Since offshore structures are operating in a hostile environment, the in-situ reparation of cracked structural parts is always difficult. An easy technique to arrest or retard the growth of fatigue cracks is the drilling of stop holes. The holes on the crack tip transform the stress singularity of the crack to a notch, reducing thus the driving force for further crack propagation. Most of the existing research works on this subject are focusing on cracks subjected to mode-I loading. However, the fatigue cracks on structural elements of offshore structures are subjected to mixed-mode loading and corrosive conditions. Aim of the present work is the prediction of mixed-mode fatigue crack growth retardation due to stop holes. The solution is based on FEM analysis of the multiaxial stress state, the principle of minimum potential energy, the S-N curve of a corroded steel, and on the implementation of a mixed mode fatigue crack growth model. A stress-based multiaxial fatigue criterion is adopted to estimate the crack initiation on the perimeter of a stop-hole. The results indicate that the corrosive environment greatly reduces the effect of the stop holes, and that stop holes of diameter 15 mm give a longer delay than stop holes of diameter 5 or 10 mm.
Cracks may form in structural components due to overloads or cyclic loading. The latter are called fatigue cracks, and will continue to propagate in the presence of cyclic loading, until a critical crack size is reached, at which the component will fracture. A simple way to stop, or at the very least to delay the crack propagation, is to drill a hole through the crack tip (Ayatollahi et.al., 2015; Chen, 2016; Murdani et.al., 2008; Pavlou, 2017). This technique is used by maintenance crews all around the world, as an emergency repair method when a crack is detected (Wu et.al., 2010). The so-called stop hole transforms the crack tip to a notch, thereby reducing the local stresses. A new crack has to be initiated at the stop hole before the crack propagation can continue, thereby extending the fatigue life of the component. This life extension or delay has been estimated for different cases by several researchers recently (e.g. Ayatollahi et.al., 2014; Ayatollahi et.al., 2015; Pavlou, 2017; Wu et.al., 2010).
