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内压圆筒斜接管结构的棘轮效应分析

Ratcheting study of pressurized lateral nozzle of cylinder

  • 摘要: 利用自行设计的圆筒斜接管面内弯曲加载装置, 采用电阻应变法, 在多轴疲劳实验机上对循环弯曲载荷作用下的20^\#钢内压圆筒斜接管结构进行了棘轮效应试验, 发现斜接管结构的锐角区存在棘轮应变, 且主要发生在第一主应变方向即指向焊缝的方向. 对于所研究的实验结构, 最大棘轮应变点出现在对称面锐角区的接管侧. 采用阶梯加载的方法确定了各考察点的棘轮边界. 选取4种典型的随动强化模型, 借助ANSYS软件的二次开发对面内循环弯矩作用下内压斜接管结构的棘轮效应进行数值模拟, 发现Ohno-Wang模型及基于Ohno-Wang模型的改进模型对棘轮应变的预测较为准确. 采用MJS(modifed Jiang-Sehitoglu)模型按等效塑性应变增量控制法得到了与实验结果基本吻合的各考察点的棘轮边界, 并根据最大棘轮应变点的数据确定了结构的棘轮边界, 可用于该结构的塑性安定性评价.

     

    Abstract: Lateral nozzle of cylinder or lateral tee piping is widely used in nuclear,electric, petroleum and chemical industries. The cylinder or the piping maysuffer plastic accumulation, namely ratcheting, due to fluid pressuretogether with seismic load and thermal expansion. Extensive and quantitativeratcheting investigation is necessary to detemine the ratcheting boundaryfor the safety of the structure. As the phenomenological cyclic plasticconstitutive models have made a great progress in the last two decades, someinvestigators have taken advantage of the advanced model to evaluate theratcheting of simple structure of pressure vessels and piping. However, fewliterature studied ratcheting and ratcheting boundary of complicatedstrctures such as lateral nozzle of cylinder or lateral tee piping.In this paper, ratcheting of pressurized lateral nozzle of cylinder made of20^\# carbon steel was experimentally studied with a multiaxial fatiguetesting system and a self-designed in-plane bending apparatus for lateralnozzle structure. The specimen, pressurized by a pumping station withadjustable pressure, was simply supported on a stiff beam, and pulled in apulsatile way by the servo-hydraulic testing machine to simulate thein-plane cyclic bending. Ratcheting strains were acquired by multi-channelstrain processors with strain gauges. The cyclic loading and the strainacquirement were controlled and processed simutaneously by a computer.Ratcheting strains were detected around the acute angle region of thestructure. It was found that ratcheting mainly occured in the direction ofthe first principle strain, which is directed to the intersecting weld. Themaximum ratcheting strain occured at the nozzle side of the acute angleregion in the symmetrical plane for the structue. Ratcheting boundaries ofgauged points were experimentally determined by step pressure loading.Numerical ratcheting analysis of structure was accomplished by secondarydevelopment of ANSYS with four typical kinematic hardening models, in whichOhno-Wang model and its modified models improved the prediction ofratcheting strain. Ratcheting boundaries of gauged points were numericallydetermined by the equivalent plastic strain increment control method withMJS model(Modifed Jiang-Sehitoglu model) and validated to be in goodagreement with that experimental results. Finally, the ratcheting boundaryof the structure was determined according to the values of maximumratcheting strain point, which may be used to evaluate the shakedown of thestructure.

     

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