A Generic Hybrid Model for Bulk Elastodynamics, With Application to Ultrasonic Nondestructive Evaluation

Monolayer two-dimensional transitional metal dichalcogenides, such as MoS₂, WS₂, and WSe₂, are direct band gap semiconductors with large exciton binding energy. They attract growing attentions for optoelectronic applications including solar cells, photodetectors, light-emitting diodes and phototransistors, capacitive energy storage, photodynamic cancer therapy, and sensing on flexible platforms. While light-induced luminescence has been widely studied, luminescence induced by injection of free electrons could promise another important applications of these new materials. However, cathodoluminescence is inefficient due to the low cross-section of the electron–hole creating process in the monolayers. Here for the first time we show that cathodoluminescence of monolayer chalcogenide semiconductors can be evidently observed in a van der Waals heterostructure when the monolayer semiconductor is sandwiched between layers of hexagonal boron nitride (hBN) with higher energy gap. The emission intensity shows a strong dependence on the thicknesses of surrounding layers and the enhancement factor is more than 500-fold. Strain-induced exciton peak shift in the suspended heterostructure is also investigated by the cathodoluminescence spectroscopy. Our results demonstrate that MoS₂, WS₂, and WSe₂ could be promising cathodoluminescent materials for applications in single-photon emitters, high-energy particle detectors, transmission electron microscope displays, surface-conduction electron-emitter, and field emission display technologies

Transient ultrasonic guided wave simulation in layered composite structures using a hybrid wave and finite element scheme

A guided wave simulation method for layered composites based on the wave and finite element scheme is presented. An approach for calculating complex displacement fields such as those generated from piezoelectric transducers is developed. The scattering of waves from different types of defects is computed. A rigorous energy based criteria is proposed for model order reduction. All calculations are carried out in the frequency domain and an inverse discrete fourier transform is performed to get the time domain result. Numerical examples of a multi-layered composite beam are performed to assess the performance and validate the methodology. Three types of damages are simulated namely a notch, a transverse crack and an internal delamination. The results are validated against finite element simulations and are found to be in excellent agreement. Moreover the approach is found to be orders of magnitude faster compared to finite element simulations

Hybrid Numerical-Analytical Scheme for Locally Inhomogeneous Elastic Waveguides

Numerical simulation of guided wave excitation, propagation, and diffraction in laminate structures with local inhomogeneities (obstacles) is associated with high computational cost due to the need for a mesh-based approximation of extended domains with a rigorous account for the radiation conditions at infinity. To obtain computationally efficient solutions, hybrid numerical-analytical approaches are currently being developed, based on linking a numerical solution in a local vicinity of the source and/or obstacles with an explicit analytical representation in the external semi-infinite domain. However, the developed methods are generally not widely spread because the possibility of such coupling with an external multimode wave field is generally not provided in standard finite-element (FE) software. We propose a scheme that allows the use of the FE software as a black box for the required correct matching of local numerical and global analytical solutions (FEM-An). The FEM is used to obtain a set of local numerical solutions that serve as a basis in the inner domain. These solutions satisfy the boundary conditions induced by guided wave modes so that they fit correctly with the modal expansion in the outer region. The expansion coefficients of both FE and modal decompositions are determined then from the condition of stress and displacement continuity at the interface between the inner and outer domains. This scheme was numerically validated against analytical solutions to test problems and FE solutions for long waveguide sections with perfect match layer absorbing conditions at the ends (FEM PML). Along the way, it turned out that the FEM-PML approach gives an incorrect result in the backward-wave bands and at high frequencies. The application of the FEM-An hybrid scheme is illustrated by examples of Lamb wave diffraction by elastic inclusions and delaminations

Spectral subtraction and enhancement for torsional waves propagating in coated pipes

