So these curves are typical of mile steel or

low carbon steel, which is typically used in buildings, bridges, cranes, et cetera.

And the proportional limit, which is this point here, where the material stop,

where the stress stops being linearly proportional to strain,

typically occurs at a stress of around 30 to 50,000 pounds per square inch or

210 to 350 approximately, megapascals.

Now that curve assumes that the area is constant.

In other words, the stress is the force divided by the area,

where the area is taken as the original curve.

And the curve that we get in this case is called a conventional stress

strained curve.

In other words, the solid red line here where it is based on the original area.

However, we can get large lateral contractions up to,

for example, this point C here.

The contraction or the thinning out of the material is very small.

However, once we go beyond that point, we can get into a region here of necking,

where the cross-sectional area of the material reduces considerably.

And if we base the Stress on the true area,

then we get the so-called true stress strain curve,

which is the dotted line here, up to E prime, the point of failure.

Now materials, which have very large plastic deformations.

In other words, this region here before they fail are called ductile materials.

And generally, a ductile material would be favorable,

because there's no sudden failure or collapse of the material.

And before failure occurs, we get very large deformations or

extensions, so we have a clear warning of the approaching failure.