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DOI: 10.4018/978-1-4666-6505-7.ch004

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TopThe stress at a point is uniquely defined by specifying the magnitude and direction of a single vector having the units of force per unit elemental area, Figure 1(a). For engineering purposes, it is more useful to define stress at a point in terms of the direct and shear components of stress acting normally and tangentially to the six faces of an elemental prism as shown in Figure 1 (b). The stress at a point is defined when the stress components, *σ _{xx}, σ_{xy}, σ_{xz}*, ... etc. are known and the gradient of stress is defined when the values of the , , , etc. are known.

It is often convenient to choose a prism so oriented that the shear components are zero, Figure 1(c). The direct stresses on such an elemental prism are called principal stresses. The stress at a point can therefore be specified either by the magnitudes and directions of the principal stresses or by the magnitudes of the corresponding conjugate stresses. Similarly, deformations may be defined in terms of direct and shear components (angular distortions) of strain denoted by ε_{x,} ε_{y}, ε_{z} and ε_{xy}, ε_{xz}, ε_{yz} respectively.

In soil mechanics, stresses which tend to cause compression and compressive strains are usually taken as positive while those that cause extension or tensile strains are taken as negative. This is the opposite of the sign convention used in structural mechanics.

In 1678 Robert Hooke on the basis of experiments on a steel wire deduced that stress is proportional to strain if the elastic limit is not exceeded. Later Poisson showed that as the length of a specimen increased under load, the diameter or lateral dimension decreased, and that for many materials the lateral and longitudinal strains are always in a fixed ratio. These two discoveries expressed as Hooke’s law and Poisson’s Ratio form the basis of the classical theory of elasticity, Timoshenko and Goodier (1951). They can be expressed as:

Following conventional definition of stress, the normal stress is defined as:

Similarly the shear stress is given by

The simplest form of a continuum is given by considering an elementary unit of material that is so small that one does not need to involve local moments to establish equilibrium. In this class of continua, the stress tensor can be written as:

For one set of axes (x, y, z), Figure 1b, the shear stresses vanish giving the principal stresses σ_{1}, σ_{2}, σ_{3}, Figure 1c. It is helpful to concentrate on principal stresses because

*1.*Most test data are from tests in which only principal stresses are applied.

*2.*We do not know much about the effects of rotation of principal axes on the stress-strain behaviour. It is therefore most often assumed that rotation of the axes, as such, has no effect on the stress-strain behaviour.

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