On Sat, 11 May 2013 15:02:17 -0700 (PDT), Ivan Vegvary
wrote:

Replaced the door hinge assemblies in SWMBO's oven (purchased oven used). Made big difference. No longer need to push chair against oven door.

Questions about springs. These are 8" long, 1/2 inch diameter, 1.25 inches elongation when in use.

My understanding from my 'strength of materials' classes 50+ years ago is that metal does not deform until you reach the elastic limit. I would guess that most springs in normal use NEVER reach that elastic limit of metal. This would include automotive valve springs, screen door springs etc.

So how, when and why does a spring actually weaken. I need an education, all responses welcome.

Ivan Vegvary

In the case of an oven door, it usually weakens from corrosion that's
accelerated by heat.

But your more pertinent question has to do with the phenomenon of
fatigue, as metallurgists use the term. It's tricky and it's
interesting, as an insight into the differences among metals.

Fatigue failure is the result of repeated cyclical loading, often
through thousands or even millions of cycles, at or even BELOW the
elastic limit of the metal. That's the nastiest kind of fatigue,
because you have to know the load and you have to understand the
material. Aluminum, for example, has just a fraction of the fatigue
strength of steel. Thus, in aircraft, it's something they watch out
for all the time. Wherever a part is cyclically loaded to a high
percentage of its yield strength, as with a spring, fatigue is an
issue, and a frequent cause of failure.

Wikipedia has a a pretty decent explanation. Yes, it really is that
complicated:

http://en.wikipedia.org/wiki/Metal_fatigue

Part failure that occurs because of cyclic loading *above* the elastic
limit actually is quite different.

--
Ed Huntress