Damage Detection of Submerged Structures Using Linear and Nonlinear Guided Waves

Metallic plates are one of the major components of liquid containment structures and are widely used in petrochemical and civil engineering. In many cases, the metallic plates have one side exposed to liquid and are subjected to different types of loads with varying amplitudes. Corrosion damage and material degradations are the two major concerns. Damage detection of the submerged plate structures plays an important role in maintaining the structural integrity and safety of high-valued infrastructures (e.g. liquid storage tanks and pipes). Guided wave testing is one of the most promising damage detection approaches. Although guided wave based techniques have been extensively studied on different structures in gaseous environments, the design and implementation for the structures immersed in liquid have not been well investigated. This research aims at enhancing the understanding of guided wave propagation and interaction with damage in submerged structures. The focus of this research is on metallic plates that have one side in contact with liquid and the other side exposed to air. The specific objectives of this thesis include the investigation on the propagation characteristics of guided waves in metallic plates with one side exposed to liquid, the development of numerical models to investigate the scattering characteristics of guided waves at corrosion pit damage, the analyses of the influence of the surrounding liquid medium on the linear and nonlinear guided waves features, and the evaluation of the sensitivity of linear and nonlinear guided waves features to different types of damage in the one-side immersed metallic plate. The main body of the thesis consists of four journal articles (Chapters 2-5). Chapter 2 discusses the propagation characteristics and sensitivity to damage of linear guided waves in a metallic plate loaded with water on one side. The targeted damage is local thickness thinning (e.g. corrosion pits) with a size of around a few millimeters. Chapter 3 further investigates and compares the guided wavefields between a plate surrounded by air and the same plate with one side partly exposed to water. The influence of the surrounding liquid medium on the guided wave propagation is demonstrated experimentally and numerically. Chapters 4 and 5 study two different nonlinear guided wave features, which are second harmonic generation and combination harmonic generation, respectively. The nonlinear guided wave features have better sensitivity to microstructural defects that precede the damage in the macroscale. The targeted damage in Chapters 4 and 5 is fatigue degradation in the early stage, where fatigue appears as multiple micro cracks and is distributed in the structural materials. The microstructural defects are too small to be detected by the linear guided wave feature. However, these small defects can distort the guided waves passing through the material, producing new wave components at frequencies other than the excitation frequency of the incident waves. This provides a way for the nonlinear guided wave technique to evaluate the earlystage damage in submerged structures.Thesis (Ph.D.) -- University of Adelaide, School of Civil, Environmental & Mining Engineering, 202

Guided Wave Based Integrated Structural Health Monitoring and Nondestructive Evaluation

