Experimental investigation of brain contusion characteristics and dynamic response in low-age children using an animal model.

INTRODUCTION: Brain contusion is a prevalent traumatic brain injury (TBI) in low-age children, bearing the potential for coma and fatality. Hence, it is imperative to undertake comprehensive research in this field. METHODS: This study employed 4-week-old piglets as surrogates for children and introduced self-designed devices for both free-fall drop impact tests and drop-hammer impact tests. The study explored the characteristics of brain contusion and dynamic responses of brain under these distinct testing conditions. RESULTS: Brain contusions induced by free-fall and drop-hammer conditions both were categorized as the coup injury, except that slight difference in the contusion location was observed, with contusion occurring mainly in the surrounding regions beneath the impact location under free-fall condition and the region just right beneath the impact location under drop-hammer condition. Analysis of impact force and intracranial pressure (ICP) curves indicated similar trends in impact forces under both conditions, yet different trends in ICPs. Further examination of the peak impact forces and ICPs elucidated that, with increasing impact energy, the former followed a combined power and first-order polynomial function, while the latter adhered to a power function. The brain contusion was induced at the height (energy) of 2 m (17.2 J), but not at the heights of 0.4, 0.7, 1, 1.35 and 1.7 m, when the vertex of the piglet head collided with a rigid plate. In the case of a cylindrical rigid hammer (cross-sectional area constituting 40 % of the parietal bone) striking the head, the brain contusion was observed under the energy of 21.9 J, but not under energies of 8.1 J, 12.7 J and 20.3 J. Notably, the incidence of brain contusion was more pronounced under the free-fall condition. CONCLUSIONS: These findings not only facilitate a comprehensive understanding of brain contusion dynamics in pediatric TBIs, but also contribute to the validation of theories and finite element models for piglet heads, which are commonly employed as surrogates for children.

Quadrupole topological phases and filling anomaly in all-dielectric Lieb lattice photonic crystals.

While higher-order photonic topological corner states typically are created in systems with nontrivial bulk dipole polarization, they could also be created in systems with vanishing dipole polarization but with nontrivial quadrupole topology, which though is less explored. In this work, we show that simple all-dielectric photonic crystals in the Lieb lattice can host a topologically nontrivial quadrupole bandgap. Through a combination of symmetry analysis of the eigenmodes and explicit calculations of the Wannier bands and their polarization using the Wilson loop method, we demonstrate that the Lieb photonic crystals can have a bandgap with vanishing dipole polarization but with nontrivial quadrupole topology. The nontrivial bulk quadrupole moment could result in edge-localized polarization and topological corner states in systems with open edges. Interestingly, the indices of the corner states show an unusual "3+1" pattern compared to previously known "2+2" pattern, and this new pattern leads to unusual filling anomaly when the corner states are filled. Our work could not only deepen our understanding about quadrupole topology in simple all-dielectric photonic crystals but could also offer new opportunities for practical applications in integrated photonic devices.

Optical polarization perturbed by shear strains of ultrasonic bulk waves in anisotropic semiconductors: Multiphysics modeling and optoacoustic validation.

Characterization of lattice properties of monocrystalline semiconductors (MS) has been rapidly advanced. Of particular interest is the use of shear strains induced by optoacoustic-bulk-waves. However, this technique has been hindered owing to the lack of quantitative correlations between optoacoustic-bulk-waves-induced shear strains and anisotropic photoelasticity of MS. Motivated by this, a multiphysics model is developed to interrogate the coupling phenomena and interaction between optical polarization and shear strains in MS. With the model, perturbation to the polarization of a monochromatic laser beam, upon interacting with optoacoustic waves in MS, is scrutinized quantitatively. Experimental results are in agreement with those from the model, both revealing the polarization perturbed by shear strains quantitatively depends on the crystal orientation and crystal-structure-related symmetry, which are jointly governed by mechanical/photoelastic/optical anisotropies of MS. The approach has paved a new way for selectively acquiring high-sensitivity shear components of optoacoustic-ultrasonic-waves for in situ, high-definition characterization of anisotropic MS.

