S0 Lamb Mode Scattering Studies in Laminated Composite Plate Structures with Surface Breaking Cracks: Insights into Crack Opening Behavior.

Ultrasonic-guided waves are attractive for rapid inspection of laminated composite structures, where cracks developed transverse to the loading direction are the severe type of damage. This paper presents studies of the interaction of fundamental symmetric S0 Lamb mode with vertical surface-breaking cracks in laminated composite plate structures. Finite element simulations and experimental investigations are used to study the effect of crack depth on S0 wave reflection behavior. Results show a monotonic rise of the reflection coefficient for different crack depths in a manner that is strongly dependent on the orientation of the plies and transverse ply location in the vicinity of the crack. Scattered wave packets in the reflection regime are captured using an in-plane laser. The S0 Lamb mode's sensitivity is numerically presented for the different crack depths in the long wavelength limit. We also observed that the reflected wave mode depicts the information of the corresponding broken interfaces. An attempt was made to show that this behavior relates to the crack-opening behavior in response to in-plane excitation. The reflection coefficient as a characteristic polynomial is proposed for various orientations. It was observed that the dispersion at receiver nodes makes the analysis challenging for distinguishing the signal from crack faces due to the smaller dimension. The study outcomes show its prospect as a promising NDE tool for crack damage detection in thin laminated plate structures.

Far-field ultrasonic imaging using hyperlenses.

Hyperlenses for ultrasonic imaging in nondestructive evaluation and non-invasive diagnostics have not been widely discussed, likely due to the lack of understanding on their performance, as well as challenges with reception of the elastic wavefield past fine features. This paper discusses the development and application of a cylindrical hyperlens that can magnify subwavelength features and achieve super-resolution in the far-field. A radially symmetric structure composed of alternating metal and water layers is used to demonstrate the hyperlens. Numerical simulations are used to study the performance of cylindrical hyperlenses with regard to their geometrical parameters in imaging defects separated by a subwavelength distance, gaining insight into their construction for the ultrasonic domain. An elegant extension of the concept of cylindrical hyperlens to flat face hyperlens is also discussed, paving the way for a wider practical implementation of the technique. The paper also presents a novel waveguide-based reception technique that uses a conventional ultrasonic transducer as receiver to capture waves exiting from each fin of the hyperlens discretely. A metallic hyperlens is then custom-fabricated, and used to demonstrate for the first time, a super-resolved image with 5X magnification in the ultrasonic domain. The proposed hyperlens and the reception technique are among the first demonstrations in the ultrasonic domain, and well-suited for practical inspections. The results have important implications for higher resolution ultrasonic imaging in industrial and biomedical applications.

Quantifying adhesive thickness and adhesion parameters using higher-order SH guided waves.

A method to quantify the interface shear stiffness, adhesive shear modulus and adhesive thickness in an aluminium-epoxy-aluminium joint is presented. Shear horizontal guided waves are considered to infer the properties. A numerical model that employs spring stiffness boundary conditions at the aluminium-epoxy interface was developed to generate dispersion curves. The sensitivity of the first four SH-like modes to epoxy thickness, interface shear stiffness, and adhesive shear modulus are analyzed. The dispersion analysis reveals that higher-order anti-symmetric modes are sensitive to all three parameters, whereas the symmetric modes are sensitive only to adhesive thickness. Hence to prevent false alarms that might arise while assessing the bond conditions, symmetric and anti-symmetric modes should be simultaneously generated. Periodic permanent magnet (PPM) electromagnetic acoustic transducers (EMATs) are used to generate and detect SH-like modes. Utilizing the constant wavelength property of PPM-EMATs, SH2-like and SH3-like modes are generated. Short-time Fourier transform (STFT) is used to separate the modes merged in the received time response. By overlaying the dispersion curves of SH2-like mode on STFT, the thickness of epoxy is quantified. The dispersion curves of SH3-like mode are generated using the measured thickness and overlaid on STFT to measure the interface shear stiffness and epoxy shear modulus. The proposed method is experimentally demonstrated on aluminium-epoxy-aluminium samples of different surface treatments. The study demonstrates a reliable nondestructive evaluation of adhesive bonds that reduces possible false alarms.

