Study On Fatigue Properties Of Welded Seam On Steel Structures Under Strong Earthquakes

The welded seam on tall steel structures under strong earthquakes are susceptible to cumulative fatigue damage, which may induce welded joints damage or entire structure collapse. However, up to now, no reliable study of fatigue behavior and mechanism of fatigue failure at low-cycle for welded seam were developed throughout the world.In this thesis, the topic of the fatigue deterioration mechanism and the fatigue life prediction methodology of welded seam on tall steel structures under earthquakes was promoted. The emphasis was placed on low-cycle plastic-strain fatigue. Firstly, the fatigue cheracteristics of welded seam were carefully investigated by a series of strain-controlled low-cycle fatigue test. Then, on the base of test data the cyclic hardening and softening mechanism, mechanical performance, energy change and fracture mode in the low cycle fatigue conditions was analyzed. Subsequently, a new energy model for low cycle fatigue life prediction was derived.The predicted results were compared with the experimental data The main contents of this thesis are shown as follows.The low cycle fatigue behavior of weld seam under stress control has been researched. Significant results are obtained, which include the laws of cyclic hardening and softening, mechanical performance, the Masing characteristic of material, and the calculation of the plastic strain energy, etc. These results provide the rimental data and analyzing base for the life prediction and damage assessment of weld seam. The stress-life (S-N) curve of weld seam,which is acquired in the analysis and design of steel fatigue has been obtained.From the standpoint of energy concept, the failure process of weld seam under stress control has been researched. In this research, the plastic strain energy is used as a key parameter connected with the life, and a new model for the life prediction of objects is proposed. Based on this model, the assessment for the low cycle fatigue test of weld seam have been performed. A good agreement is noted between the predicted and experimental results.