n {\displaystyle \lambda } T These stresses generally include an elastic ("static") stress component, that is related to the current amount of deformation and acts to restore the material to its rest state; and a viscous stress component, that depends on the rate at which the deformation is changing with time and opposes that change. To formulate the EulerâCauchy stress principle, consider an imaginary surface δ On the Ï-plane, Symmetric Stress-Energy Tensor We noticed that Noether’s conserved currents are arbitrary up to the addition of a divergence-less field. {\displaystyle \tau _{\text{n}}^{2}} n In particular, the contact force is given by. , 2 But WP claims that the symmetry of the stress tensor need only hold in the case of equilibrium: "However, in the presence of couple-stresses, i.e. In particular, the local strain rate E(p, t) is the only property of the velocity flow that directly affects the viscous stress ε(p, t) at a given point. The viscous stress tensor is a tensor used in continuum mechanics to model the part of the stress at a point within some material that can be attributed to the strain rate, the rate at which it is deforming around that point. , and where there are no normal shear stresses σ n n It can be shown that the principal directions of the stress deviator tensor , ∂ τ σ For a general vector x = (x 1,x 2,x 3) we shall refer to x i, the ith component of x. where Ev and Es are the scalar isotropic and the zero-trace parts of the strain rate tensor E, and μv and μs are two real numbers. ≠ n 1 3 λ {\displaystyle J_{1}} i {\displaystyle \Delta S} n {\displaystyle \sigma _{1}\geq \sigma _{2}\geq \sigma _{3}} u ) This also is the case when the Knudsen number is close to one, The strain rate tensor E(p, t) is symmetric by definition, so it has only six linearly independent elements. This means that the stress vector is a function of the normal vector that [2] Thus, in this case the viscosity tensor μ has only two independent parameters. can be expressed as the sum of two other stress tensors: where = ≥ . Principal stresses are often expressed in the following equation for evaluating stresses in the x and y directions or axial and bending stresses on a part. , the maximum shear stress is expressed by. ferromagnetic fluids which can suffer torque by external magnetic fields). n As with any symmetric tensor, the viscous stress tensor ε can be expressed as the sum of a traceless symmetric tensor εs, and a scalar multiple εv of the identity tensor. = {\displaystyle \Delta \mathbf {F} /\Delta S} j ) However, in the presence of couple-stresses, i.e. In any chosen coordinate system with axes numbered 1, 2, 3, this viscous stress tensor can be represented as a 3 × 3 matrix of real numbers: Note that these numbers usually change with the point p and time t. Consider an infinitesimal flat surface element centered on the point p, represented by a vector dA whose length is the area of the element and whose direction is perpendicular to it. i n is a proportionality constant, 3 1 45 T , , i.e. Let dF be the infinitesimal force due to viscous stress that is applied across that surface element to the material on the side opposite to dA. = , and This leaves the first integral, where Thus, Expanding the determinant leads to the characteristic equation. n P This is shown as: The maximum shear stress or maximum principal shear stress is equal to one-half the difference between the largest and smallest principal stresses, and acts on the plane that bisects the angle between the directions of the largest and smallest principal stresses, i.e. n = is the mean surface traction. , true stress tensor,[1] or simply called the stress tensor is a second order tensor named after Augustin-Louis Cauchy. {\displaystyle 45^{\circ }} The is a constant of proportionality, and in this particular case corresponds to the magnitudes S Viewed 541 times 2 … Active 3 years, 3 months ago. This implies that the balancing action of internal contact forces generates a contact force density or Cauchy traction field [5] is the divergence operator, σ . The magnitude of the normal stress component σn of any stress vector T(n) acting on an arbitrary plane with normal unit vector n at a given point, in terms of the components σij of the stress tensor Ï, is the dot product of the stress vector and the normal unit vector: The magnitude of the shear stress component τn, acting orthogonal to the vector n, can then be found using the Pythagorean theorem: According to the principle of conservation of linear momentum, if the continuum body is in static equilibrium it can be demonstrated that the components of the Cauchy stress tensor in every material point in the body satisfy the equilibrium equations. 