Concentric shell gradient index metamaterials for focusing ultrasound in bulk media.

Focusing of ultrasound waves is criticalto a number ofclinical andindustrial applications including biomedical and underwater imaging,nondestructive evaluation and material processing. This paper discusses the use of a novel'add-on' gradient refractive index (GRIN) metamaterial structure made ofconcentric shells,to focus ultrasonic waves generated by conventional transducers. Analysis based on the Huygen's principle and numerical simulations is used to design the geometric and material properties of the proposed structure, whose working is demonstrated through experiments. Varying the shell material or thickness is shown to offer an elegant and straightforward way to tailor the focal spot inside the target material. The concentric-shell GRIN lens proposed here has a simple design, and has a potential to be used in dynamic focusing without advanced lenses or electronic steering.

Lensing in the Ultrasonic Domain using Negative Refraction Induced by Material Contrast.

The focusing of ultrasound using topographic lenses, typically made of plates with step changes that cause an interaction between forward- and backward-propagating guided waves, has been widely studied in recent years. However, such 'step-change' lenses require precise machining and moreover, the thick-thin structure can be unstable during deployment in practical inspection applications. The work reported here follows from the insight that perhaps any approach to induce a mismatch in acoustical impedance as achieved by the step-change can also lead to focusing of ultrasonic guided waves. By carefully choosing the impedance pairing, a novel material contrast lens stacking Aluminium and Molybdenum plates in series is shown to achieve focusing of ultrasound through negative refraction. The interface between the two metals causes the interaction of the forward-propagating second symmetric Lamb mode S2 into the backward- propagating first symmetric S2b. The focusing of Lamb waves is demonstrated using numerical simulations validated by experiments. Comparison with a simple Aluminium-Aluminium plate combination brings out the underlying physics of focusing using the proposed material contrast lens. Simulation results showing super-resolution imaging using the proposed material contrast lens  are also presented, demonstrating the power of the proposed approach. This report opens up the possibilities of developing new lensing devices for use in medical imaging and nondestructive evaluation, among other possible applications.

Signal noise based transfer function approach for reliability estimation of ultrasonic inspection.

In recent years several methods for reducing the burden of experimentation for obtaining Probability of Detection (PoD) curves have been proposed, especially involving the use of numerical simulation. In particular, there is much interest in being able to estimate the PoD capabilities of a given NDE method, target embodiment and material, provided this is known for some canonical material for the same combination of method/embodiment. Ultrasonic experiments on materials with low signal to noise ratio (SNR) are often difficult and time consuming since the higher signal noise causes serious complications. Set in this context, this paper proposes an approach for transferring PoD curves among materials with different SNR values. The classical transfer function PoD approach is based on the hypothesis that the ratio of signals in related quadrants is equal, which requires large datasets for multiple quadrants. The approach proposed here directly deals with SNR instead of the ratio of signals, and thus requires only the experimental data of the parent application. The new approach is illustrated through example cases involving the prediction of PoD curves for ultrasonic inspection of an aluminium plate using the empirical PoD data for the same in austenitic stainless steel and mild steel. The approach is also demonstrated in each of the possible combinations among these three materials.

Monitoring pipe wall integrity using fiber Bragg grating-based sensing of low-frequency guided ultrasonic waves.

Recent literature shows that low-frequency ultrasonic guided waves experience mode confinement and loss of axi-symmetry in pipes with axially uniform features such as eccentricity. Considering extended wall loss as a case of uniform eccentricity, this paper proposes to monitor pipe integrity by measuring changes to the modal structure of low-frequency axisymmetric L(0,2) longitudinal guided waves. Fiber Bragg gratings are shown to be effective in detecting changes to L(0,2) modal characteristics, providing a novel route to health monitoring of pipe assets.

