Research On Calculation Method Of Stress Intensity Factor For Surface Cracks In Ship Structures

As a typical welded structure of the ship,the initial defects of the welded toe part can easily develop into semi-elliptical surface cracks.Under the alternating external loads,surface cracks will continue to expand and penetrate the steel plate,which will affect the watertightness of the hull compartment and even threaten the navigation safety.Therefore,from the perspective of damage tolerance design,it is necessary to accurately assess the stress intensity factor at the tip of the surface cracks,which lays the foundation for the fatigue life assessment and fracture analysis of the hull structure.Based on the theory of fracture mechanics,this paper studies the two-dimensional(2D)weight function method for calculating the stress intensity factor of surface cracks in arbitrary complex stress fields.The paper mainly includes the following aspects:(1)In order to expand the applicable scope of the weight function,the two-dimensional weight function of surface crack with high aspect ratio(a/c?1)is studied.In this paper,stress intensity factors of surface crack with crack depth ratio a/c=1.0~8.0 and crack depth ratio a/T=0.01~0.8 are calculated by three-dimensional finite element models,which are used as reference solutions for solving weight function.Given the reference solution,the crack surface is meshed to complete the quadrilateral element discretization and the unknown parameters of the weight function are solved with Gaussian numerical integration method by a self-programming program in MATLAB,and finally the weight function coefficient parameter formula is fitted by the least square method.(2)A variety of bi-directional stress distribution loads are designed to verify the effectiveness of the proposed two-dimensional weight function for surface cracks with high aspect ratio.The results show that comparing the weight function results of the stress intensity factors with the finite element results under different load conditions,the maximum relative error is within 10%.(3)Based on the previous work,the two-dimensional weight function of low-aspect-ratio surface cracks(a/c?1)is combined with finite element stress analysis to analyze the stress intensity factor of the surface cracks at the weld toes of typical joints in the hull.The finite element models of in-plane and out-of-plane cross joint structures and typical hull stiffened plate structures without damage are established firstly by using ABAQUS to analyze and extract the stress at the weld toe section.The extracted stress is taken as the load input of crack surface,and the stress intensity factors are calculated by weight function.Then,finite element models with surface crack damages are established to calculate stress intensity factors.Finally,the weight function results are verified by comparing the two results.