1 are the same as the principal directions of the stress tensor {\displaystyle \tau _{\text{n}}^{2}} J Thus, the stationary values (maximum and minimum values)of However, elastic stress is due to the amount of deformation (strain), while viscous stress is due to the rate of change of deformation over time (strain rate). {\displaystyle \tau _{\mathrm {n} }} The Mohr circle for stress is a graphical representation of this transformation of stresses. 3 {\displaystyle I_{3}} σ k δ I The linear transformation which transforms every tensor into itself is called the identity tensor. Cauchy's fundamental lemma is equivalent to Newton's third law of motion of action and reaction, and is expressed as. 3 The principal stresses are unique for a given stress tensor. = According to the principle of conservation of linear momentum, if the continuum body is in static equilibrium it can be demonstrated that the components of the Cauchy stress tensor in every material point in the body satisfy the equilibrium equations (Cauchy's equations of motion for zero acceleration). Following the classical dynamics of Newton and Euler, the motion of a material body is produced by the action of externally applied forces which are assumed to be of two kinds: surface forces k ) T k only, and is not influenced by the curvature of the internal surfaces. x F The tensor consists of nine components = and it can be stated as being equal to one-half the difference between the largest and smallest principal stresses, acting on the plane that bisects the angle between the directions of the largest and smallest principal stresses. 1 If the particles have rotational degrees of freedom, this will imply an intrinsic angular momentum and if this angular momentum can be changed by collisions, it is possible that this intrinsic angular momentum can change in time, resulting in an intrinsic torque that is not zero, which will imply that the viscous stress tensor will have an antisymmetric component with a corresponding rotational viscosity coefficient. {\displaystyle \left(\sigma _{ij}-\lambda \delta _{ij}\right)n_{j}=0} {\displaystyle x_{j,m}=\delta _{jm}} {\displaystyle P} k σ The resultant vector the system is singular. and {\displaystyle u_{k}} 3 {\displaystyle \lambda } . We will consider only the symmetric part of the stress tensor so only 6 of these components are independent. is parallel to the gradient of i n i The index subset must generally either be all covariant or all contravariant. {\displaystyle \tau _{\text{oct}}} The tensor relates a unit-length direction vector n to the traction vector T(n) across an imaginary surface perpendicular to n: The SI units of both stress tensor and stress vector are N/m2, corresponding to the stress scalar. {\displaystyle s_{ij}} {\displaystyle n_{2}} {\displaystyle n_{2}} ( of the stress tensor depend on the orientation of the coordinate system at the point under consideration. 1 : This equation means that the stress vector depends on its location in the body and the orientation of the plane on which it is acting. 3 Octahedral plane passing through the origin is known as the Ï-plane (Ï not to be confused with mean stress denoted by Ï in above section) . we obtain, this result can be substituted into each of the first three equations to obtain, Doing the same for the other two equations we have. I [5] Thus, the total force In three dimensions, it has three components. {\displaystyle n_{3}\neq 0} Today we prove that. n The part Ev of E acts as a scalar multiplier (like εv), the average expansion rate of the medium around the point in question. F ( S j 0 1 {\displaystyle \mathbf {T} ^{(\mathbf {n} )}} of a particular material point, but also on the local orientation of the surface element as defined by its normal vector In a smooth flow, the rate at which the local deformation of the medium is changing over time (the strain rate) can be approximated by a strain rate tensor E(p, t), which is usually a function of the point p and time t. With respect to any coordinate system, it can be expressed by a 3 × 3 matrix. j ( τ j According to the principle of conservation of angular momentum, equilibrium requires that the summation of moments with respect to an arbitrary point is zero, which leads to the conclusion that the stress tensor is symmetric, thus having only six independent stress components, instead of the original nine: where n ,[7][10] i.e., having a common tangent at {\displaystyle n_{3}} 1 2 , respectively, and knowing that K , whose properties are the same in all directions), each part of the stress tensor is related to a corresponding part of the strain rate tensor. With constitutive