Study On Ultimate Strength Of Box Girders Structure With Initial Defects

The design method based on the ultimate state of the ship is more reasonable than the traditional design method.It can better reflect the ultimate state of the ship.However,during ship navigation,decks are often subjected to external loads such as collisions,missile strikes,blast impacts,and fragment damage.As a result,the deck structure will be cracked,damaged,and deformed,causing safety problems.Therefore,when designing a ship,it cannot be limited to the ultimate strength of perfect structure,and more attention should be paid to the ultimate strength containing damage.The reinforced structure under the conventional ship deck is mostly T-girder.Once the deck is damaged,the T-girder is also easily damaged and loses its bearing capacity.In order to strengthen the ultimate strength of the deck,a new reinforced structure,Box-girder structure,was proposed.The Box-girder structure is a strong under-deck longitudinal reinforcement member capable of strengthening the longitudinal strength.It enhances the protection capability and improving the carrying capacity of the vessel.Therefore,it is important to study the ultimate strength under the condition of initial defects after the Box-girder cabin is damaged.And it is also important to explore the influence of the ultimate strength of the ship structure with initial defects.The main contents are as follows:(1)Taking the ultimate strength research method as the research object,elaborated the advantages and disadvantages of each analytical method.For the iterative algorithm in the nonlinear finite element method.Select Reckling No.23 as the cabin finite element model,and carry out the arc length method and quasi-static method to study the computational efficiency of the method.Based on the results of the numerical analysis,a loading rate that can improve the quasi-static calculation efficiency is finally obtained.(2)Taking the crack-damaged structure as the research object,the influence of different grid scales on the ultimate strength calculation results was compared and a reasonable grid scale of the cracked cabin structure was obtained;the ultimate strength of the cracked cabin structure was studied considering the effect of thickness.An ultimate strength evaluation formula for the T-girder cabin structure considering the influence of thickness under the influence of the torsional and bending loads was put forward;the ultimate strength of theT-girder structure with typical crack damage under different torsion and bending loads was calculated.Based on quasi-Newton method(BFGS)and general global optimization method,the results of the ultimate strength analysis are fitted,and the relevant parameters in the evaluation formula are obtained.(3)Using the nonlinear finite element method,the ultimate strength calculations of the perfect structure and the damage of T-girder and box-girder cabin with central cracks,double-sided cracks and unilateral cracks under bending and torsional loads are performed and compared.The variation of the ultimate strength of the T-girder and box-girder cabin under different damage modes was obtained.(4)For the difference between the Box-girder structure and the T-girder structure,introduce the initial deformation feature model based on the plate-type deformation,the column-type deformation and the side-type deformation,And the most influential initial deformation characteristics under various working conditions are obtained.Carry out the ultimate strength analysis of the cabin structure under the effects of different load cases.The comparison results obtained the effect of initial deformation on the ultimate strength of the cracked cabin.And the rationality of the anti-defect property of the box girder structure was verified.Considering the difference between the box-girder structure and the T-girder structure,an initial deformation feature model with plate-type defects,column-type defects and side-type defects is introduced,and the T-girder and box-girder cabins are completed using the nonlinear finite element method.The analysis of the ultimate strength for the ultimate strength calculation of the cabin and the damaged cabin section under bending and torsion loads.Through comparative analysis,the rule of the ultimate strength of the T-girder and box-girder cabins subjected to initial deformation under different failure modes was obtained.