4. Kinetics

Our next objective is to outline the mathematical formulas that describe internal and external forces acting on a solid.  Just as there are many different strain measures, there are several different definitions of internal force.  We shall see that internal forces can be described as a second order tensor, which must be symmetric.  Thus, internal forces can always be quantified by a set of six numbers, and the various different definitions are all equivalent.

4.1 Surface traction and internal body force

Forces can be applied to a solid body in two ways.

(i) A force can be applied to its boundary: examples include fluid pressure, wind loading, or forces arising from contact with another solid.

(ii) The solid can be subjected to body forces, which act on the interior of the solid.  Examples include gravitational loading, or electromagnetic forces.

The surface traction vector t at a point on the surface represents the force acting on the surface per unit area of the deformed solid.

Formally, let dA be an element of area on a surface.  Suppose that dA is subjected to a force .  Then

The resultant force acting on any portion S of the surface of the deformed solid is

Surface traction, like true stress,’ should be thought of as acting on the deformed solid.

Normal and shear tractions

The traction vector is often resolved into components acting normal and tangential to a surface, as shown in the picture.

The normal component is referred to as the normal traction, and the tangential component is known as the shear traction.

Formally, let n denote a unit vector normal to the surface.  Then

The body force vector denotes the external force acting on the interior of a solid, per unit mass.

Formally, let dV denote an infinitesimal volume element within the deformed solid, and let  denote the mass density (mass per unit deformed volume).  Suppose that the element is subjected to a force .  Then

The resultant body force acting on any volume V  within the deformed solid is

4.2 Traction acting on planes within a solid

Every plane in the interior of a solid is subjected to a distribution of traction.  To see this, consider a loaded, solid, body in static equilibrium.  Imagine cutting the solid in two.  The two parts of the solid must each be in static equilibrium.  This is possible only if forces act on the planes that were created by the cut.

The internal traction vector T(n) represents the force per unit area acting on a section of the deformed body across a plane with outer normal vector n.

Formally, let dA be an element of area in the interior of the solid, with normal n.  Suppose that the material on the underside of dA is subjected to a force  across the plane dA.  Then

Note that internal traction is the force per unit area of the deformed solid, like true stress’

The resultant force acting on any internal volume V with boundary surface A within a deformed solid is

The first term is the resultant force acting on the internal surface A, the second term is the resultant body force acting on the interior V.

Newton’s third law (every action has an equal and opposite reaction) requires that

To see this, note that the forces acting on planes separating two adjacent volume elements in a solid must be equal and opposite.

Traction acting on different planes passing through the same point are related, in order to satisfy Newton’s second law (F=ma).

Let  be a Cartesian basis.  Let , ,  denote the components of traction acting on planes with normal vectors in the , , and  directions, respectively.  Then, the traction components  acting on a surface with normal n are given by

where  are the components of n.

To see this, consider the forces acting on the infinitessimal tetrahedron shown in the figure.  The base and sides of the tetrahedron have normals in the ,  and  directions.  The fourth face has normal n.  Suppose the volume of the tetrahedron is dV, and let  denote the areas of the faces.  Assume that the material within the tetrahedron has mass density  and is subjected to a body force b. Let a denote the acceleration of the center of mass of the tetrahedron. Then, F=ma for the tetrahedron requires that

Recall that  and divide through by :

Finally, let .  We can show that

so

or, using index notation

The significance of this result is that the tractions acting on planes with normals in the , , and  directions completely characterize the internal forces that act at a point.  Given these tractions, we can deduce the tractions acting on any other plane.  This leads directly to the definition of the Cauchy stress tensor in the next section.

4.3 The Cauchy (true) stress tensor

Consider a solid which deforms under external loading. Let  be a Cartesian basis.  Let , ,  denote the components of traction acting on planes with normals in the , , and  directions, respectively, as outlined in the preceding section

Define the components of the Cauchy stress tensor  by

Then, the traction   acting on any plane with normal n follows as

To see this, recall the last result from the preceding section

and substitute for  in terms of the components of the Cauchy stress tensor

The Cauchy stress tensor completely characterizes the internal forces acting in a deformed solid.  The physical significance of the components of the stress tensor is illustrated in the figure:  represents the ith component of traction acting on a plane with normal in the  direction.

Note the Cauchy stress represents force per unit area of the deformed solid.  In elementary strength of materials courses it is called true stress,’ for this reason.

HEALTH WARNING: Some texts define stress as the transpose of the definition used here, so that .  In this case the first index for each stress component denotes the direction of traction, while the second denotes the normal to the plane.  We will see later that Cauchy stress is always symmetric, so there is no confusion if you use the wrong definition.  But some stress measures are not symmetric (see below) and in this case you need to be careful to check which convention the author has chosen.

4.4 Other stress measures  Kirchhoff, Nominal and Material stress tensors

Cauchy stress  (the actual force per unit area acting on an actual, deformed solid) is the most physical measure of internal force.  Other definitions of stress often appear in constitutive equations, however.

The other stress measures regard forces as acting on the undeformed solid.  Consequently, to define them we must know not only what the deformed solid looks like, but also what it looked like before deformation.  The deformation is described by a displacement vector  and the associated deformation gradient

as outlined in Section 2.1. In addition, let

We then define the following stress measures

Kirchhoff stress

Nominal (First Piola-Kirchhoff) stress

Material (Second Piola-Kirchhoff) stress

The inverse relations are also useful  the one for Kirchhoff stress is obvious  the others are

The Kirchoff stress has no obvious physical significance.