relations appropriate to a linear, isotropic fluid we obtain generalized … {\displaystyle \sigma _{ij}} = λ John Conrad Jaeger, N. G. W. Cook, and R. W. Zimmerman (2007), "Continuum Mechanics: Concise Theory and Problems", "Classical and Computational Solid Mechanics", "Theory of Elasticity for Scientists and Engineers", "Mathematical Theory of Continuum Mechanics", "Computational Elasticity: Theory of Elasticity and Finite and Boundary Element Methods", https://en.wikipedia.org/w/index.php?title=Cauchy_stress_tensor&oldid=993736386, Short description is different from Wikidata, Creative Commons Attribution-ShareAlike License, Consider a continuum body (see Figure 4) occupying a volume, The normal stress can be written in terms of principal stresses, and the other parallel to this plane, called the, This page was last edited on 12 December 2020, at 05:36. j The other two possible values for {\displaystyle n_{3}}, By multiplying the first three equations by j Any symmetric tensor can be decomposed into a linear combination of rank-1 tensors, each of them being symmetric or not. This also is the case when the Knudsen number is close to one, , respectively. is the mean stress given by. τ , the shear stress in terms of principal stresses components is expressed as, The maximum shear stress at a point in a continuum body is determined by maximizing 2 σ Just as the axial vector $\FLPtau=\FLPr\times\FLPF$ is a tensor, so also is every cross product of two polar vectors—all the same arguments apply. It isantisymmetricin the rst and second indices (say) if. {\displaystyle {\boldsymbol {\sigma }}} Using just the part of the equation under the square root is equal to the maximum and minimum shear stress for plus and minus. The characteristic equation has three real roots j j . and On an element of area It is not a vector field because it depends not only on the position n The components {\displaystyle \mathbf {F} } 1 3 n {\displaystyle \tau _{\text{n}}} This value is the same in all eight octahedral planes. = In a Newtonian medium, specifically, the viscous stress and the strain rate are related by the viscosity tensor μ: The viscosity coefficient μ is a property of a Newtonian material that, by definition, does not depend otherwise on v or σ. σ The maximum shear stress is expressed as, Assuming The state of stress at a point in the body is then defined by all the stress vectors T(n) associated with all planes (infinite in number) that pass through that point. {\displaystyle K_{n}\rightarrow 1} = The principal stresses and principal directions characterize the stress at a point and are independent of the orientation. A medium is said to be Newtonian if the viscous stress ε(p, t) is a linear function of the strain rate E(p, t), and this function does not otherwise depend on the stresses and motion of fluid around p. No real fluid is perfectly Newtonian, but   n ( The deviatoric stress tensor can be obtained by subtracting the hydrostatic stress tensor from the Cauchy stress tensor: As it is a second order tensor, the stress deviator tensor also has a set of invariants, which can be obtained using the same procedure used to calculate the invariants of the stress tensor. s which removes the volume integral in ().Hence an important property of the stress tensor is its symmetry. {\displaystyle \pi } λ Thus the zero-trace part εs of ε is the familiar viscous shear stress that is associated to progressive shearing deformation. M a symmetric sum of outer product of vectors. , The equivalent stress is defined as. can be obtained similarly by assuming. k i However, the stress tensor itself is a physical quantity and as such, it is independent of the coordinate system chosen to represent it. j 2 Internal mechanical stresses in a continuous medium are generally related to deformation of the material from some "relaxed" (unstressed) state. ) n The strain rate tensor E(p, t) can be defined as the derivative of the strain tensor e(p, t) with respect to time, or, equivalently, as the symmetric part of the gradient (derivative with respect to space) of the flow velocity vector v(p, t): where ∇v denotes the velocity gradient. {\displaystyle n_{1},\,n_{2},} S ( {\displaystyle \mathbf {n} } can be obtained similarly by assuming, Therefore, the second set of solutions for n 0 , therefore, which is satisfied at every point within the body. − $\endgroup$ – Prahar Dec 30 '14 at 23:20. ) In continuum mechanics, the Cauchy stress tensor Absent of rotational effects, the viscous stress tensor will be symmetric. {\displaystyle n_{3}} Let $\varphi$ be the electrostatic potential (a scalar field), and let $\underline{A}$ be the magnetic potential (a 3-vector) from classical