SUMMARY Variations in parameters representing natural frequency, damping and effective nonlinearities with fatigue damage are assessed. The experimental system under consideration consists of a steel beam carrying a lumped mass. The identification of the parameters is performed by exploiting and modeling nonlinear behavior of the beam-mass system and matching an approximate solution of the representative model with quantities obtained from spectral analysis of measured vibrations. The representative model and identified coefficients are validated through comparison of measured and predicted responses. Percentage variations of the identified parameters with damage progression up to the point where cracks were observed are determined. Their sensitivities to the state of damage of the beam-mass system are also quantified. The results show that damping and effective nonlinearity parameters are more sensitive to damage progression than the natural frequency. Moreover, the sensitivity of nonlinear parameters to damage is better established using a physically derived parameter rather than spectral parameters such as amplitudes, bispectra or bicoherence levels and/or amplitude ratios of harmonic components. Copyright r 2010 John Wiley & Sons, Ltd. Received 16 September 2009; Revised 25 November 2009; Accepted 24 December 2009