Crack growth analysis in ship structures using spectral methods

Paper in proceeding, 2014

The structural flexibility of modern (container) ship structures subjected to wave loading conditions has increased as a result of larger ship sizes. Cyclic variations of relatively large displacements in these structures may result in local fatigue damage problems. It is known that there are large uncertainties in the prediction of fatigue crack initiation life using fatigue design methods from classification societies. As a consequence, cracks are often observed in ship structures earlier than expected. The existence of cracks in a ship can affect the ship structure’s integrity. Depending on the location of a found crack and its length, it may propagate quite fast to a critical length which could endanger the safety of the entire vessel; cf. MSC Carla and MOL Comfort (under investigation). Furthermore, it may not be realistic to repair all cracks immediately when they have been found in a vessel; some of them may not be critical at all. Therefore, it is of great importance to have knowledge and understanding from studies on fatigue crack growth rate for different wave environments and ship operation conditions. Such studies can assist in decision making for maintenance planning of cracks repair in order to ensure a ship’s structural safety. The most commonly used methods for crack propagation analysis are often too complex to be used in a full-ship crack growth analysis. However, the current study focuses on fatigue fracture analysis methods of ship structures using linear-elastic fracture mechanics (LEFM). The objective is to propose and present a more suitable method for crack growth analysis in ships. By considering the special properties of ship response in a stationary sea state, the structure stress in a ship can be assumed to be narrow band Gaussian process. An explicit formula/simple method is proposed to estimate the crack growth rate under arbitrary sea states. Note that the narrow band assumption might be a bit conservative for the crack growth prediction. Hence, the crack propagation analyses, which are carried out using the code FRANC2D, are carried out using the narrow band approximation. Results are presented from the proposed method, which shows that the method can be efficiently used to compute the crack growth rate in ship structures. Furthermore, uncertainties associated with the crack propagation analysis, such as mean stress effect and crack closure, are also addressed.