| Fatigue damage is the main reason for the degradation of structures (components) in the fields of aerospace, ship engineering, offshore platform, bridge and petrochemical pipeline. A suitable nondestructive technique with capability to provide reliable and reproducible quantitative information for the accumulation of fatigue damage, can significantly improve the safety and reliability of the structures, reduce the risk of disasters, and avoid heavy casualties and property losses.Electro-mechanical impedance (EMI) based structural health monitoring technology has many obvious advantages, such as high sensitivity to minor damages, wide range of adaptability (metals, composites, concrete and other structures), anti-interference ability, convenient signal acquisition and processing. This technique has good prospects in the monitoring of fatigue damages. At present, the application of EMI technique in the monitoring of fatigue damages is still in the infancy. The problems mainly focus on the following aspects:(1) The development of theoretical models are not perfect, the existed piezoelectric impedance models are only applicable for the structures without damage;(2) The driving voltage of currently available device is quite low, the vibration energy is not sufficient for monitored structures, and the improvement of sensitivity to fatigue damages is obviously limited;(3) It is difficult to extract damage location information from the measured electrical impedance signals, and the location of fatigue damage can’t be measured precisely;(4) The piezoelectric sensors used in most of the EMI studies is brittle, proper packaging and protective teclinology should be developed before the application of this method to a real structure;(5) In the multi-dimensional monitoring of fatigue damages, many researches should be performed on the key technology such as the establishment of sensor network, the transmission and processing of distributed signals, from the development of theoretical models and monitoring systems.Aluminum alloy has been widely applied in the structures of aerospace, ship engineering, high-speed trains, due to the low density, high specific strength, good corrosion resistance, excellent weldability and molding manufacturability. The airframe structure material is one of the most important applications of aluminum alloy. During the flight process, the stress state of the body changes alternatively with the outside air pressure. The fatigue failure of the aluminum alloy body will lead to disastrous air disintegration accident. The microscopic observation of fatigue fracture process, the fatigue damage state detection, and the remaining life prediction of aluminum alloy have always been paid attention in the engineering and material science field. On the other hand, aluminum alloy has many advantages such as the convenient material selection and forming, the small fatigue limit, and the easily controlled fatigue process. This material has been widely used in the monitoring of fatigue damage with the EMI technique. As a basic exploration of fatigue damage monitoring principle, the aluminum alloy was also selected in this study to enhance the comparability with other researchers.In this paper, EMI technique was used for online monitoring of the entire process of fatigue failure in aluminum alloy specimens. With the manners of theoretical model, numerical simulation, and experimental testing, systematical studies were performed from three aspects, which were quantitative monitoring of the initiation and propagation process of fatigue crack, the establishment of high excitation voltage electrical impedance measurement system (HEVEIMS), and the accurate location method of fatigue crack. The major conclusions are as follows:(1) The bonding condition of piezoelectric ceramic (PZT) sensor is an important factor which affects the electrical impedance signal obviously. Theoretical simulation and experimental investigation results show that, with decrease in bonding quality, the resonant impedance peaks shifted to higher and lower frequency, respectively, in the low and high frequency ranges. There exists a critical frequency, and the shifted direction of impedance peaks changes at this point.(2) At the fatigue crack initiation stage, the trends of damage identification indicators Δ f and root mean square deviation (RMSD) increased monotonically with fatigue cycles. Before the formation of macroscopic submillimeter fatigue crack, the changing values of Δf and RMSD were0.6kHz and11.68%. The initial pre-crack damages can be real-time monitored with these two indicators.(3) At the fatigue crack propagation stage, the crack length increased gradually, and the trends of Δf and RMSD increased monotonically with fatigue cycles. The increment trend of Δf was evidently similar to that of crack length, and the RMSD values were more sensitive to the initial expansion of crack length in the first5,000cycles. When the crack length expanded to2.90mm, the changing values of Δf and RMSD were1.1kHz and7.61%, respectively. The process of crack propagation can be real-time monitored with these two indicators.(4) The selection of baseline, environmental noise, the state of structural stress, and the testing frequency band can affect the fatigue monitoring results of EMI technique obviously. The measured electrical impedance signals are seriously influenced when the frequency of environmental noise is in the selected monitoring band. The resonant peaks in the electrical impedance signals shift to lower frequency and the values of RMSD increase monotonically, when the tensile stress in the monitored specimens increases gradually. In the kHz frequency range, the sensitivity first increases and then decreases with the increase of testing frequency band, and the frequency band of130-210kHz has the highest testing sensitivity.(5) A HEVEIMS was established with ARB-1410wave generation card, impedance measurement circuit and DPO-4032digital oscilloscope. The excitation voltage of PZT sensor was extended from2V to35V, and the measurement accuracy of electrical impedance modulus, real and imaginary parts is quite high in the frequency band of10-300kHz.(6) The excitation voltage of PZT sensor is an important factor of EMI sensitivity. The driving energy of PZT sensor increases with excitation voltage. Compared with1V excitation voltage, the sensitivity to submillimeter small fatigue cracks increased145%, when the excitation voltage increased to20and25V.(7) The location of fatigue damage in aluminum alloy was measured with active Lamb wave technique, which was excited by PZT sensor bonded on the surface of the specimen. The time delay between the excitation and reflection signals was calculated according to the extracted maximum values of wavelet coefficients, and then the location of fatigue crack was determined. For the2.44mm and1.54mm fatigue cracks generated at25mm and60mm from the PZT sensor, the relative error between the measured and the real values were3.2%and-1%, respectively, and the location accuracy satisfies the engineering requirement. |
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