Flexural Modes Research Articles

Sustainable Polypropylene-Based Composites with Agro-Waste Fillers: Thermal, Morphological, Mechanical Properties and Dimensional Stability.

Natural fiber composites (NFC) are eco-friendly alternatives to synthetic polymers. However, some intrinsic natural fillers' properties hinder their widespread implementation as reinforcement in polymeric matrices and require further investigation. In the scope of this study, the thermal, rheologic, mechanical (tension and flexural modes), and morphological properties, as well as the water absorption and dimensional stability of the NF polypropylene (PP)-based injection molded composites reinforced with rice husk (rh) and olive pits (op) of 20 wt.% and 30% wt.%, respectively, were investigated. The results suggest that the higher content of the rice husk and olive pits led to a similar reduction in the melt flow index (MFI), independent of the additive type compared to virgin polypropylene (PPv). The melting and crystallization temperatures of the PPrh and PPop composites did not change with statistical significance. The composites are stiffer than the PP matrix by up to 49% and possess higher mechanical strength in the tension mode at the expense of decreased ductility. PPrh and PPop have a superior flexural modulus in the bending mode, while the flexural strength improvement was accomplished for the PP30%rh. The influence of the fibers' distribution in the bulk of the parts on their mechanical performance was confirmed based on a non-localized morphology evaluation, which constitutes a novelty of the presented research. The dimensional stability of the composites was improved as the linear shrinkage in the flow direction was decreased by 49% for PPrh and 30% for PPop, positively correlating with an increase in the filler content and stiffness. PPop was less susceptible to water sorption than PPrh due to fibers' composition and larger surface-to-area volume ratios.

Strong plasmon-mechanical coupling through standing acoustic waves and antenna enhancement, toward mass transduction

Plasmon-mechanical resonators are frequently employed in sensor development. Here, I report the coupling of plasmon and mechanical modes in a multilayer architecture. In the experiment, the multilayer comprised indium tin oxide (ITO) and silver (Ag) mirrors placed on the top and bottom of a SiO2 substrate. By activating a Fabry–Pérot hybrid mode and surface plasmon polariton in a Kretschmann configuration, I demonstrated the generation of a hybrid plasmon-mechanical mode by plasmonic dispersion and through strong coupling, which resulted in mechanical distortion at multiple frequencies. Adsorption of rhodamine G6 on Ag modifies the optomechanical spectrum; it causes eigenmodes to split, resulting in a coupling rate greater than the optical decay rate (κ∗<g). Besides, the excitonic modes were observed to interact with plasmon-mechanical modes, especially on-resonance, where the plasmon-mechanical modes exhibited the higher intensity. For example, the out-of-phase flexural mode (iii, g0=427×2πkHz) became active because of the coupling of the dye. Meanwhile, after dye absorption, the strong in-phase compressional mode (iv, g0=560×2πkHz) exhibited the widest splitting and an increase in the side peaks, which merged when the pump power was raised. The system was then analyzed to determine the ideal experimental settings for detecting mass by the oscillator in a controlled experiment. The mass sensor responsivity was evaluated to be F=1017Hzg−1. Advantages and disadvantages of this approach are presented.

Reflective mode conversions between extensional and flexural waves by ultrathin oblique anisotropic tri-component resonators

In this letter, we report a theoretical study for the conversions between extensional and flexural modes for Lamb waves. The incident extensional waves can be completely converted into reflected flexural waves, or vice versa. The used structure is oblique anisotropic tri-component resonators with locally dipolar resonators. Without special optimization designs, the thickness of conversion device is much smaller than the working wavelengths (1/150 and 1/15 for the extensional and flexural waves, respectively). Our proposed design is more feasible for low-frequency application scenarios.

