On the Nature of the Cauchy Stress Tensor

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ON THE NATURE OF THE CAUCHY STRESS TENSOR The Cauchy stress tensor, σ, is symmetric, σ =σσT, and has the following physical meaning: Consider an elementary surface, da, with unit normal n, through a typical point x in the deformed conﬁguration of a continuum. The tractions (force per unit cur- rent area), t(n), transmitted at x across da are given by, (n) t = n • σ =σσ n. (1) The total force dff(n) on the elementary vector area da = n da then is dff(n) = t(n) da =σσ da. (2) The tensor σ can be represented by its covariant, contravariant, or mixed components with respect to any coordinate system, be it curvilinear, rectilinear, nonorthogonal, or orthogonal. If we use an orthogonal Cartesian coordinate sys- = σ =σ σ tem xi, with unite base vectors, ei,i 1, 2, 3, then ij ei × ej, where ij for ﬁxed i and j is the ith component of the tractions across an element which is cur- rently normal to the xj-coordinate direction, and × denotes dyadic product. Now consider any (convected or otherwise) curvilinear coordinate system with base vectors (not necessarily either unit or mutually orthogonal), gA, and B B =δB the corresponding reciprocal base vectors, g , so that gA • g A, A, B = 1, 2, 3. Then the stress tensor σ may be represented as, σ =σAB =σ A B =σB A gA × gB AB g × g A g × gB , (3) σ σAB σB where AB, , and A are the contravariant, covariant, and mixed components of the stress tensor, σ, repectively. Therefore, the Cauchy stress tensor is a second-order symmetric tensor, independently of any coordinate system that may be used to represent its components. It is neither covariant nor contravariant. In fact such a question is rele- vant only in relation to the components of the Cauchy stress or any other tensor! For more information, please see Sections 3.1 and 3.2 of: Plasticity, A Treatise on Finite Deformation of Heterogeneous Inelastic Materi- als, by Sia Nemat-Nasser, Cambridge University Press, 2004. SNN.

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