Residual Ultimate Strength Analyses Of Cracked Ship Structures

It is of crucial importance to estimate the ultimate strength of structures during the design and service of ships. Ship structures are prone to suffering various types of accidents and damages, which may cause ultimate strength decrease. So it has been given much attention to reasonably estimate the residual ultimate strength of cracked ship structures for the assurance of service safety.Significant improvements have been made in the past in the structural integrity assessment, however, as a new research field of ship structures, residual ultimate strength assessment has not been sufficiently studied. It is essential to rationally analyze the effects of various damage forms on ultimate strength of structures and reasonably establish the structural damage analysis system. Cracking may be one of the most important types of damages in aging ships. And as the complexity of stress distribution when cracks exist in structures, the strength assessment methods of cracked structures are of great difference comparing to those for integrated ones. Systematic analyses are insufficient currently and the research methods related to crack problems are generally limited to theoretical elastic fracture mechanics which could not be conveniently applied in engineering practice or fully plastic estimation method which simplifies the failure mode too much.So this thesis aims at comprehensively reviewing the study progress of residual ultimate strength assessment of cracked ship structures, discussing the analysis methods, investigating the effects of crack damages on the ultimate strength of typical ship structural elements, providing reasonable crack-arrest proposals and presenting residual ultimate strength assessment systems of complex ship structures and ship hull girder. To achieve this purpose, the following efforts and contributions have been made in this thesis:(1) Systematically summarizing current study methods and progress on residual ultimate strength of cracked ship structures; based on existing research conclusions, discussing elastic-plastic and fully plastic failure analysis methods;(2) Conducting non-dimensional parametric FE analyses on cracked plates taking into account the parameters including material properties as yield stress and Poisson's ratio, crack characteristics as crack length, crack angle and crack position represented by two eccentric ratios, the plate's geometrical dimensions as plate thickness and aspect ratio, the boundary loading conditions of uniform stress and constant displacement. And providing reasonable empirical formulas validated by test results for residual ultimate strength calculation of cracked plate;(3) Conducting numerical analyses on residual ultimate strength of cracked stiffened panels with crack damage considering three types of damaged panels with vertical cracks, inclined cracks, crack and cope-hole; analyzing the effects of influential factors including relative crack length, yield ratio of material, plate thickness, crack angle, relative crack length combinations in main plate and stiffener, and cope-hole dimension; And providing reasonable empirical formulas validated by test results for residual ultimate strength calculation of cracked stiffened panels;(4) Conducting residual ultimate strength analyses on plates with typical multiple crack damages; investigating the effects of leading crack and disturbing cracks considering the parameters as characteristics of leading and disturbing cracks and their relative position; pointing out the failure modes and main influential factors for residual ultimate strength of plate with multiple crack damages;(5) Conducting crack initial angel analyses based on the theory of strain energy density criterion; discussing numerical calculation methods; investigating the crack-arrest method of adding a steel patch around crack tip; and proving reasonable engineering proposals on crack-arrest and structure repair;(6) Based on the current ultimate strength assessment methods of integrated complex ship structures and ship hull girder, applying the insights and results developed for structure elements above to the residual ultimate strength of cracked complex ship structures and ship hull girder.The items (2)⁶ in above-mentioned works embody the innovation researches of this thesis. Through the studies,the following main conclusions can be drawn:(1) Yield ratio of the material has a great influence on the ultimate strength of the cracked plates; but the ultimate strength will almost not be altered by Poisson's ratio; The influencing curves of crack length can be divided into two phases poly-fitted by first-order exponential decreasing function and approximate linear function; the changing tendency of ultimate strength of inclined cracked plate with varieties of crack angle can be regarded as being sinusoidal. The eccentricity ratio has a distinct influence on the ultimate strength of the cracked plates; the ultimate strength of the relatively thin steel plate sheet has a very small difference which can be neglected with varieties of plate thickness and the 2D plan stress model can be effectively adopted when conducting finite element analyses; two different loading conditions should be taken into account including the uniform stress and the constant displacement on the boundary of the plate. The difference between the two loading conditions is correlative to the aspect ratio of the plate and when the aspect ratio is sufficiently large, two different loading conditions will bring the same results but from opposite direction;(2) Crack length, crack inclination, material's yield ratio and cope hole influence the residual ultimate strength of stiffened panels significantly; for elastic-perfectly-plastic materials, the assessment method by subtracting cracking sectional areas considering net-section failure mode can give precise results; an effective crack in an un-stiffened plate can be obtained when cracks existed both in stiffener and plate of a stiffened panel then the residual ultimate strength of cracked stiffened panels can be estimated using the methods for cracked equivalent plate; the changes of effective crack length and different crack length combinations in stiffer and plate will influence the residual ultimate strength of panels to some extent; the small cope hole existing in stiffener will effectively arrest the spread of cracks from plate to stiffener and will reduce the degree of strength decrease to a certain extent;(3) When multiple cracks exist in a plate, the disturbing cracks will affect the stress distribution around leading crack tips but it sometimes will decrease the stress intensity factors of lead crack tips in a certain position and length region; the disturbing cracks will affect the residual ultimate strength of the plate only when they have relatively long lengths and are distributed relatively close to leading crack; in the effecting range, the disturbing cracks will influence the plastic zone of leading crack tips; with the increasing of tensile loads, the plastic zones of leading and disturbing cracks will connect together to approximately form a new longer crack, which will rapidly decrease the residual ultimate strength of the plate; when the disturbing cracks are near enough to change the plastic zone of leading crack tip, the horizontal distance of the leading and disturbing crack centers will affect the residual ultimate strength of plate; the disturbing cracks with no effects can be neglected in the strength analyses; the insights and conclusions can be useful for random cracks analyses;(4) Based on strain energy density theory, the initial crack growth angel can be obtained by the strain energy density curve around the crack tip as a function of the angel degree related to the local coordinate system established in the crack tip; numerical calculations can exhibit the changing tendency of elastic-plastic strain energy density around the crack tip with the increase of tensile load; for the stiffened panel with crack in a plate, the stiffener can greatly decrease the crack opening displacement but when there is a crack in a stiffener, its arrest effect will be unobvious so cracks in the stiffener should be effectively avoided in engineering structures; adding steel patches around crack tips will effectively decrease the stress intensity in the vicinity of crack tips to reduce the effects of the crack; the arrest effect of steel patch is related to its position and rigidity; the steel patch should be located around the direction of initial crack growth angel and larger rigidity will have a better effect.