Damage detection and health monitoring are critical for ensuring the structural safety in various fields, such as aerospace, civil and nuclear engineering. Structural health monitoring (SHM) performs online nondestructive evaluation (NDE) and can predict the structural remaining life through appropriate diagnosis and prognosis technologies. Among various SHM/NDE technologies, guided ultrasonic waves have shown great potential for fast and large area SHM/NDE, due to their sensitivity to small defects and capability to propagate long distances. Recent advances in guided wave based SHM/NDE technologies have demonstrated the feasibility of detecting damage in simple structures such as metallic plates and pipes. However, there remain many challenging tasks for quantifying damage, especially for damage quantification in complex structures such as laminated composites and honeycomb sandwich structures. Moreover, guided wave propagations in complex structures, and wave interactions with various types of defects such as crack, delamination and debonding damage, need to be investigated. The objective of this dissertation research is to develop guided wave based integrated SHM and NDE methodologies for damage detection and quantification in complex structures. This objective is achieved through guided wave modeling, optimized sensor and sensing system development, and quantitative and visualized damage diagnoses. Moreover, the developed SHM/NDE methodologies are used for various damage detection and health monitoring applications. This dissertation is organized in two major parts. Part I focuses on the development of integrated SHM/NDE damage diagnosis methodologies. A non-contact laser vibrometry sensing system is optimized to acquire high spatial resolution wavefields of guided waves. The guided wavefields in terms of time and space dimensions contain a wealth of information regarding guided wave propagations in structures and wave interactions with structural discontinuities. To extract informative wave signatures from the time-space wavefields and characterize the complex wave propagation and interaction phenomenon, guided wavefield analysis methods, including frequency-wavenumber analysis, wavefield decomposition and space-frequency-wavenumber analysis, are investigated. Using these analysis methods, the multi-modal and dispersive guided waves can be resolved, and the complex wave propagation and interaction can be interpreted and analyzed in time, space, frequency, and wavenumber multi-domains. In Part I, a hierarchical damage diagnosis methodology is also developed for quantitative and visualized damage detection. The hierarchical methodology systematically combines phased array imaging and wavefield based imaging to achieve efficient and precise damage detection and quantification. The generic phased array imaging is developed based on classic delay-and-sum principle and works for both isotropic and anisotropic materials. Using the phased array imaging, an intensity scanning image of the structure is generated to efficiently visualize and locate the damage zone. Then the wavefield based imaging methods such as filter reconstruction imaging and spatial wavenumber imaging are performed to precisely quantify the damage size, shape and depth. In Part II, the developed methodologies are applied to five different SHM/NDE applications: (1) gas accumulation detection and quantification in water loaded structures, (2) crack damage detection and quantification in isotropic plates, (3) thickness loss evaluation in isotropic plates, (4) delamination damage detection and quantification in composite laminates, (5) debonding detection and quantification in honeycomb sandwich structures. This dissertation research will initiate sensing and diagnosis methodologies that provide rapid noncontact inspection of damage and diagnosis of structural health. In the long run, it contributes to the development of advanced sensor and sensing technologies based on guided waves, and to providing on-demand health information at component or subsystem level for the safety and reliability of the structure

Scattering characteristics of quasi-Scholte waves at blind holes in metallic plates with one side exposed to water

Corrosion is one of the major issues in metallic structures, especially those operating in humid environments and submerged in water. It is important to detect corrosion at its early stage to prevent further deterioration and catastrophic failures of the structures. Guided wave-based damage detection technique is one of the promising techniques for detecting and characterizing damage in structures. In water-immersed plate structures, most of the guided wave modes have strong attenuation due to energy leakage into the surrounding liquid. However, there is an interface wave mode known as quasi-Scholte waves, which can propagate with low attenuation. Therefore, this mode is promising for structural health monitoring (SHM) applications. This paper presents an analysis of the capability of quasi-Scholte waves in detecting internal corrosion-like defects in water-immersed structures. A three-dimensional (3D) finite element (FE) model is developed to simulate quasi-Scholte wave propagation and wave scattering phenomena on a steel plate with one side exposed to water. The accuracy of the model is validated through experimental measurements. There is good agreement between the FE simulations and experimental measurements. The experimentally verified 3D FE model is then employed in a series of parametric studies to analyze the scattering characteristics of quasi-Scholte waves at circular blind holes with different diameters and depths, which are the simplest representation of progressive corrosion. The findings of this study can enhance the understanding of quasi-Scholte waves scattering at corrosion damage of structures submerged in water and help improve the performance of in-situ damage detection techniques.Xianwen Hu, Ching Tai Ng, Andrei Kotouso

Nondestructive Evaluation/Structural Health Monitoring of Immersed Plates by Means of Guided Ultrasonic Waves

