Local Interaction Simulation Approach Research Articles

The Luxembourg–Gorky effect for elastic shear horizontal guided waves — Analytical and numerical modelling

The Luxembourg–Gorky (L–G) effect for elastic shear horizontal guided waves is investigated. A new non-classical theoretical model of wave dissipation is proposed to explain the nonlinear modulation transfer. The nonlinear form of the Kelvin–Voigt model is used in these investigations. The analytical work is validated using numerical simulations based on the Local Interaction Simulation Approach (LISA), implemented for the nonlinear wavefield. The analytical and simulated results – that are in good agreement – not only explain the acoustic equivalent of the L–G effect but also demonstrate how the severity of damage and carrier frequencies of the excited waves influence the nonlinear modulation transfer behind this effect. The work presented is in good agreement with the analytical models that explain the original L–G effect for electromagnetic waves.

Fatigue-Crack Detection in a Multi-Riveted Strap-Joint Aluminium Aircraft Panel Using Amplitude Characteristics of Diffuse Lamb Wave Field

Structural health monitoring of riveted aircraft panels is a real challenge for maintenance engineers. Here, a diffused Lamb wave field is used for fatigue-crack detection in a multi-riveted strap-joint aircraft panel. The panel is instrumented with a network of low-profile surface-bonded piezoceramic transducers. Various amplitude characteristics of Lamb waves are used to extract information on fatigue damage. A statistical outlier analysis based on these characteristics is also performed to detect damage. The experimental work is supported by simplified modelling of wave scattering from crack tips to explain complex response features. The Local Interaction Simulation Approach (LISA) is used for this modelling task. The results demonstrate the potential and limitations of the method for reliable fatigue-crack detection in complex aircraft components.

Nonlinear Modes in Shear Horizontal Wave Propagation–Analytical and Numerical Analysis

Nonlinear shear horizontal guided wavefield is investigated analytically in plates. The major focus is on the effect of nonlinear material parameters, excitation frequency, wave number and wave amplitude on nonlinear mode generation. The analytical solution is based on the multiple-scale perturbation method and modal decomposition approach. The approximate analytical solution is validated using the Local Interaction Simulation Approach, implemented for nonlinear shear wave propagation. The results show that the analysed shear horizontal wavefield is distorted by material nonlinearity. The resulting displacement field can be interpreted in a form of nonlinear modes.

Informative Bayesian tools for damage localisation by decomposition of Lamb wave signals

Ultrasonic guided waves offer a convenient and practical approach to structural health monitoring and non-destructive evaluation, thanks to some distinct advantages. Guided waves, in particular Lamb waves, can be used to localise damage by utilising prior knowledge of propagation and reflection characteristics. Typical localisation methods make use of the time of arrival of waves emitted or reflected from the damage, the simplest of which involves triangulation (with a known wave speed). In order to obtain reflection information, it is useful to decompose the measured signal into the expected waves propagating directly from the actuation source in the absence of damage, called a baseline, and for this paper referred to as nominal waves. This decomposition allows for determination of the residual signal which contains only waves from reflection sources such as damage, boundaries or other local inhomogeneities. Previous decomposition methods make use of accurate analytical models, but there is a gap in methods of decomposition for complex materials and structures. A new method is shown here which uses a Bayesian approach to decompose single-source signals, requiring only prior information on surface displacement along the propagation path. This Bayesian decomposition has the advantage of generating a distribution of possible nominal signals and allows for quantification of the uncertainty of the expected signal. Furthermore, the approach produces inherent parametric features which correlate to known physics of guided waves, and likelihood estimates can be used to assess the quality of the decomposition. In this paper, the decomposition method is demonstrated on data from a simulation of guided wave propagation in a small aluminium plate, using the local interaction simulation approach, for a damaged and undamaged case. Analysis of the decomposition method is done in three ways; inspect individual decomposed signals, track the inherently produced parametric features along propagation distance, and use method in a localisation strategy. The localisation method is demonstrated using the decomposed signal at several sensor locations and triangulates for the source of reflected waves from damage. The Bayesian decomposition was found to work well in returning signals containing only reflected waves, as well as obtaining parametric features that can be used to assess damage and confidence in the decomposed wave. The use of these waves in the localisation method returned estimates accurate to within 1 mm in many sensor configurations. Leading on from the work shown here, the paper finishes with future work; the authors intend to extend this method to scenarios where less prior knowledge is available.

