perpendiculartension stress component sufficiently high to activate the crack initiation from the pit. Basedon the observations about the stress conditions around the pit-like cracks, the direction forcrack initiation will be a function of the ratio, due to that the normal and tangential stressesvary with the orientation angle. Thus a crack will preferentially nucleate at sites ofmaximum principal normal stress as is indicated in figure 22 (a) and corroborated by figure22 (b).(b)4. ConclusionsFrom results obtained on corrosion fatigue tests using Electrochemical Noise and SRET,it was possible to determine the pit initiation time, pit activity and pit localization onspecimen surface. The changes in dissolution current obtained from SRET