# Finite element-rod elements what is the pde

case 1)in finite element analysis of structures using simple rod elements we do the stiffness matrix and then find the displacements from loads and constraints

case 2)finite element method is a technique for solving partial differential equations. In the case1 what is the partial differential equation and what it looks like?

## Answers and Replies

HallsofIvy
Science Advisor
Homework Helper
?? The partial differential equation depends on exactly what the problem is. There are many different "structures" that will give different differential equations.

i have an example. There are three rods of different cross sectional areas
A1,A2,A3. with lengths L1,L2,L3 connected in series. What will be the pde
which when tried to solve by fem will give the stiffness matrix and force,displacements

For your simple 1D rod example, the simple relationship for Hooke's Law will provide the governing equation you require.

Stress = E* strain (from d(stress)/dx = 0)

You can then build the finite element equations from that point. Reference to any reasonably simple FEM text book will assist you to solve those fundamentals.

PerennialII
Science Advisor
Gold Member
If you've your FE equations, you can interpret FE as an application of Galerkin's method and work your way backward to the PDE with a suitable application of Gauss' theorem, integration by parts etc. Kind of like deriving BEM.

radou
Homework Helper
In most cases of structural analysis FEM, the differential equation is the equation of virtual work. Or, you can apply an energetic approach and derive potential energy equations.