Two mathematical methods are presented to analyze uniaxial stress‐strain measurements of amorphous polymeric materials at constant stretching rates. Both methods are based on a well‐known single integral constitutive equation in which the time and strain effects are separated. The first approach results in an analytical equation and is only valid for materials of which the relaxation rate and nonlinearity are restricted. The second method is based on a numerical evaluation of the integral, and is exact for infinitesimal summation steps. Both methods enable the nonlinearity of the strain tensor to be determined if the linear viscoelastic behavior of the material is known.