Development and global validation of a 1-week-old piglet head finite element model for impact simulations.

PURPOSE: Child head injury under impact scenarios (e.g. falls, vehicle crashes, etc.) is an important topic in the field of injury biomechanics. The head of piglet was commonly used as the surrogate to investigate the biomechanical response and mechanisms of pediatric head injuries because of the similar cellular structures and material properties. However, up to date, piglet head models with accurate geometry and material properties, which have been validated by impact experiments, are seldom. We aim to develop such a model for future research. METHODS: In this study, first, the detailed anatomical structures of the piglet head, including the skull, suture, brain, pia mater, dura mater, cerebrospinal fluid, scalp and soft tissue, were constructed based on CT scans. Then, a structured butterfly method was adopted to mesh the complex geometries of the piglet head to generate high-quality elements and each component was assigned corresponding constitutive material models. Finally, the guided drop tower tests were conducted and the force-time histories were ectracted to validate the piglet head finite element model. RESULTS: Simulations were conducted on the developed finite element model under impact conditions and the simulation results were compared with the experimental data from the guided drop tower tests and the published literature. The average peak force and duration of the guide drop tower test were similar to that of the simulation, with an error below 10%. The inaccuracy was below 20%. The average peak force and duration reported in the literature were comparable to those of the simulation, with the exception of the duration for an impact energy of 11 J. The results showed that the model was capable to capture the response of the pig head. CONCLUSION: This study can provide an effective tool for investigating child head injury mechanisms and protection strategies under impact loading conditions.

Transient non-Hermitian skin effect.

The discovery of non-Hermitian skin effect (NHSE) has opened an exciting direction for unveiling unusual physics and phenomena in non-Hermitian system. Despite notable theoretical breakthroughs, actual observation of NHSE's whole evolvement, however, relies mainly on gain medium to provide amplified mode. It typically impedes the development of simple, robust system. Here, we show that a passive system is fully capable of supporting the observation of the complete evolution picture of NHSE, without the need of any gain medium. With a simple lattice model and acoustic ring resonators, we use complex-frequency excitation to create virtual gain effect, and experimentally demonstrate that exact NHSE can persist in a totally passive system during a quasi-stationary stage. This results in the transient NHSE: passive construction of NHSE in a short time window. Despite the general energy decay, the localization character of skin modes can still be clearly witnessed and successfully exploited. Our findings unveil the importance of excitation in realizing NHSE and paves the way towards studying the peculiar features of non-Hermitian physics with diverse passive platforms.

Machine learning-enabled resolution-lossless tomography for composite structures with a restricted sensing capability.

Construction of a precise ultrasound tomographic image is guaranteed only when the sensor network for signal acquisition is of adequate density. On the other hand, machine learning (ML), as represented by artificial neural network and convolutional neural network (CNN), has emerged as a prevalent data-driven technique to predictively model high-degree complexity and abstraction. A new tomographic imaging approach, facilitated by ML and based on algebraic reconstruction technique (ART), is developed to implement in-situ ultrasound tomography, and monitor the structural health of composites with a restricted sensing capability due to insufficient sensors of the sensor network. The blurry ART images, as the inputs to train a CNN with an encoder-decoder-type architecture, are segmented using convolution and max-pooling to extract defect-modulated image features. The max-unpooling boosts the resolution of ART images with transposed convolution. To validate, a carbon fibre-reinforced polymer laminate is prepared with an implanted piezoresistive sensor network, the sensing capability of which is purposedly restrained. Results demonstrate that the developed approach accurately images artificial anomaly and delamination in the laminate, with inadequate training data from the restricted sensor network for tomographic image construction, and in the meantime it minimizes the false alarm by eliminating image artifacts.

Sustainable-Macromolecule-Assisted Preparation of Cross-linked, Ultralight, Flexible Graphene Aerogel Sensors toward Low-Frequency Strain/Pressure to High-Frequency Vibration Sensing.

