Prandtl-Reuss equations

The Prandtl-Reuss equations describe how materials deform plastically—that is, how they permanently change shape when subjected to forces. They relate the incremental (small changes) stresses and strains during plastic deformation, ensuring that the material's response complies with the principles of plasticity, such as the flow rule and yield condition. Essentially, these equations provide a mathematical framework to predict how metals and other materials will behave when they are pushed beyond their elastic limit and start to deform permanently under load.