Advances in nonlinear ultrasonic detection of microcracks

<p indent="0mm">A large number of key components are widely operated under extreme conditions of high temperature and high pressure in power, nuclear energy, aerospace and other important industrial fields. Microdefects are inevitably generated in these key components during the process of manufacturing. Due to the periodic loading and thermal stress, microcracks are generally initiated from these microdefects at the location with stress concentration. It has been found that the initiation and propagation of fatigue microcracks usually occur prior to 70%–90% of fatigue life. It is difficult to observe the evidence before the formation of macrocracks, which may result in catastrophic accidents. Thus, the risk of sudden failure of key components increases greatly. Therefore, the accurate detection of microcracks is crucial for guaranteeing the safe operation of key components. Based on the diffraction, reflection and transmission, traditional ultrasonic testing method is merely validated to effectively detect macroscopic defects within half a wavelength. The nonlinear ultrasonic testing method is found to be sensitive to microcracks, as high order harmonics, sub-harmonics, DC components and frequency mixing waves can be generated due to the strong nonlinear interaction microcracks and ultrasonic waves. With the outstanding effectiveness to detect microcracks, the nonlinear ultrasonic testing method has attracted much attention in recent years. In this work, the research on detection of microcracks using nonlinear ultrasonic testing method is systematically reviewed in terms of theoretical models, numerical simulations and experimental measurements. Firstly, according to the breathing effect and sliding mechanism of microcrack surfaces, we present several theoretical models of nonlinear interaction between ultrasonic waves and microcracks, namely contact acoustic nonlinearity (CAN), bi-linear stiffness model, nonlinear spring model, Hertz contact theory, and rough contact surface model. The advantages and disadvantages of these theoretical models are discussed comparatively. Two mechanisms are mainly considered for the nonlinear interaction of microcracks and ultrasonic waves, namely the macroscopic opening and closing of crack surfaces and the microscopic contact of rough microcracks surfaces. Secondly, we introduce numerical simulation studies on the nonlinear ultrasonic generation induced by microcracks. The applicability is analyzed for several simulation methods, such as finite element method, spectral finite element method, finite difference method, boundary element method and local interaction simulation approach, and the generation and propagation of nonlinear ultrasonic waves are analyzed as well. The effect of microcrack geometries on the nonlinear ultrasonic waves is then summarized. The microcrack geometries are considered mainly clouding the microcrack length, width, depth, density and direction. Furthermore, we present the experimental measurements on the generation of nonlinear ultrasonic waves induced by microcracks and on the localization and imaging of microcracks based on the nonlinear ultrasonic waves. The generation of high order harmonics, sub-harmonics, DC components and frequency mixing waves was observed in the specimens with microcracks. The nonlinear ultrasonic detection is presented to evaluate fatigue microcracks with various microcrack length, width, depth, density and direction. Based on this phenomenon, nonlinear ultrasonic mixing method, transducer array imaging method and nonlinear ultrasonic phased array method are introduced to localize the microcracks. Finally, further studies on evaluation of microcracks using nonlinear ultrasonic waves are proposed, including the comprehensive physical model for actual microcracks in engineering components, robust detection method, and quantitative evaluation of microcracks and service life.

