Continuous Function for Displacement as a Basis for Finite Elment Theory
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Finite Element: Theory, Applications & Implementation Presented By: Arthur Anconetani Barbara Gault Ryan Whitney
Finite Element Method (FEM) § A continuous function of a continuum (given domain ) having infinite degrees of freedom is replaced by a discrete model, approximated by a set of piecewise continuous functions having a finite degree of freedom.
General Example • A bar subjected to some excitations like applied force at one end. Let the field quantity flow through the body, which has been obtained by solving governing DE/PDE, In FEM the domain is subdivided into sub domain and in each sub domain a piecewise continuous function is assumed.
General Steps of the FEM § § § § § 1. Discretize & Select the Element Types 2. Select a Displacement Function 3. Define the Strain/Displacement & Stress/Strain Relationships 4. Derive the Element Stiffness Matrix & Equations 5. Assemble the Element Equations to Obtain the Global & Introduce Boundary Conditions 6. Solve for the Unknown Degrees of Freedom 7. Solve for the Element Strains & Stresses 8. Interpret the Results
Discretize & Select the Element Types § Divide the body into equivalent systems of finite elements with nodes and the appropriate element type § Element Types: § § One-dimensional (Line) Element Two-dimensional Element Three-dimensional Element Axisymmetric Element
One Dimensional Element
Select a Displacement Function § There will be a displacement function for each element
Pascal's Triangle
Define Strain Displacement & Stress/Strain Relationships § For one-dimensional; Deformation in the x-direction, strain e is related to the displacement u § [B] – Matrix relating strain to nodal displacement § Hooke's Law is used for the stress/strain relationship
sx=Eex To Stiffness Matrix
Derive the Element Stiffness Matrix & Equations § Virtual work principle of a deformable body in equilibrium is subjected to arbitrary virtual displacement satisfying compatibility condition (admissible displacement), then the virtual work done by external loads will be equal to virtual strain energy of internal stresses. § d. Ue is the element internal energy § d. We is the element external energy § Please view the integration sheet
Stiffness Matrix § fe – Element Force § ke – Element Stiffness Matrix § de – Element Displacement § § § E – Young Modulus A – Cross Section Area L - Length
Assemble Equations for Global Matrix & Introduce Boundary Conditions § Combine each element stiffness matrix into one, which is known as the global matrix § This is done by combining each [ke] into their proper location on the global matrix {F} = [K]{D} § Capital letters represent the same as the element stiffness matrix, but for global matrix
Solve for Unknown DOF's § Using the global matrix with the boundary conditions, we can now eliminate some variables and solve for the unknowns, i. e. displacements, end forces
Solve for Element Strains & Stresses Interpret Results § Solve for the stress using the equation below § To interpret the results use the FBD with your found values or use the computer program Algor
FEM Steps (Displacement Method) § Discretize into finite elements, Identify nodes & § § § elements Develop element stiffness matrices [ke] for all elements Assemble element stiffness matrices to get the global stiffness matrix Apply kinematic boundary conditions Solve for displacements Finally solve for element forces and stresses by picking proper rows
Example
§ We combine the two stiffness matrices into the global matrix.
§ Now that the displacement at u 2 has been obtained, the end forces and stress values can be obtained by reverting back to the individual element stiffness matrices § For the stress, you only need to look at the individual node of the stifness equation Reactions
Element Forces
Element Stresses
Final Notes § For this case, the calculated stresses in elements 1 & 2 are exact within the linear theory for 1 -D bar structures. Smaller finite elements will not help § For tapered bars, averaged values of the crosssectional areas should be used for the elements. § The displacements must be found first to find the stresses, since we are using the displacement based FEM
Assignment § Write the displacement functions for the following elements: § Analyze the bar shown below for: § (a) Displacement at B § (b) End Forces § (c) Average Stresses in bar AB & BC
Source: https://slidetodoc.com/finite-element-theory-applications-implementation-presented-by-arthur/
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