What does stiffness of a material refer to?

Stiffness of a material is fundamentally defined as its ability to resist deformation when a load is applied. This means that a stiffer material will not easily change its shape or size under mechanical stress. It characterizes how much a material will deflect or deform for a given load.

When a load is applied to a stiff material, it retains its original form more effectively than a less stiff material, which will deform more significantly under the same load. Thus, stiffness is critical in applications where maintaining shape and size under operational loads is essential, such as in structural beams or frameworks.

The other concepts relate to different material behavior: returning to the original shape after load removal pertains to elasticity, while retaining a deformed shape indicates plasticity. Resisting penetration relates to hardness and durability rather than stiffness specifically. Understanding the definition of stiffness helps in distinguishing it from these other characteristics of material behavior.