Formulation and Stability Analysis of a Multi-Scale Modeling Approach for Simulation of Elastic Wave Propagation

In this work, we report on the formulation and detailed stability analysis of a dynamic multi-scale scheme involving two different local computational strategies for modeling of elastic wave propagation. The coupled model involves the Local Interaction Simulation Approach and Cellular Automata for Elastodynamics, however the presented analysis approach is general and applies to other numerical techniques. This scheme is capable of coupling two numerical models with possibly dissimilar spatial discretization lengths and material properties, hence it is appealing for a multi-scale and/or multi-resolution analysis. The method developed in this paper employs an interface force–displacement coupling to yield the multi-scale model equations. It is shown that the governing equations contain a self-coupling term that affects the stability of the system, as it contributes to additional stiffness at the interface. Stability analysis is presented in terms of rotations of two vectors in [Formula: see text] space, where each vector represents individual model’s stability. Three model configurations of practical interest were investigated, analytical formulae derived and used to analyze stability. These analytical formulae were compared against results from numerical simulations and perfect agreement was observed.

Spectral analysis of guided wave propagation in discretized domains under local interactions

The paper presents a spectral analysis of wave propagation in bounded two-dimensional elastic media discretized under assumptions of two selected local computational strategies. The Local Interaction Simulation Approach and Cellular Automata for Elastodynamics are first overviewed with particular emphasis on their fundamental assumptions, treatment of boundary conditions, and resulting formulations. Each method’s numerical dispersion characteristics for bulk waves are then analyzed using a closed set of iteration equations, after which guided Lamb wave dispersion characteristics are analyzed for homogeneous and layered plates. In all cases, comparisons to finite element dispersion characteristics, re-formulated in the form of local iteration equations, provide insight into the relative advantages and disadvantages of the local formulations and reveal non-physical effects induced by numerical discretization. Finally, verification of the analytical dispersion results is presented using transient simulations with accompanying spectral analysis.

Two-Point Frequency Shift Method for Shear Wave Attenuation Measurement.

Ultrasound shear wave elastography (SWE) is an increasingly used noninvasive modality for quantitative evaluation of tissue mechanical properties. SWE typically uses an acoustic radiation force to produce laterally propagating shear waves that are tracked in the spatial and temporal domains, in order to obtain the wave velocity. One of the ways to study the viscoelasticity is through studying the shear wave phase velocity dispersion curves. Shear wave attenuation can be also characterized in viscoelastic tissues with methods that use multiple lateral data samples. In this article, we present an alternative method for measuring the shear wave attenuation without using a rheological model two-point frequency shift (2P-FS). The technique uses information related to the amplitude spectra FS of shear waves measured at only two lateral locations. The theoretical basis for the 2P-FS is derived and validated. We examined how the first signal position and the distance between the two locations affect the shear wave attenuation estimation in the 2P-FS method. We tested this new method on digital phantom data created using the local interaction simulation approach (LISA) in viscoelastic media. Moreover, we tested data acquired from custom-made tissue-mimicking viscoelastic phantom experiments and ex vivo porcine liver measurements. We compared results from the 2P-FS method with the other two techniques used for assessing a shear wave attenuation: the FS-based method and the attenuation-measuring ultrasound shear wave elastography (AMUSE) technique. In addition, we evaluated the 2P-FS algorithm with different levels of added white Gaussian noise to the shear wave particle velocity using numerical phantoms. Tests conducted showed that the 2P-FS method gives robust results based on only two measurements and can be used to measure attenuation of viscoelastic soft tissues.

Shear wave attenuation measurement of tissue-mimicking materials using a two-point frequency shift method

