Answer
Load relaxation (or creep) is the loss of spring force over time while held at a constant work height. This is primarily caused by operating the spring at temperatures where the material's yield strength is reduced, or the stress is high enough to cause microscopic plastic flow. Relaxation is predicted using the Arrhenius equation for temperature-dependent processes or by consulting material-specific relaxation curves (e.g., $17$-$7PH$ typically loses $< 5\%$ load after 100 hours at $450^{\circ}F$ at $100$ ksi stress). If troubleshooting a failed assembly where preload is lost, the engineer should measure the 'free height' of the used spring; a significant reduction compared to the original specification indicates that the spring has 'set' due to relaxation.
