How accurate is the offset yield point theory?

[ECU0406]

How accurate is the offset yield point theory?

http://www.princeton.edu/~humcomp/sophlab/m&mgl_7.htm

 

[Astronuc]

Since the 0.2%-offset method is widely used, it is considered accurate enough. Really this is used comparatively or as a proof that the material is suitable. In analyses, one would want as close actual behavior as possible. For example, one might use 0.001 (0.1%) offset.

The problem is that the true yield point could vary depending upon composition and processing, as well as testing method.

In design, most systems are designed to some fraction of yield so that there is adequate margin to permanent deformation and to allow for overload conditions which might produce high transient stresses.

 

[ECU0406]

is there any way to actually test and find the yield point for any material?

I am using a wire called DSC (Dispersion Strenghtened Copper). it starts off as a metal powder 99% copper, .5% alumin oxide, and the other .5% iron and lead. it also has a oxygen free copper cladding after the metal powder is drawn to a wire.

 

[ECU0406]

is the offset yield point just a straight line with the same slope but shift .002 strain over?

 

[FredGarvin]

The issue that arises with the yield point is that in some materials, the yield point is not very well defined. The curve is so smooth that it doesn't present itself visually. The .2% offset is the accepted method.

Take a look here:
http://www.key-to-steel.com/default.aspx?ID=CheckArticle&NM=43

 

[Astronuc]

is the offset yield point just a straight line with the same slope but shift .002 strain over?

Yes. The slope is Young's modulus or Elastic modulus, and represents a linear relationship between stress and strain, i.e.

$$\sigma\,=\,E\,\epsilon$$

http://www.mse.cornell.edu/courses/engri111/modulus.htm

http://hyperphysics.phy-astr.gsu.edu/hbase/permot3.html#c2

http://www.grantadesign.com/education/sciencenote.htm

http://www.uoregon.edu/~struct/courseware/461/461_lectures/461_lecture24/461_lecture24.html

http://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Mechanical/Tensile.htm

This might be of use
http://www.shodor.org/~jingersoll/weave/tutorial/tutorial.html

One can measure up to the proportional limit.

In addition to material composition, the yield strength and to some degree, the elastic modulus is a function of the metallurgical state (dislocation density), i.e. how much residual cold-work is present. Fully recrystallized (fully annealed) materials have minimal dislocation density and have lower strength.

Accuracy depends upon one's testing and measuring systems.