Shear wave elastography (SWE) is a method used in several clinical applications for assessment of soft tissue viscoelastic properties. SWE uses an acoustic radiation force (ARF) to produce laterally propagating shear waves to obtain the wave velocity. Here, we present a two-point shear wave attenuation coefficient measurement method without using a rheological model. The technique uses information related to the magnitude spectrum frequency shift of shear waves measured at only two lateral locations. We examined how the distance from the start of the ARF push and the distance between the two locations affected the attenuation coefficient estimates. We tested this method on digital phantom data created using local interaction simulation approach in viscoelastic media and on data acquired from phantom and ex vivo liver experiments. We compared the results from the two-point method with other two techniques used for assessing shear wave attenuation: the frequency-shift-based (FS) method and attenuation measuring ultrasound shearwave elastography. In comparison with the FS method, our technique does not assume that the shape parameter of the Gamma function is constant over a lateral distance. Tests conducted showed that the proposed method is feasible to provide robust attenuation estimates based on two measurement points in tissue-mimicking materials and ex vivo liver tissue.Shear wave elastography (SWE) is a method used in several clinical applications for assessment of soft tissue viscoelastic properties. SWE uses an acoustic radiation force (ARF) to produce laterally propagating shear waves to obtain the wave velocity. Here, we present a two-point shear wave attenuation coefficient measurement method without using a rheological model. The technique uses information related to the magnitude spectrum frequency shift of shear waves measured at only two lateral locations. We examined how the distance from the start of the ARF push and the distance between the two locations affected the attenuation coefficient estimates. We tested this method on digital phantom data created using local interaction simulation approach in viscoelastic media and on data acquired from phantom and ex vivo liver experiments. We compared the results from the two-point method with other two techniques used for assessing shear wave attenuation: the frequency-shift-based (FS) method and attenuation measu...

Two Point Method For Robust Shear Wave Phase Velocity Dispersion Estimation of Viscoelastic Materials

Ultrasound shear wave elastography (SWE) is an imaging modality used for noninvasive, quantitative evaluation of tissue mechanical properties. SWE uses an acoustic radiation force to produce laterally propagating shear waves that can be tracked in spatial and temporal domains in order to obtain the wave velocity. One of the ways to study the viscoelasticity is through examining the shear wave velocity dispersion curves. In this paper, we present an alternative method to two-dimensional Fourier transform (2D-FT). Our unique approach (2P-CWT) considers shear wave propagation measured in two lateral locations only and uses wavelet transformation analysis. We used the complex Morlet wavelet function as the mother wavelet to filter two shear waves at different locations. We examined how the first signal position and the distance between the two locations affect the shear wave velocity dispersion estimation in 2P-CWT. We tested this new method on a digital phantom data created using the local interaction simulation approach (LISA) in viscoelastic media with and without added white Gaussian noise to the wave motion. Moreover, we tested data acquired from custom made tissue mimicking viscoelastic phantom experiments and ex vivo porcine liver measurements. We compared results from 2P-CWT with the 2D-FT technique. 2P-CWT provided dispersion curves estimation with lower errors over a wider frequency band in comparison to 2D-FT. Tests conducted showed that the two-point technique gives results with better accuracy in simulation results and can be used to measure phase velocity of viscoelastic materials.

Low reflection effect by 3D printed functionally graded acoustic black holes

The acoustic black hole (ABH) effect produces drastic wave compression and energy trapping in thin-walled structures. Recent research has shown that the reflected wave in laboratory ABH structures can be reduced by attaching a thin absorbing layer in specific wave concentration areas. In this paper, a novel design and implementation of one-dimensional (1D) functionally graded acoustic black hole (FG-ABH) is presented, as an alternative to achieve low reflection effects. Two configurations of the FG-ABHs are demonstrated, i.e., an axially graded ABH and a thickness graded ABH. The FG-ABH beams are manufactured by 3D printing technology using an Objet Connex 500 printer. Materials with varying levels of viscoelastic characteristics are produced using two base materials called TangoPlus and VeroWhitePlus. The FG-ABH beams have both diminishing thickness and elastic modulus in the tip of the ABH wedge. Numerical investigation of wave propagation, attenuation and reflection are conducted utilizing a viscoelastic code: University of Michigan’s Local Interaction Simulation Approach (UM/LISA). The damping effect of the materials is included based on Kelvin-Voigt viscoelastic theory. Finally, the process of wave propagation in FG-ABHs, traditional ABH and uniform beam are experimentally investigated using a Scanning Laser Doppler Vibrometer (SLDV) system. The reflection coefficients, i.e. the ratios between the amplitude of reflected wave and incident wave, are computed. Results indicate that the FG-ABHs enhance the ABH effect, and lower reflection is observed when compared to a traditional ABH structure.

