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Elasticity/Minimizing a functional

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Minimizing a functional in 1-D

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In 1-D, the minimization problem can be stated as

Find such that

is a minimum.

We have seen that the minimization problem can be reduced down to the solution of an Euler equation

with the associated boundary conditions

or,

Minimizing a Functional in 3-D

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In 3-D, the equivalent minimization problem can be stated as

Find such that

is a minimum.

We would like to find the Euler equation for this problem and the associated boundary conditions required to minimize .

Let us define all our quantities with respect to an orthonormal basis .

Then,

and

Taking the first variation of , we get

All the nine components of are not independent. Why ?

The variation of the functional needs to be expressed entirely in terms of . We do this using the 3-D equivalent of integration by parts - the divergence theorem.

Thus,

Substituting in the expression for , we have,

For to be minimum, a necessary condition is that for all variations .

Therefore, the Euler equation for this problem is

and the associated boundary conditions are