elastic strain limits

[MarkUMSU]

I'm comparing different metals that could be used to make a particular spring/flexure.

In this case the amount of strain possible without plastic deformation is an important parameter.

Does anyone know of a material that could be comparable to NiTi (Which I believe can undergo about 8% strain without yielding)

What other metals can take a high strain without a permanent deformation? How much strain can they take?

Thanks!

 

[israelkk]

Not that I know, all metals are considered linear (without plastic deformation) up to ~0.2% deformation. The TINI can go 8% deformation only few cycles. For large number of cycles it is practically 3-4% to avoid fracture due to fatigue. Anyway, this deformation is only in the "soft" phase of the TINI when it is heated it undergoes phase transformation and in this state it is as every metal and then it becomes a "spring" with the designed forces.

 

[Compositepro]

Why do you need 8% strain in the material? Springs are designed so that the combination of geometry and physical properties gives the required spring constant.

It seems that what you are asking for is in the realm of elastomeric spings (as in urethane).

 

[salmon2]

0.2% is the offset, not the total strain.

There are two popular ways to defined yield limit for steel, 0.2% offset and 0.5 total elongation, which I assume they are interchangeable. So for most steel, your requirement 8% is way too high.

 

[MarkUMSU]

Hi All, thanks for your responses.

Irsrealkk,
Can you give me an approximate number cycles when you talk about fatigue properties of NiTi?
10's, 1000's, 10^6?

The 8% is not so much a requirement, but more of a benchmark. The requirement is closer to a 3% minimum. I'm hoping to find some other materials that may be better than NiTi in terms of their other properties...

Copmositepro,
This project can be simplified to bending a beam or wire over certain radius but having it recoil back to being straight once the force is removed.

Of course there are many factors to consider, but it would be helpful to know what *metals* could be feasible for this.

The Machinist's handbook lists some steel, copper, and nickel based alloys that are used in springs, but it does not include the maximum strain data that would allow me to calculate a minimum bending radius for a particular section. -And there may be materials that are not listed in the handbook...

I would appreciate any other suggestions for metals that are known to recoil from high amounts of strain.
Thanks!

 

[israelkk]

MarkUMSU

Assuming a shape memory alloy is what you need it can operate on the magnitude of 10^6 with 3% strain.
I have the impression you do not understand how TINI or shape memory alloys are used and how they work and what can be accomplished with them.

You can take a spring wire and roll it over a mandrel and it will become straight again when you release the load. It depends on the wire diameter and mandrel diameter but it may not give you any useful force.

It is better you tell us what you really after, what forces, deflections or radiuses, etc, you are after.

There is no point talking vaguely without real numbers.

 

[TVP]

Mark,

You will not be able to successfully design a complex spring using Machinery's Handbook as your reference for material properties, spring behavior, etc. You need to review some authoritative references, discuss this directly with a reputable spring manufacturer, or provide the information that israelkk mentioned previously.

 

[MarkUMSU]

Thanks for your suggestions thus far, but this should not be so complicated....

Here's some specifics.

The wire will have a diameter between 0.4mm and 0.5mm and is bent around a radius of about 8mm less than 100 times.

It must spring back on its original shape after being deformed.

It must be conductive, or have a conductive coating/plating such as silver.

I've already made a worksheet where I can vary parameters and solve for the forces, stress, strain, etc. Now it is a matter of finding a material that fits the requirements.

It appears that nitinol can meet these requirements, but it is difficult to apply silver plating in a reel to reel process. This is why I'm asking if anyone might suggest other elastic metal alternatives. Any Ideas?

Thanks