Resolution of any stress system into uniform tension and shearing stress:

Note that the quantity is invariant with respect to transformations from one set of rectangular axes to another, so that we can call the quantity the “mean tension at a point”.

For a stress system with uniform normal pressure , this quantity is . Using this concept we can resolve any stress system into components characterized by the existence and non-existence of mean tension, e.g.

(3.17)

The stress system involves no mean-tension, furthermore, we can choose co-ordinates such that the normal tractions corresponding to these axes vanish.

In a word, this stress system involves no mean tension. Hence, we can say that any stress system at a point is equivalent to uniform tension in all directions and tangential traction across three planes which cut each other at right angles.

Consider a body of an arbitrary shape is subjected to a constant pressure p which remains constant in all directions, such that,

(3.18)

and the state of strain in the body is such that,

(3.19)

The quantity is a measure of its cubicle compression and the quantity, is called the modulus of compression. It is obtained by dividing the mean tension at a point by cubicle dilation at that point.