Metal fatigue, springs
 

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

Metal fatigue, springs
 

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

Metal fatigue, springs
 

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 an oven application, due to heat. Or poor materials.

Metal fatigue, springs
 

Ivan Vegvary wrote in
:

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.

As others have stated, it's probably due to the environment rather than
the design of the spring per se.

There's always lots of discussion in firearms circle about whether
magazine & hammer springs fatigue. IF they are properly designed &
manufactured, they shouldn't.

That said, I have an ar-15 based target rifle with a highly regarded (and
expensive) after-market trigger. The hammer spring will weaken over the
course of ~ 9 months if it is stored cocked. At that point, it will no
longer reliably fire military ammo, which tends to have harder primers.
If you compare it with a new spring, it definitely deformed a bit under
the constant tension. The manufacturer of the trigger makes their own
springs, and I suspect they do not fully understand the finer points of
spring fatigue.

Doug White

Metal fatigue, springs
 

Ed, thanks for your reply and the wonderful wiki link.
Ivan

Metal fatigue, springs
 

On Sun, 12 May 2013 13:38:50 GMT, Doug White
wrote:

Ivan Vegvary wrote in
:

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.

As others have stated, it's probably due to the environment rather than
the design of the spring per se.

There's always lots of discussion in firearms circle about whether
magazine & hammer springs fatigue. IF they are properly designed &
manufactured, they shouldn't.

That said, I have an ar-15 based target rifle with a highly regarded (and
expensive) after-market trigger. The hammer spring will weaken over the
course of ~ 9 months if it is stored cocked. At that point, it will no
longer reliably fire military ammo, which tends to have harder primers.
If you compare it with a new spring, it definitely deformed a bit under
the constant tension. The manufacturer of the trigger makes their own
springs, and I suspect they do not fully understand the finer points of
spring fatigue.

Doug White

There are a couple of factors that lead to sress-relaxation in springs
kept under load. Creep is one of them. I forget the other.

In any case, it isn't fatigue, as metallurgists and other engineers
apply the term.

--
Ed Huntress