As far as I can tell, stress and traction seem to refer to similar ideas. According to Chapter 8 of Twiss and Moore's Structural Geology, traction is
Force per unit area on a surface of a specified orientation (Twiss, 129)
while stress is
A pair of equal and opposite tractions acting across a surface of specified orientation (Twiss, 129)
I don't understand this latter concept. Why have a term at all if the forces are equal but opposite? Then they should cancel out and be irrelevant right?
What is stress and how is it different from traction?
Best Answer
Your book is giving you an oversimplified description (because it is written for neophytes), and that is part of what is confusing you. Stress is a 2nd order tensor entity (called the stress tensor), and, in component form, requires 6 numbers to specify the state of stress at a specific location in space. The stress tensor can be used to determine the traction acting on any surface of specified orientation. So, once you know these 6 components, you can determine the normal and tangential traction on a surface. The components of the stress tensor can be arranged in a symmetric 3x3 matrix and, when matrix multiplied by a 3x1 column vector representing a unit normal to a specified surface, delivers a 3x1 column vector representing the components of the traction exerted by the material on one side of the surface acting on the material on the other side of the surface. This is called the Cauchy stress relationship. I hope that this makes some kind of sense to you.