Fatigue damage represents a key failure mode in ship structures. Such damage typically begins at vulnerable points in the structure, like welded joints, stress concentration areas, and cracks. Cyclic loading, particularly from waves, encountered by ships during their operational life is a major cause of fatigue damage, which is the main focus of this study. There are various methods to address different sea state conditions, though they can sometimes be approximate. This paper aims to review the most commonly used methods to highlight their strengths and weaknesses and to provide essential background knowledge for developing reliable theoretical and numerical models for predicting the fatigue life of ship structures exposed to various sea states over their lifetime. The primary theoretical approaches discussed include energy spectral methods in both time and frequency domains, which are used to quantify wave-related energy and amplitude characteristics and evaluate wave loads for predicting the fatigue life of structures and welded joints. The discussion also covers the determination of cyclic stress in specific structural details of the hull girder and welded joints to identify the relevant maximum stress range for subsequent fatigue studies conducted using finite element analysis.

Isogeometric Analysis (IgA) is an emerging numerical procedure conceived as a useful advancement of the traditional Finite Element Method (FEM). The essence of IgA is to bridge the current communication gap between Computer-Aided Design (CAD) and Computer-Aided Engineering (CAE) modules by employing high-order smooth spline basis functions both for geometry modelling and the construction of the solution space of analysis. As such, the method enables major benefits, namely a simplified pre-processing workflow and enhanced accuracy per degree of freedom associated with the capability of by-passing the time-consuming mesh creation stage and preserving exact geometry regardless of grid refinement. Furthermore, a user’s prerogative of selecting bases with adequate interelement continuity allows for a direct discretization of high-order differential operators, thereby avoiding stress recovery algorithms at the patch level during post-processing. In this contribution, the isogeometric approach is introduced to the detailed analysis of a ship’s structural member. In this regard, an in-house MatLab code has been developed for the structural assessment of linearly elastic isotropic continua, defined by either one or multiple NURBS patches. Within the IgA framework, the displacement approach of the mechanical problem is treated in its equivalent weak form and the obtained variational statement is discretized by means of an isogeometric Navier solid element formulation. Patch tests, involving in-plane loaded steel plates, are preliminarily performed to verify the accuracy of the numerical prediction, delivered by the present code, with respect to its analytical counterpart. A typical T-beam with associated plating is then analysed under uniform loading. It is demonstrated that the proposed approach can be recommended for engineering practice.

Crossing cutouts are commonly practised in marine beams to permit the passage of intersecting structural members. Apertures of this kind are characterized by distinctive curvilinear profiles of various shape to accommodate the cross-sectional geometry of the crossing element while providing scallops and limiting the use of backing strips during welding at the same time. Yet, despite the fairness of their outline, marine cutouts may act as relevant stress concentrators in modern ship design. The detection of stress gradients near such small-scale structural details notoriously suffers from geometric discretization errors and the need for careful meshing in conventional finite element approaches. Thanks to an integrated design-to-analysis workflow empowered by a spline solution space of improved interelement continuity, the isogeometric paradigm has recently established itself for the ability of preserving the exact accuracy of the original geometry description, enabling thus comparatively enhanced precision and convergence rates during analysis. In this paper, the extent of stress concentrations around crossing cutouts of varying shape is explored within the elastic isogeometric analysis of a marine stiffener under static loading.