Fatigue crack origination

Figure 10.2 A family of short, transverse corrosion-fatigue cracks originating on the external surface.

The fatigue crack originates at the surface, then the crack embryo begins to grow. In my opinion, a movement of vacancies to the tip of the crack plays a dominant role in crack growth. 

The process of fatigue cracks origin results from the formation of critical dislocation clusters in front of structural barriers. 

The vibratory mode most frequently encountered is of the plate type and involves either the shroud or the disc. Fatigue failure generally originates at the impeller outside diameter, adjacent to a vane often dne to the vibratory motion of the shroud or disc. The fatigue crack propagates inward along the nodal line, and finally a section of the shrond or disc tears out. 

In aluminum alloys exposed to aqueous chloride solutions, corrosion fatigue cracks frequently originate at sites of pitting or intergranular corrosion. Initial crack propagation... 

Fig. 7.122 Corrosion-fatigue-crack-growth rate as a function of stress-in-tensity range for X-65 line pipe steel in air and in 3.5% NaCl solution under cathodic coupling to zinc. Cycled at indicated frequencies and R = 0.2. Coupled potential = -800 10 mV (SHE). (Note Original reference includes data on which these lines are based.) Source Ref 1 69...

The report of the inquiry [111] criticised the design and fabrication of the alterations made to the original pontoon. The actual cause of the accident was the failure of some tie bars in the detail around the jacking points. The failure was due to brittle fracture which initiated from severe notches such as a small radius curve at the fillet between the spade end and the shank of the tie bar. Weld defects and fatigue cracks were also present in tie bars subsequently recovered from the sea bed. The tie bars had been flame cut to shape and had weld repairs visible to the eye. There had been no post welding heat treatment of the steel. The steel complied with the original specification but tests showed low Charpy V notch impact values. Photo elastic tests indicated a stress concentration factor of 7 at the fillet between the spade end and the shank. The fracture was initiated in the opinion of the inquiry tribunal by the low ambient temperature of around 3°C. 

We have seen in Section 17.1 that the basis of the crack origination is the deformation process. The foundation of the fatigue crack propagation is the rapture of interatomic bonding. 

---