This PDF file contains the front matter associated with SPIE Proceedings Volume 11381, including the Title Page, Copyright information, and Table of Contents.

Fiber-reinforced polymer matrix composites have excellent in-plane stiffness and strength properties, and are therefore ideal for usage in panels of aircraft wings or fuselage as well as launch vehicle case segments. Those thin plates or shell structures are often stiffened with many locally increased thickness regions, or beams of various cross-sectional shapes such as flat or T-shaped. Small defects in any of those stiffened regions would greatly reduce the structural performance as a whole. Locating such defects is time consuming because of the large extend of the panels as well as the number of stiffeners. Guided ultrasonic wave-based techniques could be applied for damage detection in large areas. However, the scattering characteristics of stiffeners are complex. In particular, when multiple stiffeners are present, the incident Lamb wave signal is altered with the passing of each stiffener. Thus, the goal of this work is to efficiently model Lamb wave propagation when multiple stiffeners are present, with and without defects, in an effort to identify useful signal features for damage detection. To this end, the so-called global-local method is used for Lamb wave modeling. The global functions are used to represent the nominal composite region – parameters are obtained by means of waveguide finite element (WFE) method – and the stiffened region is represented by finite element discretization. With a recently developed coupling technique, a source problem, representing a surface-mounted transducer is coupled with multiple stiffener-scattering models to examine the transmission characteristics. The global-local model is validated by laboratory waveform measurements on a stiffened composite plate. The results from global-local method can then be used to efficiently determine the maximum number of stiffeners before the transmitted Lamb waves become too weak to identify defects.

Composites have been extensively used in aerospace engineering due to their advantages of light weight, high strength, and engineering design flexibility. Manufacturing defects such as wrinkle and porosity can affect the performance of the composites and may lead to failure in the end, while damage such as delamination, fiber fraction, and matrix cracking can directly cause failure of the composites. In this paper, a Lamb wave based nonintrusive nondestructive evaluation system, which employs piezoelectric transducer for actuation and scanning laser Doppler vibrometer for wavefield sensing, is presented for typical composite defect and damage inspection and evaluation. Two composite panels with different geometry (flat or curved) and with various embedded defects (wrinkle and delamination) are inspected using the nonintrusive Lamb wave system. Both the wrinkles and delamination are detected from the wavefield and approximately quantified through wavefield imaging methods.

Additive manufacturing (AM) requires a new paradigm for quality assurance testing. A nondestructive test setup has been integrated into an AM chamber. A pulsed laser generates Rayleigh waves that are then received by a laser interferometer. Two levels of interrogation are investigated; detection of defects using linear ultrasonic methods and tracking changes in microstructure using nonlinear ultrasonic methods. One of the challenges in AM chambers is surface roughness, which affects both light collection for reception as well as the Rayleigh wave propagation characteristics. This paper describes the laser ultrasonic system, its integration into an AM chamber, and some sample results.

In this paper, we discuss the current status of the development of an elastic wave analyzer for icy sub-surface (EWAIS). We begin with an introductory overview of the necessity to develop an active elastic wave transmitter that will enable reaching distances of kilometer thru ice. It is being developed to be driven by piezoelectric transducers with a bouncing constrained mass that impacts a transmission head. The EWAIS transmits the waves at two distinct frequencies; the primary frequency of the actuator at ~10 kHz for high resolution, and the secondary converted low frequency (LF) in the range of 100 to 500 Hz for long distance transmission. The sonar equation was used for the estimation of required source level, and an impact force sensor is used for the performance measurement of EWAIS prototype.

Combining low weight and high strength, carbon fiber reinforced composites are widely used in the aerospace industry, including for primary aircraft structures. Barely visible impact damage can compromise the structural integrity and potentially lead to failures. Edge Illumination (EI) X-ray Phase Contrast imaging (XPCi) is a novel X-ray imaging technique that uses the phase effects induced by damage to create improved contrast. For a small cross-ply composite specimen with impact damage, damage detection was compared to ultrasonic immersion C-scans. Different defect types could be located and identified, verified from the conventional ultrasonic NDE measurement.

Adhesive junctions or co-infusion of skin and stiffeners represent efficient manufacturing processes for aircrafts composites stiffened panels leading to weight saving, although they have not been widely adopted yet due to certification issues and the lack of well-established design tools and procedures. Airworthiness requirements for composite structures pose major challenges to the certification of adhesively bonded or co-infused stiffened structures. FAA Advisory Circular 20-107B prescribes the methods for substantiating the limit load capacity of any bonded stiffener, the failure of which would result in catastrophic loss of the airplane. Today, composites primary structures that work mostly under compressive loads are designed following the no-buckling criteria up to Ultimate Load. Such a design approach leads to stiffer and heavier structures if compared to letting the compressed skins work in post-buckling until failure. In order to exploit the full structural potentiality of this type of structures under compressive loads new design approaches, mostly based on Finite Element Modelling, have to be developed and validated with experimental results to correctly predict the nonlinear mechanisms of load absorption beyond skin buckling onset. Furthermore state-of-the art Non-Destructive- Techniques and Structural Health Monitoring Systems can be employed for a continuous monitoring of the joints health status. In this context, the joining technique of the stringers to the skin has a particular importance; indeed, although different joining processes barely influence the linear behavior of a stiffened plate until its first instability load, they are responsible for relevant differences in the ultimate failure load. This paper presents numerical and experimental activities carried out to study the behavior of compressed stiffened plates obtained by different manufacturing processes as well as monitoring techniques of the health status of the panels by classical NDT and guided waves based SHM systems. The numerical problem has been modelled with approaches of increasing complexity, from “classical” FE models to predict the first buckling load, to post-buckling analyses up to more refined techniques including the behavior of the skin-stiffener interface.

Guided wave-based damage evaluation has been regarded as a promising method in the area of structural health monitoring. The main obstacle for the practical application of these guided wave-based monitoring methods is the reliability of damage evaluation under time-varying ambient conditions. In this paper, an analytical model and a semi-analytical finite element (SAFE) method are proposed to study the effect of temperature and load on guided wave propagation in an isotropic plate respectively including the dispersion curves and waveform. In the presented models, the temperature and load dependent elastic constants are considered to study the variations of guided wave properties. The result shows that the phase velocity gradually decreases with the incremental temperature. It is also observed that the phase velocity gradually decreases with the incremental load. Finally, the analytical model and SAFE method are validated through the experimental data. It shows that the results obtained from the theoretical model match well with the experimental results.