Nonlinear scattering and mode conversion of Lamb waves at breathing cracks: An efficient numerical approach

This article presents an efficient numerical approach to the investigation of nonlinear scattering and mode conversion phenomena of Lamb waves as they interact with breathing cracks. A Local Interaction Simulation Approach (LISA) is adopted, which possesses the versatility to capture arbitrary damage profiles. The stick-slip contact dynamics is implemented in the LISA model via the penalty method, which captures the nonlinear interactions between Lamb waves and breathing cracks. The LISA framework achieves remarkable computational efficiency with its parallel implementation using Compute Unified Device Architecture (CUDA) executed on powerful GPUs. A small-size LISA model with absorbing boundaries is tailored for the purpose of extracting the Lamb wave scattering and mode conversion features. Due to the explicit parallel CUDA implementation and the small-size model setup, the computation is highly efficient. Numerical case studies on nonlinear scattering of Lamb waves from breathing cracks are given. Distinctive higher harmonic generation and selective mode conversion phenomena are presented using the complex-valued Wave Damage Interaction Coefficients (WDICs) containing both amplitude and phase information of the scattered wave field. The effect of oblique incident angle on nonlinear scattering phenomenon is investigated. The rough crack surface feature with initial openings and closures is also considered to better approximate fatigue cracks in practical engineering scenarios. In addition, the wave amplitude effect on the nonlinear scattering and mode conversion is studied. This research may provide guidelines for the effective design of sensor arrays utilizing nonlinear Lamb waves for fatigue crack detection.

Nonlinear features of guided wave scattering from rivet hole nucleated fatigue cracks considering the rough contact surface condition

This article presents the investigation of nonlinear scattering features of guided waves from rivet hole nucleated fatigue cracks considering the rough contact surface condition. A small-size numerical model based on the local interaction simulation approach is developed, enabling the efficient analysis of the contact acoustic nonlinearity during the wave crack interactions. The study starts with an idealized breathing crack model possessing smooth, perfectly kissing contact surfaces. Then, the nature of rough crack surfaces is considered with randomly distributed initial openings and closures. Several distinctive aspects of the nonlinear scattering phenomenon are discussed: (1) the amplitude effect, which renders significantly different nonlinear response under various levels of excitation wave amplitudes; (2) the directivity and mode conversion features, which addresses the scattering direction dependence of the fundamental and superharmonic wave mode components; (3) the nonlinear resonance phenomenon, which maximizes the nonlinear response during the wave crack interactions at certain excitation frequency ranges. This study demonstrates that these nonlinear features are substantially influenced by the crack surface condition and differ much between an idealized breathing crack and a rough crack in most practical cases. Fatigue tests on a thin aluminum plate with a rivet hole is conducted to induce cracks in the specimen. An active sensor array surrounding the crack zone is implemented to generate and receive ultrasonic guided waves in various directions. Current work emphasizes on using a highly efficient numerical model to explain the nonlinear features of scattered waves from fatigue cracks considering the rough crack surface condition. These special features may provide insights and guidelines for nonlinear guided wave based nondestructive evaluation and structural health monitoring system design. The paper finishes with discussion, concluding remarks, and suggestions for future work.

Phase velocity dispersion estimation of viscoelastic materials by two-point wavelet-based method

Ultrasound shear wave elastography (SWE) is a promising imaging modality used for noninvasive, quantitative evaluation of tissue mechanical properties. This method uses an acoustic radiation force to produce laterally propagating shear waves that can be tracked to obtain the wave velocity. One of the ways to explore the viscoelasticity is through examining the shear wave velocity dispersion curves. In this paper, we present an alternative method to the classical two-dimensional Fourier transform (2D-FT), a two-point wavelet transformation method. We use the Morlet wavelet function as the mother wavelet to filter two shear waves at different locations. We examined how starting distance from the push and distance between the two locations affected the shear wave velocity dispersion. We tested this method on digital phantom data created using local interaction simulation approach (LISA) in viscoelastic media and on data acquired from phantom experiments. We compared results from the two-point method with the 2D-FT technique. The two-point method provided dispersion curves estimation with lower errors over a wider frequency band. Tests conducted showed that the two-point technique gives results with better accuracy in simulation results and can be used to measure phase velocity of viscoelastic materials.

