Definition of Elastic Hysteresis
Elastic hysteresis describes the lagging effect of strain behind stress in a material undergoing cyclical loading and unloading. Instead of following the exact same path, the stress-strain curve forms a loop, indicating that some mechanical energy is absorbed and dissipated as heat during the deformation cycle, rather than being fully recovered.
Causes and Mechanism
This phenomenon arises from internal friction within the material, such as molecular rearrangements, viscous flow at a microscopic level, or thermal effects. When a material is loaded, energy is stored; however, during unloading, these internal processes may not fully reverse instantaneously or without internal resistance, leading to a residual strain or a different stress-strain path.
A Practical Example
A common example is a rubber band. When you stretch a rubber band (loading) and then let it contract (unloading), the force required to stretch it is slightly greater than the force it exerts as it returns to its original length at any given elongation. The area enclosed by the loading and unloading curves on a force-elongation graph (analogous to a stress-strain curve) represents the energy lost as heat.
Importance and Applications
Understanding elastic hysteresis is crucial in engineering and materials science. It impacts the design of components subject to cyclic loading, such as tires, shock absorbers, and vibration dampers, where energy absorption is desired. Conversely, in systems where energy efficiency and minimal heat generation are critical, materials with low hysteresis are preferred.