Ultralight and highly flexible aerogel sensors, composed of reduced graphene oxide cross-linked by sustainable-macromolecule-derived carbon, are prepared via facile freeze-drying and thermal annealing. The synergistic combination of cross-linked graphene nanosheets and micrometer-sized honeycomb pores gives rise to the exceptional properties of the aerogels, including superior compressibility and resilience, good mechanical strength and durability, satisfactory fire-resistance, and outstanding electromechanical sensing performances. The corresponding aerogel sensors, operated at an ultralow voltage of 0.2 V, can efficiently respond to a wide range of strains (0.1-80%) and pressures (13-2750 Pa) even at temperatures beyond 300 °C. Moreover, the ultrahigh-pressure sensitivity of 10 kPa-1 and excellent sensing stability and durability are accomplished. Strikingly, the aerogel sensors can also sense the vibration signals with ultrahigh frequencies of up to 4000 Hz for >1 000 000 cycles, significantly outperforming those of other sensors. These enable successful demonstration of the exceptional performance of the cross-linked graphene-based biomimetic aerogels for sensitive monitoring of mechanical signals, e.g., acting as wearable devices for monitoring human motions, and for nondestructive monitoring of cracks on engineering structures, showing the great potential of the aerogel sensors as next-generation electronics.

Surface/sub-surface crack-scattered nonlinear rayleigh waves: A full analytical solution based on elastodynamic reciprocity theorem.

High-order harmonics and sub-harmonics that are engendered upon interaction between surface Rayleigh waves and material flaws have been exploited intensively, for characterizing material defects on or near to waveguide surfaces. Nevertheless, theoretical interpretation on underlying physics of defect-induced nonlinear features of Rayleigh waves remains a daunting task, owing to the difficulty in analytically modeling the stress and displacement fields of a Rayleigh wave in the vicinity of defect, in an explicit and accurate manner. In this study, the Rayleigh wave scattered by a surface or a sub-surface micro-crack is scrutinized analytically, and the second harmonic triggered by the clapping and rubbing behaviors of the micro-crack is investigated, based on the elastodynamic reciprocity theorem. With a virtual wave approach, a full analytical, explicit solution to the micro-crack-induced second harmonic wavefield in the propagating Rayleigh wave is ascertained. Proof-of-concept numerical simulation is performed to verify the analytical solution. Quantitative agreement between analytical and numerical results has demonstrated the accuracy of the solution when used to depict a surface/sub-surface crack-perturbed Rayleigh wavefield and to calibrate the crack-induced wave nonlinearity. The analytical modeling and solution advance the use of Rayleigh waves for early awareness and quantitative characterization of embryonic material defects that are on or near to structural surfaces.

Imaging damage in plate waveguides using frequency-domain multiple signal classification (F-MUSIC).

Earlier, an ameliorated MUSIC (Am-MUSIC) algorithm is developed by the authors [1], aimed at expanding conventional MUSIC algorithm from linear array-facilitated nondestructive evaluation to in situ health monitoring with a sparse sensor network. Yet, Am-MUSIC leaves a twofold issue to be improved: i) the signal representation equation is constructed at each pixel across the inspection region, incurring high computational cost; and ii) the algorithm is applicable to monochromatic excitation only, ignoring signal features scattered out of the excitation frequency band which also carry information on structural integrity. With this motivation, a multiple-damage-scattered wavefield model is developed, with which the signal representation equation is constructed in the frequency domain, avoiding computationally expensive pixel-based calculation - referred to as frequency-domain MUSIC (F-MUSIC). F-MUSIC quantifies the orthogonal attributes between the signal subspace and noise subspace inherent in signal representation equation, and generates a full spatial spectrum of the inspected sample to visualize damage. Modeling in the frequency domain endows F-MUSIC with the capacity to fuse rich information scattered in a broad band and therefore enhance imaging precision. Both simulation and experiment are performed to validate F-MUSIC when used for imaging single and multiple sites of damage in an isotropic plate waveguide with a sparse sensor network. Results accentuate that effectiveness of F-MUSIC is not limited by the quantity of damage, and imaging precision is not downgraded due to the use of a highly sparse sensor network - a challenging task for conventional MUSIC algorithm to fulfil.