Effect of ply orientation and through-thickness position of delamination on the reflection of fundamental symmetric S0 Lamb mode in GFRP composite plate structures.

This paper studies the interaction of the fundamental symmetric Lamb wave mode S0 with delaminations in 8-layered glass fibre reinforced polymer (GFRP) laminated composites, using numerical models validated against experiments. The validation is performed using experiments on the eight layer quasi-isotropic GFRP composite laminate [0/90/45/-45]S. Further numerical studies were performed on the laminates [0/45/-45/90]S, [0/45/90/-45]S, [90/45/-45/0]S, [90/-45/0/45]S, [45/-45/0/90]S, and [45/90/-45/0]S to study the effect of ply layups on the reflection of S0 Lamb waves. The reflection coefficients calculated for these laminates followed a similar trend as the laminate [0/90/45/-45]S. Physical insight was obtained into the reflection behaviour, based on an analysis of wave dispersion in the sub-laminates. It was difficult to detect the delamination situated at the mid plane of the laminate, where the shear stress was found to be zero for all ply layups. The delaminations present at a depth greater than DL12 (>0.42 mm) from the source of excitation, were found to be accessible for the laminates [0/90/45/-45]S, [0/45/-45/90]S and [0/45/90/-45]S. The sensitivity of the S0 mode reflection depends upon the position of the delamination along the thickness direction and ply layup orientation of the composite laminate. The S0 mode showed higher sensitivity to DL23 and DL34 (0.42 mm to 1.26 mm) in all the laminates that were inspected.

Bayesian synthesis for simulation-based generation of probability of detection (PoD) curves.

This paper examines the feasibility of using Bayesian synthesis to reduce the number of experimental cases and trials required for generation of probability of detection (PoD) curves. A Bayesian framework is developed for the data-level combination of experimental and simulated datasets, in the context of the inspection of back-wall breaking notches in metallic samples by bulk ultrasonic shear waves. PoD curves generated using the proposed approach, where results from a reduced number of experimental defect cases and trials are used in combination with simulated datasets, are shown to compare well with those from the conventional approach using a large number of experiments. Finally, the framework is also shown to be versatile for generating PoD curves for complex defects (illustrated through the example of an inclined notch) using simulations for canonical defects (vertical notches).

Deep subwavelength ultrasonic imaging using optimized holey structured metamaterials.

This paper reports the experimental demonstration of deep subwavelength ultrasonic imaging of defects in metallic samples with a feature size of λ/25 using holey-structured metamaterial lenses. Optimal dimensions of the metamaterial's geometric parameters are determined using numerical simulation and the physics of wave propagation through holey lenses. The paper also shows how the extraordinary transmission capacity of holey structured metamaterials comes about by the coupling of higher frequencies in the incident ultrasonic wave field to resonant modes of the lens.

Ultrasonic bent waveguides approach for distributed temperature measurement.

This paper describes novel techniques for simultaneous measurement of temperatures at multiple locations using two configurations (a) a single transducer attached to multiple waveguides of different lengths (each with a single bend) and (b) single waveguide with multiple bends connected to single transducer. These techniques improve upon the earlier reported studies using straight waveguides, where the non-consideration of the effect of temperature gradients was found to be a major limitation. The range of temperature measurement is from room temperature to maximum utility temperature of the waveguide material. The time of flight difference of reflected ultrasonic longitudinal guided wave modes (L(0,1)) from the bend, which is the reference signal, and another signal from the end of the waveguide, is utilized to measure the local temperature of the surrounding media. Finite element simulations were employed to obtain the appropriate dimensions and other design features of the multiple bent waveguide. This work is of interest to several industrial applications involving melters and furnaces.

Longitudinal guided waves confined in radius filler regions of composite joints.

