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.

Fatigue failure is a failure mechanism that results from the formation and growth of cracks under repeated cyclic stress loading, leading to fracture. It can result in failure at stress levels well below the material yield or ultimate strengths.

The video below discusses the mechanisms behind fracture, and how S-N curves can be used to predict the fatigue life of a component. It also discuss the effect of having a tensile mean stress, and how we can use techniques like the Rainflow counting method and Miner’s rule to consider complex cyclic stresses.