The elastic property identification of composite materials is of great interest for the conditional monitoring of lightweight structures, such as modern aircrafts (e.g., B737, A380, etc…). Because of the uncertainties in manufacturing processes for composite materials, it is important to develop a way to measure the properties of the final part in a manner that is non-destructive and can be applied in-situ on an existing structure. The first part of this paper examines the possibility of property characterization of quasi-isotropic multi-layered composite laminates using pseudo-experimental data (phase velocity dispersion curves with true elastic constants). An inversion scheme is proposed by minimizing the discrepancy between phase velocity dispersion curves for the ‘true’ elastic constants and those for “trial” constants, determined from the Semi-Analytical Finite Element (SAFE) method. Of interest are the three fundamental Lamb modes (the axial mode -S₀, the flexural mode- A₀, and the shear horizontal mode -SH₀). A simulated annealing optimization (SA) is exploited to fast locate the global optimum with the smallest discrepancy. The second part of this paper implements data-driven matched field processing to locate the defects on the same type of composite with complicated geometry, including skin and stringer. Using a full matrix data acquisition system, two defect locations are investigated, one is on the skin, the other is the stringer.

Guided ultrasonic wave-based methods are promising for structural health monitoring of isotropic and composite materials and structures. The technology has seen a lot of attention in the research community over the past decades, and many analytical and numerical methods have been developed to describe different aspects of guided wave propagation and scattering phenomena as well as damage detection. However, very little research was geared towards the influence of the uncertainty in the material properties for the calculation of the dispersion curves. The lack of knowledge of the exact material properties together with manufacturing tolerances could lead to erroneous conclusions. Hence, in this study, an uncertainty analysis for the material properties of fiber-reinforced composites is conducted to quantify the effect of uncertain material constants on the dispersion curves. A fuzzy arithmetical approach based on the Transformation Method is used to generate the dispersion curves with uncertain parameters in conjunction with a root-finding algorithm. The uncertain parameters are modeled as linear fuzzy numbers. Using triangular membership functions, both the nominal value and the worst-case interval are adequately combined into one fuzzy number. Furthermore, it is shown that the measure of influence for the uncertain material parameters on the group velocities of the considered Lamb waves is not equally weighted. These findings might allow for the development of efficient, nondestructive material characterization techniques in the future.

During the last couple of decades, ultrasonic guided waves have been shown to be increasingly capable of interrogating long bones in the human skeleton in order to characterize osteoporosis. Their diagnostic role is promising as the established gold-standard diagnostic techniques of dual-energy x-ray absorptiometry (DXA) and quantitative computed tomography (QCT) do not provide information about the material properties. Ultrasonic guided waves can provide information about the material properties as well as the geometry (i.e., cortical bone thickness) and cracks. Wave propagation in cortical bone is much different than in soft tissue. Likewise, there are similarities and differences between wave propagation in bone and mechanical components such as pipes and plates. While steel pipes and plates typically are homogeneous, prismatic, isotropic, uniform thickness, and essentially lossless; long bones are heterogeneous, non-prismatic, anisotropic, variable thickness, and very lossy. Thus, guided wave propagation in long bones is quite complicated, and yet it is not uncommon to use Lamb wave propagation as a surrogate for wave propagation in long bones. The aim of this work is to compare and contrast wave propagation in a long bone with that in a plate to point out where the surrogate Lamb wave analog is useful and where it is not. The semi-analytical finite element (SAFE) method is used to obtain dispersion curves for a cross-section of the plate, and mid-diaphyseal cross-section of the tibial cortex. The frequency domain finite element (FDFE) method is used to account for the non-prismatic nature of the bone and damping.

The way in which objects interact with various fields often allows us to detect their presence. Thus, imaging techniques based on reflections, echoes and shadows, as well as any form of scattering and localization more generally, inform us on the location, shape and composition of an object even if buried deep under thick layers of materials. Conversely, to hide an object, we say to “cloak” it, we must suppress any interaction it might have with probing fields. Care must of course be taken so that the cloaking device itself is cloaked as well. In this talk, we show how to cloak solids against stress waves.

Carbon fiber reinforced polymer (CFRP) composites have been widely used in aerospace structures due to their high specific strength and stiffness, resistance to corrosion, and lightweight. However, it has posed new challenges for implementing guided wave-based structural health monitoring (SHM) techniques due to the general anisotropic behavior and complicated wave-damage interaction scenarios in composites. This paper presents a new methodology for detecting various types of composite damage such as simulated delamination and actual impact damage using pure shear horizontal (SH) wave generated by adjustable angle beam transducers. For the first time, angle beam transducers were successfully applied to excite pure SH0 wave in a 2-mm quasi-isotropic composite plate. The pure SH0 wave excitation was verified by a three-dimensional (3D) finite element (FE) simulation. SH0 wave propagation and interaction with the delamination were further investigated numerically and strong trapped waves within the delamination region were observed. Experimental validations were conducted to detect simulated delamination by Teflon insert using pure SH0 wave. A good match between the FE simulation and the experiment was achieved. Pure SH0 wave was also utilized to detect actual impact damage in the quasi-isotropic CFRP composite plate. It can be found that the SH0 wave is sensitive to both delamination and impact damage, and a significant amplitude drop is observed due to the presence of different types of damage. Both numerical and experimental results demonstrated the effectiveness of pure SH0-wave detection of various damage types in composites using angle beam transducers.

Carbon fiber reinforced composites are widely used in the aerospace industry, but barely visible impact damage can lead to delamination and compromise the structural integrity. The scattering of the fundamental anti-symmetric guided wave mode (A₀ Lamb mode) at an artificial circular delamination in a quasi-isotropic laminate was investigated experimentally. A 5 cycle Hanning windowed wave pulse was used as the excitation signal for the experiments. Fast Fourier Transform was employed to identify the guided wave amplitude of the scattered field along various directions. The experimental wavefield was captured using a laser Doppler vibrometer. Experimental results are presented for the scattering pattern and scattering amplitude as a function of distance from the damage. The results of this study can help to improve delamination detection techniques using guided waves and to gain physical insights into the scattering of guided waves at a delamination.

