Due to crack-induced nonlinearity, ultrasonic wave can distort, create accompanying harmonics, multiply waves of different frequencies, and, under resonance conditions, change resonance frequencies as a function of driving amplitude. All these nonlinear ultrasonic features have been widely studied and proved capable of detecting fatigue crack at its very early stage. However, in noisy environment, the nonlinear features might be drown in the noise, therefore it is difficult to extract those features using a conventional spectral density function. In this study, nonlinear spectral correlation is defined as a new nonlinear feature, which considers not only nonlinear modulations in ultrasonic waves but also spectral correlation between the nonlinear modulations. The proposed nonlinear feature is associated with the following two advantages: (1) stationary noise in the ultrasonic waves has little effect on nonlinear spectral correlation; and (2) the contrast of nonlinear spectral correlation between damage and intact conditions can be enhanced simply by using a wideband input. To validate the proposed nonlinear feature, micro fatigue cracks are introduced to aluminum plates by repeated tensile loading, and the experiment is conducted using surface-mounted piezoelectric transducers for ultrasonic wave generation and measurement. The experimental results confirm that the nonlinear spectral correlation can successfully detect fatigue crack with a higher sensitivity than the classical nonlinear coefficient.
This paper presents a damage visualization technique using a fully noncontact laser ultrasonic measurement system and a synchronized scanning strategy. The noncontact laser ultrasonic measurement system is composed of a Q-switched Nd:YAG laser for ultrasonic wave generation and a laser Doppler vibrometer (LDV) for ultrasonic wave detection. The laser beams for ultrasonic wave generation and detection are shot on the target structure with a constant and tiny distance, and these two laser beams are synchronously moved over the scanning area. Compared with conventional laser scanning strategies, the ultrasonic responses detected through the synchronized scanning strategy owns a much higher and more stable signal to noise ratio and the scanning time can be significantly reduced with less time averaging. By spatial comparison in the scanning area, damage can be detected and visualized without relying on baseline data obtained from the pristine condition of the target structure. In this paper, the developed technique is validated by visualization hidden corrosion in a steel straight pipe and a steel elbow pipe.
Fatigue crack and its precursor often serves as a nonlinear source, and the nonlinear ultrasonic features created by a fatigue crack have a much higher sensitivity compared with linear features. This paper presents a fatigue crack visualization technique based on noncontact laser ultrasonics and state space techniques. Under a broadband laser pulse excitation, defect nonlinearity exhibits modulation at multiple frequency peaks in a spectral plot due to interactions among various input frequency components of the broadband input. These modulations are weak and hardly discernable in both the frequency and time domains. In order to detect the nonlinear changes caused by fatigue cracks in the time domain, a state space attractor is reconstructed using a single laser pulse response and its geometrical deviations from the baseline data obtained from the pristine condition of a target structure are computed. Through scanning tests using a Q-switched Nd:YAG laser and laser Doppler vibrometer (LDV), the proposed method can be used for visualizing fatigue cracks in metallic plates.
Fatigue crack is one of the main culprits for the failure of metallic structures. Recently, it has been shown that nonlinear
wave modulation spectroscopy (NWMS) is effective in detecting nonlinear mechanisms produced by fatigue crack. In
this study, an active wireless sensor node for fatigue crack detection is developed based on NWMS. Using PZT
transducers attached to a target structure, ultrasonic waves at two distinctive frequencies are generated, and their
modulation due to fatigue crack formation is detected using another PZT transducer. Furthermore, a reference-free
NWMS algorithm is developed so that fatigue crack can be detected without relying on history data of the structure with
minimal parameter adjustment by the end users. The algorithm is embedded into FPGA, and the diagnosis is transmitted
to a base station using a commercial wireless communication system. The whole design of the sensor node is fulfilled in
a low power working strategy. Finally, an experimental verification has been performed using aluminum plate specimens
to show the feasibility of the developed active wireless NWMS sensor node.
This paper presents a fatigue crack detection technique based on visualization of nonlinear ultrasonic wave modulation produced by a fatigue crack. When distinctive low frequency (LF) and high frequency (HF) inputs are generated and applied to a structure, the presence of a fatigue crack can provide a mechanism for nonlinear ultrasonic modulation and create spectral sidebands around the frequency of the HF signal. In this study, the two input signals are created by two air-coupled transducers (ACT), and the corresponding ultrasonic responses are scanned over a target specimen using a 3D laser Doppler vibrometer (LDV). The crack-induced spectral sidebands are isolated using a combination of linear response subtraction (LRS), and continuous wavelet transform (CWT) filtering. Then, the extracted spectral sideband components are visualized near the fatigue crack. The effectiveness of the proposed non-contact scanning technique is tested using an aluminum plate with a real fatigue crack.