Generalised semi-analytical method for excitability curves calculation and numerical modal amplitude analysis for Lamb waves

The paper presents an efficient and exact method for the calculation of excitability curves and for the analysis of numerical models for guided waves. The proposed procedure makes use of the wave equation and through-thickness-only discretisation of anisotropic, layered plates to obtain the Lamb wave amplitude characteristics. Thereby, the method can be used for any arbitrarily selected anisotropic (layered structures, such as composites) material in a straightforward manner. A general framework for the proposed analysis is given, along with application examples. For this purpose, orthogonality conditions based on the reciprocity theorem are derived for the numerical model. Although the developed mathematical framework is verified and application examples are given based on the local interaction simulation approach for elastic waves propagation, the proposed methodology can easily be adopted for other numerical methods (eg, finite elements and cellular automata for elastodynamics). The method can also be used to study the influence of discretisation parameters on excitability curves estimates.

Acoustic emission source characterisation using evolutionary optimisation

When a crack initiates and grows in a metal or composite structure, for example, due to high cycle fatigue, the crack propagation gives rise to acoustic emissions (AE)-ultrasonic waves travelling through the structure. Because the presence and rate of growth of any cracks are important pieces of information about the condition or health of the structure, the monitoring of AE activity using sensors mounted on its surface is a potentially useful technique of structural health monitoring. In tests, acoustic emissions are often simulated by breaking a pencil lead against the surface of the structure in a standardised way (a Hsu-Nielsen source), but the forces that this imparts are not well understood at present. The current paper proposes a new evolutionary optimisation-based approach to source characterisation. The principle is to introduce a parametrised representation of a general source and then identify the parameters that allow the source to best match responses measured elsewhere on the structure. The predicted responses are modelled using a local interaction simulation approach (LISA) algorithm to simulate the propagation of the ultrasonic waves. The approach is validated here using experiments on AE propagation in thin plate-like structures, where the ultrasound propagates as Lamb waves. Three separate case studies are proposed here. In the first case, an idealised point source is simulated using laser-generated ultrasound, and the optimisation algorithm uses a two-dimensional LISA model. A differential evolution optimisation scheme is used to find the optimal profile of forcing to match the simulation with experiment. In the second case, the two-dimensional LISA approach is used to characterise the forces associated with standard pencil lead breaks. The final study addresses the full three-dimensional wave propagation. Because of the computational expense of the latter calculation, the LISA algorithm is implemented using a CUDA graphics card computer system.

Modelling nonlinearity of guided ultrasonic waves in fatigued materials using a nonlinear local interaction simulation approach and a spring model

Modelling and numerical simulation – based on the framework of the Local Interaction Simulation Approach – was developed to have more insight into nonlinear attributes of guided ultrasonic waves propagating in fatigued metallic materials. Various sources of nonlinearity were considered in this modelling work, with particular emphases on higher-order harmonic generation and accumulation of nonlinearity along wave propagation. The material hyper-elasticity was considered in the model using an energy density approach based on the Landau–Lifshitz formulation; and the “breathing” motion pattern of a fatigue crack in the material was interrogated using a spring model. Upon the successful validation with the model prepared in the commercial software based on the Finite Element Methods, two scenarios were comparatively investigated, i.e. the lower and higher frequency regime. In the first case propagation of a basic symmetric mode pair was simulated using the model to observe a cumulative characteristic of the second harmonic mode with nonlinear hyper-elastic material definition upon appropriate selection of excitation frequency. In the second case, the higher-order symmetric mode pair was excited according to the “internal resonance” conditions, revealing a strong dependence of manifested nonlinearity on numerical parameters. Moreover, it was shown that with the use of the wave from the low frequency regime it was easier to differentiate later stages of the crack development, being in contrary to waves in the high frequency regime, which allowed to clearly observe early stages of the crack expansion. Such outcome lays the foundation to develop the damage detection and monitoring scheme in the field of Structural Health Monitoring based on utilising the nonlinear features of guided ultrasonic waves.