Due to the extensive use of composite materials in aerospace structures, they are highly susceptible to various types of damage including barely visible impact damage (BVID) due to low-velocity impact events such as bird strikes and runway debris. Implementation of guided-wave based structural health monitoring (SHM) for BVID detection in composites has been extremely challenging due to the anisotropic nature of composites and the complex wave-damage interaction with impact damage. This paper presents a new methodology for the detection of BVID in composites using selectively pure shear horizontal (SH) guided wave mode generated by angle beam transducers (ABT) and received by phased array transducers (PAT). In this methodology, pure SH0 wave excitation was achieved using the angle beam transducers and variable angle wedges and after propagation in the composite it was received by the phased array transducer. First, the semi-analytical finite element (SAFE) method was used to generate the guided wave dispersion curves and the corresponding tuning angle for the SH0 mode excitation was obtained based on Snell’s law. Then, pitch-catch experiments using the ABT-PAT transducer pair were conducted to validate the pure SH0 mode excitation. After this, impact damage experiments were conducted on multiple quasi-isotropic carbon fiber reinforced polymer (CFRP) composite plates having different thicknesses by conforming to the ASTM D7136 standard to generate controlled impact damage size of 1". Next, impact damage detection experiments using pure SH0 mode at 500 kHz, were conducted using the ABT-PAT transducer pair. It was observed that pure SH0 mode was sensitive to the impact damage as a significant decrease in the signal amplitude and mode conversion was observed. With our experiments, we demonstrated the validity and usefulness of this technique for impact damage detection in composite structures. An invention disclosure describing the use of angle beam transducers for SH wave excitation method has been filed and is in the process of becoming a provisional patent.

Typically, material properties originate from destructive tensile tests and are used in computational models in the design and analysis process of structures. This approach is well-established in relation to isotropic homogeneous materials. However, if this approach is used for composite laminates, inaccuracies can arise that lead to vastly di erent stress distributions, strain rates, natural frequencies, and velocities of propagating elastic waves. In order to account for this problem, the alternative method is proposed, which utilizes Lamb wave propagation phenomenon and optimization techniques. Propagating Lamb waves are highly sensitive to changes in material parameters and are often used for structural health monitoring. In the proposed approach, the elastic constants, which are utilized to determine dispersion curves of Lamb waves, are optimized to achieve a good correlation between model predictions and experimental observations. In the first step of this concept, parametric studies have been carried out in which the influence of mass density, Young's modulus, Poisson's ratio of reinforcing fibers as well as a matrix of composite laminate and volume fraction of reinforcing fibers on dispersion curves of Lamb waves was investigated. The dispersion curves of Lamb waves were calculated by using the semi- analytical spectral element method considering the variability of properties of composite constituents. The resulting dispersion curves were also compared with experimental measurements of full wavefield data conducted by scanning laser Doppler vibrometer and processed by 3D Fourier transform. It allowed formulating fitness function which will be next used in a genetic algorithm for optimization and identification of elastic constants.

We investigate the extreme wave event in mechanical metamaterials composed of Triangulated Cylindrical Origami (TCO). Specifically, we focus on the realization of so-called rogue waves by employing a homogeneous one-dimensional chain constructed from the TCO unit cells. Our numerical simulation shows the significant energy focusing in the very limited number of unit cells. The wave localization behavior varies as a function of the geometrical parameters of the TCO, which enables us to manipulate the wave localization. The configurability of the wave localization using the TCO can be leveraged for energy harvesting purposes in engineering applications.

Inspired by the presence of a Dirac cone in the band structure of graphene, researches have studied and produced phononic crystals with Dirac-like behaviors. Using Dirac physics, researchers have developed topological systems, zero-index materials, and one-way wave propagation. However, the interesting phenomena produced by these structures produce narrow band results and are difficult to physically implement. Using a reconfigurable origami structure, this research aims to illuminate the effects of phononic lattice perturbation on lattice degeneracies.

We investigate the impact behavior of the Triangulated Cylindrical Origami (TCO) architecture to determine if it can effectively protect a payload dropped from a height. The TCO architecture inherently exhibits coupled longitudinal and rotational motions. TCO is highly tunable and can offer monostable or bistable characteristics based on its initial geometric configurations such as the height and rotational angle. When monostable TCO unit cells are combined in a chain, they can exhibit interesting rarefaction behavior under impact. Specifically, if one end of the chain is impacted, the initial compressive wave can be overtaken by a tensile wave, such that the other end of the chain can feel tension instead of compression in a counter-intuitive manner. In this study, we begin by designing monostable TCO unit cells that are numerically shown to exhibit rarefaction behavior when constructed in chains. Then, we fabricate plastic TCO unit cell prototypes and apply static compression to these prototypes to verify their monostability and to determine their mechanical properties. These unit cells are then organized in a chain of multiple unit cells that are connected mechanically. The chain is then tested dynamically by dropping it using a custom-made drop tower apparatus. We measure the impact felt by the TCO system by using accelerometers and digital image correlation systems. We find that this origami-based system offers a tunable way to mitigate impact applied to the proof mass, showing great potential as a novel payload impact mitigation system for space applications.

Blood pressure is recognized as an important sign for managing hypertension and cardiovascular diseases. It is thus important and essential to continuously monitor blood pressure in our daily life. In this study, a novel flexible piezo-composite ultrasonic transducer is reported for continuous blood pressure monitoring in radial artery. The flexible transducer is theoretical designed and fabricated by using PZT-5H/PDMS 1-3 composite. The performance of blood pressure monitoring is characterized, which is based on detecting blood vessel diameter variations in the cardiac cycle.

In earlier work, our research group developed a technology to sort microparticles in a microfluidic system fabricated within a rigid polymethyl methacrylate (PMMA, acrylic) prism, sandwiched by lead-zirconium-titanate (PZT) wafers that operate in through-thickness mode (with inertial backing) to produce standing bulk waves. The overall configuration is compact, solid, and mechanically robust. The “tilted angle” geometry (orienting the microfluidic channel skew with respect to the standing wave node lines) enables robust separation, whereby particles of different sizes can separate by multiples of the node line spacing; the microparticles display undulating trajectories, where deviation from the straight path increases with particle diameter and excitation voltage. Our earlier work involved particles with large size differences, exemplified by separating 2 μm particles from 15 μm particles, or by separating red blood cells from white blood cells. In this paper we describe our recent results separating 2 μm particles (polystyrene spheres) from 6 μm particles. We describe challenges faced by researchers working with these experimental systems, including the following examples: internal reflections can produce standing wave nodes additional to those predicted by a simple model; particles are observed through a microscope, but the limited depth-of-field places some particles in focus and others out of focus; the conditions in the microfluidic channel vary over relatively short time intervals, including thermal variations produced by the excitation process; and, there exist tradeoffs in choosing an acoustic couplant between the PZT wafers and the PMMA prism. Nonetheless, we show effective separation of microparticles of different sizes.

