Car components are exposed to the random/harmonic/impact excitation which can result in component failure due to vibration fatigue. The stress and strain loads do depend on local stress concentration effects and also on the global structural dynamics properties. Standardized fatigue testing is long-lasting, while the dynamic fatigue testing can be much faster; however, the dynamical changes due to fatigue are usually not taken into account and therefore the identified fatigue and structural parameters can be biased. In detail: damage accumulation results in structural changes (stiffness, damping) which are hard to measure in real time; further, structural changes change the dynamics of the loaded system and without taking this changes into account the fatigue load in the stress concentration zone can change significantly (even if the excitation remains the same).
This research presents a new approach for accelerated vibration testing of real structures. The new approach bases on phase locked harmonic excitation and can be used for identification of natural frequencies and damping while the damage due to vibration is being accumulated. At the same time a real-time control loop takes into account the structural changes and adequately changes the excitation to preserve the constant amplitude of the stress at the fatigue zone of the tested structure.
An example of a real aluminum (A-S8U3) part excited close to the 4th natural frequency is presented. Due to high testing frequency the 1×10^6 cycles are reached in approx. 20 minutes. The presented measurements show the change of natural frequency and damping during the test, while the stress amplitude at the fatigue zone is kept constant.
Ladisk, Faculty of Mechanical Engineering, University of Ljubljana
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