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Material nonlinearity is explored for the assessment of structural integrity. Crack–wave interactions are of particular interest. The major focus is on higher-order harmonics, generated in propagating shear horizontal (SH) waves. These harmonics are generated due to global material nonlinearity and local effects such as fatigue cracks. The theoretical background of the proposed method is explained. The method is examined using numerical simulations and experimental tests. The former involves the Local Interaction Simulation Approach (LISA), implemented for the nonlinear shear horizontal wavefield. The latter is based on a high-frequency shear excitation approach. Experimental tests are conducted using a series of beam specimens with fatigue cracks. Low-profile, surface-bonded piezoceramic shear actuators are used for excitation. The excitation frequency is selected to minimize the number of generated modes in the examined specimens. Nonlinear ultrasonic responses are collected using a non-contact laser vibrometer. The results show that higher-order harmonic generation—based on shear horizontal wave propagation—can be used for crack detection in the presence of global material nonlinearity. [ABSTRACT FROM AUTHOR]

Harmonic generation has always been of fundamental interest in the study of the nonlinear nature of any physical system. In the present study, particle-in-cell simulations have been carried out to explore the harmonic generation of electromagnetic (EM) waves in a magnetized plasma. EM wave propagation is chosen to be parallel to the applied external magnetic field. The simulations show the excitation of odd higher harmonics of right circularly polarized waves and left circularly polarized waves when the incident wave is linearly polarized. The harmonic generation is maximum when the incident EM wave frequency matches the electron cyclotron frequency. When the incident EM wave has a circular polarization, no harmonics get excited. A theoretical understanding of these observations has also been provided. The studies thus show that by appropriately tailoring plasma parameters, EM waves of higher frequencies and the desired nature of circular polarization can be generated. [ABSTRACT FROM AUTHOR]

Harmonic generation measurement is recognized as a promising tool for inspecting material state or micro-damage and is an ongoing research topic. Second harmonic generation is most frequently employed and provides the quadratic nonlinearity parameter (β) that is calculated by the measurement of fundamental and second harmonic amplitudes. The cubic nonlinearity parameter ( β 2 ), which dominates the third harmonic amplitude and is obtained by third harmonic generation, is often used as a more sensitive parameter in many applications. This paper presents a detailed procedure for determining the correct β 2 of ductile polycrystalline metal samples such as aluminum alloys when there exists source nonlinearity. The procedure includes receiver calibration, diffraction, and attenuation correction and, more importantly, source nonlinearity correction for third harmonic amplitudes. The effect of these corrections on the measurement of   β 2 is presented for aluminum specimens of various thicknesses at various input power levels. By correcting the source nonlinearity of the third harmonic and further verifying the approximate relationship between the cubic nonlinearity parameter and the square of the quadratic nonlinearity parameter ( β ∗ β ), β 2 ≈ β ∗ β , the cubic nonlinearity parameters could be accurately determined even with thinner samples and lower input voltages. [ABSTRACT FROM AUTHOR]

We consider a quasi-one-dimensional Bose–Einstein condensate with contact and long-range dipolar interactions, under the action of the time-periodic modulation applied to the harmonic-oscillator and optical-lattice trapping potentials. The modulation results in generation of a variety of harmonics in oscillations of the condensate's width and centre-of-mass coordinate. These include multiple and combinational harmonics, represented by sharp peaks in the system's spectra. Approximate analytical results are produced by the variational method, which are verified by systematic simulations of the underlying Gross–Pitaevskii equation. This article is part of the theme issue 'New trends in pattern formation and nonlinear dynamics of extended systems'. [ABSTRACT FROM AUTHOR]

