The Complex Structure Of The Local Dynamic Effects And Fatigue Analysis

Presently, Finite element method is commonly used in dynamic analysis of structures. The magnitude of dynamic internal force and dynamic displacement can be obtained by solving dynamic equations with this method, but the effects of structural inertia forces induced by continuous masses on the internal forces of elements, i.e. the effects of local dynamic responses, are neglected. However, it is proved theoretically and practically when density of the material is high, external damping is very large, whiplash happens or solid-liquid couples with structural system, there are local dynamic responses on the internal forces of elements and it cannot be neglected in the structural analysis.Researches on structural fatigue reliability analysis of sea platform can be found in many opening literatures. But researches regarding fatigue analysis and evaluation of platform in ice sea area are reported less. One of the main ideas in this paper is the pseudo excitation method is introduced in the fatigue analysis of sea platform under actions of ice forces. With using this method, the computational time can be decreased greatly without loss of numerical precision.In this paper, aiming at the developmental conditions of the local dynamic responses at present, further discussions on local dynamic responses of structures under random excitation are proceeded based on achievements at home or abroad. The mathematic formulas of local dynamic responses of beam element, plate element and shell element under random excitation are deduced. And then computational models for local dynamic responses are established. Based on the mathematic formulas deduced and program(?) in existence, the corresponding computer codes of local dynamic responses are written and compiled for complicated structural analysis. Then, using the extended program, the local responses of sea platform (MSW) under random excitation are computed and analyzed. Comparisons of results with and without consider local dynamic responses are acquired. Finally, the fatigue life of Beigaodian JZ20-2 platform is evaluated using the developed program based on the theories and methods of fatigue analysis.The numerical results show that the local dynamic responses have great effects on some members of platform under the actions of environmental loads, thus local dynamic responses have dominating effects on the design of platform. They also show that high computational efficiency can be achieved by introducing the pseudo excitation method to replace the CQC or SRSS method.It is suggested that the local dynamic responses should be considered in the dynamic analysis of structures and the pseudo excitation method can be used to improve the computational efficiency in spectrum analysis.