# Why use proof of stress?

1. Apr 2, 2014

### uzman1243

I dont really understand why use proof of stress.

One reason I can come up with is: For materials without a clear distinct yield point, yield strength is stated as stress at which permanent deformation of 0.2% of original dimension will result. (a dictionary definition)

Other than that, why we use proof stress as a measure of yielding in the case of some materials? and how well does it show the material's yielding behavior?

2. Apr 6, 2014

### Astronuc

Staff Emeritus
Yield strength/stress or 'proof stress' allows for some uncertainty in testing. The uncertainty arises from some variation in composition (various elements have a range of content), microstructure, metallurgical state, and testing method.

Many materials are used in their elastic range, well below the elastic limit or yield point.

Hopefully, I'll be able to dig up some more resources, or others may contribute from their experience.

3. Apr 6, 2014

### AlephZero

For materials that do not have a clear yield point, the idea of "proof stress" is a simple way to test samples of material to check their properties. You apply a load that generates the correct stress level, remove the load, and measure the change in length of the test piece. If it is bigger than 0.2%, the test failed.

That sort of testing was done before there was any mathematical theory of stress and strain. In fact one meaning of "to prove" in English is "to test". For example, guns are tested by firing a "proof charge" which is bigger than the normal amount of explosive, and then checking the dimensions of the gun barrel. This may also create some locked-in plastic stresses in the barrel which improve its resistance to the loads in normal use.

A bit of British history: http://www.gunproof.com/

A similar type of "proof testing" is sometimes done on rotating machinery, where it is deliberately run above its normal operating speed and then checked for the amount of permanent deformation.

Last edited: Apr 6, 2014
4. Apr 10, 2014

### satchurated

The 0.2% yield is a usefull value in maritime industry because allows you to understand how much energy a hull ship or a platform leg can withstand without permanent damage