Two-dimensional Shape Functions
Shape functions are selected to fit as exact as possible the Finite Element Solution. If this solution is a combination of polynomial functions of n^{th} order, these functions should include a complete polynomial of equal order.
That is, a complete polynomial of n^{th} order can be written as:
with: the number of terms.
More specifically:
polynomial order n | number of terms p | |
---|---|---|
Constant: | ||
Linear: | ||
Quadratic: |
A quick way to easily obtain the terms of a complete polynomial is by using the Pascal's triangle:
order n | new polynomial terms | number of terms p |
---|---|---|
Linear | ||
Quadratic | ||
Cubic | ||
Quartic |
Contents |
Derivatives of the shape functions
For isoparametric elements (those using the same shape functions to interpolate the geometry and the unknowns), we have:
To obtain the derivatives of the shape functions:
with the Jacobian matrix with a determinant
From here is easy to obtain:
Shape Functions for Triangular Elements
The Three Node Linear Triangle
- The solution for each triangular element can be approached by their corresponding to be expressed using the shape functions:
- If the shape functions are lineal polynomials (three-node triangular element, n=3), and remembering:
- this expression can be written as:
- with the element area and
- And the system of equations is:
- The element area is computed as the half of the determinant of the coordinates matrix:
- Finally, the different parameters can be expressed in terms of the nodal local coordinates as:
- with
Areal Coordinates
In order to generalise the procedure to obtain the shape functions, the areal coordinates is a very useful transformation.
In a triangle, areal or barycentric coordinates are defined as each of the partial subareas obtained by dividing the triangle in three sections.
That is, if we use a inner point P of the triangle of area A as the common vertex of the three subareas A_{1}, A_{2} and A_{3}, then:
Note that:
- A_{1} + A_{2} + A_{3} = A
- L_{1} + L_{2} + L_{3} = 1
- If P is the Centroid or Center of Mass of the triangle, then L_{1} = L_{2} = L_{3} = 1/3
For the Finite Element Method is also interesting to note that:
with x_{p} and y_{p} the coordinates of P or any other point inside the triangle (x,y). This is equivalent to the following system of equations:
that is exactly the shape functions for a triangular element of three nodes.
Natural Coordinates
It is usual to define the triangle in terms of a normalised geometry (natural coordinates) as is showed in the figure:
Therefore:
For isoparametric elements, we have:
To obtain the Jacobian of these shape functions: