Research On Fatigue Strength Of Weld Structure And Application

Welding is the most important connection method for wagon body structures. However, fracture accidents often occur at welded structure, where90%of accidents are the fatigue failure. The Main factors influences fatigue strength of welded structures, such as weld defects, weld joint types, weld shapes and residual stress. Quantitative study on these factors is benefit for fatigue strength and fatigue life prediction of welded structures.This thesis is supported by the railways development technology develop project "study on basic theory of Daqin line heavy train-experiment study for load spectrum of train". The intensity factors, Residual stress and welding root crack stress, are focused. The research findings are applied in fatigue strength study of C70gondola car body's sleeper beam cover and also in C80gondola end-supported beam connecting region. The main work involves:1. The refined finite element model of T-butt joint is built for residual stress analysis. The effects of geometry factors on welding residual stress of T-butt joint's root region are studied. Geometry includes five types, which are weld seam length, web plate thickness, base plate thickness, weld leg length, weld angle.2. The finite element models of T-butt joint's weld root crack is built by singular element method. The convergence and availability of finite model are tested in this thesis. The effects of crack depth ratio, crack shape ratio, weld leg ratio and weld angle on magnification factors of stress intensity factor is studied, and the mathematical models of relationship between four geometric parameters and stress intensity magnification factors is built.3. C70gondola car body's sleeper beam cover crack is analyzed by statistic analysis, material analysis, assembly analysis and fracture analysis. The residual stress, static stress and dynamic stress of cover crack are measured. Assembly clearance is divided into composing and contact clearance. The relationship between residual stress and composing clearance and also the relationship among stress, dynamic stress and contact clearance is studied with finite element method.4. The relationship between C70gondola car body's sleeper beam cover crack initiation life and composing clearance, contact clearance, fatigue strength reducing coefficient and also the relationship between crack growth life and composing clearance, contact clearance are studied with actual load spectrum. The crack propagation model is built under the influence of residual stress. Crack shape ratio and non-dimensional crack growth rate are studied as crack propagation.5. Simulation model of C80gondola end-supported beam connecting region weld root crack is built with the incomplete penetration defect ate welding root. The relationship between crack growth life and crack depth, crack shape ratio, weld leg ratio, weld angle is studied with actual operating condition.