This paper presents a methodology for structural health monitoring of fatigue cracks under stationary random loads. The methodology is composed of two parts: an empirical correlation to monitor the evolution of damage indices and a numerical scheme for fatigue life estimation. The damage indices are based on the variation of the vibrational response of the structure due to crack propagation. Two damage metrics are studied, and the methodology is verified for a cantilever beam subjected to random base excitation. The case study validates the effectiveness of the methodology and shows that both metrics have the potential for damage detection. For the numerical estimation of the fatigue life, the proposed framework uses the probability density function of the stress, which is obtained from frequency-domain methods, and an equivalent stress approach based on the Walker's equation for fatigue crack growth. Excellent agreement is found between the predicted fatigue life and the experimental values.
This work aims to contribute to the development of SHM systems based on vibration methods to be applied on sandwich structures. The main objective is focused on experimental damage identification via changes in the Frequency Response Function FRF with the usage of damage metrics. Specimens of sandwich structures made from skins of epoxy resin reinforced by glass fiber and a core of PVC foam are manufactured. First, preliminary nondamped Finite Element FE models are performed, and results obtained are used to define the frequency range of interest for the experimental procedure. After that, vibration experimental analyses are carried out on undamaged specimens. The natural frequencies are compared to the preliminary FE results. Second, experimental analyses are performed on damaged specimens with and without piezoelectric sensors. Then, damage metric values are calculated based on FRFs for damaged and undamaged structures, which were obtained from experimental and FE analyses with damping effects. In addition, a new procedure is proposed to improve the quality of results provided by the damage metric. It is shown that the new procedure is very effective to identify the damage using both amplitude and phase from FRFs. Lastly, it is discussed the potential and limitations of the FE model to predict damage metric values, comparing to experimental data.
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