Relaxation Performance Of Welding Residual Stress Under Alternate Loadings

Ship is a steel-welded structure and has the complex shapes. During the entire life cycle, the working condition of Ship is serious, and the load is very complex. Generally, it needs not only to be under various static loads, but also to be in the face of external cyclic loads. Under the chronic and atrocious stress state, the internal stress distribution condition of the welded specimen will be changed continuously. For the all-welded vessels, the safety and fatigue strength of ship structures will be directly related to the variation of the structural residual stress.So it is very important to make a careful studies and evaluations on the ship structures strength, which is also the key to ensure the ship's normal use during the whole working cycle. Based on this background, this paper attempts to explore the relationship between structures strength and the relaxation and redistribution of the welding residual stress.First, literature materials are presented. Characteristic and significance of the structural welded residual stress relaxation, under cyclic loads, were introduced in this paper, meanwhile, analyzing and discussing the current status. Then the structure optimization development and its application in project are introduced. On this basis, the main content and the searching method of this study are shown.Second, the paper introduces the measuring method of X-ray stress testing apparatus in details and the related definition and theories about the relaxation of the residual stress were reviewed.Third, the experiment is carried out on a thick welded plate of 100mm. With the fatigue test machine, cyclic compressive loads were imposed on the welded specimen. In the testing progress, at every calibrated cyclic number, using X-ray stress testing apparatus, the real time values of the residual stress for each measuring point were measured and recorded. After all cycles ended, based on the Levenberg-Marquardt theory, the experimental data were synthesized and analyzed by the 1Stopt software. Then, the relaxation model about the time-varying redistribution on the surface of the welded specimen was presented. Research results show that the relaxation of the welded residual stress is related to the value of the initial stress and the action directions of the external cyclic loads.Fourth, the time-varying redistribution model of the welding residual stress is extended to the material basic property for the first time. With the FE-SAFE software, the fatigue analysis also is carried out. Under three typical cyclic loads(cyclic compressive load, cyclic tensile load and alternating load), the different influencing mechanism of the residual stresses relaxation, for welded structures, were discussed within the framework of fatigue failure mechanism, fatigue strength evaluation and fatigue life prediction. Moreover, for the different types of the cyclic loads, the applicative criterion about how to apply the stress relaxation effect was briefly discussed. The final aim is to advocate the reasonable use of stress relaxation. Results show that the influence of the stress relaxation is related to the detailed forms of the imposed loads. For the cyclic compressive loads, the relaxation behavior has a negative influence on the structural fatigue strength. However, for the tensile loads and the alternating loads, the relaxation behavior has positive effects.Finally, from the diversification of the welded specimen, the paper made an prospect about the experimental research on the relaxation of the residual stress.Through the study about the relaxation effects of welded residual stress, this dissertation aims to provide theory and method guide for the engineering practice.