Ultrasonic guided waves are routinely used for inspection of pipelines. The technique is well established for uncoated pipes where attenuation is very low. However, when the pipe is coated, buried or immersed, sound energy will be absorbed by the coating or radiate into the surrounding medium. Attenuation will increase and the scanning distance will be significantly reduced. The noise level can also increase when the condition of the coating material degrades with age and the bonding condition between pipe and coating becomes unevenly distributed. The increase of attenuation ratio and noise level therefore makes the inspection of ultrasonic waves propagating in coated and buried pipelines particularly difficult. It is often desirable to identify small features amongst the noise floor. To improve signal to noise ratio under these conditions, two techniques are proposed for the study of the propagation of torsional waves in Denso Tape coated pipes. A frequency domain, backward wave cancelling algorithm is used to eliminate the reflected waves coming from the backward direction and clean up the signal. On this basis, a spectral subtraction method is used, which requires knowledge of a small section of pipe that includes no real features, so that the signal from this region provides the characteristic noise signature of the pipe itself. The spectrum of the noise signature is calculated and then subtracted from the total signal using a sliding window technique. Furthermore, a signal region, for instance the reflected signal from a pipe weld or end, is specified. This represents the characteristic of the incident signal and any signal similar in shape will be enhanced using the sliding window technique. These two techniques serve to reduce the noise floor and enhance small signals that may be buried in it. This is important for ultrasonic non-destructive testing applications in coated and buried pipes.The Innovate UK project (Ultrasonic Nuclear Inspection

Modelling of Pencil-Lead Break Acoustic Emission Sources using the Time Reversal Technique

In Acoustic Emissions (AE), Hsu-Nielsen Pencil-Lead Breaks (PLB) are used to generate sound waves enabling the characterization of acoustic wave speed in complex structures. The broadband signal of a PLB represents a repeatable emission, which can be applied at different regions of the structure, and therefore can be used to calibrate the localization algorithms of the AE system. In recent years, the use of Finite Element Method (FEM) has flourished for modelling acoustic Lamb wave propagation, which is present in thin plate-like structures. The primary challenge faced by the AE community is the lack of a well-known mathematical function of a PLB signal that can be applied in numerical simulations. This study makes use of a Time Reversal (TR) approach to identify the emission source of the PLB on a 7075-T651 aluminum plate. An ABAQUS CAETM model with piezoelectric actuators and sensors was developed. In order to avoid edge reflections, absorbing boundaries based on the Stiffness Reduction Method (SRM) were considered. The captured PLB signals were used as input to the FEM and was time-reversed. Furthermore, a band-limited white noise signal was used to calibrate the contribution of the broadband frequencies found in the transmitted wave packet. Preliminary results indicate that the TR approach can be used to understand the shape and function of the original transmitted signal

Tomography applied to Lamb wave contact scanning nondestructive evaluation

The aging world-wide aviation fleet requires methods for accurately predicting the presence of structural flaws that compromise airworthiness in aircraft structures. Nondestructive Evaluation (NDE) provides the means to assess these structures quickly, quantitatively, and noninvasively. Ultrasonic guided waves, Lamb waves, are useful for evaluating the plate and shell structures common in aerospace applications. The amplitude and time-of-flight of Lamb waves depend on the material properties and thickness of a medium, and so they can be used to detect any areas of differing thickness or material properties which indicate flaws. By scanning sending and receiving transducers over an aircraft, large sections can be evaluated after a single pass. However, while this technique enables the detection of areas of structural deterioration, it does not allow for the quantification of the extent of that deterioration. Tomographic reconstruction with Lamb waves allows for the accurate reconstruction of the variation of quantities of interest, such as thickness, throughout the investigated region, and it presents the data as a quantitative map. The location, shape, and extent of any flaw region can then be easily extracted from this Tomographic image. Two Lamb wave tomography techniques using Parallel Projection tomography (PPT) and Cross Borehole tomography (CBT), are shown to accurately reconstruct flaws of interest to the aircraft industry. A comparison of the quality of reconstruction and practicality is then made between these two methods, and their limitations are discussed and shown experimentally. Higher order plate theory is used to derive analytical solutions for the scattering of the lowest order symmetric Lamb wave from a circular inclusion, and these solutions are used to explain the scattering effects seen in the Tomographic reconstructions. Finally, the means by which this scattering theory can be used to develop Lamb wave Tomographic algorithms that are more generally applicable in-the-field, is presented