This paper investigates delamination detection in composite pipes using higher harmonic generation technique for the first time. The semi-analytical finite element (SAFE) method is developed to analyze the dispersion characteristics of the composite pipe. Then, a flexural mode at low frequency is selected to detect delamination. The three-dimensional (3D) finite element (FE) model is developed to simulate the flexural mode propagating in the composite pipe and its interaction with delamination. By comparing the simulated radial displacement signals with and without delamination, it is observed that the delamination can contribute to the higher harmonic generation. Subsequently, the ultrasonic measurements are conducted to demonstrate the numerical results. Using a relatively simple piezoelectric transducer (PZT) configuration, an almost pure flexural mode can be excited in the experiment. More importantly, experimental results indicate the higher harmonics induced by the delamination can be observed obviously. Finally, extensive parametric studies are performed to reveal the sensitivity of higher harmonics to the delamination with various sizes and at different interface. The proposed higher harmonic generation of flexural waves is promising for the practical application of delamination detection in composite pipes. [ABSTRACT FROM AUTHOR]

When two rough surfaces are loaded together contact occurs at asperity peaks. An interface of solid contact regions and air gaps is formed that is less stiff than the bulk material. The stiffness of a structure thus depends on the interface conditions; this is particularly critical when high stiffness is required, for example in precision systems such as machine tool spindles. The rough surface interface can be modelled as a distributed spring. For small deformation, the spring can be assumed to be linear; whilst for large deformations the spring gets stiffer as the amount of solid contact increases. One method to measure the spring stiffness, both the linear and nonlinear aspect, is by the reflection of ultrasound. An ultrasonic wave causes a perturbation of the contact and the reflection depends on the stiffness of the interface. In most conventional applications, the ultrasonic wave is low power, deformation is small and entirely elastic, and the linear stiffness is measured. However, if a high-powered ultrasonic wave is used, this changes the geometry of the contact and induces nonlinear response. In previous studies through transmission methods were used to measure the nonlinear interfacial stiffness. This approach is inconvenient for the study of machine elements where only one side of the interface is accessible. In this study a reflection method is undertaken, and the results are compared to existing experimental work with through transmission. The variation of both linear and nonlinear interfacial stiffnesses was measured as the nominal contact pressure was increased. In both cases interfacial stiffness was expressed as nonlinear differential equations and solved to deduce the contact pressure-relative surface approach relationships. The relationships derived from linear and nonlinear measurements were similar, indicating the validity of the presented methods. [ABSTRACT FROM AUTHOR]

Nickel and nickel-based composites are of vital importance in many fields, while temperature loading can greatly influence the strength and performance of the materials. Nondestructive evaluation and characterization of such thermal damage can be used to predict the failure of metallic structures, thermal barrier coatings and so on, especially in a non-contact way under certain strict circumstances, such as testing at high temperature or in radiative environment. Herein, a contactless ultrasonic technique employing electromagnetic acoustic transducers (EMATs) combined with the resonance ultrasound spectroscopy is applied to make up the low energy transition efficiency of EMATs and enhance the signal-to-noise ratio of ultrasonic testing signals. The method is adopted to assess the thermal damages of different levels in artificially heat loaded nickel plates. The damage sensitivity of third order harmonics generated from shear waves is discussed, along with linear ultrasonic features including wave velocity and attenuation. Experimental results show that the proposed nonlinear electromagnetic acoustic resonance (EMAR) technique can be used to evaluate the thermal damage in ferromagnetic material with improved reliability and sensitivity over linear ones. [ABSTRACT FROM AUTHOR]

When two rough surfaces are loaded together contact occurs at asperity peaks. An interface of solid contact regions and air gaps is formed that is less stiff than the bulk material. The stiffness of a structure thus depends on the interface conditions; this is particularly critical when high stiffness is required, for example in precision systems such as machine tool spindles. The rough surface interface can be modelled as a distributed spring. For small deformation, the spring can be assumed to be linear; whilst for large deformations the spring gets stiffer as the amount of solid contact increases. One method to measure the spring stiffness, both the linear and nonlinear aspect, is by the reflection of ultrasound. An ultrasonic wave causes a perturbation of the contact and the reflection depends on the stiffness of the interface. In most conventional applications, the ultrasonic wave is low power, deformation is small and entirely elastic, and the linear stiffness is measured. However, if a high-powered ultrasonic wave is used, this changes the geometry of the contact and induces nonlinear response. In previous studies through transmission methods were used to measure the nonlinear interfacial stiffness. This approach is inconvenient for the study of machine elements where only one side of the interface is accessible. In this study a reflection method is undertaken, and the results are compared to existing experimental work with through transmission. The variation of both linear and nonlinear interfacial stiffnesses was measured as the nominal contact pressure was increased. In both cases interfacial stiffness was expressed as nonlinear differential equations and solved to deduce the contact pressure-relative surface approach relationships. The relationships derived from linear and nonlinear measurements were similar, indicating the validity of the presented methods.

