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Metody obliczeniowe stateczności i nośności granicznej konstrukcji kadłuba okrętowego

Taczała M.

Prace Naukowe Politechniki Szczecińskiej. Wydział Techniki Morskiej

2008

Nr 605 (2)

3-165

Celem pracy jest przedstawienie propozycji metody określania zależności pomiędzy odkształceniem i naprężeniem panelu ściskanego poddanego dowolnej kombinacji obciążeń: rozciągania i ściskania w dwóch kierunkach oraz ścinania. W pracy omówiono także metody wyznaczania nośności granicznej kadłuba oraz zaproponowano przybliżoną metodę analizy nośności z uwzględnieniem zginania, ścinania i skręcania.

A ship hull is a complex structure composed of plating and stiffeners, subject to the action of diverse loads (the weight of the hull, cargo, ballast, equipment, buoyancy forces and wave forces), inducing both local and global effects, with the following cross-sectional forces: vertical and horizontal bending, vertical and horizontal shear and torsion. An analysis of longitudinal strength is a fundamental aspect in the evaluation of a ship hull structural strength, particularly overall bending due to the vertical bending moment. Other forces are less significant, though some are important for specific types of ships, an example being torsion of open-deck ships, particularly containerships, and shear in the case of bulk-carriers, where large shear forces appear between empty and full holds. Longitudinal strength is analysed by way of the method of permissible stresses, which are defined in the rules of classification societies. In recent years, requirements concerning the ultimate capacity of a ship hull have also appeared in the rules, resulting in the necessity for development of analysis methods. Ultimate limit states are the subject of this dissertation, including the ultimate capacity of ship hull structural elements subject to loads being an effect of the distribution of internal forces in the hull beam, as well as the ultimate capacity of the ship hull. A case of destruction due to yielding and buckling is considered under the following assumptions: structural material is ductile; excessive stress concentrations causing fatigue damage do not appear; the material is plastic; manufacturing failures related to welding which can result in cracking are disregarded. Computational methods applied in the analysis of the ultimate capacity of the ship hull and its structural elements are presented in the dissertation. The principal subject is the approximate methods applied due to the size and complexity of the real object. The investigations are focused mainly on the ultimate capacity of the ship hull subject to bending moments. The analysis of hull bending, assuming a flat cross-section hypothesis, is relatively simple. Shear and torsion of the ship hull are investigated less, for two reasons: these effects generally induce smaller stresses, and the analysis is much more complex since the out-of-plane deformations of the cross-section must be considered. The objective of the dissertation is to propose a method for evaluation of the stress-strain relationship of the ship panel subject to an arbitrary combination of loading: two-directional tension/compression and shear. The methods for evaluating the ship hull ultimate capacity are also presented and an approximate method including bending, shear and torsion is proposed. Chapter 1 introduces the problems of ship hull ultimate capacity. A brief description of loads acting on the ship hull and the methods of evaluation of their characteristics using the rules of classification societies are given in Chapter 2. The formulation and solution to the problem of elastic stability of stiffened ship panels employing two deformation models of stiffener web are given in Chapter 3. In the first model - rigid web - the assumption is accepted that the web is undeformed in cross-section while in the second -flexible web - such deformations are allowed. A comparison of results for both models is important, as the flexible web model requires much more computational effort. Elastic buckling stress is used for evaluating the panel ultimate capacity employing approximate formulations, as well as for evaluation of the stress-strain relationship for the panel. The proposed method is referred to as analytical-numerical because assumptions are made concerning the deformation mode, while the numerical approach is used for the solution to the problem. When applying approximate methods for analysis of the ship hull ultimate capacity, an important issue is to compute buckling stresses and evaluate the stress-strain relationship subject to complex loading. By employing the finite element method, arbitrary problem can be solved but the related computational effort is significant; therefore, approximate methods have become attractive. These methods are described in Chapter 4, where the procedure for construction of the stress-strain relationship for compressed panels, originally derived by Gordo and Soares, is presented. The method is - as are other approximate approaches - incapable of dealing with complex loading which is necessary for the analysis of ship hulls subject to bending, shear and torsion. Hence an approximate method for the evaluation of the response of ship panels subject to compression and shear is presented, based on the concept of the stiffened plate finite element. In this approach, identical displacement functions are employed for the plate and stiffeners. In Chapter 5 a proposition is presented for a method for evaluation of the ship hull ultimate capacity subject to all internal forces in the hull beam. A computational model of a thin-walled beam is applied, accepting the basic assumptions of one of the approximate methods -the Smith method - that the panels buckle between longitudinal and transversal girders. It implies division into structural elements - longitudinally and transversally stiffened panels. The stress-strain relationships according to the approach presented in Chapter 4 are evaluated for the panels and applied in the analysis of the ultimate capacity of the ship hull instead of constitutive relationships. Verification of the method, as applied in the analysis of a number of examples, proved that the results are consistent with those available in the literature. The dissertation is summarized in Chapter 6 where future investigations are also presented. The methods developed in the dissertation and their numerical implementation can be an effective approach to investigation of the ultimate capacity of a ship hull. The method can be implemented in computer codes supporting the process of designing and verifying ship structures, developed by the classification societies.