A reverse time migration-based multistep angular spectrum approach for ultrasonic imaging of specimens with irregular surfaces.

We develop a new ultrasonic imaging framework for non-destructive testing of an immersed specimen featuring an irregular top surface and demonstrate its capability of accurately depicting the lower surfaces of multiple damages hidden in the specimen. Central to the framework is a multistep angular spectrum approach (ASA), via which the forward propagation wavefields of wave sources and backward propagation wavefields of the received wave signals are calculated. Upon applying a zero-lag cross-correlation imaging condition of reverse time migration (RTM) to the obtained forward and backward wavefields, the image of the specimen with an irregular surface can be reconstructed, in which hidden damages, if any and regardless of quantity, are visualized. The effectiveness and accuracy of the framework are examined using numerical simulation, followed with experiment, in both of which multiple side-drilled holes, at different locations in aluminum blocks with various irregular surfaces, are characterized. Results have proven that multiple damages in a specimen with an irregular surface can be individually localized, and the lower surface of each damage can further be imaged accurately, thanks to the RTM-based algorithm in which multiple wave reflections from the specimen bottom are taken into wavefield extrapolation. The proposed imaging approach presents higher computational efficiency, compared to conventional RTM, and enhanced imaging contrast over prevailing total focusing methods.

Development, validation, and application of ligamentous cervical spinal segment C6-C7 of a six-year-old child and an adult.

BACKGROUND AND OBJECTIVE: The cervical spine is one of the primary regions that is easily injured in traffic accidents. Although adult cervical spine finite element models have been widely adopted to investigate the cervical injury, few efforts have been made with respect to the development and application of FE models of the pediatric cervical spine, especially that of a six-year-old child. The objective of this study is to develop and validate high quality cervical spinal segment C6-C7 FE models of a six-year-old child and an adult, and to further investigate the differences of C6-C7 between the child and adult under different loading conditions. METHODS: The cervical spinal segment C6-C7 FE models were developed by a structured multiblock method, and were verified under flexion, extension, axial rotation, and lateral bending conditions. The validated models were used to investigate the differences of C6-C7 between the child and adult under different loading conditions. RESULTS: The global angular displacement of C6-C7, the ligament elongation ratio, and the maximum effective strain of annulus fibrosus of the child were obviously larger than those of the adult under the same loading conditions. Regarding the loading forms, the flexion angular displacement of C6-C7 of the child was obviously larger than those of the extension and lateral bending, while for the adult cervical segment C6-C7, no obvious differences existed. The elongation ratio of different ligaments was highly dependent on the types of loadings. The maximum effective strain of annulus fibrosus under flexion, extension and lateral bending loads occurred at the compressive region of the front, rear, and one compressive lateral side, in which the annulus fibrosus was more susceptible to injury under the lateral bending condition, compared with those of the flexion and extension conditions. CONCLUSIONS: Both the developed child and adult cervical spinal segment C6-C7 FE models exhibited high biofidelity. The responses (angular displacement, the ligament elongation ratio, and the maximum effective strain of annulus fibrosus) of the child and adult are dependent on the loading types, and the responses of the child were obviously larger than those of the adult under the same loading conditions.

Diffuse ultrasonic wave-based structural health monitoring for railway turnouts.