This paper studies the feasibility of using ultrasonic guided waves for fast inspection of conformal deltoid radius filler or "Noodle" regions of joints in stringer composite structures. Semi-analytical finite element simulations, supported by experiments and three-dimensional finite element models, are used to demonstrate the existence of a longitudinal guided ultrasonic mode confined or trapped in the Noodle regions. Studies reveal that this mode has attractive properties for rapid screening of Noodle joints, including strong energy concentration, low dispersion, and attenuation. Discussing the physics of mode confinement in light of material differences and geometry, the phenomenon is shown to be related to feature-guiding effects noted in literature recently.

Anisotropic effects on ultrasonic guided waves propagation in composite bends.

Ultrasonic guided waves have proven to be attractive to the long-range testing of composite laminates. As complex-shaped composite components are increasingly incorporated in high-performance structures, understanding of both anisotropic and geometric effects on guided waves propagation is needed to evaluate their suitability for the non-destructive testing (NDT) of such complex structures. This paper reports the Semi-Analytical Finite Element (SAFE) simulations revealing the capability of energy confinement carried by two types of guided modes in 90° carbon fiber/epoxy (CF/EP) bends. Existence of the phenomenon is cross-validated by both 3D Finite Element (FE) modeling and experimental measurements. The physics of such energy trapping effect is explained in view of geometric variation and anisotropic properties, and the frequency effect on the extent of energy concentration is discussed. Finally, the feasibility of using such confined guided waves for rapid inspection of bent composite plate structures is also discussed.

Shear horizontal feature guided ultrasonic waves in plate structures with 90° transverse bends.

Antisymmetric and symmetric Lamb-type feature guided waves (FGW) have recently been shown to exist in small angle plate bends. This paper reports Semi-Analytical Finite Element (SAFE) method simulations revealing the existence of a new family of Shear Horizontal (SHB) type of FGW mode in 90° bends in plate structures. Mode shapes and velocity dispersion curves are extracted, demonstrating the SH-like nature of a bend-confined mode identified in studies of power flow across the bend. The SHB mode is shown to have reduced attenuation in the higher frequency range, making it an ideal choice for high-resolution inspection of such bends. Further modal studies examine the physical basis for mode confinement, and argue that this is strongly related to FGW phenomena reported earlier, and also linked to the curvature at the bend region. Wedge acoustic waves discussed widely in literature are shown as arising from surface-limiting of the SHB mode at higher frequencies. The results are validated by experiments and supported by 3D Finite Element (FE) simulations.

Ultrasonic guided waves in elliptical annular cylinders.

This paper studies the influence of cross-section ovalness or ellipticity on lower order axisymmetric guided wave modes in thin pipes. The second longitudinal mode L(0,2) and the fundamental torsional mode T(0,1) are studied, as these are of interest to current pipe inspection. The semi-analytical finite element (FE) method is mainly used, with three-dimensional FE simulations for visualization and cross-validation of results. The studies reveal that even a small degree of ovalness can affect mode shapes and velocities. The effect is more pronounced on the L(0,2) mode than on T(0,1) and this may be important for practical inspection applications.

Low frequency axisymmetric longitudinal guided waves in eccentric annular cylinders.

This paper studies the effect of axially uniform eccentricity on the modal structures and velocities of the lower order axisymmetric guided wave mode L(0,2) in circular tubes or pipes. The semi-analytical finite element method is mainly used, supported by fully three-dimensional finite element models and validated using experiments. The studies show that even a small eccentricity in the pipe can cause a loss in the L(0,2) mode axisymmetry, leading to its confinement in the thinned side of the pipe cross-section and also a reduction in mode velocities. The physics of this phenomenon is related to the feature-guiding and mode confinement effects noted in recent years in the literature, particularly studies on waveguides with local cross-section variations and curvature.

Symmetric low-frequency feature-guided ultrasonic waves in thin plates with transverse bends.

