Von Mises stress is a theoretical measure used in engineering to determine the yield point of materials under complex loading conditions. It helps predict when a material will deform plastically, meaning it will not return to its original shape after the load is removed. This stress criterion is particularly useful for ductile materials, which can undergo significant deformation before failure. The Von Mises criterion states that yielding occurs when the Von Mises stress exceeds a certain threshold, typically the yield strength of the material. This allows engineers to assess safety and performance in structures and components subjected to various forces, ensuring they can withstand operational conditions without failing.