The nominal stress tensor can be regarded as the internal force per unit undeformed area acting within a solid, as follows

1.      Visualize an element of area dA in the deformed solid, with normal n, which is subjected to a force  by the internal traction in the solid;

2.      Suppose that the element of area dA has started out as an element of area  with normal  in the undeformed solid, as shown in the figure;

3.      Then, the force  is related to the nominal stress by

To see this, note that one can show that

Recall that the Cauchy stress is defined so that

Substituting for  and rearranging shows that

The material stress tensor can also be visualized as force per unit undeformed area, except that the forces are regarded as acting within the undeformed solid, rather than on the deformed solid.  Specifically

1.      The infinitesimal force  is assumed to behave like an infinitesimal material fiber in the solid, in the sense that it is stretched and rotated just like an small vector dx in the solid

2.      This means that we can define a (fictitious) force in the reference configuration  that is related to  by  or .

3.      This fictitious force is related to material stress by

To see this, substitute into the expression relating  to nominal stress to see that

Finally multiply through by , note , and rearrange to see that

where we have noted that

In practice, it is best not to try to attach too much physical significance to these stress measures.  Cauchy stress is the best physical measure of internal force  it is the force per unit area acting inside the deformed solid.  The other stress measures are best regarded as generalized forces (in the sense of Lagrangian mechanics), which are work-conjugate to particular strain measures.  This means that the stress measure multiplied by the time derivative of the strain measure tells you the rate of work done by the forces.  When setting up any mechanics problem, we always work with conjugate measures of motion and forces.

Specifically, we shall show later that the rate of work  done by stresses acting on a small material element with volume  in the undeformed solid (and volume  in the deformed solid) can be computed as

where  is the stretch rate tensor,  is the rate of change of deformation gradient, and  is the rate of change of Lagrange strain tensor.  Note that Cauchy stress (and also Kirchhoff stress) is not conjugate to any convenient strain measure  this is the main reason that nominal and material stresses need to be defined.  The nominal stress is conjugate to the deformation gradient, while the material stress is conjugate to the Lagrange strain tensor.

4.5 Stress measures for infinitesimal deformations

For a problem involving infinitesimal deformation (where shape changes are characterized by the infinitesimal strain tensor and rotation tensor) all the stress measures defined in the preceding section are approximately equal.

To see this, write the deformation gradient as ; recall that , and finally assume that for infinitesimal motions .  Substituting into the formulas relating Cauchy stress, Nominal stress and Material stress, we see that

4.6 Principal Stresses and directions

For any stress measure, the principal stresses  and their directions , with i=1..3 are defined such that

Clearly,

1. The principal stresses are the (left) eigenvalues of the stress tensor
2. The principal stress directions are the (left) eigenvectors of the stress tensor

The term left’ eigenvector and eigenvalue indicates that the vector multiplies the tensor on the left. We will see later that Cauchy stress and material stress are both symmetric.  For a symmetric tensor the left and right eigenvalues and vectors are the same.

Note that the eigenvectors of a symmetric tensor are orthogonal.  Consequently, the principal Cauchy or material stresses can be visualized as tractions acting normal to the faces of a cube. The principal directions specify the orientation of this special cube.

One can also show that if , then  is the largest normal traction acting on any plane passing through the point of interest, while  is the lowest.  This is helpful in defining damage criteria for brittle materials, which fail when the stress acting normal to a material plane reaches a critical magnitude.

In the same vein, the largest shear stress can be shown to act on the plane with unit normal vector  (at 45o to the  and  axes), and its magnitude is .  This observation is useful for defining yield criteria for metal polycrystals, which begin to deform plastically when the shear stress acting on a material plane reaches a critical value.

4.7 Hydrostatic and Deviatoric Stress; von Mises effective stress

Given the Cauchy stress tensor , the following may be defined:

The Hydrostatic stress is defined as

The Deviatoric stress tensor is defined as

The Von-Mises effective stress is defined as

The hydrostatic stress is a measure of the pressure exerted by a state of stress.  Pressure acts so as to change the volume of a material element.

The deviatoric stress is a measure of the shearing exerted by a state of stress. Shear stress tends to distort a solid, without changing its volume.

The Von-Mises effective stress can be regarded as a uniaxial equivalent of a multi-axial stress state.  It is used in many failure or yield criteria.  Thus, if a material is known to fail in a uniaxial tensile test (with  the only nonzero stress component) when , it will fail when  under multi-axial loading (with several  )

The hydrostatic stress and von Mises stress can also be expressed in terms of principal stresses as

The hydrostatic and von Mises stresses are invariants of the stress tensor  they have the same value regardless of the basis chosen to define the stress components.

4.8 Stresses near an external surface or edge  boundary conditions on stresses

Note that at an external surface at which tractions are prescribed, some components of stress are known.  Specifically, let n denote a unit vector normal to the surface, and let t denote the traction (force per unit area) acting on the surface.  Then the Cauchy stress at the surface must satisfy

For example, suppose that a surface with normal in the  direction is subjected to no loading.  Then (noting that  ) it follows that . In addition, two of the principal stress directions must be parallel to the surface; the third (with zero stress) must be perpendicular to the surface.

The stress state at an edge is even simpler.  Suppose that surfaces with normals in the  and  are traction free.  Then , so that 6 stress components are known to be zero.