What is the significance of 0.2% in determining proof load?

[chandran]

I got a material property data for a steel as follows.

1.0.2% of proof load 358kg

2.0.2% of proof stress 28.64(kg/sqmm)


My questions are

1)what is proof load

2)what is 0.2% of proof load. Why 0.2%? Why not 0.3%

3)How this proof load is arrived while testing the specimin in a universal tensile testing machine?

 

[FredGarvin]

Proof load is usually another term for the load associated with they yield point. Since it is a steel, it is customary to establish the elastic portion of the stress-strain curve and thus the yield point at a .2% offset. It is my understanding that .2% was agreed upon to use as the offset because not all materials have a distinct yield point.

Take a look here (scroll down to measures of yielding): http://www.key-to-steel.com/Articles/Art43.htm

 

[Astronuc]

Fred nailed this one right on!

If one checks a glossary in a reference book, one will find -
proof load
A predetermined load, generally some multiple of the service load, to which a specimen or structure is submitted before acceptance for use.

However,
proof stress
(1) A specified stress to be applied to a member or structure to indicate its ability to withstand service loads. (2) The stress that will cause a specified small permanent set in a material.

And it is the second definition which applies here.

IIRC, 0.1% strain offset has also been used in addition to the 0.2% offset, in the US. According to ASM's "Atlas of Stress-Strain Curves", in the UK (and perhaps outside of North America), offset yield strength is referred to as proof stress, and 0.1% or 0.5% is used.

The point of choosing 0.2% is that it is easier that 0.1% and not as much as 0.5%. Using the offset "avoids the practical difficulties of measuring the elastic or proportional limit" of a material. This is not so much a problem these days with digitally controlled tensile test systems, but it was significant 25 or more years ago before high quality digital electronics were available.

The idea was to get as close as reasonably possible to the elastic limit.

 

[Pyrrhus]

Well, when one a material doesn't has an obvious fluency point, and it has great unit strains after it has exceeded its proportional limit, you can get an arbitrary fluency stress by that method, the offset method. It's basically a straight line on the stress-strain diagram, parallel to the initial linear part of the curve, but it displaces certain standard unit strain, in this case 0.2%. The intersection of the displaced line with the curve defines the fluency stress. This stress is obtained arbitraly and it's not a physical property of the material. However, it can be used as the fluency stress of the material.