OPTIMIZATION OF STRESS-STRAIN STATE IN CHORD JOINTS OF BOX GIRDERS IN BRIDGE CRANES

Purpose. To enhance the fatigue limit and plasticity of chord joints in bridge crane box girders by optimizing their post-welding stress-strain state through rational combination of structural and technological parameters. Research methods. The mechanical method of determining actual stresses by drilling blind holes was used to analyze residual stresses in heat-affected zones of welded joints. The finite element numerical method was employed for modeling the technological state of welded joints. Methods of mathematical experiment planning and statistical processing of experimental data were utilized. Metallographic methods were applied to investigate the structure of the heat-affected zone in welded joints. Results A theoretical model of the post-welding stress-strain state of chord joints in box crane girders was developed, taking into account phase and structural transformations in heat-affected zones. Empirical dependencies on the influence of technological manufacturing parameters and structural parameters of girders on the post-welding state of their chord joints were established. Patterns of redistribution of residual stresses and plastic deformations during the first loading cycles were determined. A criterion for optimizing the post-welding state was proposed – the final yield strength of the area of maximum residual stresses after maximum operational loading. Scientifically-based structural and technological recommendations were developed, allowing up to 1.23 times increase in the fatigue limit of chord joints in box girders of bridge cranes. Scientific novelty The study proposes a theoretical model of the post-welding stress-strain state of chord joints that accounts for phase and structural transformations in heat-affected zones. Empirical dependencies and patterns of the influence of manufacturing technology parameters and design on the post-welding state were established. Patterns of redistribution of residual stresses and plastic deformations during the first loading cycles of cranes were determined. Practical value. Structural and technological recommendations were developed to increase the fatigue limit of chord joints in box girders. Characteristics of actual stress cycles were obtained, which take into account the level of residual stresses and plastic deformations of the first operational cycles. Recommendations for determining stresses and modeling the post-welding state of metal structures were developed.