A real spring does not always follow the same path while it is loaded and returned. The difference can contain useful information—but only when the test method and fixture are controlled.
Two paths through one cycle
The loading curve records increasing test displacement. The unloading curve records the return. At the same displacement, the two paths may show different force. This difference can arise from material damping, internal friction, coil contact, end seating, guides and fixtures.
Hysteresis energy
On a force–deflection plot, the area enclosed by the loading and unloading paths represents energy not returned during the cycle. Software can integrate the two branches to report work applied, work returned and a hysteresis measure.
The result is meaningful only when sampling order, units and branch selection are preserved. Sorting points by displacement can destroy the time order and merge separate strokes incorrectly.
Force difference at a position
A simpler comparison is the force difference between loading and return at a defined displacement, often near the middle of the working range. The location must be stated; the largest difference anywhere on the curve answers a different question.
Repetitions and conditioning
Early cycles may settle the spring or fixture. If the procedure specifies repetitions, retain the complete set and identify which cycle is evaluated. Do not silently discard an inconvenient repetition or combine points from different strokes.
Before blaming the spring
- Check platen or hook seating and alignment.
- Check guide friction and fixture movement.
- Confirm load and position calibration.
- Confirm the recipe used the intended speed, dwell and target.
- Review the raw time-ordered trace for interruptions or steps.
This guide provides general engineering information. Apply the governing drawing, licensed standard, risk assessment and approved procedure for the actual test.