Probabilistic Model Of Crack Propagation In Stiffened Panel And Ship Structural Reliabiliy Analysis

With the progress of globalization and the awareness of low-carbon economy,the ship size has kept increasing and high strength steels have been widely used,which makes the ship hull structure fatigue problem unprecedentedly serious.The S-N curve methods,which being adopted by most of the classification societies nowadays,mainly focuses on the assessment integral structure while pays little attention to structures with initial imperfections.With the development of fracture mechanics and increasing requirement of accuracy of ship hull structure fatigue strength assessment,fatigue assessment based on the structure fatigue crack propagation theory has become a hot research topic lately.An extensive study of propagation characteristics of fatigue cracks in ship structure stiffened panels has been carried out.Fatigue crack propagation of stiffened panels under tension loads and bending loads have been investigated experimentally and also numerically via a finite element method,based on the results,together with those obtained with an analytical model,a probabilistic model of crack propagation in stiffened panel is proposed aiming at establishing a method for structural reliability analysis of ship hull structures.The main work includes:(1)The material tensile tests and crack propagation tests were conducted in order to investigate the basic characteristics of fatigue crack propagation of the material,the common crack propagation test process,and also to explore the new method for observing crack length.In order to obtain the crack propagation material parameters and their statistic characteristics,the least square method and the incremental polynomial method are adopted to fit the test data.The specimens' thickness on the results for the stress intensity factor is discussed,and the principle for test load design,specimen scaling as well as other special particulars requiring special attention in experimentation are summarized.(2)For stiffened panels with correlated initial cracks in the plate and also the web,a series of fatigue crack propagation tests under tension loads as well bending loads are conducted,with an emphasis placed on the fatigue crack propagation characteristics.A linear formula is derived to describe relationship between the crack propagation length in the plate and that in the web.Meanwhile,a variety of concerning aspects in experimentation,namely,the specimen design,parametic configuration,and feasibility analysis,are discussed in depth.The standard process for experiment design and implementation is also given.(3)Based on the linear elastic fracture mechanics theory and a finite element method,the crack propagation process of stiffened panel has been simulated.By adopting the Faulkner residual stress model,the effect of welding residual stress on the crack propagation has been discussed.Based on the Paris crack propagation law,an APDL module,accounting for the influence of welding residual stress on the stress strength factor and the propagation rate of cracks,has been developed for simulate fatigue crack propagation of stiffened panels of various sizes and under load of various forms.(4)A probabilistic model of crack propagation in stiffened panels with correlated cracks under tension,bending and tension-bending combined loads are derived.The crack propagation process of stiffened panels in a mid-ship section is simulated using the Monte-Carlo Method.Based on the code module developed for simulating the propagation process of a single crack in stiffened panels,a FORTRAN and APDL code named CrackGo is developed for simulating the propagation process of cracks in midship sections.The influences of initial crack length,geometry correction coefficient and material crack propagation parameters are taken into account in the code.(5)Based on the probabilistic model for crack propagation in stiffened panels and also on the critical crack size determined by failure assessment,a structural reliability analysis method accounting for the effect of stiffened panel crack propagation on the longitudinal bending strength is proposed.A computer program named CrackGo-Pro has been developed,in which,the section modulus is calculated in real time and updated together with the neutral axis and also the load as the crack propagates.Taking the uncertainty of critical section modulus,a reliability assessment method based on the stiffened panel crack propagation theory and ship hull longitudinal bending strength has been established.The limit state function has been derived taking into account the influence of crack propagation in stiffened panels.Based on the influence of crack propagation in stiffened panels on the longitudinal bending moment,a method accounting for the uncertainty of section modulus is proposed for analyzing the structural reliability of ship beams.To summerize,as the main components of the National Natural Science Fund,the research of this paper based on the fracture mechanics theroy is mainly focusing on the crack progagation of ship hull structure stiffened panels with correlated initial crack.Combined the finite element numerical method and reliability theroy,the structural reliability analysis method considering the effects of crack propagation in stiffened panel to the ship hull structure longitudinal bending strength have been established.Compared with the traditional S-N curve method,in which the crack initiation life was taken as the structure fatigue life,fracture mechanics based assessment method complies better with the real failure mode of ship structure.Additionally,it can also provide valuable reference data for inspection and maintenance of ship structure saving the cost of ship operation and maintenance effectively.The ship hull structure reliability assessment method based on fracture mechanics and reliability theroy has significant practical value and extensive application prospects.