Proof Stress

 

To know about proof stress, first understand the behavior of materials. Like ductile and brittle materials. We all know the ductile is elastic and show permanent deformation under external load. Meanwhile the brittle doesn't show much deformation. 

But now the question is how much deformation will be considered as under ductile category or in brittle category.

To distinguish between ductile and brittle materials.

proof stress show significant role to find the category of materials. i.e. ductile or brittle.

Stress-strain curve for x material is shown below.

To find the material X is ductile or brittle 

proof stress point should be located on stress strain curve of the material.

For many ductile materials other than mild streel, eg. aluminum, copper, high tensile steel etc. no definite yield point is obtained. for such materials, the shape of the stress-strain diagram is given below. In such a case, the yield stress is obtained by measuring the stress corresponding to some residual strain in the material. 0.2 percent offset strain is commonly used for this purpose. the yields stress determined in this way is more often called the proof stress.

To find the proof stress  

Draw the parallel line (yellow dotted line shown in figure) which is 0.2% of strain offset. The point where yellow line crosses the blue line is marked as point of proof stress.

Now if the blue line doesn't cross yellow line then the material is considered as brittle and if the blue line touches or cross the yellow line, then material is considered as ductile.

now to find proof stress, project the point of proof stress from curve to stress line(x-axis), the will give the value of proof stress for material.