Flexural behavior and load-bearing capacity calculation methodology for wooden beams strengthened using aluminum plates attached with self-tapping screws

ABSTRACT To investigate the flexural failure mode and behavior of wooden beams strengthened using aluminum (Al) plates, which were attached with self-tapping screws and structural adhesive, ten specimens were tested in bending. The experimental parameters were the thickness of the bottom Al plate, the longitudinal and transverse grain spacing of the self-tapping screws. The experimental results revealed that (1) compared with unreinforced beams, the reinforced beams exhibited ultimate load capacity and displacement ductility coefficients that were 18.4%–54.3% and 6.4%–43.1% higher, respectively; (2) for the same Al plate thickness in the tensioned area, the initial stiffness of the reinforced wooden beams increases as the longitudinal grain spacing of the screw grains decreases; (3) the plane-section assumption was satisfied by the strain in the middle section of the Al plate-reinforced wooden beam along the height; (4) the strains at the same position in the wooden beam and Al plate were similar, thus indicating that the wooden beam and Al plate behaved collectively. Furthermore, a formula was proposed for calculating the flexural capacity of an Al plate-reinforced wooden beam, thus providing a valuable reference when designing wooden beam reinforcements.

Seismic experiments and shear resistance prediction of multi‐celled corrugated‐plate CFST walls

AbstractMulti‐celled corrugated‐plate CFST walls (MC‐CFST walls) are innovative composite members for load bearing and energy dissipation. The corrugated steel plates effectively enhance the confinement capacity of the infilled concrete and reduce steel consumption. In this study, the seismic behavior of MC‐CFST walls was investigated through experimental, numerical, and theoretical analyses. Eight specimens were designed considering different key parameters, including the width and depth of the corrugated cell, the amplitude of the corrugated steel plate, and the type of boundary columns. These specimens were tested under axial compression and horizontal cyclic loading. The test results indicated that MC‐CFST walls exhibited excellent energy dissipation capacity and ductility. Specimens with boundary columns exhibited shear failure modes, while those without boundary columns exhibited flexural failure modes. Increasing the width of the corrugated cell or reducing the effective depth of the composite wall had a significant negative impact on its seismic performance. Subsequently, the finite element (FE) model was established and verified against experimental results. Finally, based on the full‐sectional plasticity assumption and the superposition principle, theoretical formulas for predicting the compression‐bending and shear capacities of MC‐CFST walls were proposed and validated against experimental data. The results showed that both theoretical formulas could effectively and accurately predict the shear resistance of MC‐CFST walls, providing valuable references for practical designs.

A convergence‐enhanced Timoshenko beam element for the stochastic nonlinear analysis of reinforced concrete components

AbstractAn effective and precise physical model is generally required for the performance assessment of reinforced concrete (RC) components. In this study, a convergence‐enhanced Timoshenko beam element considering finite deformation is proposed for the stochastic nonlinear analysis of RC components. The unified framework of the rank 2 correction matrix is obtained by approximating the iterative matrix through the linearly independent basis vectors, and two types of Broyden–Fletcher–Goldfarb–Shanno (BFGS) operators are constructed. Then, a consistent solution scheme with superior numerical stability and efficiency is proposed based on the second kind of BFGS operator and improved quasi‐Newton method. Accurate discretization of the displacement fields and description of nonlinear sectional behavior are achieved by the high‐order Lagrangian interpolation functions, concrete damage‐plasticity model and J2 plasticity model. In an updated Lagrangian formulation, these shape functions and constitutive relationships are used to develop a complete secant stiffness with higher‐order terms in the strain tensor and residual force vector. To validate the proposed Timoshenko beam element, numerical applications involving material nonlinearity and geometrical nonlinearity are conducted. The numerical stability and efficiency of the solution scheme, the shear‐locking free technique, and the oscillation of axial force are thoroughly examined and discussed by solution of RC columns, cantilever beam and progressive collapse of planar RC frame. Finally, the parametric study considering the geometric and material randomness indicates that the RC column designed for flexural failure mode suffered considerable degradation of loading capacity and a larger scatter of residual load.