Real-time damage evaluation is a critical step to warrant the integrity of turnout systems in railway industry. Nevertheless, existing structural health monitoring (SHM) approaches, despite their proven effectiveness in laboratory demonstration, are restricted from in-situ implementation in engineering practice. Based upon the continued endeavors of the authors in developing SHM approaches and exploring real world applications, an in-situ SHM approach, exploiting active diffuse ultrasonic waves (DUW) and a benchmark-less method, has been developed and implemented in a marshalling station in China. When trains passing a railway turnout, the train-induced loads on the rail track can lead to the growth of defects in the rail, and such growth disturbs the ultrasound traversing at the defect and gives rise to discrepancies between the DUW signals acquired before and after the train's passage. On this basis, a damage index, making use of the defect growth-induced changes in DUW signals, is proposed to identify the presence of defect. The probability of defect growth induced by the train-related load can be used to assess the severity of the defect. Via an online diagnosis system, conformance tests are implemented in Chengdu North Marshalling Station, in which defects in switch rails are identified and the health status of in-service rail tracks are continuously monitored. The results have demonstrated the effectiveness and reliability of DUW-driven SHM towards real world railway turnout applications.

A tunable bidirectional SH wave transducer based on antiparallel thickness-shear (d15) piezoelectric strips.

Guided wave based defects inspection is very promising in the field of structural health monitoring (SHM) and nondestructive testing (NDT) due to its less dissipation and thus long distance coverage. In comparison with the widely used Lamb waves, shear horizontal (SH) waves are relatively simple but less investigated probably due to the traditional notion that SH waves were usually excited by electromagnetic acoustic transducers (EMAT). In this work, we proposed a tunable method to excite single-mode bidirectional SH waves in plates using antiparallel thickness-shear (d15) piezoelectric strips (APS). The proposed SH wave driving mechanism here is similar to that by using the periodic permanent magnetics (PPM) based EMAT with the period of strips equal to half of the wavelength. Both finite element simulations and experiments were conducted to validate this transducer in excitation of bidirectional SH waves. Results show that the Lamb waves excited by single piezoelectric strip can be suppressed very well. The radiation angle of the excited bidirectional SH wave can be reduced by extending the strip length, increasing the driving frequency or using more strips. Moreover, the APS transducer can selectively excite SH1 wave and suppress the SH0 wave at 174 kHz and 273 kHz in a 10 mm-thick aluminum plate. Considering its simple structure, flexible design and low excitation energy, the APS SH wave transducer is expected to be widely used in near future.

A Spray-on, Nanocomposite-Based Sensor Network for in-Situ Active Structural Health Monitoring.

A new breed of nanocomposite-based spray-on sensor is developed for in-situ active structural health monitoring (SHM). The novel nanocomposite sensor is rigorously designed with graphene as the nanofiller and polyvinylpyrrolidone (PVP) as the matrix, fabricated using a simple spray deposition process. Electrical analysis, as well as morphological characterization of the spray-on sensor, was conducted to investigate percolation characteristic, in which the optimal threshold (~0.91%) of the graphene/PVP sensor was determined. Owing to the uniform and stable conductive network formed by well-dispersed graphene nanosheets in the PVP matrix, the tailor-made spray-on sensor exhibited excellent piezoresistive performance. By virtue of the tunneling effect of the conductive network, the sensor was proven to be capable of perceiving signals of guided ultrasonic waves (GUWs) with ultrahigh frequency up to 500 kHz. Lightweight and flexible, the spray-on nanocomposite sensor demonstrated superior sensitivity, high fidelity, and high signal-to-noise ratio under dynamic strain with ultralow magnitude (of the order of micro-strain) that is comparable with commercial lead zirconate titanate (PZT) wafers. The sensors were further networked to perform damage characterization, and the results indicate significant application potential of the spray-on nanocomposite-based sensor for in-situ active GUW-based SHM.

A high-resolution structural health monitoring system based on SH wave piezoelectric transducers phased array.

Guided wave based structural health monitoring (SHM) has been regarded as an effective tool to detect the early damage in large structures and thus avoid possible catastrophic failure. In recent years, Lamb wave phased array SHM technology had been intensively investigated while the inherent multi-mode and dispersive characteristic of Lamb waves limits its further applications. In comparison, the fundamental shear horizontal (SH0) wave is non-dispersive with uncoupled displacements and thus more promising for defect detection. In this work, we proposed an SH0 wave linear phased array SHM system based on our recently proposed omni-directional SH wave piezoelectric transducer (OSH-PT). Firstly, the working principle of the phased array system was presented and the total focusing method (TFM) was employed for imaging. Then the SH0 wave mode generated by the OSH-PT was confirmed in a defect-free plate. Finally, experiments were carried out to examine the performances of this SHM system. Results showed that the proposed system can detect a through-thickness hole as small as 2 mm in diameter with the location error only about 6.3 mm. Moreover, the proposed phased array system can also detect multi-defects. Due to its low working frequency and thus low attenuation, the proposed phased array system is capable of monitoring large structures. This work will lay the foundations of SH wave based phased array SHM.

