แง่การคำนวณของไฟไนต์เอเลเมนต์แบบผสมในการดัดของแผ่นหนา / ภาคภูมิ วานิชกมลนันท์ = Computational aspects of mixed finite elements in bending of thick plates / Parkpoome Vanitkamonnunt
By using the conventional finite element method in which the displacement fields are assumed and the stiffness matrices formulated from the principle of minimum potential energy, the obtained displacement solutions are generally found to be accurate but the stress solutions contain significant errors, especially when a coarse mesh is used. As a result, a mixed finite element method employing multifield variables has been developed wherein the displacement fields and stress or strain fields are generally assumed. The stiffness matrix is formulated from Hellinger-Reissner principle with two fields of variables or Hu-Washizu principle when three variable fields are used. In thick plate analysis using Mindlin plate theory taking the effects of transverse shear strains into account, the stress results contain terms in bending and shear which, in turn, affect the accuracy of the stress results significantly. Therefore mixed finite elements can be used to improve the accuracy of the stress solutions. However, shear locking problems may arise when using Mindlin plate theory, particularly when plate thickness becomes very small. Consequently these problems must be addressed in formulating the element stiffness matrices. This research explores several computational aspects of the following mixed finite elements: the 8-node PLAT8 and PLAT8H elements, the 5-node HMPL5 element, the 4-node BUBBLE4 and MiSP4 elements and the 9-node BUBBLE9 element. Each element formulation is summarized and the efficiency tested in plates with various types of supports. An 8-node plate bending element in GTSTRUDL was used as a benchmark and all of the elements examined with regard to accuracy of displacement, moment and shear stress solutions. Also included in the investigation are the effects of mesh refinement and variation of span length-to-thickness and element aspect ratios, the effects of element distortion, invariance, computational efficiency and, lastly, the patch test. It was found that the mixed elements yield displacement solutions as accurate as conventional elements, whereas moment and shear stress solutions are closer to the exact solutions. The tests also showed that plates with more restraint tend to produce better results. Attempts to improve the HMPL5 and BUBBLE4 elements led to an increase in accuracy of displacements but at the expense of reduced accuracy in moments and shears. All things considered, the HMPL5 element is the most suitable element for analyzing thick plates in bending.