Higher-order shear deformation theory for accurate prediction of vibration behavior of thick piezoelectric disks and design of efficient surface electrodes

A modified higher-order shear deformation plate theory (MHSDT) is proposed to predict the resonance frequencies and related mode shapes of piezoelectric disks of various thicknesses. The theory explicitly accounts for the effects of the electrical potential on the displacement of the disk. The equations of motion for the disk are derived using Hamilton's principle in conjunction with a variational approach. It is shown analytically that the distribution of the electrical field on the surface of the disk is quantitatively similar to the distribution of the sum of the total stresses within the disk. Thus, the deformation of the disk can be approximated directly from the stress distribution on the disk, thereby greatly simplifying the solution process. The validity of the proposed MHSDT is confirmed by comparing the solutions for the natural frequencies, displacement fields, and electric field distributions of the disk for various radius-to-thickness ratios and boundary conditions with those obtained using Reddy’s third-order plate theory and COMSOL finite element simulations, respectively. The results show that the solutions obtained from the MHSDT method are consistently closer to the exact solutions than those determined using Reddy’s third-order method. Finally, the linear relationship between the electric field and the flexural displacement of the disk is leveraged to design surface electrodes capable of exciting specific flexural mode shapes of the disk under a free boundary condition. The efficiency of the designed electrodes is experimentally demonstrated using an amplitude-fluctuation electronic speckle pattern interferometer (AF-ESPI).

Impact response of stainless steel tube locally-strengthened by concrete-filled steel tube

This paper proposes a novel stainless-steel tube locally strengthened by a concrete-filled steel tube to resist impact loads. A drop hammer impact test system was used to determine the failure modes, impact forces, and displacement responses of the strengthened tubes under an impact load. The effects of the strengthening length and thickness of the concrete and outer steel tubes on the impact responses of the strengthened tubes were investigated. The impact resistance was found to improve with an increase in the thickness of the concrete and outer steel thickness. The test results also indicated that the strengthened tubes exhibited a flexural failure mode with a combination of local and global deformations. An analytical model for predicting the dynamic responses of the locally strengthened tubes was established based on an equivalent two-degree-of-freedom (TDOF) model. The analytically predicted results were compared with the experimental results, and the good consistency between these two results demonstrated the accuracy of the established analytical model.

Evanescent wave quartz-enhanced photoacoustic spectroscopy employing a side-polished fiber for methane sensing

We present an all-fiber-based laser gas analyzer (LGA) employing quartz-enhanced photoacoustic spectroscopy (QEPAS) and a side-polished fiber (SPF). The LGA comprises a custom quartz tuning fork (QTF) with 0.8 mm prong spacing, two acoustic micro-resonators (mR) located on either side of the prong spacing, and a single-mode fiber containing a 17 mm polished section passing through both mRs and QTF. The SPF polished face is positioned to enable the evanescent wave (EW) to create a photoacoustic wave and excite the fundamental flexural mode of the QTF. Sensor performance was demonstrated using methane in nitrogen gas mixtures, with CH4 mixing ratios ranging from 75 ppmv to 1% (by volume), measured with an accumulation time of 300 ms, and a minimum detection limit of 34 ppmv subsequently determined. The EW-QEPAS sensor is ideal for miniaturization, as it does not contain any free-space optics and is suitable for gas sensing in harsh environments and where mobility is required.

Measuring Vibrational Modes in Living Human Cells

The question of whether human living cells in physiological conditions exhibit mechanical resonances has remained unresolved for more than 70 years. We here construct a theoretical model to predict how these hypothetical vibration modes of a living cell can modify the stochastic response of a supporting micromechanical resonator. The key signatures that demonstrate the existence of these vibration modes are then searched for experimentally. To this aim, single human breast epithelial cells were attached to the surface of compliant small microcantilevers. Close examination of the frequency spectra of the thermal fluctuations of the microcantilever over a high-frequency range of 1 kHz–1 MHz revealed anomalies that are consistent with our theoretical predictions. We identify the stochastic coupling of the first and third vibration modes of the cell with the torsional and flexural vibration modes of the cantilever, respectively. These mechanical resonances are very broad and are located at 10–30 kHz and 150–180 kHz, respectively. The analysis of the experiments allows us to obtain information on the temporal evolution of several physical properties of the cell, in addition to the resonance frequencies, during the adhesion process—such as the mass, viscoelasticity, and contact area. These results open multiple avenues in our understanding of single cell mechanobiology, vibrational spectrometry of living cells and mechanical destruction of cancerous cells by targeting specific cell frequencies. Published by the American Physical Society 2024

Measurement of Nanofiber Mechanical Flexural Modes Based on Near-Field Scattering.

