Research On Prediction Theory Of The Fatigue Strength And Life For Welded Steel Structures

This dissertation applies 3-D finite element theory to development of the shape functions and stiffness matrixes of the triangular prism isoparameter elements (6 to 15 nodes). A calculating method for Gaussian integral in triangular prism is presented. The stress field in welded steel structures can be computed by the triangular prism isoparameter elements (6 to 15 nodes) and hexahedron isoparameter elements (8 to 21 nodes). The stress fields in misaligned welded joints, "T" type of plate welded joint, flanged beam and perpendicular flanged beam are calculated respectively, and their stress concentration factors formulation are obtained.Based on the line spring model of Rice and Levy, this paper sets up eight nodes line spring element and its stiffness matrixes and works out a program consist of prism element, hexahedron element and line spring element. The stress intensity factor for surface crack in "T" type of plate welded joint is computed, and the computation shows that agreement is achieved between the calculation and practical measured result.By introducing the concept of the worst-case notch, the theory prediction model of the effective stress concentration factor for welded steel structures are established for symmetrical and unsymmetrical stress cycle respectively. What is taken account of includes the effects of the material, geometrical shape, manner of loading, ratio of cycle stress, residual stress and short crack in weld toe on fatigue strength. The effectiveness of predicting model is proved by means of comparison between predictions and experiments for a few types of welded steel structures.Area method and stress field intensity approach to estimate the fatigue strength of welded steel structures are presented in the paper. By means of area method and stress field intensity approach and computational analysis of finite element, the fatigue strength of misaligned welded joint, "T"type of plate welded joint and cruciform joint are estimated. Estimations are consistent with fatigue testes.Based on the predicting model of effective stress concentration factor, estimating methods of S-N curve for welded steel structures are set up under symmetrical and unsymmetrical stress cycle respectively. The effectiveness of the method was verifiedby tests. Adopting the method in this dissertation to obtain S-N curve can reduce fatigue experiments and helpful in gaining economic efficiency.One point method, mean square curve method and the maximum likelihood principle to predict P-S-N curve for welded steel structures for arbitary survival rate are presented. P-S-N curve can be obtained by predicting S-N curve and adding to a few fatigue tests. These methods can be reference for engineering application.Energy estimating approach of fatigue crack initiation life for welded steel structures is obtained by using Molski-Glinka energy density equation, introducing the worst-case fatigue notch factor, and considering the effects of residual stress on fatigue. The approach has more predicting precision than Neuber's method through comparison between the calculation and tests.Studying the rule of geometric shape of surface crack during propagation by means of fracture mechanics theory, a predicting method of fatigue crack propagation life and it's numerical calculation theory are presented.