内压圆筒斜接管结构的棘轮效应分析

RATCHETING STUDY OF PRESSURIZED LATERAL NOZZLE OF CYLINDER

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.

Lateral nozzle of cylinder or lateral tee piping is widely used in nuclear,electric,petroleum and chemical industries.The cylinder or the piping may suffer plastic accumulation,namely ratcheting,due to fluid pressure together with seismic load and thermal expansion.Extensive and quantitative ratcheting investigation is necessary to detemine the ratcheting boundary for the safety of the structure.As the phenomenological cyclic plastic constitutive models have made a great progress in the last two decades,some investigators have taken advantage of the advanced model to evaluate the ratcheting of simple structure of pressure vessels and piping. However,few literature studied ratcheting and ratcheting boundary of complicated strctures such as lateral nozzle of cylinder or lateral tee piping. In this paper,ratcheting of pressurized lateral nozzle of cylinder made of 20~# carbon steel was experimentally studied with a multiaxial fatigue testing system and a self-designed in-plane bending apparatus for lateral nozzle structure.The specimen,pressurized by a pumping station with adjustable pressure,was simply supported on a stiff beam,and pulled in a pulsatile way by the servo-hydraulic testing machine to simulate the in-plane cyclic bending.Ratcheting strains were acquired by multi-channel strain processors with strain gauges. The cyclic loading and the strain acquirement were controlled and processed simutaneously by a computer. Ratcheting strains were detected around the acute angle region of the structure.It was found that ratcheting mainly occured in the direction of the first principle strain,which is directed to the intersecting weld.The maximum ratcheting strain occured at the nozzle side of the acute angle region in the symmetrical plane for the structue.Ratcheting boundaries of gauged points were experimentally determined by step pressure loading. Numerical ratcheting analysis of structure was accomplished by secondary development of ANSYS with four typical kinematic hardening models,in which Ohno-Wang model and its modified models improved the prediction of ratcheting strain.Ratcheting boundaries of gauged points were numerically determined by the equivalent plastic strain increment control method with MJS model（Modifed Jiang-Sehitoglu model） and validated to be in good agreement with that experimental results.Finally,the ratcheting boundary of the structure was determined according to the values of maximum ratcheting strain point,which may be used to evaluate the shakedown of the structure.