Studies conducted in the last two decades have demonstrated the effectiveness of guided ultrasonic waves (GUWs) for the nondestructive evaluation (NDE), as well as for structural health monitoring (SHM) of waveguides, such as pipes, plates, and rails. Owing to the ability of travelling relatively large distances in dry structures with little attenuation, GUWs allows for the inspection of long waveguides, locating cracks and notches from few monitoring points, while providing full coverage of the cross section. Laser pulses are one of the most effective methods to generate ultrasonic bulk and guided waves in dry structures. In this dissertation we propose a non-contact NDE method based on the generation of broadband ultrasonic signals by means of laser pulses to inspect underwater structures. The waves are then detected by means of an array of immersion transducers and analyzed by means of statistical analysis to search for damages on the wet structure of interest. In this study we first investigated the effect of water’s depth, temperature, and pressure, and the laser energy and wavelength on the amplitude of the laser-induced ultrasonic waves. The results showed that the 0.532 μm wavelength is the most suitable for our applications. A good range of nominal laser energy is comprised between 160 mJ and 190 mJ. Furthermore, the variations of temperature and pressure have minimal effects on the ultrasonic signals. The following phase showed the ability of the technique to detect various types of defects in an immersed plate, which we achieved by building in house A B-scan system, controlled by National Instrument PXI running under LabVIEW. We designed two series of tests in which the number of transducers, their spatial arrangement, as well as the types of features extracted from the time, the frequency and time-frequency domain varied. By developing two unsupervised algorithms based on outlier analysis, we revealed that the method is capable of successfully detecting a crack and a hole-through. Next, the variation of the energy peak in the time-frequency space was shown to decrease with a dependence on the plate thickness. A range of peak energy was experimentally tabulated and the experimental group velocities of the first fundamental symmetric mode were calculated for six plates of different thickness, varying between 1 mm and 10 mm. Finally, the ability of a focused transducer to interrogate the damage state of the original aluminum plate was shown. As predicted, our multivariate algorithm successfully detected all the five defects devised on the plate. This work concluded with a comparison between the two methods. The results showed that both the hybrid laser-immersion transducer technique and the focusing technique can be successfully used for the noncontact monitoring of immersed plates

On the Processing of Highly Nonlinear Solitarywaves and Guided Ultrasonic Waves for Structural Health Monitoring and Nondestructive Evaluation

The in-situ measurement of thermal stress in civil and mechanical structures may prevent structural anomalies such as unexpected buckling. In the first half of the dissertation, we present a study where highly nonlinear solitary waves (HNSWs) were utilized to measure axial stress in slender beams. HNSWs are compact non-dispersive waves that can form and travel in nonlinear systems such as one-dimensional chains of particles. The effect of the axial stress acting in a beam on the propagation of HNSWs was studied. We found that certain features of the solitary waves enable the measurement of the stress. In general, most guided ultrasonic waves (GUWs)-based health monitoring approaches for structural waveguides are based on the comparison of testing data to baseline data. In the second half of the dissertation, we present a study where some baseline-free signal processing algorithms were presented and applied to numerical and experimental data for the structural health monitoring (SHM) of underwater or dry structures. The algorithms are based on one or more of the following: continuous wavelet transform, empirical mode decomposition, Hilbert transform, competitive optimization algorithm, probabilistic methods. Moreover, experimental data were also processed to extract some features from the time, frequency, and joint timefrequency domains. These features were then fed to a supervised learning algorithm based on artificial neural networks to classify the types of defect. The methods were validated using the numerical model of a plate and a pipe, and the experimental study of a plate in water. In experiment, the propagation of ultrasonic waves was induced by means of laser pulses or transducer and detected with an array of immersion transducers. The results demonstrated that the algorithms are effective, robust against noise, and able to localize and classify the damage

Sea Level Fluctuations

This thematic issue presents 11 scientific articles that are extremely useful for understanding the processes and phenomena of the interacting geospheres of the Earth. These processes have an important impact on the biosphere and many human activities. The results of scientific research presented in this book are fully united by the common theme "investigation of the fundamental foundations of the emergence, development, transformation, and interaction of hydroacoustic, hydrophysical and geophysical fields in the World Ocean." The book is recommended to a wide range of readers, as well as to specialists in the field of hydroacoustics, oceanology, and geophysics

