Engineering Rock Mechanics

400 Appendix A: Stress and strain analysis

Nota tion

Stress is a property that needs three values to fully describe it in the two-
dimensional case: the magnitude of the force, the direction of the force and
the area it acts on.
It is known as a tensor property, cf. scalars with one value and vectors
with two. Stress analysis is only possible if we work in components,
like this:

Force applied at Force resolved Shear component
an arbitrary angle into normal and resolved into two
to the face of an shear components. Cartesian
element. components.

After we have resolved the force into three Cartesian components,
we can define the associated stresses, and not before. In this example
we have

and in general we have:

I normal stresses I

act normal to face

shear stresses

act along a face, parallel to
an edge of the element.

Only the stresses on the visible

faces have been shown here

This is the geomechanics, or compression-positive, convention for
right-handed axes.