What is the term for the phenomenon of continuous stretching under load even when the stress is below the yield point?

The phenomenon of continuous stretching under load, even when the stress remains below the yield point, is referred to as creep. Creep occurs primarily in materials under sustained stress at high temperatures, although it can also happen at room temperature in some materials over a prolonged period.

During this process, the material experiences a gradual and time-dependent deformation. This behavior is crucial to understand in engineering applications, particularly when designing components that will be subjected to constant loads over time. It's important to note that creep is distinct from elasticity, which is the immediate reversible deformation, and ductility, which refers to a material's ability to undergo significant plastic deformation before rupture. Plasticity, similarly, involves permanent deformation but typically occurs under stresses that exceed the yield point.

In practical terms, considering creep is essential when designing structures or components that will experience long-term loads, ensuring safety and longevity.