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Fatigue crack propagation and computational remaining life assessment of ship structures

 

Y. SUMI

 

Department of Naval Architecture and Ocean Engineering, Yokohama National University, 79-5 TokiWadai, Hodogaya-ku, Yokohama 240-8501, Japan

 

Abstract: The fitness for serviceability of structural members of marine structures in which fatigue cracks might be found during in-service inspection is investigated in order to prevent instantaneous failures of ships, as well as a loss of serviceability such as the oil- and/or watertightness of critical compartments. The essential features of fatigue crack propagation and the remaining life assessment are discussed in the first part of the paper, where the effects of weldment, complicated stress distributions including stress biaxialities at three-dimensional structural joints, structural redundancy, and crack curving are found to be of primary importance. The second part of the paper contains a discussion of an advanced numerical simulation method for the remaining life assessment, in which the above-mentioned effects of fatigue crack propagation are taken into account. The simulated crack paths and the fatigue crack propagation lives are found to be in fairly good agreement with the experimental results.

 

Key words: ship structures, fatigue crack propagation, remaining life assessment, computer simulation, welding residual stress, structural redundancy, biaxial stress, crack curving

 

Address correspondence to: Y. Sumi

Received for publication on July 8, 1997; accepted on Feb. 2, 1998

 

Introduction

 

Fatigue strength is one of the most important factors for a strength evaluation of marine structures, which generally consists of a strength assessment of buckling, plastic collapse, instantaneous fracture, and fatigue. Fatigue cracks are sometimes found at the discontinuities of structural details during in-service inspections after a period of service. In order to avoid potentially hazardous situations, it is essential to assess the remaining crack-propagation lives so that appropriate counter measures can be adopted in a systematic way for the structural maintenance of the ship. For this purpose, it is necessary to adopt a proper inspection and maintenance procedure based on a damage-tolerance analysis of cracked components, in which the remaining life assessment is important to prevent instantaneous failure and also to evaluate the ship's fitness for service.1,2

In the first part of this paper, typical features of fatigue cracks observed in the structural details of a ship are discussed with regard to stress analyses of typical initiation sites, together with simple estimates of stress intensity factors for small cracks. Then the growth of relatively long fatigue cracks are categorized into several stages to evaluate fitness for serviceability and also to prevent catastrophic failures. Various complicated stress analyses of fatigue cracks, which could be necessary for the assessment of crack propagation lives, are discussed, including arbitrarily shaped crack path predictions. It will be seen that various aspects of fatigue crack growth must be taken into consideration, e.g., loading spectrum, structural redundancy, welding residual stress, stress biaxiality, and noncollinear crack-paths.

In the second part of the paper, attention is focused on a remaining life assessment of a welded structural component. The fatigue crack-propagation law is modified to take into account the effect of welding residual stress. A step-by-step finite-element approach is employed for fatigue crack-path prediction, where a two-dimensional cracked domain is automatically mesh-generated by the modified quadtree algorithm, and the stress field ahead of the current crack tip is analyzed by taking account of the higher-order stress field parameters.3,4 An incremental curved crack extension is determined using the first-order perturbation solution together with the local symmetry criterion, and the procedure is successively repeated. Furthermore, the precise effect of structural redundancy is taken into

 

 

 

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