The exotic plasmonic properties of highly doped graphene monolayer have opened new doors to develop advanced nanophotonic devices. It has been successfully experienced that the incident field can become substantially intense enough to convert the fundamental laser frequency into both even and odd high-order harmonics via a nonlinear phenomenon. In this work, using highly doped graphene nanorings with semimetallic behaviour, we analyzed the formation of high-harmonic generation using both finite-element method (FEM) and finite-difference time-domain (FDTD) calculations. Employing the inherent anharmonic charge-carrier dispersion of graphene and choosing accurate geometrical sizes for the atomic scale nanorings allow for the generation of third-harmonic (∼730 nm) with high intensity, substantial quality factor ( Q -factor), and significant conversion efficiency up to η THG ∼0.0014%. By realizing the hybridization of excited plasmons, we showed that the unique spectral response of graphene allows for the generation of gate-tunable and efficient higher order harmonic formation along the visible spectrum. We believe that the proposed atomic-scale platform has a strong potential to be used for advanced and integrated photonic devices. [ABSTRACT FROM AUTHOR]

The development of kW-class diode-pumped picosecond laser sources emitting at various wavelengths started at the HiLASE Center four years ago. A 500-W Perla C thin-disk laser with a diffraction limited beam and repetition rate of 50-100 kHz, a frequency conversion to mid-infrared (mid-IR), and second to fifth harmonic frequencies was demonstrated. We present an updated review on the progress in the development of compact picosecond and femtosecond high average power radiation sources covering the ultraviolet (UV) to mid-IR spectral range at the HiLASE Center. We also report on thin-disk manufacturing by atomic diffusion bonding, which is a crucial technology for future high-power laser development. [ABSTRACT FROM AUTHOR]

Nondestructive testing in concrete is complex due to its nonhomogeneous ingredients. The presence of varying sizes of aggregates in concrete results in nonlinear characteristics. This paper aims to compare mortar and normal concrete with varying aggregate sizes. The specimens are damaged gradually with loading and unloading paths. Fast Fourier transform (FFT) of the recorded time domain waveforms is conducted to show the frequency spectra. The amplitude of the second harmonic frequency is used as a reference to the internal damage. The analysis of the nonlinear ultrasonic parameter second harmonic amplitude is used according to each loading branch with two damage levels: total and incremental. Results showed that the magnitude of the total damage produced by mortar at the last loading branch was relatively lower than all of the concrete mixes with different sizes of aggregates. The magnitude of the incremental damage produced by mortar in every step load consistently increased as the load increased. For the normal concrete with different sizes of aggregates, the highest total damage was obtained when mixed with large aggregates. According to a previous study using a finite-element model, increasing the aggregates' size in concrete increased the crack width. This could have led to the increase in the magnitude of the normalized second harmonic ratio. On the other hand, the incremental damage of concrete with different sizes of aggregates varied per loading branch. These results indicated the complexity of concrete as a heterogeneous material where it grew differently through formation of microcracks that varied from one loading branch to another. [ABSTRACT FROM AUTHOR]