In this work, we theoretically design and numerically realize topological pumpings in 2D and 3D spatially modulated acoustic metamaterials. By adiabatically changing the pumping parameters along the synthetic dimension, we observe topologically protected states, which are shown to be pumped from one edge to the other in 2D materials, and from edge to edge and corner to corner in 3D materials. This spatially modulated metamaterials could pave the way for acoustic topological wave trapping and manipulation and serve as a platform for the study of higher-dimensional topological effects among different physical systems.

Unlike conventional elastic waveguides, topologically protected wave transmission in topological metamaterials is immune to backscattering and localization from lattice imperfections and sharp corners. Topologically protected waveguides can be formed by breaking space inversion symmetry within the unit cell of a hexagonal lattice, creating an elastic realization of the quantum valley Hall effect. Recent studies have demonstrated the achievement of tunable topological edge states through the application of an external bias, such as a mechanical, thermal, or magnetic load. These initial studies demonstrate the capability to modify topological edge states through oftentimes complex realizations of truss-like lattice structures or external stimuli. However, a comprehensive reconfigurable topological metamaterial that enables real-time adaptation of both frequency and spatial characteristics of topological properties in an easily integrable manner has yet to be developed. Thus, to advance the state of the art, this research introduces an electromechanical metamaterial with the capability to adjust the frequency range for topological edge states and instantaneously create or eliminate topological interfaces through the integration of piezoelectric circuitry with a continuous mechanical substrate. The metamaterial is comprised of inductor circuitry connected to a thin piezoelectric plate in a periodic manner which produces a hexagonal lattice pattern of electromechanical resonators. The plane wave expansion method is used to reveal a tunable Dirac cone in the band structure of the lattice unit cell and indicate how perturbations to the circuit inductance can open topologically distinct bandgaps. Numerical simulations identify edge modes located at frequencies within the topological bandgap and demonstrate adaptive topologically protected elastic wave transmission.

Acoustic metamaterial consisting of an array Helmholtz resonators has been revealed to have the band gap phenomenon which implies a great potential for noise reduction. However, the application of the conventional acoustic metamaterial is often limited by the narrow band gap. In this paper, an acoustic metamaterial containing a series of membrane-coupled Helmholtz resonators is proposed to produce mult iple band gaps in the low-frequency regime for achieving broadband noise reduction. First, a theoretical model is developed to describe the dynamic of the membrane-coupled Helmholtz resonator system. The membrane is assumed to be loaded with a small mass at the centre for tuning its natural frequencies, thus the resonances of the coupled system and the band gaps of the proposed acoustic metamaterial. Subsequently, based on the theory of acoustic wave propagation together with the derived theoretical model for the membrane-coupled Helmholtz resonator system, a theoretical analysis of the proposed acoustic metamaterial is performed. Using the Bloch’s theorem, the dispersion relation of the proposed acoustic metamaterial is derived. Multiple band gaps are observed in the band structure. A corresponding finite element model of the proposed metamaterial is built to confirm the predictions from the theoretical analysis.

In this talk, I will present our practical design of a three-dimensional (3D) acoustic DZIM, which is made of a cubic lattice of metal rods. We demonstrate a four-fold degenerate point with conical dispersion can be induced at the Brillouin zone center, at which the effective mass density and compressibility simultaneously turning zero. To demonstrate the extraordinary functionalities of the DZIM, we have fabricated an acoustic "periscope" with two 90° turns to achieve almost perfect tunneling of a normally incident plane wave. The experimental results agree well with our theoretical predictions and numerical simulations.

We investigate combinatorial search and optimizations on tessellations of Tachi-Miura Polyhedron (TMP), origami-based auxetic mechanical metamaterials composed of bellows-like structures. One of the unique features of TMP is that TMP tessellations can achieve various rigid foldable configurations out of one same design of a TMP cell. In this study, we search for the possible combinations of TMP cells by implementing an efficient search algorithm based on graph theory. This combinatorial search method can be exploited to design programmable structures that are adaptive to varying situations of the outer environment.

Considering the diminishing dimensions of modern silicon wafers (50-100 μm thick and their coating layers are in an nm scale) and potential defect sizes, spatial resolutions for the corresponding non-destructive evaluation (NDE) solutions should be in the order of sub-μm or nm. However, the spatial resolutions of current NDE techniques are often in the range of μm or mm. In this study, an ultrafast ultrasonic measurement system is developed using a femtosecond pulse laser. The proposed ultrafast ultrasonic measurement system can generate ultrasonic waves up to several THz (10¹² Hz), and measure the corresponding responses with a sampling rate up to PHz (10¹⁵ Hz). In this system, the femtosecond pulse laser beam is split into pump and probe pulses for a pulse-echo ultrasonic measurement. The pump pulse produces ultrafast ultrasound onto the target silicon wafer, and the ultrasound travels through the thickness direction. Then, the waves reflected from the coating layer are measured using an optically delayed probe pulse. Using the proposed system, the coating thicknesses of silicon wafers in the range of 50 nm to 200 nm were successfully estimated.

Acoustic source localization (ASL) technique is an important step for structural health monitoring (SHM). ASL in three dimensional (3D) structures is more challenging. The 3D acoustic source localization technology not only has important significance for the non-destructive monitoring of large-scale 3D structures, but also is indispensable for the spatial sound source localization problems. More unknown parameters, large number of sensors, limited known properties of the 3D structure and complex nonlinear equations greatly hinder the application and development of 3D acoustic source localization. Besides, when the acoustic signal propagates through different media in a 3D heterogeneous structure, it is refracted at the interfaces following Snell's law which makes the ASL in heterogeneous structures even more difficult. In this paper, the basic theoretical research on the acoustic source localization in 3D heterogeneous structures is carried out considering the refraction effect. A localization technique is first proposed based on the triangular pyramid sensor cluster using only the time difference of arrival (TDOA) and the location information of clusters. This technique can predict the velocity and acoustic source location with a relatively small number of sensors. A 3D finite element based numerical model of a heterogeneous structure made of two materials was analyzed to verify the proposed acoustic source localization technique. The results show the reliability of the proposed technique.

