Solid Mechanics: Stress  
The Definition of Stress  
The concept of stress originated from the study of strength and failure of solids. The stress field is the distribution of internal "tractions" that balance a given set of external tractions and body forces. First, we look at the external traction T that represents the force per unit area acting at a given location on the body's surface. Traction T is a bound vector, which means T cannot slide along its line of action or translate to another location and keep the same meaning. In other words, a traction vector cannot be fully described unless both the force and the surface where the force acts on has been specified. Given both DF and Ds, the traction T can be defined as
The internal traction within a solid, or stress, can be defined in a similar manner. Suppose an arbitrary slice is made across the solid shown in the above figure, leading to the free body diagram shown at right. Surface tractions would appear on the exposed surface, similar in form to the external tractions applied to the body's exterior surface. The stress at point P can be defined using the same equation as was used for T. Stress therefore can be interpreted as internal tractions that act on a defined internal datum plane. One cannot measure the stress without first specifying the datum plane.  
The Stress Tensor (or Stress Matrix)  
Surface tractions, or stresses acting on an
internal datum plane, are typically decomposed into three mutually
orthogonal components. One component is normal to the surface and
represents direct stress. The other two components are
tangential to the surface and represent shear stresses.
What is the distinction between normal and tangential tractions, or equivalently, direct and shear stresses? Direct stresses tend to change the volume of the material (e.g. hydrostatic pressure) and are resisted by the body's bulk modulus (which depends on the Young's modulus and Poisson ratio). Shear stresses tend to deform the material without changing its volume, and are resisted by the body's shear modulus.
Defining a set of internal datum planes aligned with a Cartesian coordinate system allows the stress state at an internal point P to be described relative to x, y, and z coordinate directions. For example, the stress state at point P can be represented by an infinitesimal cube with three stress components on each of its six sides (one direct and two shear components). Since each point in the body is under static equilibrium (no net force in the absense of any body forces), only nine stress components from three planes are needed to describe the stress state at a point P. These nine components can be organized into the matrix:
where shear stresses across the diagonal are identical (i.e. s_{xy} = s_{yx}, s_{yz} = s_{zy}, and s_{zx} = s_{xz}) as a result of static equilibrium (no net moment). This grouping of the nine stress components is known as the stress tensor (or stress matrix). The subscript notation used for the nine stress components have the following meaning:
 
Equations of Equilibrium 
Consider the static equilibrium of a solid subjected to the body force vector field b. Applying Newton's first law of motion results in the following set of differential equations which govern the stress distribution within the solid,
In the case of two dimensional stress, the above equations reduce to,

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