• A mechanical metamaterial for enhancing dispersive higher harmonic generation is proposed. • Phase matching conditions for enhancing dispersive third harmonic generation are derived. • Higher harmonic generation and enhancement in mechanical metamaterial is experimentally demonstrated. Higher harmonic generation (HHG) of elastic waves is beneficial for various applications, including engineering nondestructive testing, imaging, therapy, etc. Due to the ubiquity of dispersion in most materials and structures, however, it is usually difficult to simultaneously satisfy the conditions of phase-matching and non-zero energy flux that are needed for high-efficient HHG. Here we propose a mechanical metamaterial with coupled translation-rotation motion and explore its rich dispersion property for the enhancement of two HHGs in arbitrary regions of the acoustic band, i.e., the dispersive second harmonic generation (SHG) and the dispersive third harmonic generation (THG). Besides the phase-matching condition for enhancing SHG, we analytically obtain two phase-matching conditions for THG and reveal that THG is a complex process involving the interaction of multiple waves. By tailoring the dispersion of the metamaterial, the enhanced dispersive SHG and HHG are shown to be able to propagate at an arbitrarily slow group velocity, or even zero velocity, giving rise to localization. The analytical predictions are validated by numerical simulations and experimental tests. This work shows that by combining nonlinearity, mechanical metamaterials can be designed to have advanced capabilities of wave manipulation. [Display omitted] [ABSTRACT FROM AUTHOR]

We study analytically and numerically the second-harmonic generation in a one-dimensional nonlinear acoustic metamaterial, composed of an air-filled waveguide periodically loaded by clamped elastic plates. Based on the transmission line approach, we derive a nonlinear dynamical lattice model which, in the continuum approximation, leads to a nonlinear dispersive wave equation. By applying the perturbation method to the latter, we derive the analytical expressions for the first- and second-harmonics, which are in excellent agreement with the numerical simulations of the nonlinear dynamical lattice model. Apart from the case of dispersionless nonlinear propagation and the Fubini solution, special attention is payed to the role of dispersion. In that regard, it is found that, once dispersion comes into play, second-harmonic beatings in space due to phase-mismatch can be identified. Our results provide many opportunities for the development of new periodic acoustic structures featuring both nonlinearity and dispersion. [ABSTRACT FROM AUTHOR]

The development of kW-class diode-pumped picosecond laser sources emitting at various wavelengths started at the HiLASE Center four years ago. A 500-W Perla C thin-disk laser with a diffraction limited beam and repetition rate of 50–100 kHz, a frequency conversion to mid-infrared (mid-IR), and second to fifth harmonic frequencies was demonstrated. We present an updated review on the progress in the development of compact picosecond and femtosecond high average power radiation sources covering the ultraviolet (UV) to mid-IR spectral range at the HiLASE Center. We also report on thin-disk manufacturing by atomic diffusion bonding, which is a crucial technology for future high-power laser development.

Interest in using the higher harmonic generation of ultrasonic guided wave modes for nondestructive evaluation continues to grow tremendously as the understanding of nonlinear guided wave propagation has enabled further analysis. The combination of the attractive properties of guided waves with the attractive properties of higher harmonic generation provides a very unique potential for characterization of incipient damage, particularly in plate and shell structures. Guided waves can propagate relatively long distances, provide access to hidden structural components, have various displacement polarizations, and provide many opportunities for mode conversions due to their multimode character. Moreover, higher harmonic generation is sensitive to changing aspects of the microstructures such as to the dislocation density, precipitates, inclusions, and voids. We review the recent advances in the theory of nonlinear guided waves, as well as the numerical simulations and experiments that demonstrate their utility. [ABSTRACT FROM AUTHOR]

This paper presents an investigation of self-focusing of a q-Gaussian laser beam and its effect on harmonic generation in a preformed collisionless parabolic plasma channel. In the presence of a q-Gaussian laser beam, the carriers get redistributed from high field region to low field region on account of ponderomotive force as a result of which a transverse density gradient is produced in the channel which in turn generates plasma wave at pump frequency. Generated plasma wave interacts with the incident laser beam and generate higher harmonics of the incident laser beam. Moment theory has been used to derive differential equation for the spot size of laser beam propagating through the channel. The differential equation so obtained has been solved numerically. The effect of the intensity of laser beam, deviation of intensity distribution of laser beam along its wave front from Gaussian distribution, plasma density and depth of channel on beam width of laser beam and harmonic yield has been investigated. The effect of order of higher harmonic on harmonic yield has been also investigated. [ABSTRACT FROM PUBLISHER]