E&M. Knowing that the stress tensor of point O (Figure 6) in the principal axes is. the stress vector on an octahedral plane is then given by: The normal component of the stress vector at point O associated with the octahedral plane is, which is the mean normal stress or hydrostatic stress. ( n {\displaystyle T_{i}^{(n)}=\sigma _{ji}n_{j}} remains unchanged for all surfaces passing through the point ) At every point in a stressed body there are at least three planes, called principal planes, with normal vectors This decomposition is independent of the coordinate system and is therefore physically significant. Stress tensor is symmetric. moments per unit volume, the stress tensor is non-symmetric. j σ {\displaystyle \Delta S} On the other hand, the relation between E and ε can be quite complicated, and depends strongly on the composition, physical state, and microscopic structure of the material. {\displaystyle \mathbf {T} ^{(\mathbf {n} )}} {\displaystyle S} It can be attributed to friction or particle diffusion between adjacent parcels of the medium that have different mean velocities. The tetrahedron is formed by slicing the infinitesimal element along an arbitrary plane with unit normal n. The stress vector on this plane is denoted by T(n). not imaginary due to the symmetry of the stress tensor. 2 σ The normal and shear components of the stress tensor on these planes are called octahedral normal stress , following Euler's equations of motion, internal contact forces and moments are transmitted from point to point in the body, and from one segment to the other through the dividing surface λ 1 i λ where σ11, σ22, and σ33 are normal stresses, and σ12, σ13, σ21, σ23, σ31, and σ32 are shear stresses. − . n The stress tensor F 1.10.1 The Identity Tensor . σ σ A second set of solutions is obtained by assuming {\textstyle s_{ij}={\frac {1}{3}}I} {\displaystyle \sigma _{ij}} where d is the k:th Cartesian component of 2 A tensor aij is symmetric if aij = aji. 2 Expanding this equation we have, This proves that the stress tensor is symmetric. S i i σ d 1 = 2 In any material, the total stress tensor σ is the sum of this viscous stress tensor ε, the elastic stress tensor τ and the hydrostatic pressure p. In a perfectly fluid material, that by definition cannot have static shear stress, the elastic stress tensor is zero: where δij is the unit tensor, such that δij is 1 if i = j and 0 if i ≠ j. exerted at point P and surface moment Knowing that in the equation , we have. {\displaystyle \mathbf {n} } In continuum mechanics, the Cauchy stress tensor, true stress tensor, or simply called the stress tensor is a second order tensor named after Augustin-Louis Cauchy.The tensor consists of nine components that completely define the state of stress at a point inside a material in the deformed state, placement, or configuration. The eigenvalues are the roots of the characteristic polynomial. τ If the fluid is isotropic as well as Newtonian, the viscosity tensor μ will have only three independent real parameters: a bulk viscosity coefficient, that defines the resistance of the medium to gradual uniform compression; a dynamic viscosity coefficient that expresses its resistance to gradual shearing, and a rotational viscosity coefficient which results from a coupling between the fluid flow and the rotation of the individual particles. {\displaystyle \sigma _{\text{n}}=\sigma _{ij}n_{i}n_{j}=\sigma _{1}n_{1}^{2}+\sigma _{2}n_{2}^{2}+\sigma _{3}n_{3}^{2}} = .[3][4]:p.66â96. Thus. max where (symmetric) stress tensor is proportional to the symmetric eij but that is something we have to demonstrate. i 2 The EulerâCauchy stress principle states that upon any surface (real or imaginary) that divides the body, the action of one part of the body on the other is equivalent (equipollent) to the system of distributed forces and couples on the surface dividing the body,[2] and it is represented by a field Assuming a material element (Figure 2.3) with planes perpendicular to the coordinate axes of a Cartesian coordinate system, the stress vectors associated with each of the element planes, i.e. J For most general cases, stress tensor need not be symmetric in fluid mechanics. 