A method and an experimental setup to study the nonlinear behavior of fatigued and intact metal samples under high-frequency stresses has been developed and tested. The method is based on the measurement of the vibration velocity of stepped bar samples flexurally and/or extensionally vibrating at resonance. The vibration velocity signal, picked up by a laser vibrometer, is automatically acquired and analyzed by classical fast Fourier transform (FFT) methods for different excitation levels. The experimental setup was designed in such a way that it can be used for fatiguing and for testing the material samples. The tests have been done with samples of titanium and aluminium alloys. Results before and after fatigue crack nucleation are compared. A strongly nonlinear, nonclassical behavior is observed for fatigued samples. Theoretical interpretations of the different nonlinear behavior of standing waves in fatigued and intact samples are explored, and the results of the models are qualitatively compared with the experimental data. The models' approach provides an interpretation about the origin of nonlinearity in fatigued samples.