Rob has provided a very lucid description of concepts. Unfortunately there
is a wide variation in the usage of these terms, even among the widely
read continuum mechanics textbooks. My experience is that it is best to
focus on the concepts, instead of the terms. Also, I have found that
Wikipedia offers the opportunity to see how a term is used across a
variety of fields. I recommend reading the Stress(Mechanics) entry....
Best,
Mark
-----Original Message-----
From: "Robert J. Twiss" <[log in to unmask]>
Reply-To: Tectonics & structural geology discussion list
<[log in to unmask]>
Date: Thu, 29 Sep 2011 19:26:24 -0700
To: <[log in to unmask]>
Subject: Re: About stress and traction
>Several points on the subject of stress and traction:
>
>1) Traction is a force per unit area on a surface. It is a vector
>quantity.
>
>2) Surface stress is a pair of equal and opposite tractions acting on a
>surface. Technically, it is NOT a vector because each component of the
>surface stress is a pair of equal and opposite traction components. That
>is why we must define a different sign convention for surface stress
>components (compressive stress is either positive or negative). We
>generally represent the surface stress by one of its tractions, but ONLY
>after we agree on a convention for choosing which side of the surface,
>relative to the reference coordinate system, we will use as a reference
>(see '3)' below). Using one side or the other as a reference results in
>the tensile stress being either positive (engineering convention) or
>negative (geologic convention).
>
>3) The complete stress, the stress tensor, or what we commonly refer to
>as simply the stress, is a combination of the three surface stresses
>acting on three mutually perpendicular surfaces. Again, we choose a sign
>convention for the stress by choosing to represent the stress by the
>traction components that act either on the positive side of the surface
>(the side for which the outward unit normal to the surface points in a
>positive coordinate direction ‹ this gives the engineering sign
>convention) or on the negative side of the surface (the side for which
>the outward unit normal to the surface points in a negative coordinate
>direction ‹ this gives the geologic sign convention).
>
>I think it is very confusing and misleading to refer to the surface
>stresses as vector quantities. We can ONLY represent them by the
>tractions (a vector quantity) AFTER we have adopted a convention for
>choosing which tractions (those on the positive or those on the negative
>side of the surface) to use in representing the surface stresses.
>
>This is all summarized in Table 7.1 in "Structural Geology" by Twiss &
>Moores, and is discussed in detail in Section 7.1.
>
>Cheers,
>
>Rob Twiss
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