Finite elements/Axial bar finite element solution

Axially loaded bar: The Finite Element Solution edit

The finite element method is a type of Galerkin method that has the following advantages:

  1. The functions   are found in a systematic manner.
  2. The functions   are chosen such that they can be used for arbitrary domains.
  3. The functions   are piecewise polynomials.
  4. The functions   are non-zero only on a small part of the domain.

As a result, computations can be done in a modular manner that is suitable for computer implementation.

Discretization edit

The first step in the finite element approach is to divide the domain into elements and nodes, i.e., to create the finite element mesh.

Let us consider a simple situation and divide the rod into 3 elements and 4 nodes as shown in Figure 6.

Figure 6. Finite element mesh and basis functions for the bar.

Shape functions edit

The functions   have special characteristics in finite element methods and are generally written as   and are called basis functions, shape functions, or interpolation functions.

Therefore, we may write


The finite element basis functions are chosen such that they have the following properties:

  1. The functions   are bounded and continuous.
  2. If there are   nodes, then there are   basis functions - one for each node. There are four basis functions for the mesh shown in Figure 6.
  3. Each function   is nonzero only on elements connected to node  .
  4.   is 1 at node   and zero at all other nodes.

Stiffness matrix edit

Let us compute the values of   for the three element mesh. We have


The components of   are


The matrix   is symmetric, so we don't need to explicitly compute the other terms.

From Figure 6, we see that   is zero in elements 3 and 4,   is zero in element 4,   is zero in element 1, and   is zero in elements 1 and 2. The same holds for  .

Therefore, the coefficients of the   matrix become


We can simplify our calculation further by letting   be the shape functions over an element  . For example, the shape functions over element   are   and   where the local nodes   and   correspond to global nodes   and  , respectively. Then we can write,


Let   be the part of the value of   that is contributed by element  . The indices   are local and the indices   are global. Then,


We can therefore see that if we compute the stiffness matrices over each element and assemble them in an appropriate manner, we can get the global stiffness matrix  .

Stiffness matrix for two-noded elements edit

For our problem, if we consider an element   with two nodes, the local hat shape functions have the form


where   is the length of the element.

Then, the components of the element stiffness matrix are


In matrix form,


The components of the global stiffness matrix are


In matrix form,


Load vector edit

Similarly, for the load vector  , we have


The components of the load vector are


Once again, since   is zero in elements 2 and 3,   is zero in element 3,   is zero in element 1, and   is zero in elements 1 and 2, we have


Now, the boundary   is at node 4 which is attached to element 3. The only non-zero shape function at this node is  . Therefore, we have


In terms of element shape functions, the above equations can be written as


The above shows that the global load vector can also be assembled from the element load vectors if we use finite element shape functions.

Load vector for two-noded elements edit

Using the linear shape functions discussed earlier and replacing   with  , the components of the element load vector   are


In matrix form, the element load vector is written


Therefore, the components of the global load vector are


Displacement trial function edit

Recall that we assumed that the displacement can be written as


If we use finite element shape functions, we can write the above as


where   is the total number of nodes in the domain. Also, recall that the value of   is 1 at node   and zero elsewhere. Therefore, we have


Therefore, the trial function can be written as


where   are the nodal displacements.

Finite element system of equations edit

If all the elements are assumed to be of the same length  , the finite element system of equations ( ) can then be written as


Essential boundary conditions edit

To solve this system of equations we have to apply the essential boundary condition   at  . This is equivalent to setting  . The reduced system of equations is


This system of equations can be solved for  ,  , and  . Let us do that.

Assume that  ,  ,  ,  , and   are all equal to 1. Then  ,  ,  ,  , and  . The system of equations becomes


Computing element strains and stresses edit

From the above, it is clear that the displacement field within an element   is given by


Therefore, the strain within an element is


In matrix notation,


The stress in the element is given by


For our discretization, the element stresses are


A plot of this solution is shown in Figure 7.

