Research On Fracture And Crack Arrest In Ship Structures

The linear elastic and elastic-plastic fracture problems for the stiffened cracked plate in ship structures are studied in this thesis. The loading carrying capacity of the cracked plates locally reinforced using stiffeners are studied and the efficiency of the stiffeners for retarding the crack propagation are evaluated. The relation between the fracture parameters and the loading carrying capacity are obtained. It is an important problem to study the fracture behavior of the stiffened cracked plate in view of the application of damage tolerant design concepts. The work has been developed from both experiment and theory.Firstly experiments are conducted to study the crack growth of the stiffened cracked plate according the model of the deck in ship structure. The efficiency of the stiffener for retarding the crack propagation is studied. Experiment studies show that the structure lateral strain decrease because of the effect of the stiffener. The local reinforce can retard the crack propagation effectively.Secondly the linear elastic fracture problem of the finite stiffened cracked plate is studied. Based on the fundamental and basic equation for solving problem of elasticity established by Muskehelishvili and the solution of the Hilbert problem, the resultant forces, the displacements and the stresses are given by the mode of analytical function and . Considering the condition of compatibility of displacement between the plate and stiffener, the boundary problem is determined by using the boundary collocation technique. Case studies are presented to illustrate the influence of geometric parameters and material parameters on the stress intensity factor. Case studies show that the stress intensity factors decrease considerably with the structures' height breadth ratio increasing. Many materials used for the practical engineering structure have good toughness, with post yield regime appears around the crack tip. It is impossible to characterize the fracture behavior by linear elastic fracture mechanics. The Dugdale model for finite cracked plate with stiffened edges under tension is studied in this thesis. A discrete stiffened plate model is constructed for investigation. The boundary tangential traction along the two sides of the plate is approximately expressed by a discretization. Considering the compatibility condition of the displacement between the edge of the cracked plate and the stiffener, the boundary tangential traction is obtained. The relation between the plastic zone length on the crack tip and the applied stress is approximately evaluated, and the fracture parameter CTOD (crack tip opening displacement) is obtained. The numerical results show that the effect of the stiffened edge varies with the crack length. Sequentially the linear elastic fracture problem is studied for a cracked plate reinforced locally subject to mode I loading. Based on the complex function method, the stress intensity factors are calculated and the efficiency of the stiffeners for retarding the crack propagation is evaluated in this condition. Both the longitudinal rigidity and the transverse rigidity of the stiffeners are involved. It is assumed that the shear stresses are lumped at finite number of locations, considering the compatibility condition of the displacement at the locations, the solution is obtained, and the final result is obtained by summation. The influence of the stiffener location and the stiffener relative stiffness on cracked plate is included. Case studies show that the stress intensity factor is reduced significantly with the increasing of relative stiffness between the stiffener and the cracked plate. The stiffener longitudinal stiffness plays a significant role in the reduction of stress intensity factor. The next this paper presents a solution to the ductile fracture problem of cracked plate locally reinforced subjected to remote normal stress. The Dugdale model is constructed for this condition. The Dugdale solutions for the plastic zone size and CTOD (Crack Tip Opening Displacement) a...