# nodal displacement finite element analysis

e And we express the displacement in each of these brick elements, as a function of the nodal point displacements, of the displacements of the corners of the bricks, then we can, of course, express the total displacement in the body as a function of the nodal point displacement. Symmetry or anti-symmetry conditions are exploited in order to reduce the size of the model. In this particular case, I know that there's a discontinuity in area here and for that reason, intuitively, I will put one element from here to there with a constant area. And that is the point that I'm looking at. We have only one displacement component. In the previous two articles, I have addressed the fundamental idea behind direct stiffness method for decomposing a structure with pre-defined individual sub-domain or an “element”. In fact, two former matrix multiplications. And here, I dropped the hat already, just for convenience of writing, and from now on, when we have this vector, U here, then that means that we're talking about the concentrated nodal point displacements, or the actual note point displacements of the finite element mesh. e Proper support constraints are imposed with special attention paid to nodes on symmetry axes. On the left hand side, I have the following part. If we have specific geometries, we might use cylindrical coordinates systems for certain elements, Cartesian coordinate systems for other elements, and so on. And that is, of course, a very important computational aspect. j   {\displaystyle \delta \ \mathbf {r} } We notice, however, that the element below it here-- if I take my pen here, and draw in another element, we notice that that element has the same node as the top element. Second time around, imposing a unit displacement at the second degree of freedom, all the others being 0. We will use the {\displaystyle {R}_{k}^{o}} An element shape function related to a specific nodal point is zero along element boundaries not containing the nodal point. That is, the solution for the element nodal point displacements is performed as usual, the element nodal point forces are calculated as usual, and then a simple procedure is employed to calculate the element stresses from the nodal point forces using the principle of virtual work. We have here, typically, a support that prevents displacements in any direction. And that's what we have done here. Now, in dynamic analysis, of course, the loads are time dependent and if we are considering a truly dynamic analysis, then we have to include inertia forces. Nodes will have nodal (vector) displacements or degrees of freedom which may include translations, rotations, and for special applications, higher order derivatives of displacements. In general, there are a lot of Finite Element types. And we are satisfying, of course, that the elements remain together, so no gaps opening up. The first equation we solved for the Ua now, of course, in this particular case, we would now have all bars on there. 1. Here, we have the surface forces with components in the x, y, and z directions. These virtual displacements over the body give us also virtual displacements on the surface of the body, which are listed in here. o That is the third condition where that equilibrium condition is embodied in the principle of virtual work. So what I'm doing here is I express the displacements of element m as a function of all the nodal point displacements, and I'm listing here in u hat these displacements for N, capital N nodal points. Notice also that I've written here, Um, of course, but that Um here, for our specific case is simply this displacement, Vm. Then we have the following relationship-- and this is the important assumption of the finite element discretization. So stress strain law is satisfied, compatibility is satisfied, both of them exactly. In our finite element analysis, we are proceeding in the following way-- we say, well, let us idealize this complete body as an assemblage of elements, and what I've done here is to draw one typical element. Using an iterative method, we increase the number of elements along each side and solve. If you take that body and subject it to any arbitrary virtual displacement that satisfy the displacement boundary conditions, then the external virtual work is equal to the internal virtual work. Our R vector is simply, in this particular case, 0, 0, 100. We now can, of course, express our accelerations in the element in terms of nodal point accelerations again, and we are using here the same Hm matrix that we use already for the displacement interpolations. Topics: Formulation of the displacement-based finite element method. So notice that our epsilon bar m, is a virtual strain, is given in this way. Well, here we have the body forces that I applied to the body, the surface forces that I applied to the body. Well, what we will be doing is we will be applying this principle of virtual displacements for our finite element discretization, which means that in an integral sense, we satisfy equilibrium. All I've done is since our total body is idealized as a sum of volumes, namely the volumes over the elements, I can rewrite the total integral as a the sum over the element integrals. q {\displaystyle \mathbf {R} ^{o}} So here is a problem once again. B Hence, the displacement of the structure is described by the response of individual (discrete) elements collectively. 5. We rewrote this in terms of the nodal point displacements and element interpolation matrices that we use for our finite element discretization. R Notice I use the transpose, the capital T here, to denote the transpose of a vector. {\displaystyle {q}_{i}^{e}} In this problem, displacement u at node 1 = 0, that is primary boundary condition. For unit displacement at this end of the element, this is the interpolation. Let's assume that we know the stresses, at this point. Well, we will not derive these boundary conditions and the governing differential equations in this approach, but rather what we do is we invoke this principle, we set del pi equal to 0, and that gives us the principle of virtual displacements. Here, I have on the left hand side-- let's go through this equation in detail. Select a Displacement Function -Assume a variation of the displacements over each element. Since this principle here shall hold for any arbitrary virtual displacements that satisfy the displacement boundary conditions, we can now invoke this principle n times. Lecture 3: The Displacement-Based Finite Element Method, Finite Element Procedures for Solids and Structures, General effective formulation of the displacement-based finite element method, Discussion of various interpolation and element matrices, Physical explanation of derivations and equations, Example analysis of a nonuniform bar, detailed discussion of element matrices. Since we do know the initial stress, we put that one, of course, on the right hand side. + ME 1401 - FINITE ELEMENT ANALYSIS. There is no more assumption in this step. So this is the changing area in this domain. Of such brick elements that they remain compatible under deformations OCW materials at your own.... In detail six such known strains, and notice that these u,,! Life-Long learning, or to teach others is modeled by a beam element Results 2 would these. Certain manner dictated by the dynamic finite equation.. 2 numerical technique for approximate! 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