We systematically investigated the intrinsic mechanical flexural modes of tapered optical fibers (TOFs) with a high aspect ratio up to 3×10^{4}. Based on the near-field scattering of the hemispherical microfiber tip to the vibrating TOF evanescent field, we detected more than 320 ordered intrinsic mechanical modes through the TOF transmission spectra which was enhanced by 72dB compared to without near-field scattering. The trend of the vibration amplitude with the mode order was similar to pendulum waves. Our results open a pathway to study the mechanical modes of photonic microstructures-nanostructures that are expected to be used in waveguide QED, cavity optomechanical, and optical sensing.

Subwavelength negative refraction and flexural wave lens design via resonant double-negative piezoelectric metamaterial

We report the concept and demonstration of a double-negative, resonant metamaterial characterized by both dynamic negative mass and stiffness for negative refraction of flexural wave modes by means of a lens designed using this concept. The negative equivalent material properties are obtained in the subwavelength regime by concurrently exploiting both the effect of mechanical resonators (negative mass) and of piezoelectric patches with inductive resonant shunts (negative stiffness), leading to double-negative behavior. Following the theoretical foundations based on a modal framework, we analytically derive the frequency-dependent mass and stiffness properties as a function of the electromechanical parameters. The findings are corroborated by numerical computation of dispersion properties and simulations showing the focusing of a point source. As a case study, energy harvesting performance enhancement by exploiting the piezoelectric effect at the focal spot is also discussed.

Mechanical characteristics and stability analysis of an innovative type of steel–concrete composite support in shallow-buried excavation tunnels

Under the influence of the principle of New Austrain Tunnelling Method (NATM), the initial support structure of shallow-buried tunnels in urban soft rock mainly adopts the forms of section steel arches or lattice girder arches combined with bolt-mesh-shotcrete. In order to solve the problems of tunnel deformation and collapse caused by untimely support or insufficient support strength during the construction process, the concept of “timely and high-strength support” was put forward, the high-strength steel pipe grid (HSPG) arch was innovatively designed, and the steel pipe grid bolt-shotcrete (SPGB) structure was established. Next, the flexural bearing performance and failure modes of the joint component and jointless component of the HSPG arch were analyzed, and the design scheme was also optimized by finite element simulation. Then, the full-scale model test of steel pipe grid concrete component (SPGC) under the pure bending load was conducted to explore the coupling bearing effect in a laboratory. In addition, relying on the underground excavation tunnel project of urban subway, the characteristic curves and stability coefficient of surrounding rock and SPGB structure were given by using the principle of convergence-confinement method. Theoretical calculations and on-sit measurements showed that SPGB structure had good adaptability in weak surrounding rock tunnel. In general, the research results were of great significance for clarifying the bearing mechanism and applicable conditions of SPGB structure, and also provided a reference for its further application in the tunnel and underground engineering.

Simultaneous dual-harmonic detection for LITES with a multi-pass configuration based on combined flexural modes of a quartz tuning fork

The simultaneous dual-harmonic detection for light-induced thermoelastic spectroscopy (LITES) with a multi-pass configuration based on combined flexural modes of a quartz tuning fork (QTF) is demonstrated. To enhance the photothermal energy absorbed by a custom QTF, the laser beam is designed to pass through the QTF multiple times. With dual-frequency modulation methodology, the second harmonic (2f_fun) signal and the first harmonic (1f_ove) signal are simultaneously inspired at resonant frequencies of fundamental and overtone flexural modes of the QTF, respectively. Thus, precise light intensity and gas concentration can be retrieved concurrently with corresponding 1f and 1f-normalized 2f signals using frequency division multiplexing technology. In comparison to traditional LITES under the same condition sensor, our triple-pass configuration performs 1.84 times better in signal-to-noise ratio (SNR). According to Allan variance analysis, 1f-normalized 2f signal enables a longer integration time of 1100 s and a lower detection limit of 0.07 ppm. The corresponding normalized noise equivalent absorption coefficient (NNEA) is 0.8 × 10−9 cm−1W/Hz1/2.

