Continuum Damage Mechanics is a promising tool for the failure prediction of structural components. Nevertheless, it is not a simple task to do a mathematically correct and physically realistic description of the strain-softcning behavior due to the degradation of the microstructure. In the case of metallic bars, the cyclic plastic deformation induces a strain-hardening and also a degradation of the structure (fatigue damage). In the other hand, the degradation of the structure induces a softening behavior in the engineering stress-strain curve. Hence, it is very important to model the coupling between plasticity and damage in order to perform an adequate lifetime prevision. Many attempts to describe this type of behavior have been unsatisfatory. The mathematical problem is, in general, ill posed and a numerical approximation of the solution is incorrect from the physical point of view. In this cases the phenomenon of strain localization due to strain-soflzening is mesh dependent. In the present work a new Damage theory for elasto-plastic materials that overcome this problem is proposed. The theory has a strong thermodynarnic basis and take into account the softening behavior. One basic difference from the others models is that the scalar variable D related with damage is taken as an independent kinematic variable, similarly as in the theories of continua with microstructure. The effectiveness and usefulness of the theory is checked by comparing numerical simulations of cyclic uniaxial tests in Aluminiun bars and 316L stainless steel bars with experimental results.