When numerically solving the time-dependent Schrödinger equation for the electrons in an atom or molecule, the Coulomb singularity poses a challenge. The solution will have limited regularity, and high-order spatial discretisations, which are much favoured in the chemical physics community, are not performing to their full potential. By exploiting knowledge about the jumps in the derivatives of the solution we construct a correction, and show how this improves the convergence rate of Fourier collocation from second to fourth order. This allows for a substantial reduction in the number of grid points. The new method is applied to the higher harmonic generation from atomic hydrogen. [ABSTRACT FROM AUTHOR]

Highlights: [•] Harmonic generation from fundamental modes in plate is numerically investigated. [•] Role of material and geometric nonlinearities in harmonic generation is assessed. [•] It was found that group velocity matching is not necessary for harmonic generation. [•] Phenomenon of mode-interaction is demonstrated. [Copyright &y& Elsevier]

The frequency dependence of the second-harmonic generation in Lamb waves is studied theoretically and numerically in order to examine the role of phase matching for sensitive evaluation of material nonlinearity. Nonlinear Lamb wave propagation in an isotropic plate is analyzed using the perturbation technique and the modal decomposition in the neighborhood of a typical frequency satisfying the phase matching. The results show that the ratio of the amplitude of second-harmonic Lamb mode to the squared amplitude of fundamental Lamb mode grows cumulatively in a certain range of fundamental frequency for a finite propagation distance. It is also shown that the frequency for which this ratio reaches maximum is close but not equal to the phase-matching frequency when the propagation distance is finite. This feature is confirmed numerically using the finite-difference time-domain method incorporating material and geometrical nonlinearities. The fact that the amplitude of second-harmonic mode becomes high in a finite range of fundamental frequency proves robustness of the material evaluation method using second harmonics in Lamb waves. [ABSTRACT FROM AUTHOR]

The use of nonlinear ultrasonics to characterize microstructural evolution is investigated with the aim of enabling earlier remaining useful life prediction and thereby greatly improving condition based maintenance. Higher harmonic generation is sensitive to microstructural features, whose evolution is indicative of ongoing damage processes. Localized plastic deformation is controlled in an aluminum sample by varying the notch length, which dictates the extent of the plastic zone. The essentials of higher harmonic generation analysis for ultrasonic guided waves are highlighted to provide a means to select a primary mode that generates a strong higher harmonic. Experimental methods to use magnetostrictive transducers for third harmonic generation measurements are described. Experimental results on aluminum plates indicate that plastic deformation increases the third harmonic by up to a factor of five and that the harmonic amplitude ratio $$A_{3}$$ / $$A_{1}^{3}$$ is sensitive to the plastic strain magnitude. These initial results show that when the plastic strain is localized, the $$A_{3}$$ / $$A_{1}^{3 }$$ ratio appears to be proportional to the plastic zone-to-propagation distance ratio. [ABSTRACT FROM AUTHOR]

The characteristic features of the propagation of finite-amplitude elastic waves in a model one-dimensional unconsolidated granular medium are investigated. The model medium is represented by a linear chain of 80 steel balls with a diameter of 6.5 mm each, this chain being preliminarily loaded with an external static force F. The elastic properties of the model are analyzed. The theoretical dependences of the coefficients of elasticity of the second, third, and fourth orders on the force F are obtained. The experimental setup is described. The results of studying the nonlinear effects, namely, the higher harmonic generation and the wave generation at combination frequencies, which accompany the acoustic wave propagation in the chain, are presented. For the chain of balls under study, a structural phase transition from the 1D structure to a 2D one is observed with an increase in the external compression force F applied to the balls. The results of the study are analyzed using the Hertz theory of contact interactions. [ABSTRACT FROM AUTHOR]

