# For the last time, are metals elastic?

I’m getting sick and tired of primary schools books that show helical springs that demonstrates a metal’s “elasticity.” For Pete’s sake, in a spring shaped like a helix, the metal is bent continuously. When the spring is stretched, the elbow bends flex. There is absolutely no way that metal in a helical or even a leaf spring can stretch and thereby demonstrate elasticity.

Everything is elastic, to some degree. Some metals can be quite elastic, e.g. - YouTube

What was the question ?

The connection is that the elastic range of the strain stress curve is the straight line slope at the front, where it obeys Hooke’s law, which is ALSO the spring law.

The difference is that the spring shape lets the material under go a lot of strain for not much stress. Note it still remains in Hookes law and so does demonstrate elasticity.

Its wrong to show spring shapes for elasticity, as with most mechanical situations we want maximal stress that remains in the elastic range, and yet a minimal strain for that stress. Eg the head bolts in the engine head… we don’t want that head lifting off the block… and we don’t want the bolt being damaged (exceed elastic range ).

When you bend any piece of metal (including the bending that happens when a spring is stretched), the metal on the outer edge of the bend stretches - the metal on the inner edge of the bend compresses. If it returns to shape after being bent, then there is elasticity happening.

Or to put it another way:

Get a springy metal bar and mark a series of dots along the length on one face, and the same on the opposite face.

Bend the rod - if the dots on the outside of the bend are further apart than they were originally, or the dots on the inside of the bend are closer together, something is stretching/compressing, and if it returns to shape when released, that is elasticity.

If the dots on both sides of the bar remain exactly the same spacing as before, then the bar isn’t bending at all.

But you are referring to strain on the spring, not the metal.

Then you’ll see, that it is not the bar that bends, it is only yourself.

Nothing with thickness can bend without either stretching/compressing in places, or breaking.

In regard to the content in your post, I’ll offer that a helical spring provides an excellent model of the elasticity of metal, in that a spring and a block of metal both exhibit a linear relationship between deformation and applied force. Whereas a spring has a spring constant “K” that relates force and displacement, metal has moduli of elasticity that relate stress (the applied force per unit of cross sectional area) and strain (the percent change in shape or volume of the metal sample).

It also happens that a helical metal spring absolutely cannot deform unless metal itself is elastic. In fact, the force/position behavior of the helical spring (i.e. the spring constant “K”) can be related back to the elastic properties of the metal it’s made of. The formula is here, under the heading “Compression Springs- Formulae, b)deflection.” The relevant metal property is “G”, or the shear modulus, which describes how metal behaves when subjected to shear loading.

Another good example of metal elasticity would be guitar strings. Choose one of the plain strings if you’re afraid of the winding on the wound strings influencing the results. Each string has somewhere around 20 pounds of tension on it. If you loosen a string a bit, it doesn’t just flop around. It simply plays at a lower frequency.

RESOLVED; metals are elastic.

Also, helical springs when a load is applied on them experience torsion, not bending.

Some are very elastic.

At the car washes I operate, the conveyors house a robust carbon steel chain that has rollers spaced on it every three and a half feet. The chain stretches and loosens on the sprockets over time due to the stresses put upon it by the weight of pulling cars through the tunnel and we periodically have to tighten the chain by removing a couple links and then reconnecting it.

Elastic.

From Wikipedia:

“In physics, elasticity is a physical property of materials which return to their original shape after they are deformed.”
A spring is a great example of this definition.
Note that deformation doesn’t only mean stretching like a rubber band.

MTS and Instron make their living on the fact that metals are elastic.

Not only that, but the lengthening of roller chains is almost never due to plastic deformation, but instead is due to wear of the pins and bushings. If you measure the full length of an outer link plate (not the pins’ center-to-center distance, but the larger distance between the front and rear edges of the plate), when a chain is new, and then after it’s been in service for a while and has worn, you find that that distance hasn’t changed, even though the total length of several joined links is longer (because of wear at the pins/bushings).

Another example is “Newton’s Cradle” - Newton's cradle - Wikipedia - it works the way it does, because of the elasticity of the metal balls (a Newton’s cradle with low elasticity ball (for example, putty or wax) would behave quite differently)

Huh. Learn a new thing every day.

There is certainly wear on the pins that hold the links together as they deform over time, however the links themselves stretch too. This is evidenced by the fact that we have to take links out more frequently when the chain is new before it “settles” into not needing tightening as often. The pins take a couple years before they are quite noticeably deformed.

We use a D-88 steel pintle chain at one location just like this one: SONNYSPARTS.COM

This chain has a “master link” section that’s 3.5 feet long and is the only place where you can remove links (except taking out a whole “regular” section from roller to roller) by removing the cotter pin and the larger pin as the pins on the rest of the chain are all fused together. You have to save the links you take out to put back in once you run out of master links. You basically have to re-create that section over time and then remove a regular section and replace it with your rebuilt master link section less one or two links once you get to that point. The chain will stretch about six feet or so over its working lifetime, which is generally 4 years give or take. The chain is 240’ long.

Our other store uses an x-348 chain where you can take links out anywhere as this chain has rotating pins that you can remove simply by creating slack in the chain with a come-along and rotating the pin out. It looks like this (although we use a more robust version of this chain): http://shop.tommycarwash.com/Tommy-Store/Tommy-AVW-common-parts/X348-Conveyor-Chain

So there’s your car wash chain lesson! Why? I don’t know!

RESOLVED then. Thanks folks.