Inspired by the optical and acoustic non-Hermitian 𝒫𝒯 symmetric system, a 𝒫𝒯 symmetric metamaterial beam for flexural waves is proposed here, based on shunted piezoelectric patches. Positive and negative shunting resistances are the key to constructing the balanced loss and gain components. The associated asymmetric flexural wave scattering, 𝒫𝒯 phase transition and exceptional points are then investigated in both analytical and numerical ways. In addition, properties of exceptional points are tunable in our proposed system, simply owing to the variable shunting parameters. Our design may contribute to asymmetric wave control, enhanced sensing, amplification, and localization of flexural waves.

Featuring hundreds of craters, cracks and diverse microscopic defects disorderedly scattered over a wide region, the pitting damage in a typical Whipple shield of spacecraft induces highly complex wave scattering. Due to the dispersive and multimode natures, only nonlinear ultrasonic waves (NUWs) with exact phase-velocity matching condition are generally used to evaluate the microstructural material deterioration. Targeting accurate, holistic evaluation of pitting damage, semi-analytical finite element (SAFE) approach is adopted to identify the internal resonant conditions and to select an efficient mode pair for characterizing pitting damage. To explore the feasibility of pitting damage evaluation by using the selected mode pair and fully utilize its unique merits, the cumulative effect of second harmonics is analyzed using numerical simulations and corroborated by experiment. Regardless of the selection of mode pair (i.e., S1-s2 and S0-s0), the amplitude of second harmonics obtained in the pitted plate is observed to increase significantly after the probing GUWs traverse the pitted region, upon interacting with the pitting damage. This phenomenon is remarkable particularly when the probing GUW does not satisfy the requirement of internal resonance. The mode pairs S0-s0 with different degrees of phase-velocity mismatching are further analyzed. Results show that the hypervelocity impact-induced pitting damage in the rear wall of Whipple shields can be detected accurately using the mode pair S0-s0, and a relatively higher excitation frequency is preferred due to its higher degree of phase-velocity mismatching, leading to standing out of the pitting damage-induced CAN.

The paper presents the experimental evaluation of the intrinsic nonlinearity of macroscopically pristine rail specimens using Rayleigh waves. A second harmonic is produced in the wave as a result of lattice-anharmonicity of Rail. The spectral amplitudes of the fundamental and generated second harmonics are then used to calculate the intrinsic nonlinearity of rail using a new amplitude-based nonlinearity parameter. The material nonlinearity of the rail steel evaluated using the experimental measurements and amplitude-based parameter are further compared with the nonlinear elasticity equations. The close agreement of the results shows the effectiveness of the proposed method in measuring the intrinsic nonlinearity of the rail steel.

Piezoelectric materials provide a flexible way to control effective material properties in both space and time domain. In this work, the band structures of guided waves in a periodically piezoelectric composite plate with a mirror plane in thickness direction are studied by using the supercell plane wave expansion method. The symmetric and anti-symmetric Lamb modes are uncoupled and can be separated because of the symmetry of geometry and material properties. The guided waves including symmetric Lamb modes, anti-symmetric Lamb modes and shear horizontal modes are calculated, respectively. Results show that the band structure for each mode can be changed drastically by the position and material properties of the scatterers. Moreover, the material properties modulated by periodical piecewise structure in both space and time domain are discussed to investigate one-way wave propagation in such spatiotemporal phononic crystal waveguides theoretically. After the calculation by using finite element method, the displacement fields are conducted by two-dimensional Fourier transform to obtain numerical dispersion curves, which shows excellent agreement with theoretical results. This work provides a powerful tool to analyze the guided waves with complex modes in piezoelectric phononic crystal plate, and it has extensive potential applications in engineering structure design for modulating wave propagation.

The effective damage detection does not only require the location of damage but also the size. When a point or circular damage expands in size under stress state, it becomes thin and elongated to transform into a thin rectangular or elliptical slot. The methods introduced in this research use piezoelectric transducers (led zirconate titanate, PZT) to excite and sense the Lamb waves in composite laminate, which are highly sensitive to transverse cracks. The PZT transducers are arranged as a network of cells, and damage is estimated inside each cell separately. The Gabor wavelet-based continuous wavelet transformation (CWT) is used to process the complicated damage-scattered signals. The concept of centroid is used to locate the damage in two stages: the first stage locates the damaged subcell, and then the damage is accurately located in two-stage detection inside the damaged subcell. Based on accurately estimated location, the damage size is calculated using cubic spline curve (CSC) and elliptical parametric (EP) methods. These methods utilize the same arrangement of transducers, and the same data processing technique at all detection stages. The results indicate that EP method could quantify the damage size with more accuracy than the CSC method..

The potential of topological data analysis (TDA) to aid acoustic emission (AE) in revealing early signs of corrosion in prestressed concrete has recently been demonstrated by the authors. This paper serves to extend the experimental investigation of this structural health monitoring potential. The topological method was evaluated in accelerated corrosion testing of control and weathered prestressed concrete specimens. The results highlight the potential of TDA to aid in extracting corrosion information from AE data. Further, with TDA aiding traditional AE monitoring, there is potential for early and reliable indication of concrete cracking, prior to the appearance of external visual signs. In addition, the results demonstrate the potential generalizability of the method toward existing in-service prestressed concrete structures. Lastly, the AE-based corrosion indicators are combined with a hidden Markov modeling framework. The capability of the framework for automated corrosion diagnostics is demonstrated through training and testing between the two specimens.

In the last two decades, nonlinear ultrasonic testing is getting more attention due to their sensitivity to microcracks among a variety of NDT techniques used in infrastructure. Vibro-Acoustic Modulation (VAM) technique is one of the practical methods, that does not need the expensive hardware components required for the conventional nonlinear methods. This method is capable of identifying damage growth using the correlation of the level of nonlinearity to the severity or density of the damage. To be able to determine the sensitivity of VAM technique in comparison with other conventional nondestructive testing methods, Acoustic Emission (AE) technique as a global method and Ultrasonic Testing (UT) and Eddy current Testing (ET) techniques as local methods are investigated in an identical testing condition for similar specimens. The comparison has been conducted by testing a typical steel material used in the steel bridges under cyclic tension load. All these methods have some features in common and some differences. A comprehensive comparison study of these techniques sheds light on their practicality for various applications. Unlike the AE technique that listens to the structure for the received signal of the released elastic energy from the defects, VAM introduces the signals to the specimen and monitors the signal that was modulated by the vibration to get information about the crack. VAM and AE have some similarities such as no need for positioning sensors on the cracks and capability of detecting the crack in the early stages. On the other hand, local techniques such as UT and ET are more accurate than the VAM technique in terms of localization but less sensitive in terms of how soon they detect cracks.