Monitoring load levels in multi-wire steel strands is relevant to ensuring the proper structural performance of post-tensioned concrete structures, suspension bridges, and cable-stayed bridges. This article investigates the use of ultrasonic nonlinearity as a means to determine the level of load applied to the strands. Since an axial load on a multi-wire strand generates proportional contact stresses between adjacent wires, ultrasonic nonlinearity from the inter-wire contact must be related to the level of axial load. This article shows that the higher harmonic generation of ultrasonic guided waves propagating in individual wires of the strand indeed changes monotonically with the applied load, with smaller higher harmonic amplitudes with increasing load levels. This trend is consistent with known studies on higher harmonic generation from ultrasonic plane waves incident on a contact interface under a changing contact pressure. The article presents experimental studies on free strands and embedded strands, and numerical studies (nonlinear Finite Element Analysis) on free strands. [ABSTRACT FROM AUTHOR]

Abstract: High intensity femtosecond extreme-ultraviolet (XUV) pulse interactions with large xenon clusters have been studied at a wavelength of 38nm. XUV radiation approaching 1011 W/cm2 is produced by high-order-harmonic conversion from a 35fs, near-infrared terawatt laser. Resulting ion spectra show high charge states with a hydrodynamic based energy distribution from cluster explosions. It is concluded that continuum lowering in the created cluster nanoplasma makes single photoionization to the high charge states possible. These experiments look toward high intensity cluster interaction experiments in the x-ray range on the Linac Coherent Light Source (LCLS) under development at SLAC. [Copyright &y& Elsevier]

In this work, the key delamination features of orthotropic CFRP laminates are investigated by the higher harmonic generation (HHG) method based on experimental and numerical approaches. A laser scanning vibrometer (LSV) is adopted for the non-contact experimental setup and an explicit 3D FEM model is proposed to facilitate the numerical simulations of underlying mechanism of HHG in CFRP. The 2D contour images are established to identify the delamination existence and the maximum response amplitude (MRA) is proposed to quantify the delamination condition. The effect of key features of delamination including the size, depth and relative orientation respect to the wave front on the nonlinear response is subsequently studied and found that the approximate location of delamination can be identified by the HHG approach and the MRA of higher harmonics is highly sensitive to the variation of delamination features. In addition, the effect of delamination features on MRA variation is briefly analyzed from the perspective of physical origin of nonlinear response. The work will enhance the detection of damage in composite structures and demonstrates the applicability of HHG technique for decoding the causality and complexity of nonlinear elasticity in delaminated composites and its potential for the comparative evaluation in defect information. [ABSTRACT FROM AUTHOR]

Observations of horizontal and vertical variations in piezometric head in a homogeneous, laboratory aquifer are presented and discussed. The observed fluctuations are induced by a simple harmonic oscillation in the clear water reservoir acting across a sloping boundary. The data qualitatively supports existing theories in that higher harmonics are generated in the active forcing zone and that a significant increase in the inland, asymptotic watertable over height (relative to that found for the vertical boundary case) is observed. The observed overheight is shown to be accurately reproduced by existing small-amplitude perturbation theory. Detailed measurements in the vicinity of the sloping boundary reveal that the signal of generated higher harmonics is strongest near the sand surface and that vertical flows are significant in this region. The aquifer is of finite-depth and is influenced by capillary effects, the experimental data therefore exposes limitations of theories which are based on the assumption of a shallow aquifer free of capillary effects. The dispersive properties of the measured pressure wave in the aquifer are comparable to those found from field observations and likewise do not agree with those predicted by the capillary free, shallow aquifer theory. Although some improvement is obtained, discrepancies between the data and theory persist even when a finite-depth aquifer and capillary effects are considered in the theoretical model. Further sand column experiments eliminate a truncated capillary fringe as a possible contributor to these discrepancies. However, the neglect of horizontal flows in the fringe may have caused the discrepancies. [Copyright &y& Elsevier]

