Illustration of simple normal stress on a straight bar with uniformcross section under axial tensile load.
This is a slightly modified version of File:Axial_stress.svg; the main changes are the negative signs on the downward forces and stresses, omission of the "avg" subscript on the stress sigma, suppression of the "look" arrow and of the "approx" sign. (The caveat about non-uniform stress distribution is important, but need not be made upfront.)
Own work
Jorge Stolfi