Determination of Yield Strength in Ductile Materials

In the United States the offset yield strength is usually specified as a strain of 0.2 percent (e = 0.002).

The yield strength obtained by an offset method is commonly used for design and specification purposes because it avoids the practical difficulties of measuring the elastic limit or proportional limit.

In many materials, the yield stress is not very well defined and for this reason a standard has been developed to determine its value. The standard procedure is to project a line parallel to the initial elastic region starting at 0.002 strain. The 0.002 strain point is often referred to as the $0.2  \%$ offset strain point. The intersection of this new line with the stress-strain curve then defines the yield strength as shown in Figure 7.

Figure 7: Determination of yield strength.

Quick Quiz

Question 1: True or false? The yield strength is the stress required to produce a small-specified amount of plastic deformation.

True
False

Question 2: A certain steel sample has a Young's Modulus of E = 2.25e+7 ksi. Find the offset yield strength using US standards.

45 ksi
56 ksi
38000 psi
124 ksi