Optical higher harmonic generation, including second harmonic generation and third harmonic generation, leaves no energy deposition to its interacted matters due to an energy-conservation characteristic, providing the “noninvasiveness” nature desirable for biological studies. Combined with its nonlinearity, higher harmonic generation microscopy provides excellent three-dimensional (3D) sectioning capability, offering new insights into the studies of embryonic morphological changes and complex developmental processes. By choosing a laser working in the biological penetration window, here we present a noninvasive in vivo light microscopy with sub-micron 3D resolution and millimeter penetration, utilizing endogenous higher harmonic generation signals in live specimens. Noninvasive imaging was performed in live zebrafish (Danio rerio) embryos. The complex developmental processes within >1-mm-thick zebrafish embryos can be observed in vivo without any treatment. No optical damage was found even with high illumination after long-term observations and the examined embryos all developed normally at least to the larval stage. The excellent 3D resolution of the demonstrated technology allows us to capture the subtle developmental information on the cellular or sub-cellular levels occurring deep inside the live embryos and larvae. This technique can not only provide in vivo observation of the cytoarchitecture dynamics during embryogenesis with submicron resolution and millimeter penetration depth, but would also make strong impact in developmental and structural biology studies. [Copyright &y& Elsevier]

We propose a new method for the derivation of the first-order perturbation theoretic operator solution (free from secular terms) of the generalized anharmonic oscillator. We apply the solution to study the possibilities of observing different non-linear and quantum optical phenomena observable in the (m-1)th-order non-linear medium interacting with coherent light. In particular, we discuss the possibilities of observing higher harmonic generation, squeezing, bunching and anti-bunching of photons. We also report an exact analytic expression for the Aharonov–Anandan non-adiabatic geometric phase for the (m-1)th-order non-linear medium interacting with coherent light. These generalized expressions are then used to study some particular cases of physical interest. The quantum mechanical system in which an intense laser beam interacts with the third-order non-linear non-absorbing medium of inversion symmetry is studied with particular importance. [Copyright &y& Elsevier]

Low efficiency of energy transition is generally considered to be the only weakness of electromagnetic ultrasonic transducers (EMATs). Electromagnetic acoustic resonance (EMAR) technique has been successfully used to overcome this disadvantage with a combination of the EMATs with the ultrasonic resonant method. In this paper, a nonlinear EMAR technique has been proposed to evaluate the thermal damage of metallic materials, which combines the feature of EMAR with the merit of higher harmonic generation that provides an effective indicator to material damage. The use of contactless EMATs can isolate the material nonlinearity and maintain the coupling condition consistently on measurements of higher harmonics generated. EMAR provides high enough signal magnitude for higher harmonic generated. An experimental scheme is proposed and applied to assess the thermal damage in aluminum and nickel plates. In addition, conventional EMAR techniques based on the measure of shear wave velocity and attenuation within a certain frequency range, are also carried out for the specimens. The experimental results show a monotonic relationship between the normalized amplitude of higher harmonic generated and the artificial thermal loading time, while no stable trends are observed by conventional linear EMAR approaches. The results in this paper indicate that nonlinear EMAR technique proposed can be used to assess the thermal damage in both nonferromagnetic and ferromagnetic materials, with improved reliability and sensitivity over linear one. [ABSTRACT FROM AUTHOR]

Implanted biomaterials often fail because they elicit a foreign body response (FBR) and concomitant fibrotic encapsulation. To design clinically relevant interference approaches, it is crucial to first examine the FBR mechanisms. Here, we report the development and validation of infrared-excited nonlinear microscopy to resolve the three-dimensional (3D) organization and fate of 3D-electrospun scaffolds implanted deep into the skin of mice, and the following step-wise FBR process. We observed that immigrating myeloid cells (predominantly macrophages of the M1 type) engaged and became immobilized along the scaffold/tissue interface, before forming multinucleated giant cells. Both macrophages and giant cells locally produced vascular endothelial growth factor (VEGF), which initiated and maintained an immature neovessel network, followed by formation of a dense collagen capsule 2-4 weeks post-implantation. Elimination of the macrophage/giant-cell compartment by clodronate and/or neutralization of VEGF by VEGF Trap significantly diminished giant-cell accumulation, neovascularization and fibrosis. Our findings identify macrophages and giant cells as incendiaries of the fibrotic encapsulation of engrafted biomaterials via VEGF release and neovascularization, and therefore as targets for therapy., Competing Interests: Competing Financial Interests: the authors do not have competing financial interest to declare.