3 At the same time, according to the principle of conservation of angular momentum, equilibrium requires that the summation of moments with respect to an arbitrary point is zero, which leads to the conclusion that the stress tensor is symmetric, thus having only six independent stress components, instead of the original nine. The Voigt notation representation of the Cauchy stress tensor takes advantage of the symmetry of the stress tensor to express the stress as a six-dimensional vector of the form: The Voigt notation is used extensively in representing stressâstrain relations in solid mechanics and for computational efficiency in numerical structural mechanics software. {\displaystyle P} 0 When the body is subjected to external surface forces or contact forces , The area of the faces of the tetrahedron perpendicular to the axes can be found by projecting dA into each face (using the dot product): and then substituting into the equation to cancel out dA: To consider the limiting case as the tetrahedron shrinks to a point, h must go to 0 (intuitively, the plane n is translated along n toward O). : , u n This tetrahedron is sometimes called the Cauchy tetrahedron. , called the traction vector, defined on the surface In matrix form this is, Expanding the matrix operation, and simplifying terms using the symmetry of the stress tensor, gives. σ Their values are the same (invariant) regardless of the orientation of the coordinate system chosen. {\displaystyle \tau _{\text{n}}} is the velocity and , . is the k:th Cartesian coordinate, Considering the solution where j For example, For example, a vector is a simple tensor of rank one. Thus, the characteristic equation is. kil: Antisymmetric tensors are also calledskewsymmetricoralternatingtensors. {\displaystyle \tau _{\text{n}}} / k This is a constrained maximization problem, which can be solved using the Lagrangian multiplier technique to convert the problem into an unconstrained optimization problem. n The antisymmetric second-rank tensor being referenced is the electromagnetic field tensor. 1 n σ The (inner) product of a symmetric and antisymmetric tensor is always zero. j The viscous stress tensor is only a linear approximation of the stresses around a point p, and does not account for higher-order terms of its Taylor series. To obtain a nontrivial (non-zero) solution for {\displaystyle s_{3}} n λ u and having the same normal vector p t I'll use the notations in your answer. In viscoelastic materials, whose behavior is intermediate between those of liquids and solids, the total stress tensor comprises both viscous and elastic ("static") components. 1 The Cauchy stress tensor obeys the tensor transformation law under a change in the system of coordinates. , called principal directions, where the corresponding stress vector is perpendicular to the plane, i.e., parallel or in the same direction as the normal vector i {\displaystyle \sigma _{3}=\min \left(\lambda _{1},\lambda _{2},\lambda _{3}\right)} n τ {\displaystyle S} n n | ) . ) ( j face of the cube has three components of stress so there are 9 possible components of the stress tensor. I S , and In general, every tensor of rank 2 can be decomposed into a symmetric and anti-symmetric pair as: $T_{ij} = \frac{1}{2}(T_{ij} + T_{ji}) + \frac{1}{2}(T_{ij} - T_{ji})$ This decomposition is not in general true for tensors of rank 3 or more, which have more complex symmetries. we have, The other two possible values for {\displaystyle n_{j}} = n = ) {\displaystyle \lambda _{i}} equal to zero, of three linear equations where Six independent components of the stress tensor. σ Therefore, the viscosity tensor μ has only 6 × 9 = 54 degrees of freedom rather than 81. = λ {\displaystyle \Delta \mathbf {F} } . In non-Newtonian fluids, on the other hand, the relation between ε and E can be extremely non-linear, and ε may even depend on other features of the flow besides E. 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Plane of the elastic medium at a point and are independent change of intrinsic angular momentum density with time Dec... A }$, each of them being symmetric or not regardless of characteristic. Of rotational effects, the contact force is given by ( 1999 ) with time these tensors always! Cross product, I need at least two vectors particle diffusion between adjacent parcels of the stress tensor will symmetric... Combination of rank-1 tensors that is something we have, this proves that the stress tensor non-symmetric... Must generally either be all covariantor all contravariant different mean velocities any symmetric tensor can be attributed to or! Noticed that Noether ’ s conserved currents are arbitrary up to the characteristic equation has components! G. Thomas Mase and George E. Mase ( 1999 ) × 3 diagonal matrix with equal values along the axis! Stresses normal to these principal planes are called principal stresses of the tensor... A divergence-less field all contravariant be perpendicular to that plane, i.e. ; X2 ) linear equations n! Noether ’ s conserved currents are arbitrary up to the hydrostatic pressure axis i.e ;... Right-Hand-Side of the stress tensor,, the stress acting on that surface element change...