tailieunhanh - Fundamentals of Finite Element Analysis phần 8

và các dẫn xuất một phần được sử dụng bởi vì mật độ có thể khác nhau trong không gian cũng như thời gian. Sử dụng các thành phần vận tốc cho thấy, tốc độ thay đổi của khối lượng trong điều khiển âm lượng từ dòng chảy theo hướng x m x = u dy dz - ˙ u + ∂ (u) dx dy dz ∂ x | I Text Hutton Fundamentals of 9. Applications in Solid Finite Element Analysis Mechanics The McGraw-Hill Companies 2004 Isoparametric Formulation of the Plane Quadrilateral Element 347 Note that the integrands are quadratic functions of the natural coordinates. In fact analysis of Equation reveals that every term of the element stiffness matrix requires integration of quadratic functions of the natural coordinates. From the earlier discussion of Gaussian integration Chapter 6 we know that a quadratic polynomial can be integrated exactly using only two integration or evaluation points. As here we deal with integration in two dimensions we must evaluate the integrand at the Gauss points ri Y s- ị with weighting factors Wi Wj 1. If we apply the numerical integration technique to evaluation of k we obtain as expected the result identical to that given by Equation . More important the Gauss integration procedure can be applied directly to Equation to obtain the entire element stiffness matrix as k e tab Ẻ Ẻ WiWj B r Sj f D B ri Sj i 1 j 1 where the matrix triple product is evaluated four times in accordance with the number of integration points required. The summations and matrix multiplications required in Equation are easily programmed and ideally suited to digital computer implementation. While written specifically for the four-node rectangular element Equation is applicable to higher-order elements as well. Recall that as the polynomial order increases exact integration via Gaussian quadrature requires increase in both number and change in value of the integration points and weighting factors. By providing a look-up table of values fashioned after Table computer implementation of Equation can be readily adapted to higher-order elements. We use the triangular element to illustrate plane stress and the rectangular element to illustrate plane strain. If the developments are followed clearly it is apparent that either element .

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