Plane stress is an assumption used in solid mechanics to simplify the analysis of a component by considering that all of the stresses acting on an object are in a single plane. If the system being analysed is sufficiently close to meeting this condition, a three-dimensional problem can be turned into a two-dimensional one, making it much easier to solve.
The plane stress assumption can be used when one of the three principal stresses is negligible. This is often the case for thin objects because the through-thicknesses stresses will be very small, so long as all of the loads are acting in the same plane as the object.
The video below explains the plane stress assumption in more detail, and looks at three examples where it can be applied – thin plates, thin-walled pressure vessels, and thin gears.