Reference work entry
The stress ellipsoid is a useful mental tool for geometrically representing the state of three-dimensional stress at a point. Stress is a force per unit area, and the stress applied to a specific plane can be represented by a vector (a quantity that has both magnitude and direction). The stress vector associated with a plane need not be perpendicular to the plane; if it is not, the stress vector can be resolved into a normal component (perpendicular to the plane) and a shear component (parallel to the plane). At any point in a body of rock, one can imagine that there are an infinite number of planes, each with a different orientation, and if that body of rock is subjected to a compressive stress, a pair of inward-pointing stress vectors will be associated with each plane. The members of each pair must be equal in magnitude and opposite in orientation to their partner; otherwise, the rock body could not remain at rest. If all the stress vectors associated with the infinite number of...
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