Ultrasonic Nondestructive Evaluation Using Non-Contact Air-Coupled Lamb Waves

Ultrasonic Lamb waves have been proved as an effective nondestructive evaluation (NDE) method due to their ability to propagate a long distance with less energy loss as well as their sensitivity to various defects on the surface or inside the structure. However, there are still challenges towards using them as a rapid inspection method for complex structural geometries, various damage types, and harsh environments, such as efficiency in Lamb wave actuation and sensing, understanding of the complicated multi-modal Lamb wave propagation, and robust detection algorithms for damage quantification and evaluation. To address these challenges, this dissertation focuses on developing a fully non-contact Lamb wave inspection system and appropriate damage detection algorithms and their applications to various structural components. Toward it, fundamental studies of the Lamb wave propagation are first conducted to understand the complicated Lamb waves (part I). Next, the single-mode Lamb wave inspection system with non-contact ACT actuation and SLDV sensing is configurated and damage detection algorithms are developed for quantitative damage detection (part Ⅱ). Finally, the applications for damage detection are applied to both isotropic metallic structures and anisotropic composite structures (part Ⅲ). In Part I, the fundamental Lamb wave dispersion curves are calculated by solving the Rayleigh-Lamb wave equations. Besides, the plate structure and material influence on the dispersion curves are studied and the Lamb wave modeshapes are also theoretically calculated and studied. Next, to understand the Lamb wave propagation, the 1D and 2D analytically modeling of Lamb waves with both in-plane actuation and out-of-plane actuation are performed. To validate our analytical modeling with in-plane actuation, the Lamb wave modeling in the case of PWAS excitation is conducted. Then experimental work using the PWAS for in-plane actuation and SLDV for out-of-plane velocity is carried out for analytical modeling validation. Similarly, to validate our analytical modeling with out-of-plane actuation, the Lamb wave modeling in the case of pulsed laser excitation is performed, then experiments are conducted using the pulsed laser for out-of-plane excitation and SLDV for out-of-plane velocity measurement for the analytical modeling validation. After that, the Lamb wave interaction with a discontinuity is then conducted using the finite element method to understand the wave behavior when interacting with discontinuity. In Part Ⅱ, the fully non-contact system is constructed by using a non-contact air-coupled piezoelectric transducer (ACT) for actuation and remote scanning laser Doppler vibrometer (SLDV) for sensing. Extensive studies on ACT configuration, actuation, and calibration are conducted to operate ACT actuation with optimal parameters. Based on Snell’s law, single-mode ACT Lamb wave actuation is conducted with incident angle θ. The result shows that a single fundamental antisymmetric Lamb wave mode (A0) can be obtained. To obtain the optimal Lamb wave signal, the ACT actuation setup is optimized by incident angle tuning. Various sensing schemes, line scan, or area scan, are investigated to obtain adequate information regarding wave propagation. The multi-dimensional Fourier transform method is adopted to analyze the multi-dimensional Lamb wave data giving the wave information in either time, space, frequency, or wavenumber domain for characterization analysis. The characterization result verifies the single A0 mode and shows the ACT actuated Lamb wave propagation is highly directional with its strongest wave intensity along the ACT axis direction. Other than that, to quantitatively evaluate the damage, and an improved cross-correlation principle-based imaging method using the scattered waves of all directions is proposed for damage imaging inspection. In Part Ⅲ, applications of the ACT-SLDV system and single A0 mode method are explored for both nuclear-spent fuel dry cask structures and composite structures. To address the complex multilayered structures in spent fuel casks, systematic studies of detections of machined crack, simulated damage growth monitoring are preliminary conducted. Then the detections of fatigue-induced crack inspection as well as crack growth monitoring are implemented. Towards the complicated multi-layered dry cask structure application, crack inspection in multilayered plate structures is conducted. Other than that, the noncontact acoustic emission testing by the ACT is conducted on various metallic structures as well as composite structures. In composite structures, the most typical delamination and impact damage are inspected using the ACT-SLDV Lamb wave method. Other than that, composite manufacturing defects, such as weak-bond quality and composite wrinkle defects are inspected