基于旋转场曲率的二维剪切梁单元建模

对二维剪切梁单元进行研究,利用平面旋转场理论推导了精确曲率模型.采用几何精确梁理论构建了剪切梁单元弹性力矩阵.通过绝对节点坐标方法建立了系统的非线性动力学方程,提出基于旋转场曲率的二维剪切梁单元,并分别引入经典二维剪切梁单元和基于位移场曲率的二维剪切梁单元进行比较研究.首先,静力学分析证明了所提模型的正确性;其次,特征频率分析验证了模型可与理论解符合,收敛精度高,并且能准确地预测单元固有频率对应的振型;最后,在非线性动力学问题上,通过与ANSYS结果对比分析,证明了该模型可有效处理柔性大变形问题,并且与经典二维剪切梁单元相比具有缓解剪切闭锁的优势.因此,本文提出的基于旋转场曲率的二维剪切梁单元在处理几何非线性问题中具有较大的应用潜力.

Analysis of planar shear deformable beam using rotation field curvature formulation

In recent years, research on space debris removal technique has received wide attention in aerospace field. Many novel concepts on active flexible debris remover have been proposed, such as flexible flying net, tethered cable manipulator. In view with the high flexibility and large deformation of this kind of structure, the implementation of attitude control is challenging. An accurate dynamic model of highly flexible structure is important and needed. The beam element is the most common element adopted in flexible remover models. So, in this investigation, a rotation field-based curvature shear deformable beam using absolute nodal coordinate formulation (ANCF) (named RB-curvature ANCF beam) is proposed and its geometrically nonlinear characteristic under large deformation motion is studied. Curvature is first derived through planar rotation transformation matrix between the reference frame and current tangent frame of beam centerline, and written as an arc-length derivative of the orientation angle of the tangent vector. Using the geometrically exact beam theory, the strain energy is expressed as an uncoupled form, and the new curvature is adopted to formulate bending energy. Based on the ANCF, the dynamic equation of beam is established, where mass and external force matrices are constant. In order to validate the performance of proposed beam element, other two types of beams are introduced as the comparative models. One is the classical ANCF fully parameterized shear deformable beam derived by continuum mechanics theory, and the other is position field-based curvature ANCF shear deformable beam (named PBcurvature ANCF beam). The PB-curvature model is evaluated by differentiating unit tangent vector of beam centerline with respect to its arc length quoted from differential geometry theory. A series of static analysis, eigenfrequency tests and dynamic analysis are implemented to examine the performance of the proposed element. In static analysis, both small and non-small deformation cases show that the proposed RB-curvature ANCF beam achieves the faster speed, higher precision and good agreement with analytical solution in the case of cantilever beam subjected to a concentrated tip force, which is compared with other two beam models. The eigenfrequency analysis validates RB-curvature ANCF beam in a simply supported beam case that converges to its analytical solution. Meanwhile, the mode shapes of the proposed ANCF beam could be correctly corresponded to vibration state of element with respect to each different eigenfrequency. In the dynamics test, a flexible pendulum case is used and simulation results show that the proposed RB-curvature ANCF beam accords well with ANSYS BEAM3, classical ANCF shear beam and PB-curvature ANCF beam in vertical displacements of tip point and middle point. Since deformation modes are uncoupled in the cross section of proposed beam element, its shear strain is achieved with much better convergence in the case of lower elastic modulus, and shear locking is significantly alleviated, compared with classical ANCF beam. Therefore, RB-curvature ANCF shear deformable beam element proposed in this paper is able to describe accurately geometric nonlinearity in large deformation problem, and can be a potential candidate in the modeling of flexible/rigid-flexible mechanisms.