Cracks do exist in a concrete building, and the reasons for cracks can be attributed to grouting, force actions, and concrete properties. Detecting existing cracks and monitoring their growth conditions is an important issue in structure health monitoring. This study proposed an integrated approach to fuse the thermal infrared images, high-resolution image, and acoustic tracking. Thermal infrared images can be used to record the surface temperatures. Those defect areas usually have their surface temperatures different from their neighborhoods. Those surface temperature differences can be an important clue to identify the defects. From the high-resolution images, cracks do occur at those discontinuities, and those discontinuities can be treated as the boundaries of the segmented regions. In this study, an approach based on considering the distributions of the segmented regions to segment the given high-resolution images to locate the boundaries of the segmented regions. Then, the defect map is generated by overlaying the thermal infrared image on the segmented regions such that the map can reveal the defect locations, and the surface temperatures can be another evidence to show the defects. Acoustic tracking is introduced to verify the results. All the defect information is stored in a 3D model such that a defect model can be established.

The method proposed to perform the parametric identification in this paper is the frequency-domain recursive hybrid genetic algorithm. For this method, time histories of the measurement were divided into a series of time intervals, and then the model of equivalent linear system was employed to identify the modal parameters of the system for each time interval. This method was applied to the three-story steel frame structures with and without circular added-damping-andstiffness devices mounted on the shaking table. The ground motion records used for these structures are time histories of El Centro earthquake adjusted to different intensities. According to the result of identified parameters using the above records, the conclusions can be made as follows: When the PGA value becomes larger: (1) the error index will become larger. (2) the range of the identified modal damping ratios and modal frequencies will become larger. (3) The maximum values of three modal damping ratios will become larger. (4) The minimum value of three modal frequencies will become smaller.

Applications of motion magnification has been seen as an effective way to extract pertinent structural health monitoring data without the use of instrumentation. In particular, phase-based motion magnification (PMM) has been adopted to amplify subtle motions that cannot be seen clearly without further processing. For large infrastructure, this tool can be helpful in identifying the dynamic range of motion and modal frequencies. The use of accelerometers poses a problem for structures that contain large geometry, due to the complexities that arise when attempting to setup a modal test. Optically, one can identify singular points or regions of interest that capture a large range of motion for a structure. These regions of interest ultimately provide the dynamic information that is needed to perform structural health monitoring (SHM) of a complex system. This paper aims to identify a shift in frequency and operational deflection shapes due to varying loading scenarios while using PMM. The ability to capture multiple points without being limited by a data acquisition system permits further analysis of structural health. For example, the ability to apply varying loading scenarios can provide warnings as to how a frequency shifts while sustaining a particular force. Due to the plethora of loading conditions, the variation in external loading makes SHM a more conclusive process. For instance, it was applied many different scenarios for loading conditions and damages to observe the shifts in the frequencies due to each factor. It was also done testing with different sensing techniques and with traditional sensing to verify the reliability of PMM. The tests were done in laboratory structures and in real structures to prove the applicability of PMM and to verify what information is needed to identify damage in the structure.

The present study is based on performing 3D vibrations studies of large structures using digital image correlation (DIC) technique. A 3D-DIC setup is developed for measuring in-plane and out-of-plane 3D displacements as well as natural frequencies of a vibrating structures (small to large scale). The DIC setup is first validated with laboratory experiments. The first indoor lab experiment is based on the application of the developed DIC setup in measuring pure in-plane and out-of-plane displacements. The in-plane and out-of-plane translations have been carried using a micrometer and a Vernier caliper, respectively. The developed DIC setup is able to accurately measure these in-plane and out-of-plane static displacements. With this success, in another indoor experiment, the developed DIC setup is used for performing vibration study of a PVC pipe (length 1m, inner diameter 56.6mm, outer diameter 60.6mm). The DIC setup is able to measure the dynamic displacements of the pipe in all three axes. Apart from this, the natural frequency measurement is also accurate. The first fundamental frequency of the pipe is 4.167 Hz. After successful validation and application in the indoor experiments, the developed DIC method is used for a field experiment. In the field experiment, the DIC technique is applied to perform vibration study of a light pole (length 3m, diameter 75.5mm, made of iron). The cameras are placed at a distance of 14.6m from the pole, whereas the distance between the cameras is 8.5m. A large size calibration board is fabricated and used for calibrating the stereovision system. The developed DIC method is able to produce 3D displacements of the vibrating pole as well as accurately measure its natural frequency. The fundamental natural frequency of the light pole is 4.9Hz. An accelerometer was also used during the field experiment for the validation of the developed DIC-based non-contact 3D vibration testing method.

In the United States, buildings’ heating, ventilation, and air conditioning (HVAC) systems account for approximately 40% of the global energy consumption, and contribute between 40% and 48% of the CO2 emissions. One of the largest causes for this type of energy consumption is heat loss. Energy loss can be attributed to inefficiency and sub-surface defects in the structure that reduce the thermal resistance and facilitate unwanted heat transmission within the environment. In recent years, infrared (IR) thermography has proven to be an excellent tool for detecting heat loss from a building envelope. At the same time, advancements in computer vision systems have enabled the use of Structure from Motion (SfM) as a photogrammetry technique. SfM is used for the generation of three-dimensional (3D) building information modeling (BIM). It has the potential to facilitate remote visual inspection of large-scale structures and infrastructure systems by creating virtual renderings. The results of an experiment performed on a lab-scale building using both IR and SfM techniques is analyzed. In particular, the accuracy of SfM in generating a 3D virtual rendering of a target object by using images acquired from a low-cost IR camera is discussed. IR images of a building are recorded and are used for reconstructing a digital model of the structure. The goal is to characterize the viability of SfM in detecting sub-surface defects that can yield to significant heat loss and reduce thermal efficiency in the façade of the building.

