This paper deals with the harmonic oscillations of periodically excite
d nonlinear systems where hysteresis is simulated via fractional opera
tor representations. Employing a diophantine version of the fractional
operational powers, the energy constrained Lindstedt-Poincare perturb
ation procedure is utilized to establish the harmonic solution. The co
nstrained perturbation procedure was employed since it allows for the
handling of strong damping and exciting forces over the full span of t
he driving frequency range. Based on the approach taken, the long time
behavior of the fractionally damped Duffing's equation is studied in
detail. Of special interest is the determination of the influence of f
ractional order on the frequency amplitude response behavior.