How many times do you hear that a coil spring or set of coil springs have gotten old and soft? Sounds reasonable, but is it?
The real truth is that the stiffness of coil springs is governed by their geometry, the type of metal that are made from, and the state of that material as it originally was when it first came out of the box. It is not affected by use/age. I’ll tell you why in just a minute.
There are many ways a coil spring can fail. Here are some:
It can corrode (rust) and thus lose material and/or become cracked,
It can be overloaded beyond what it was designed to take,
It can wear out from constant rubbing on something,
It can lose its characteristics or even fail by being exposed to temperatures higher than it was designed to meet,
It may take a “set” early in its use (essentially change its geometry) if that isn’t taken care of before it is installed for use, and there are ways to prevent that,
It can buckle (not remain straight) if it is not designed correctly for its application. We had that happen on a large spring used in the Apollo Back Pack and had to redesign the spring. Fortunately, we found the problem long before anyone had to use the Back Pack.
It can develop a “fatigue” crack from being flexed a significantly number of times beyond its planned life. Oddly enough, this will not necessarily change its operation or stiffness. We had a spring once that was cracked half way through, and it did not affect the stiffness of the spring at all. I could explain why, but it’s complicated for a non-engineer (no insult meant).
If the ends are not ground flat properly, or not ground flat at all, the life of the spring before actual failure will be reduced.
The primary characteristic of interest that determines a spring’s stiffness once its geometry has been settled is a thing inherent in the actual material, its “modulus”. The modulus is sort of a spring stiffness that is a natural characteristic of the material itself. It can be affected by temperature, but not by very much in metals. And, except for temperature, it stays the same regardless of the list I gave above.
So, unless a spring actually falls into one or more pieces, it does not get soft in use.
So, the next time someone tells you that the coils springs on your car have gotten soft, just laugh.
Now, if you want to make a spring out of a non-metal, you have my sympathy.