Glass fiber reinforced polymer (GFRP) materials are recently widely applied in marine industry, especially for hulls structures. The high popularity of composites and safety requirements result in development of Structural Health Monitoring (SHM) systems. One of the sensors types that can be permanently integrated with a composite structure and used as a part of SHM system are fiber Bragg grating (FBG) sensors. FBG sensors advantages like small size, high corrosion resistance, and absence of electric current are especially important in marine applications. The goal of the paper is to present FBG sensors application to SHM of a composite fast patrol boat. An array of the sensors was embedded into a boat hull structure. Moreover, accelerometer was utilized to determined maximum accelerations during the power boat operations. The analyses were performed during normal exploitation of the boat performed on the Gulf of Gdansk (southeastern bay of the Baltic Sea) under different sea states and wind speeds.

Threads on bolts and other fasteners are formed accomplished by rolling a smooth blank between a pair of linear dies at room temperature. The current method to control the transverse dimensions of the bolts during thread rolling fails to provide the needed tolerances for bolts with nominal dimensions of 1.0 mm or less. Towards addressing the shortcomings of the currently used roll threading manufacturing process, a mechanism to actively control the transverse dimension of the bolts is presented along with the corresponding control methodology. The proposed manufacturing process has been designed and, with its control methodology, has also been implemented on a roll threader. Comparison of bolts produced using the old system of dies and the newly developed methodology is presented using photographs taken using an optical microscope and a scanning electron microscope. These photographs clearly indicate the advantage of the newly implemented manufacturing process control system. With the new proposed system, the thread surface has clearly been improved and lateral tolerances less than five microns have been achieved on 1.0 mm diameter bolts.

Peridynamics is a non-local theory proposed for the effective handling of discontinuities such as propagation, branching, and coalescing of cracks. Collective information of the displacements of peridynamic particles is used to calculate the deformation gradient, and the equation of the motion is naturally integrodifferential. Using the integrodifferential equations, peridynamics can describe the discontinuities without additional criteria, while the classical continuum mechanics, finite element method (FEM), requires the conditions to predict the discontinuities. However, the computational efficiency of peridynamics is expensive compared to FEM because peridynamics calculates stress fields using the collective information of the displacements of the neighboring particles within the certain distance of the target particle, while FEM limits the interaction to adjacent nodes in the element. Therefore, coupling peridynamics and FEM provides both advantages of peridynamics in solving the discontinuities without additional criteria as well as the high computational efficiency of FEM.

Nowadays, deep learning (DL) has become a rapidly growing and provides useful tools for processing and analyzing big machinery data. Many research projects achieved success in failure classification from machinery data using convolutional neural networks (CNNs), one of the most extensive study aspects of DL. On this trend, we constructed a crack detection system of POM (Polyoxymethylene) gears using a deep convolutional neural network (DCNN). In our work, vibration data collected from plastic gears was visualized and labelled as crack data or non-crack images. A DCNN based on pre-trained VGG16, which firstly pre-learned from ImageNet’s data and then re-learned from the labelled images, is utilized to classify crack or non-crack situations of plastic gears. In this case of study, the image quality distortions of the dataset such as blur, noise or contrast are stable and do not affect the performance of the DCNN. However, the image size, which keep a vital role to reach high performance of the detection system, has been unknown. Hence, this paper reveals an optimized size of images created from vibration data for high-accuracy of learning.

This paper presents a natural frequency-based method for concentrated mass localization in beam-like structures. The localization of a concentrated mass is achieved by developing a novel index using relative natural frequency change (RNFC) curves and measured RNFCs. The index shows the exact location of the concentrated mass for asymmetrical beams and shows general locations of the concentrated mass including the actual location and its fake counterpart at the symmetrical position for symmetrical beams. With the general locations known, some local inspection methods can be used to identify the specific location. The effectiveness of the proposed method is confirmed by experimental investigation of a simply-supported beam and a clamped-free beam carrying a concentrated mass. In the experiment, the natural frequencies of the beam before and after adding a concentrated mass are measured by non-contact measurement using a scanning laser Doppler vibrometer.

Additive manufacturing (AM) is a common name for a group of techniques that are applied for constructing three-dimensional (3D) objects in a layer-by-layer process. One of the AM methods is multi-jet printing, which offers high accuracy of printed polymeric elements and the possibility of manufacturing products with complex shapes. Elements manufactured using AM are exploited in many industrial branches, e.g. energetic, medicine or aerospace. It results in development of structural health monitoring (SHM) systems. Among a variety of sensors applied for SHM systems fibre Bragg grating sensors (FBG) are a very promising solution. Their advantages, such as small size, multiplexing capabilities and no calibration requirements, allow them to be embedded into different material types. The goal of the paper is to analyze possibility of FBG sensors embedding into an AM polymeric material. The analyzes will be related to the influence of the manufacturing process on FBG sensors as well as influence of temperature on the sensors after finished embedding process.

Structural health monitoring (SHM) is a proven method for damage detection of static, slowly changing structures. However, there is a need to monitor structures in dynamic environments where damage may occur at much faster rates requiring evaluation in the microsecond to millisecond timescale (far below current measurement speeds). In recent years, advances in new SHM methods (along with data acquisition hardware and software) have opened the door to the measurement speeds necessary for continuously evaluating structures in dynamic environments. The electromechanical impedance (EMI) method offers excellent damage detection ability, and new electrical impedance measurement techniques have dramatically reduced the time of measurement. There are many areas that require further research to develop a fully functional microsecond SHM program using the EMI method. However, the current state of the art allows for preliminary testing for developing the algorithmic process necessary for a SHM program for continuous monitoring. This work discusses and develops the algorithmic process in a proof of concept program with LabVIEW. The program is tested in with an experiment to simulate damage as a change in boundary condition of a beam. Results indicate the program successfully completed the algorithmic process from raw data at an average of 20.47 ms on an Intel® Core™ i7-4470 processor on a 64-bit Windows 10 operating system (OS) with 8.00 GB of random-access memory (RAM) with a LabVIEW 2018 SP1 64-bit program, yielding tremendous promise and a foundation for more efficient future programs.