Optimization of acousto-ultrasonic sensor networks using genetic algorithms based on experimental and numerical data sets

Aircraft structural damage detection is becoming of increased importance. Technologies such as acousto-ultrasonic have been suggested for this application; however, an optimization strategy for sensor network design is required to ensure a high detection probability while minimizing sensor network mass. A methodology for optimizing acousto-ultrasonic transducer placement for adhesive disbond detection on metallic aerospace structures is presented. Experimental data sets were acquired using three-dimensional scanning laser vibrometry enabling in-plane and out-of-plane Lamb wave components to be considered. This approach employs a novel multi-sensor site strategy which is difficult to achieve with physical transducers. Different excitation frequencies and source–damage–sensor paths were considered. A fitness assessment criterion which compared baseline and damaged data sets using cross-correlation coefficients was developed empirically. Efficient sensor network optimization was achieved using a bespoke genetic algorithm for different network sizes with the effectiveness assessed and discussed. A comparable numerical data set was also produced using the local interaction simulation approach and optimized using the same methodology. Comparable results with those of the experimental data set indicated a good agreement. As such, the numerical approach demonstrates that acousto-ultrasonic sensor networks can be optimized using simulation (with some further refinement) during an aircraft design phase, being a useful tool to sensor network designers.

Local Interaction Simulation Approach for Efficient Modeling of Linear and Nonlinear Ultrasonic Guided Wave Active Sensing of Complex Structures

This paper presents the local interaction simulation approach (LISA) for efficient modeling of linear and nonlinear ultrasonic guided wave active sensing of complex structures. Three major modeling challenges are considered: material anisotropy with damping effects, nonlinear interactions between guided waves and structural damage, as well as geometric complexity of waveguides. To demonstrate LISA's prowess in addressing such challenges, carefully designed numerical case studies are presented. First, guided wave propagation and attenuation in carbon fiber composite panels are simulated. The numerical results are compared with experimental measurements obtained from scanning laser Doppler vibrometry (SLDV) to illustrate LISA's capability in modeling damped wave propagation in anisotropic medium. Second, nonlinear interactions between guided waves and structural damage are modeled by integrating contact dynamics into the LISA formulations. Comparison with commercial finite element software reveals that LISA can accurately simulate nonlinear ultrasonics but with much higher efficiency. Finally, guided wave propagation in geometrically complex waveguides is studied. The numerical example of multimodal guided wave propagation in a rail track structure with a fatigue crack is presented, demonstrating LISA's versatility to model complex waveguides and arbitrary damage profiles. This article serves as a comprehensive, systematic showcase of LISA's superb capability for efficient modeling of transient dynamic guided wave phenomena in structural health monitoring (SHM).

Predictions of defect detection performance of air-coupled ultrasonic rail inspection system

A rail inspection system based on ultrasonic guided waves and non-contact (air-coupled) ultrasound transduction is under development at the University of California at San Diego. The system targets defects in the rail head that are major causes of train accidents. Because of the high acoustic impedance mismatch between air and steel, the non-contact system poses severe challenges and questions on the defect detection performance. This article presents an extensive numerical study, conducted with a local interaction simulation approach, to model the ultrasound propagation and interaction with defects in the proposed system. This model was used to predict the expected detection performance of the system in the presence of various defects of different sizes and positions, and at varying levels of signal-to-noise ratios. When possible, operating variables for the model were chosen consistently with the field test of an experimental prototype that was conducted in 2014. The defect detection performance was evaluated through the computation of receiver operating characteristic curves in terms of probability of detection versus probability of false alarms. The study indicates that despite the challenges of non-contact probing of the rail, quite satisfactory inspection performance can be expected for a variety of defect types, sizes, and positions. Beyond the specific cases examined in this article, this numerical framework can also be used in the future to examine a larger variety of field test conditions.