Numerical modeling of riveting process on steam turbines blades

ABSTRACT The structural fragility due to the riveting process has been investigated in several components, such as in aeronautical structures. This process affects the reliability of the equipment when subjected to the operation loads. The high-pressure rotor under study operated about 3000 hours after re-pair maintenance of blades until the failure detected. This failure is related to the techniques used in the riveting process of the shroud in the blade, which ended up generating cracks in the radius of tenon blade. Thus, the present study numerically evaluates the mechanical behavior of the riveting process by means of a nonlinear analysis using the finite element method (FEM). For the geometry, under analysis (punch, tenon, and shroud) a 2D configuration with axisymmetric behavior was adopted, using the plane183 element for the components and the contac172 and targe169 elements for the contacts. It was adopted the elastoplastic behavior of the material in order to investigate the residual stresses after the riveting process. The results of the simulation presented good correlation with the blades failures, indicating a stress concentration in the radius of the tenon. The stresses on the tenon radius are about 1200 MPa and 180 MPa for the maximum principal stress and minimum principal stress, respectively. However, future studies using other models, such as fracture mechanics, would be applied in order to avoid failure of the joint when requested during the operating phase.

摘要
已有多项研究针对航空结构等多类构件，探讨了铆接工艺引发的结构脆弱性问题。该工艺会降低设备在承受运行载荷时的可靠性。本次研究中的高压转子在完成叶片修复维护后累计运行约3000小时，随后发生失效。此次失效与叶片围带铆接工艺所采用的技术直接相关，最终在叶片榫头圆角处引发了裂纹。据此，本研究采用有限元法（Finite Element Method, FEM）开展非线性分析，对铆接工艺的力学行为进行数值评估。针对本次分析的几何模型（冲头、榫头与围带），本研究采用具备轴对称特性的二维构型，构件单元选用plane183单元，接触单元则采用contac172与targe169单元。为探究铆接工艺完成后的残余应力分布，本研究采用材料的弹塑性本构关系。仿真结果与叶片失效现象吻合度良好，表明榫头圆角处存在应力集中现象。榫头圆角处的最大主应力与最小主应力分别约为1200 MPa与180 MPa。不过，后续可采用断裂力学等其他模型开展相关研究，以避免该连接结构在运行阶段发生失效。