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In the process of finite element stress analysis, elastic body is first divided into individual connected parts, that are the finite elements of the structure. Elements which are connected at points are the nodes. The process of dividing the domain into elements is called discretization and the pattern of elements is the mesh. Later on, using a stiffness matrix, and the loads applied to the nodes of a single element and the resultant nodal displacements of the structure. The final step is to combining together all the stiffness matrices of each and individual elements into a single large matrix which is the global stiffness matrix. Now we shall illustrate this process using linear elastic springs.
The below figure shows a shows a system of springs, each of stiffness k but with a distribution of forces. In the particular spring which is identified by the nodes 1 and 2 in that system of springs, and there are the forces F1 and F2 respectively. At equilibrium F1 + F2 = 0 or F2 = −F1 . System at equilibrium condition, which consists of a pair of springs, with different stiffnesses k1 and k2 . As it follows that
From above diagram, We can write the equation for each spring in matrix form:
As we start assembling the equilibrium equations, which describing the actual behavior of the system of two springs, and the displacement compatibility conditions, which will relate element displacements to the system displacements, and it is written as:
And therefore, substituting these displacements in the above equation:
In these above two equations, we use the notations for “f” to represent the amount of force exerted on the element “j” and at node “i“.
By expanding the each equation in matrix form:
Now summing member by member:
Let we refer to the freebody diagrams of each of the three nodes:
From the above equations, we can write the final form of the equation
Where the stiffness matrix [K] is:
Conditions and brief record of points has written down for system stiffness matrix is:
Hence, these is the calculations and conditions for system of two springs in Finite Element Theory.
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