Experimental and numerical analysis of the Microstructure and mechanical properties of unidirectional glass fiber reinforced epoxy composites

This paper proposes a numerical model to predict the pattern and features of the fracture surface and to understand the mechanisms and cause of the failure. Various fiber orientations of 0°, 90°, 0°/90° and ± 45° were utilized in the production of unidirectional glass-fiber-reinforced epoxy composites via the vacuum bagging technique. The mechanical properties of the manufactured composites were evaluated by measuring parameters including tensile strength, compressive strength, flexural strength and interlaminar shear strength. High-resolution scanning electron microscopy was used to examine the mechanisms of fracture in laminates. The mechanical properties of the composite material were found to be considerably improved when a unidirectional 0° fiber orientation was used, as compared to other fiber orientations. This was true for tensile, compressive, flexural and interlaminar shear loading modes. The composite laminates demonstrated several modes of breaking, including fibers pulling away from the matrix, holes in the matrix and river flow lines, depending on the direction of the fibers. The outcomes of the numerical simulation demonstrate a high level of fidelity with experimental findings. This study provides a valuable reference for predicting numerical analysis of the microstructure and mechanical properties.

Different Design Aspects of L-Shaped Earth Retaining Structures

The L-shaped retaining walls (L-SRWs) are considered one of the important earth retaining structures (ERSs). The investigation of L-SRWs mainly depends on factors directly related to the design process. Reviewing these factors represents the key matter in the design of ERSs, and this is the main purpose of this paper. This review showed that, in the design of L-SRWs, there are important issues to be considered. Some of them are related to the backfill and foundation soils e.g. the dependency of soil stiffness on the state of stresses, information related to soil behavior, inclination and quality of backfill, and settlement of foundation soil. Other issues are related to the wall itself such as the wall dimensions, failure mechanism, and flexural failure mode. In the design processes of L-SRWs, structural and geotechnical design requirements should be considered. L-SRWs should be designed to be proportional in their dimensions, the width of the wall base and its height should be designed to meet a specific ratio. Considerations related to the "conjugate rupture planes", the displacement pattern, and flexural failure mode should be included in the design process of L-SRWs.

Effect of voltage on mechanical properties of carbon fiber reinforced concrete

Under various voltages, the performance variations and compressive and flexural failure modes of carbon fiber concrete were investigated. The findings indicate that while the general concrete will display a cone upon failure, the compressive carbon fiber concrete specimen maintains its integrity under the influence of voltage, displaying a state of surface cracking with similar-sized crack spacing. With an increase in voltage level, the compressive strength of carbon fiber reinforced concrete first rises and then falls. The compressive strength of carbon fiber concrete at a voltage level of 15V is 66% higher than that of carbon fiber concrete without voltage. Similar to the flexural strength test phenomenon of uncharged carbon fiber concrete, the flexural strength test phenomenon of concrete beams when applying voltage is immediate fracture. The maximum strength of carbon fiber reinforced concrete is only 10% higher, and the flexural strength is only marginally improved, fluctuating.

Role of four-phonon processes in thermal conductivity of two-dimensional materials and thermal-transport enhancement arising from interconnected nanofiller networks in polymer/nanofiller composites.