For development of advanced low-cost techniques for health monitoring of biosystem to serve the mankind in holistic manner, extensive research is being carried out across the world to deploy smart materials like piezo sensors those are light weight, portable and less expensive. One such application is the non-bonded piezo sensor (NBPS) configuration based on the electro-mechanical impedance (EMI) technique addressed as bone electro-mechano gram (BEMG) in this paper for the purpose of detecting osteoporotic condition in bones. This technique is proposed to substitute currently available dual energy x-ray absorptiometry (DEXA), which is based on X-rays, whose radiation can cause harmful effects to human being. Using artificial human bones, this paper presents the experimental work done for achieving better mechanical interaction between the BEMG and the bone, resulting in improved efficacy of the configuration. In earlier research work, electrical strain gauges (ESG) and digital multimeter (DMM) were employed to establish maximum tightening and better contact to acquire the repeatability of conductance signatures, whereas in this paper the use of more expedient torque wrench (2.5-11 Nm) based tightening has been demonstrated for effective tightening and attainment of good repeatability of the conductance signature. BEMG is performed on two bones, one representing healthy bone and other the osteoporotic bone, by employing jubilee clamps and torque wrench. A high value of Coefficient of correlation equal to 0.99 is obtained between any two signature in healthy state, thus demonstrating excellent repeatability of signatures. The conductance signatures acquired depict significant difference between healthy and osteoporotic bone conditions. This tightening method was found user friendly, cost effective and eliminating the use of complex electrical equipments.

The safety of pressure vessels has been a concern in recent years. Old pressure vessels are susceptible to failure due to fatigue damage after several years of usage. Fire and spill of hazardous liquid or gas due to pressure vessel failure can cost human life and cause property damage. Therefore, monitoring these critical structures has become a problem of considerable interest. Nondestructive testing and structural health monitoring play an important role for both manufacturing and periodic inspection of pressure vessels. Acoustic emission technology is a popular and widely used technology for pressure vessel monitoring. The acoustic source localization (ASL) technique developed for the two-dimensional planar structures is applied to the surface of a cylindrical pressure vessel. The ASL on the cylindrical pressure vessel surface is performed by the time difference of arrival (TDOA) method without knowing the acoustic properties of the material. In the experiment six sensors are placed in two clusters. The location of the acoustic source is unknown, and the arrival time of the acoustic signal to each sensor is measured. After analyzing the measured data as discussed in the paper one can calculate the acoustic source position. The method is experimentally verified. The results show that the above technique can quickly and accurately locate the acoustic source position on the surface of a cylindrical pressure vessel without having the complete knowledge of the structural properties of the material.

Safety and efficacy of additive manufactured porous implants is a growing concern due to several, recent recalls. The safety of bone implants depends on the effects of implantation and partial bone ingrowth on stress and strain. Finite element analysis, using two new algorithms to simulate bone ingrowth, was verified against histology results for an ovine condylar critical sized defect model. Implants were manufactured from Ti6Al4V using selective laser sintering. Results showed that partial bone formation reduces stress concentrations to safe levels, improving the long-term fatigue resistance. Higher bone ingrowth was predicted for implants made from lower modulus Titanium-tantalum alloy.

The number and scale of tunnels around the world are continuously increasing, but various disease problems during the operation period have also followed, and they have become one of the important problems facing tunnels at present. Many detection methods have been proposed in the field of tunnel detection, such as traditional manual detection method, ultrasonic detection method, ground-penetrating radar method, laser scanning method and inspection method based on image processing technology. However, due to the high cost of equipment, single test content, strict test environment and other reasons, most of the current tunnel routine inspection is still manual inspection. In order to solve the existing problems in tunnel detection, a method for rapid detection and treatment analysis of cracks in tunnel linings based on deep learning is proposed. Firstly, lining cracks were selected as the main research objects, and their causes and treatment measures in different parts were analyzed. Secondly, the AlexNet convolutional neural network based on the Caffe framework was used to identify the cracks. The crack images were collected to establish a data set, and the network parameters were modified and trained. Then use MATLAB to extract the crack length and width, and design a human-machine interactive tunnel lining crack detection program in MATLAB GUI. Finally, the content and results of this paper are discussed.

Icing conditions are threatening for a variety of engineering applications. When accreting on the aircraft surface, ice may lead to catastrophic accidents while its deposition on turbine blades can overload the components reducing power and inducing damage. The first way to protect engineering constructions from icing issues deals with early detection of ice. Ultrasound has been demonstrated to be effective for detecting changes of the structure which it is propagating through. Guided waves interact with emerging aw and discontinuity, including ice accreting on the surface. This paper introduces an experimental campaign carried out on composite structures subject to icing. The building up of planar ice onto surface structure is analysed. Furthermore, a parametric investigation is carried out by finite element modelling to look into the interaction between wave and ice while varying its dimension. Experimental findings demonstrate the ability of guided waves to detect ice layers accreting onto surface. In addition, the ice is likely to be identified from the scattering of guided waves resulting in different transmission and reflection characteristics. Instead, numerical outcomes show how the interaction of guided waves depends upon the thickness and length of the ice.

In this work, we present the design and realization of a scaled rudder stock sample with structural embedded strain gauges and piezoelectric transducers. While the strain gauges register the loading, the electro-mechanical impedance technique is used for the assessment of the structural conditions. The experimental data acquisition is based on a dedicated multichannel EMI device that supports high frequencies (up to 1 MHz) and high currents (up to 2A) so that highly attenuated materials such as the proposed CFRP rudder stock can be inspected. Experimental results will be presented for this combined load and structural health monitoring (L-SHM) system from a laboratory investigation where the rudder stock sample was loaded with bending and torsional loads. A damage indicator is defined in the frequency domain and used for the analysis of different structural conditions. The diagnostic content of multiple piezoelectric transducers arranged on the circumference will be evaluated and compared.

A beneficial feature in guided-wave SHM systems is represented by their additional capability for acoustic data communications. Here, information about the structural integrity might be transferred between sensor nodes across the monitored mechanical waveguide itself without the need for cabling. The transferred information can be given e.g. by a numerical damage indicator which is crucially needed for the diagnostic capability inherent to the respective SHM system. In such application scenarios, the installed piezoelectric transceivers transmit encoded bit sequences which are subsequently reconstructed at the receiving piezoelectric transceiver. This combined inspection and communication approach has been recently presented in a metallic plate, as well as the effectiveness of communication involving an orthotropic composite plate has also been analyzed. The present work extends recent studies demonstrating the effective deployment of elastic guided waves (GWs) for multiple-in and multiple-out (MIMO) data transmission in the framework of an SHM application. Customized and miniaturized low-power communication nodes have been developed for this purpose. They are positioned in a spatially distributed and permanently installed network. Cable-free exchange of encoded information across a stiffened carbon-fiber reinforced plastics (CFRP) panel is studied. A combination of square-wave excitation sequences and frequency-division multiplexing (FDM) is explored for simultaneous communication with multiple nodes aiming at energy-aware application scenarios.