Recent research by the authors shows that bends in plates can act as features that can concentrate and guide ultrasonic energy along their axis. At low frequencies, two feature-guided modes are identified when the bent plate is subjected to 'in-plane' or axial excitation applied uniformly along a through-thickness line bisecting the bent edge. Of these, the slower mode has properties similar to the A0 (fundamental antisymmetric) Lamb mode in flat plates. This paper focuses on the faster bend-guided mode that is similar to the S0 (fundamental symmetric) Lamb mode in flat plates. Using 3D finite element (FE) simulation validated with experiments, this mode is shown to be more strongly generated in smaller angle bends. Features of the mode including velocity, attenuation and modal structure are considered in detailed studies. Results are discussed in light of simple modal studies using the Semi Analytical Finite Element (SAFE) method, suggesting a relationship of bend-guided waves to modes of curved bars.

Interaction of low-frequency axisymmetric ultrasonic guided waves with bends in pipes of arbitrary bend angle and general bend radius.

The use of ultrasonic guided waves for the inspection of pipes with elbow and U-type bends has received much attention in recent years, but studies for more general bend angles which may also occur commonly, for example in cross-country pipes, are limited. Here, we address this topic considering a general bend angle φ, a more general mean bend radius R in terms of the wavelength of the mode studied and pipe thickness b. We use 3D Finite Element (FE) simulation to understand the propagation of fundamental axisymmetric L(0,2) mode across bends of different angles φ. The effect of the ratio of the mean bend radius to the wavelength of the mode studied, on the transmission and reflection of incident wave is also considered. The studies show that as the bend angle is reduced, a progressively larger extent of mode-conversion affects the transmission and velocity characteristics of the L(0,2) mode. However the overall message on the potential of guided waves for inspection and monitoring of bent pipes remains positive, as bends seem to impact mode transmission only to the extent of 20% even at low bend angles. The conclusions seem to be valid for different typical pipe thicknesses b and bend radii. The modeling approach is validated by experiments and discussed in light of physics of guided waves.

Antisymmetric feature-guided ultrasonic waves in thin plates with small radius transverse bends from low-frequency symmetric axial excitation.

The influence of bends constituting annular polygonal structures on ultrasonic guided waves propagating along their axis is investigated. Considering a single bend as a bent plate connects this problem to the better-understood physics of guided waves in straight plates. Using a three-dimensional finite element simulation validated with experiments, bends in plates are shown to act as features that can concentrate and guide ultrasonic energy along their length. Two interesting feature-guided modes are identified when the bent plate is subjected to "in-plane" or axial excitation applied uniformly along a through-thickness line bisecting the bent edge. Of these, the faster traveling mode has properties similar to, but travels at group velocities lower than, the S0 (fundamental symmetric) Lamb mode in flat plates. This paper however focuses on the slower bend-guided mode that is similar to the A0 (fundamental anti-symmetric) Lamb mode in flat plates. This mode is shown to be more strongly generated in smaller angle bends where it has a low attenuation. The results are discussed in light of simple modal studies performed using the Semi-Analytical Finite Element method.

A generic hybrid model for bulk elastodynamics, with application to ultrasonic nondestructive evaluation.

Practical ultrasonic inspection requires modeling tools that enable rapid and accurate visualization; because of the increasing sophistication of practical inspection, it is becoming increasingly difficult to use a single modeling method to represent an entire inspection process. Hybrid models that utilize different or interacting numerical schemes in different regions, to use their relative advantages to maximal effect, are attractive in this context, but are usually custom-made for specific applications or sets of modeling methods. The limitation of hybrid schemes to particular modeling techniques is shown here to be related to their fundamental formulation. As a result, it becomes clear that a formalism to generalize hybrid schemes can be developed: an example of the construction of a generic hybrid modeling interface is given for the abstraction of bulk ultrasonic wave phenomena, common in practical inspection problems. This interface is then adapted to work within a prototype hybrid model consisting of two smaller finite element model-domains, and explicitly demonstrated for bulk ultrasonic wave propagation and scattering examples. Sources of error and ways to improve the accuracy of the interface are also discussed.