stress is 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. stress is what causes rocks to deform, so understanding the concepts of stress is essential to structural geology. 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, commonly abbreviated o1, o2, and o3, 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 structures in rocks, because it drives the deformation. However, the mean stress, given by
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, stress is reported in Pascals (Pa), which is equal to one Newton per meter squared (N/m2).
The sign convention that geologists use considers compressive stresses to have a positive sign.
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