Invariants of tensors
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where is the identity tensor and is the polynomial's indeterminate (it is important to bear in mind that a polynomial's indeterminate may also be a non-scalar as long as power, scaling and adding make sense for it, e.g., is legitimate, and in fact, quite useful).
The first invariant of an n×n tensor A () is the coefficient for (because the coefficient for is always 1), the second invariant () is the coefficient for , etc., the nth invariant is the free term.
The definition of the invariants of tensors and specific notations used throughout the article were introduced into the field of rheology by Ronald Rivlin and became extremely popular there. In fact even the trace of a tensor is usually denoted as in the textbooks on rheology.
The first invariant (trace) is always the sum of the diagonal components:
The nth invariant is just , the determinant of (up to sign).
The invariants do not change with rotation of the coordinate system (they are objective). Obviously, any function of the invariants only is also objective.
Calculation of the invariants of symmetric 3×3 tensors
Most tensors used in engineering are symmetric 3×3. For this case the invariants can be calculated as:
(the sum of principal minors)
where , , are the eigenvalues of tensor A.
Because of the Cayley–Hamilton theorem the following equation is always true:
where E is the second-order identity tensor.
A similar equation holds for tensors of higher order.
A scalar valued tensor function f that depends merely on the three invariants of a symmetric 3×3 tensor is objective, i.e., independent from rotations of the coordinate system. Moreover, every objective tensor function depends only on the tensor's invariants. Thus, objectivity of a tensor function is fulfilled if, and only if, for some function we have
A common application to this is the evaluation of the potential energy as function of the strain tensor, within the framework of linear elasticity. Exhausting the above theorem the free energy of the system reduces to a function of 3 scalar parameters rather than 6. Within linear elasticity the free energy has to be quadratic in the tensor's elements, which eliminates an additional scalar. Thus, for an isotropic material only two independent parameters are needed to describe the elastic properties, known as Lamé coefficients. Consequently, experimental fits and computational efforts may be eased significantly.
Since the invariants are constant in any reference, functions of invariants are also constant. Some sources define the three invariants of the 3×3 tensors as
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- SPENCER, A. J. M. Continuum Mechanics. Longman, 1980.