Official Answer
Answer
Relaxation is the loss of load $P$ over time while the spring is held at a constant work height $H$. At elevated temperatures, the thermal energy allows dislocations in the crystal lattice to move more easily, resulting in plastic deformation even if the nominal stress is below the yield point. The rate of relaxation follows the Arrhenius equation. In failure analysis, this is observed as a 'loss of free height' $H_f$. If a spring designed for $400^{\circ}F$ is accidentally used at $600^{\circ}F$, it may lose $20-30\%$ of its load within hours, leading to assembly looseness and failure of the preloaded bearing.
