DEFINITION OF STRESS

The total force exerted by all the atoms on one

side of an arbitrary plane upon the atoms immediately on

the other side of the plane. Body forces are those that act

from a distance (e.g., gravity) and are proportional to the

amount of material present. Surface forces are those that

act across surfaces of contact between parts of bodies,

including all possible internal surfaces. There are two kinds

of surface forces, including normal (compressive and tensile),

that act perpendicular to the surface, and shear

(clockwise and anticlockwise), that act parallel to the surface.

The state of stress equals the force divided by the area

across which it acts.

For any applied force, it is possible to find a choice of

coordinate axes such that all shear stresses are equal to zero,

and only three perpendicular principal stresses have nonzero

values. The principal stresses are commonly abbreviated s1,

s2, and s3. These three principal stresses are parallel to the

semimajor axes of an ellipsoid called the stress ellipsoid, parallel

to the coordinate axes chosen such that they are the only

nonzero stresses.

The deviatoric stress, or the difference between the principal

stresses, is most important for forming most structures

in rocks, because it drives the deformation. However, the

mean stress (sum of [s1 + s2 + s3] / 3) is important for

determining which deformation mechanisms operate, and the

strength of materials.

Stress has dimensions of force per unit area. In the SI

system, we use the Pascal (Pa) which is 1 Newton per meter

squared (N/m2). The sign convention that geologists use considers

compressive stresses to have a positive sign.

See also STRUCTURAL GEOLOGY.

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