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Consequences of cracks on ship structural integrity - An investigation of hull ultimate strenght and crack propagation on ship structures

Da Wu ; Qiaojian Ye
Göteborg : Chalmers tekniska högskola, 2015. 132 s. Report. X - Department of Shipping and Marine Technology, Chalmers University of Technology, Göteborg, Sweden, 2015.
[Examensarbete på avancerad nivå]

The ship structural integrity plays a significant role in ship structural safety and environmental protection. There are four types of limit states in ship design, serviceability limit states (SLS), ultimate limit states (ULS), fatigue limit states (FLS), and accidental limit states (ALS). This thesis will address two of these limit state assessment, namely ULS and FLS. In the ULS analysis, the incremental iterative approach is applied to estimate the ultimate strength of a ship’s hull girder. In this method, some factors which can affect the accuracy and calculation time and are further studied in the project. The analysis for buckling of a deck plate was carried out using both finite element method (FEM) and Common Structure Rules (CSR) for the accuracy and reliability comparison. For the FLS study the effect of fatigue cracks presence in ships on her structural safety and serviceability is investigated. A great number of survey report the existence of crack on structural components among the commercial ship fleets. Owing to the fact that it is not possible to repair all cracks immediately after they have been identified when the ship is on the sea, it is essential to predict the crack propagation speed under different operation conditions and encountered weather environment. At the same time the reduction of the ultimate strength of the hull girder due to the presentation of crack can be used as a criterion to evaluate the status of the ship’s structural safety. Consequently, the approach to solve the problem is divided into two parts, 1) evaluation of the ultimate strength and 2) crack propagation simulation. In the crack propagation simulation part, analytical handbook method, conventional finite element methods and extended finite element methods are used. Besides, both two dimensional and three dimensional geometries were considered. The stress intensity factors were extracted and used in Paris law to simulate the crack growth speed in the FRANC2D / 3D analysis; likewise the direct cyclic approach in ABAQUS was used in XFEM based fatigue analysis to plot the cycle number versus crack length curve. The results show that the crack propagation speed in 2-D case is faster than 3-D case under the same loads. If making a comparison between FRANC3D and ABAQUS analysis, the XFEM gives a more realistic result with much slower crack propagation speed. On this account, the results can be used as a reference for simplified the problem and provide a quantification of difference between different crack simulation methods in solving marine structural integrity problems. What is more, it provides a route to build a fast crack propagation prediction method in ship structures.

Nyckelord: buckling; crack propagation; FEM; fracture mechanics; progressive



Publikationen registrerades 2015-07-03. Den ändrades senast 2015-07-03

CPL ID: 219365

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