Alternatives to quasi-static and dynamic constitutive relationships have been investigated with respect to a previously developed energy-based fatigue lifing method for various load profiles, which states: the total strain energy dissipated during both a quasi-static process and a dynamic process are equivalent and a fundamental material property. Specifically, constitutive relationships developed by Ramberg–Osgood and Halford were modified for application to the existing energy-based framework and were compared to the lifing method originally developed by Stowell. Extensive experimentation performed on Titanium 6Al-4V (Ti-64) combined with experimental data generated for Aluminum (Al) 6061-T6 at various temperatures were utilized in support of this investigation. This effort resulted in considerable improvements to the accuracy of the lifing prediction for materials with an endurance limit through application of a modified-Halford approach. Additionally, the relative equality in predictive accuracy between the modified-Stowell approach the modified-Ramberg–Osgood approach was demonstrated.