The analysis of viscoelastic homogeneous bars subjected to harmonic distributed loadings is presented. The material behavior is given in terms of the Boltzmann superposition principle. The equation of motion derived for the elastica, and by including changes in the bar's length, is an integro-differential variation of the Duffing equation. The classical tools of nonlinear dynamics, such as the phase plane portrait, the Poincare map, the Fourier spectrum and the Lyapunov exponents analysis, are applied in order to investigate the different kinds of behaviors observed