Figure 7(a). FEM vs exact solutions for displacements of an axially loaded bar.
Figure 7(b). FEM vs exact solutions for stresses in an axially loaded bar.

Matlab code edit

The finite element code (Matlab) used to compute this solution is given below.

    function AxialBarFEM
      A = 1.0;
      L = 1.0;
      E = 1.0;
      a = 1.0;
      R = 1.0;
      e = 3;
      h = L/e;
      n = e+1;
      for i=1:n
        node(i) = (i-1)*h;
      for i=1:e
        elem(i,:) = [i i+1];
      K = zeros(n);
      f = zeros(n,1);
      for i=1:e
        node1 = elem(i,1);
        node2 = elem(i,2);
        Ke = elementStiffness(A, E, h);
        fe = elementLoad(node(node1),node(node2), a, h);
        K(node1:node2,node1:node2) = K(node1:node2,node1:node2) + Ke;
        f(node1:node2) = f(node1:node2) + fe;
      f(n) = f(n) + R;
      Kred = K(2:n,2:n);
      fred = f(2:n);
      d = inv(Kred)*fred;
      dsol = [0 d'];
      fsol = K*dsol';
      p0 = plotDisp(E, A, L, R, a);
      p1 = plot(node, dsol, 'ro--', 'LineWidth', 3); hold on;
      legend([p0 p1],'Exact','FEM');
      for i=1:e
        node1 = elem(i,1);
        node2 = elem(i,2);
        u1 = dsol(node1);
        u2 = dsol(node2);
        [eps(i), sig(i)] = elementStrainStress(u1, u2, E, h);
      p0 = plotStress(E, A, L, R, a);
      for i=1:e
        node1 = node(elem(i,1));
        node2 = node(elem(i,2));
        p1 = plot([node1 node2], [sig(i) sig(i)], 'r-','LineWidth',3); hold on;
      legend([p0 p1],'Exact','FEM');

    function [p] = plotDisp(E, A, L, R, a)

      dx = 0.01;
      nseg = L/dx;
      for i=1:nseg+1
        x(i) = (i-1)*dx;
        u(i) = (1/(6*A*E))*(-a*x(i)^3 + (6*R + 3*a*L^2)*x(i));
      p = plot(x, u, 'LineWidth', 3); hold on;
      xlabel('x', 'FontName', 'palatino', 'FontSize', 18);
      ylabel('u(x)', 'FontName', 'palatino', 'FontSize', 18);
      set(gca, 'LineWidth', 3, 'FontName', 'palatino', 'FontSize', 18);

    function [p] = plotStress(E, A, L, R, a)

      dx = 0.01;
      nseg = L/dx;
      for i=1:nseg+1
        x(i) = (i-1)*dx;
        sig(i) = (1/(2*A))*(-a*x(i)^2 + (2*R + a*L^2));
      p = plot(x, sig, 'LineWidth', 3); hold on;
      xlabel('x', 'FontName', 'palatino', 'FontSize', 18);
      ylabel('\sigma(x)', 'FontName', 'palatino', 'FontSize', 18);
      set(gca, 'LineWidth', 3, 'FontName', 'palatino', 'FontSize', 18);

    function [Ke] = elementStiffness(A, E, h)

      Ke = (A*E/h)*[[1 -1];[-1 1]];
    function [fe] = elementLoad(node1, node2, a, h)

      x1 = node1;
      x2 = node2;

      fe1 = a*x2/(2*h)*(x2^2-x1^2) - a/(3*h)*(x2^3-x1^3);
      fe2 = -a*x1/(2*h)*(x2^2-x1^2) + a/(3*h)*(x2^3-x1^3);
      fe = [fe1;fe2];

    function [eps, sig] = elementStrainStress(u1, u2, E, h)

      B = [-1/h 1/h];
      u = [u1; u2];
      eps = B*u
      sig = E*eps;