What is the term for the limit beyond which the force is no longer proportional to extension?

The correct term for the limit beyond which the force is no longer proportional to extension is known as the limit of proportionality. This concept is crucial in understanding the behavior of materials when they are subjected to stress.

When a material is stretched, it initially obeys Hooke's Law, which states that the force exerted by the material is directly proportional to the extension (or strain) up to a certain point. This point denotes the limit of proportionality. Beyond this limit, the relationship between force and extension becomes non-linear, meaning that if the material is further stretched, the increase in force does not correspond to a proportional increase in extension. At this stage, the material may begin to exhibit plastic behavior, where it does not return to its original shape once the force is removed.

Recognizing the limit of proportionality helps in understanding material properties and their applications, particularly in engineering and design, where maintaining material integrity under load is critical.