Reliability Analysis Of Ultimate Strength Of Hull With Defective Plate

Structural defectiveness occurs in the process of building and use of hull in all probability,such as initial distortion,concavity,hole,crackle and so on. Because of the requirements of design and use,some holes need to be drilled in the plate.The decrease of the reliability of hull structure is induced by those factors.And the danger may occur as a result of some defectiveness.Indeed,the accident may be also induced.Therefore,considering the safety,how to accurately estimate the actual carrying capacity of the hull and the ultimate remnant v after the defectiveness occurring,and also how to carry out the design and computation of the marine structure more scientifically and reasonably,all of these have been a pop topic in the field of ship mechanics.The work concerning these problems has been done in this paper,such as follows:1.Combining the nonlinear control equation of plate and the results of the FEM,the expressions of ultimate strength of the plate grid,which has initial defectiveness(initial distortion and welded stress),concavity,hole,or crackle,are discussed separately.Taking the ultimate strength of integral structure and the defectiveness as parameters,the formulae of ultimate strength of plate grid are given.Consequently,it is convenient for establishing the safety margins of defective structure,and all those provide the theoretical basis for reliability analysis of defective structure.2.The sensitivity analysis method of the structural reliability is given.The influence of every kind of defectiveness(initial distortion,concavity,hole,and crackle) on the reliability index is analyzed.The analysis indicates that,(1) for the plate grid with initial defectiveness,whatever the flexibility is,the use of that with severe initial distortion should be declined,and the use of that with general initial distortion should be controlled strictly,and also the use of that with slight initial distortion should be limited furthest.When the flexibility of plate grid is less,the welded remnant stress should minish furthest,and when the flexibility is larger, the welded remnant stress should be controlled strictly;(2) for the plate grid with concavity,the diameter of concavity should be controlled strictly,and the depth of that should be controlled reasonably;(3) for the plate grid with hole,the dimension of the hole should be controlled strictly and be less than a certain range, in order to ensure the plate grid with enough reliability reserves.If the circinal hole can be made,do it.Or if not,the dimension of the hole should be minish furthest;(4) for the plate grid with initial crackle,whatever the load and the form of crackle are,the dimension of crackle should be controlled strictly.3.Combining the formula of ultimate strength of plate grid with defectiveness,the reducing stiffness matrix and the obvious expression of reverse node force of the plate unit with defectiveness are given,which provides reference to mechanics analysis of the structural system in the progress of failure,and makes the mechanics analysis of the remanent structure more coincident with engineering practice.4.The general model of stochastic reliability analysis of marine structure is established.The basic variables in the calculation of structural mechanics,such as displacement,elastic modulus,geometrical parameter of the unit and so on,are taken into the safety margin directly.The mathematic model of safety margin is made more close to its original state(not be simply described as the relationship between resistance and internal force,such as Z=R-S,where,R is resistance and S is internal force).The obvious expression of safety margin with basic variables is given,and the calculational precision of the structural system reliability is increased.Combining the structural form of the certain ship and considering the factors of initial defectiveness,concavity,hole,and initial crackle etc,the system reliability analysis of one cabin block of the certain marine structure is carried out,and the cabin block is compared with the structure system which has no defectiveness.The analysis indicates that,because of the defectiveness,the reliability index of structure system of the cabin block reduces obviously,and especially,that of the plate with supposed geometrical distortion reduces more.Thus,the size of initial defectiveness should be controlled strictly and be made in the general range.For the important structure,the size of defectiveness must be controlled in the small range.