A finite-element model can be thought of as a system of solid springs. When a load is applied to the structure, all elements deform until all forces balance. For each element in the model, equations can be written relating displacements and forces at the nodes.
The element shown here, for example, is a 2D quadrilateral having four nodes. Each node has two degrees of freedom associated with it (displacements in X and Y directions), so that the element has a total of eight degrees of freedom. There must also be a nodal force for each nodal degree, so there are also eight nodal forces for the element.
These displacements and forces are identified by a coordinate numbering system for entry the computer program. For example, dxi1 is the deflection in the X direction for element i at node 1, while dyi1 is the deflection in the Y direction for the same node in the same element. Forces are identified in a similar manner, so that Fxi1 is the force in the X direction for element i at node 1.
An equation relating displacements and forces for the element takes the form of basic spring equation, F = kd. For four nodes:
- k11dxi1 + k12dyi1 + k13dxi2 + k14dyi2 + . . . + k18dyi4 = Fxi1
- k21dxi1 + k22dyi1 + k23dxi2 + k24dyi2 + . . . + k28dyi4 = Fyi1
- k31dxi1 + k32dyi1 + k33dxi2 + k34dyi2 + . . . + k38dyi4 = Fxi2
- k81dxi1 + k82dyi1 + k83dxi2 + k84dyi2 + . . . + k88dyi4 = Fyi4
If degrees of freedom and nodal forces are consecutively numbered (dxi1 = d1, dyi1 = d2, Fxi1 =F1, Fyi1 = F2, and so forth), the matrix can be renumbered to show how stiffness coefficients relate nodal forces and deflections.
When a structure is modeled, individual sets of matrix equations are automatically generated for each element. The elements in the model share common nodes so individual sets of matrix equations can be combined into a global set of matrix equations. This global set relates all the nodal degrees of freedom to the nodal forces, and the nodal degrees of freedom are solved simultaneously from the global matrix. When displacements for all nodes are known, the state of deformation of each element is known. And, when deformation of each element is know, the stress and strain within the element are also known.
For simple static analysis, the finite-element method is a two-step process. Nodal displacements are first simultaneously calculated from the element stiffness and the nodal forces, both internal and external. Next, stresses are calculated, generally at the each element's centroid. Because displacements are calculated for only a finite number of points in the structure, the finite-element method is a numerical approximation rather than an exact solution.
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