This is done to assure that the specimen will fail in a specific region.  The specimen is evaluated within this region by marking a gage length and observing how much change occurs during the loading.  The gage length is usually 2 inches, and the diameter (for round specimens) is .505 inches.  The dimensions are for convenience.  A diameter of .505 has a cross sectional area of .2 square inches.  Because the area is reduced in this section of the specimen, the stress will be greater (remember S=P/A).    So we now have a convenient way of observing changes in length by comparing how much the material stretches when a load is applied.  This change can be plotted on a stress-strain diagram or a load-elongation diagram as shown below.
 
 
Strain can be calculated using the following formula: 
  e = (lf - lo ) / (lo) 

Example:  A standard specimen is loaded to 2000 lbs and the gage length measures 2.003, what is the strain at this load? 

Solution:  e = (2.003 in. - 2.000 in.) / (2.000 in) = .0015 in./in.
 
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