A mechanical system's modal parameters change when fatigue loading is applied to the system. In order to perform an accelerated vibration-based fatigue test these changes must be taken into account in order to maintain constant-stress loading. This paper presents an improved accelerated fatigue-testing methodology based on the dynamic response of the test specimen to the harmonic excitation in the near-resonant area with simultaneous monitoring of the modal parameters. The measurements of the phase angle and the stress amplitude in the fatigue zone are used for the real-time adjustment of the excitation signal according to the changes in the specimen's modal parameters. The presented methodology ensures a constant load level throughout the fatigue process until the final failure occurs. With the proposed testing methodology it is possible to obtain a S-N point of the Woehler curve relatively quickly and to simultaneously monitor the changes of the specimen's natural frequency and damping loss factor. The presented methodology with real-time control is verified on an aluminium Y-shaped specimen (1.000.000 load cycles are achieved in 21 minutes) and is applicable to a specimen with arbitrary geometry. Besides the faster completion of the fatigue test the methodology can be adopted for the validation of the vibrational fatigue analysis.
See also related paper:
[Frequency-Domain Methods for a Vibration-Fatigue-Life Estimation - Application to Real Data],