Selective modes of guided Lamb waves are generated in a laminated aluminum plate for damage detection using a broadband piezoelectric transducer structured with a rigid electrode. Appropriate excitation frequencies and modes for inspection are selected from theoretical and experimental dispersion curves. Dispersion curves are obtained experimentally by short time Fourier transform of the transient signals. Sensitivity of antisymmetric and symmetric modes for delamination detection are investigated. The antisymmetric mode is found to be more reliable for delamination detection. Unlike other studies, in which the attenuation of the propagating waves is related to the extent of the internal damage, in this investigation, the changes in the time-of-flight (TOF) of guided Lamb waves are related to the damage progression. The mode conversion phenomenon of Lamb waves during progressive delamination is investigated. Close matching between the theoretical and experimentally derived dispersion curves and TOF assures the reliability of the results presented here.
"And then, Lamb wave monitoring signals can be acquired on-line when the structure is in service. The state of the structure can be determined by using some SHM algorithms       to measure the difference between the Lamb wave baseline signal and the Lamb wave monitoring signals. Such a process can be implemented well in laboratory for the steady structural boundary condition. "
[Show abstract][Hide abstract] ABSTRACT: Damage monitoring under time-varying structural boundary condition is one of the most difficult tasks in piezoelectric transducers (PZTs) and Lamb wave based SHM methods for engineering applications. Because the structural boundary changes such as variations in the tightness of bolts between structures can lead to false monitoring result even the structure is in health state. This paper proposes a Lamb wave based on-line continuous updating Gaussian Mixture Model (GMM) to study the problem. Based on the baseline GMM constructed by features of Lamb wave signals in structural health state, an on-line continuous updating GMM is studied to learn the dynamic changes of Lamb wave monitoring signals without any prior knowledge of damage patterns. The Kullback–Leibler (KL) divergence is used as a degradation index to estimate the structural damage by measuring the difference between the baseline GMM and the on-line GMM. The proposed method is validated on an aircraft steel beam. The validation results show that the method is effective for bolt hole crack growth monitoring under the time-varying changes in the tightness degree of the bolts.
EWSHM-7th European Workshop on Structural Health Monitoring; 07/2014
"In many cases the investigation by the means of the guided waves are performed using only fundamental A 0 and S 0 wave modes. In recent years some works have been done in the investigation of these waves broadband excitation using piezoelectric wafer active sensors (PWAS) –, evaluating such factors as the size of the element, selective excitation and mode control schemes which are basically used in SHM applications. The objective of this research was to investigate the use of piezoceramic ultrasonic transducers for the broadband generation of Lamb waves in different thickness isotropic plates and to determine how efficiency of the generated guided waves asymmetric and symmetric modes changes due the change of the object thickness. "
[Show abstract][Hide abstract] ABSTRACT: In the case of non-destructive testing or structural health monitoring of plate-like structures measurements are often performed using broadband ultrasonic transducers for generation and reception of guided waves. In some cases, the measurements could be performed having access just from one side of the object and it affects the excitation efficiency of generated asymmetric and symmetric guided waves modes. The experimental investigation was performed in order to evaluate how the change of the thickness of the object under investigation will influence the efficiency of generated guided waves asymmetric and symmetric modes.
"Essentially, such technique consists in transmitting long linear or non-linear frequency modulated signals, so that the pulse energy is stretched in time, but the resolution is not compromised thanks to the broad frequency spectrum of the signal itself. Broadband chirped techniques have been previously applied to ultrasonic non-destructive testing  , but are unusual in GWs-based applications because of their dispersive detrimental effect (see for instance ). "
[Show abstract][Hide abstract] ABSTRACT: A non-linear Lamb wave signal processing strategy aimed at extending the
capability of active-passive networks of PZT transducers for defect
detection is proposed. In particular, the proposed signal processing
allows to use chirp shaped pulses in actuation, instead of classically
applied spiky pulses, requiring thus lower input voltages. To such aim,
the acquired Lamb waves are processed by means of a two-step procedure:
a warped frequency transform (WFT) to compensate for the dispersion due
to the traveled distance, followed by a compression procedure to remove
from the signals the induced chirp frequency modulation. Next, the
resulting signals are exploited to feed an imaging algorithm aimed at
providing the position of the defect on the plate. The potential of the
procedure is demonstrated and validated by analyzing experimental Lamb
waves propagating in an aluminum plate where defects were emulated by
posing an added mass on the plate. The proposed automatic procedure is
suitable to locate defect-induced reflections and can be easily
implemented in real applications for structural health monitoring.
Mechanical Systems and Signal Processing 08/2013; 39(1-2):20-31. DOI:10.1016/j.ymssp.2012.10.018 · 2.26 Impact Factor
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