What is the term for a material that deforms slowly when a force is applied to it for an extended period of years, even though the force is too small to cause failure in a short period of time?

The correct term for a material that undergoes slow deformation under a sustained force over an extended period of time is creep. Creep is a time-dependent strain that occurs when a material is subjected to a constant load or stress, especially where the load is not strong enough to cause immediate failure. This phenomenon is particularly significant in materials at high temperatures or in structural applications where long-term loading conditions are expected, leading to gradual changes in shape or length.

In contrast, fatigue refers to the failure of a material after repeated loading cycles, typically involving stress or strain that may be lower than the material's yield strength but can lead to crack formation and eventual failure over time. Expansion pertains to the increase in size or volume of a material due to thermal effects or moisture absorption, while elongation is a measure of the increase in length of a material when subjected to a tensile force and is typically assessed during a tensile test. Thus, while these terms describe different properties of materials under various conditions, creep specifically addresses the phenomenon of long-term deformation under a persistent load.