Interaction of Lamb Wave Modes with Weak Material Nonlinearity: Generation of Symmetric Zero-Frequency Mode.

The symmetric zero-frequency mode induced by weak material nonlinearity during Lamb wave propagation is explored for the first time. We theoretically confirm that, unlike the second harmonic, phase-velocity matching is not required to generate the zero-frequency mode and its signal is stronger than those of the nonlinear harmonics conventionally used, for example, the second harmonic. Experimental and numerical verifications of this theoretical analysis are conducted for the primary S0 mode wave propagating in an aluminum plate. The existence of a symmetric zero-frequency mode is of great significance, probably triggering a revolutionary progress in the field of non-destructive evaluation and structural health monitoring of the early-stage material nonlinearity based on the ultrasonic Lamb waves.

Structural damage detection using finite element model updating with evolutionary algorithms: a survey.

Structural damage identification based on finite element (FE) model updating has been a research direction of increasing interest over the last decade in the mechanical, civil, aerospace, etc., engineering fields. Various studies have addressed direct, sensitivity-based, probabilistic, statistical, and iterative methods for updating FE models for structural damage identification. In contrast, evolutionary algorithms (EAs) are a type of modern method for FE model updating. Structural damage identification using FE model updating by evolutionary algorithms is an active research focus in progress but lacking a comprehensive survey. In this situation, this study aims to present a review of critical aspects of structural damage identification using evolutionary algorithm-based FE model updating. First, a theoretical background including the structural damage detection problem and the various types of FE model updating approaches is illustrated. Second, the various residuals between dynamic characteristics from FE model and the corresponding physical model, used for constructing the objective function for tracking damage, are summarized. Third, concerns regarding the selection of parameters for FE model updating are investigated. Fourth, the use of evolutionary algorithms to update FE models for damage detection is examined. Fifth, a case study comparing the applications of two single-objective EAs and one multi-objective EA for FE model updating-based damage detection is presented. Finally, possible research directions for utilizing evolutionary algorithm-based FE model updating to solve damage detection problems are recommended. This study should help researchers find crucial points for further exploring theories, methods, and technologies of evolutionary algorithm-based FE model updating for structural damage detection.

Dispersed Sensing Networks in Nano-Engineered Polymer Composites: From Static Strain Measurement to Ultrasonic Wave Acquisition.

Self-sensing capability of composite materials has been the core of intensive research over the years and particularly boosted up by the recent quantum leap in nanotechnology. The capacity of most existing self-sensing approaches is restricted to static strains or low-frequency structural vibration. In this study, a new breed of functionalized epoxy-based composites is developed and fabricated, with a graphene nanoparticle-enriched, dispersed sensing network, whereby to self-perceive broadband elastic disturbance from static strains, through low-frequency vibration to guided waves in an ultrasonic regime. Owing to the dispersed and networked sensing capability, signals can be captured at any desired part of the composites. Experimental validation has demonstrated that the functionalized composites can self-sense strains, outperforming conventional metal foil strain sensors with a significantly enhanced gauge factor and a much broader response bandwidth. Precise and fast self-response of the composites to broadband ultrasonic signals (up to 440 kHz) has revealed that the composite structure itself can serve as ultrasound sensors, comparable to piezoceramic sensors in performance, whereas avoiding the use of bulky cables and wires as used in a piezoceramic sensor network. This study has spotlighted promising potentials of the developed approach to functionalize conventional composites with a self-sensing capability of high-sensitivity yet minimized intrusion to original structures.