Since I presently design products that sometimes use springs, I have some passing familiarity with spring failure modes that may apply to an aircraft feel system. They are:
If you operate the spring into the yield range of stress, you can expect loss of strength over its operating life-This is a gradual degradation.
Otherwise, sufficient cycles can fracture a spring eventually depending on the applied loads. Once a coil type compression spring fractures, the broken ends slide past each other and the spring will shorten, decreasing the effective spring length. This would be the most hazardous failure mode in an aircraft feel system.
In an aircraft feel system such as used on the A330, the springs are enclosed in housings that limit ultimate compression and protect against FOD. The design would avoid stress levels that exceed the Soderberg criterea, thus providing reliable service over the lifetime of the aircraft.
All that you probably ever wanted to know about spring failures can be found here:
Mechanical-Spring-Failure-Modes