Analysis of fatigue crack initiation and propagation in ship structures

Ships structures are subjected to various types of cyclic loads from waves, wind and cargo operations that cause fatigue damages in the structures. There exist rules to regulate the structural design with sufficient fatigue strength to survive their service period. However, fatigue cracks do occur earlier than expected in numerous locations of e.g. ocean-crossing container vessels. The presence of fatigue cracks greatly affects a ship’s safety and serviceability. Managing how initiated cracks grow is an important task to ensure a safety and cost-effectiveness ship transportation. The objective of this study is to develop a robust method, which can be used to predict crack growth and crack maintenance in ship structures. A longitudinal stiffener of a 2800TEU container vessel with full-scale measurements of e.g. strain signals and operation conditions are used in the study. Firstly, a spectral S-N fatigue analysis is adopted to predict when the first crack occurs in the ship’s most fatigue-critical region. Then the crack’s growth characteristics are modeled by the code FRANC2D. Finally, the time needed for the crack initiation and growth are studied in terms of structural maintenance plan, i.e. crack growth management