Recent research has shed light on the importance of four-phonon scattering processes in the thermal conductivity (k) of 2D materials. The inclusion of 4 phonon scattering processes from first-principles has been shown to lead to a thermal conductivity of ∼1290 W m-1 K-1 in graphene at 300 K, significantly lower than the values predicted to be in excess of 4000 W m-1 K-1 based only on 3 phonon scattering processes. Four phonon processes are shown to be most significant for flexural ZA phonon modes, where the reflection symmetry selection rule (RSSR) is less restrictive for 4-phonon than 3-phonon scattering processes. This combined with the low frequencies of ZA phonon modes, leading to high populations, leads to higher 4-phonon than 3-phonon scattering of low frequency ZA phonon modes in graphene at 300 K. In this review, the role of parameters such as atomic structure, phonon dispersion and temperature on 4-phonon scattering processes in a wide range of 2D materials is reviewed. Materials such as graphene nanoplatelets (GnPs) have been extensively investigated for enhancement of the thermal conductivity of polymer composites. However, such enhancement is limited by the poor interfacial thermal conductance between the polymer and filler material. Interconnected filler networks overcome this issue through highly efficient continuous percolative heat transfer paths throughout the composite. Such 3D networks have been shown to enable ultra-high polymer thermal conductivities, approaching ∼100 W m-1 K-1, and even exceeding those of several metals. In this review, different techniques used to achieve such interconnected 3D filler networks, namely, aerogels, foams, ice-templating, expanded graphite, hot pressing of filler coated polymer particles, the synergistic effect between multiple fillers, and the stitching of filler sheets, are discussed and their impact on thermal conductivity enhancement are presented.

Four-phonon scattering and thermal transport in 2H–MoTe2

Among two-dimensional (2D) transition metal dichalcogenides, molybdenum ditelluride (MoTe2) is of great interest to achieving unique functionalities in diverse electronic, photonic, and phononic devices. Despite extensive studies over the past decade, the thermal transport properties and mechanisms of MoTe2 remain largely unclear. Here, we grow few-layer (7L, 10L, and 17L) single-crystalline 2H–MoTe2 and measure their in-plane thermal conductivities using an optothermal Raman technique. The room-temperature values range from 4 to 9 W m−1 K−1, substantially lower than theoretical predictions considering only three-phonon scattering. By employing first-principles calculations and machine learning-assisted molecular dynamics simulations, we reveal that four-phonon scattering could suppress the thermal conductivity of 2H–MoTe2 by 80%, which agrees with experimental measurement and is mainly attributed to the flexural acoustic modes. Thickness-dependent suppression is observed owing to the varying reflection symmetry. Our work highlights higher-order phonon-phonon interactions in 2D materials and offers guidance for the thermal design of emerging devices.

Seismic behavior and modeling of T‐shaped reinforced concrete walls under high axial force ratios

AbstractThis paper presents large‐scale quasi‐static tests conducted to investigate the seismic behavior of slender T‐shaped walls under a high axial compression force ratio of approximately 0.19. Four specimens were designed per various codes and design methods for comparison. All specimens failed in a flexural mode, characterized by the crushing of concrete and buckling or fracture of boundary longitudinal rebars at the web toe. The specimens designed per the US code ACI 318‐19 provisions and designed using displacement‐based method exhibited satisfactory deformation capacity with an ultimate drift exceeding 2.0%, which validates the effectiveness of the provisions and displacement‐based method for T‐shaped walls under high axial force ratios. However, the specimen designed per the Chinese code GB 50011‐2010 experienced a sudden drop in strength after 1.0% drift in flange‐in‐tension loading, indicating insufficient design of the special boundary element at the web toe and the necessity of improvement of the current Chinese code provisions. The flexural strength of the T‐shaped wall specimens can be accurately estimated using cross‐sectional analysis. Analysis of the test data indicated that the effective stiffness of the T‐shaped RC walls with an axial force ratio of no greater than 0.1 was significantly lower than 0.35EcIg, while that of the T‐shaped walls with an axial force ratio of 0.19 reached 0.5–0.7EcIg. Existing equations did not provide accurate estimate of the effective stiffness of T‐shaped walls. The lateral drift limit formulation developed by Segura and Wallace could reasonably estimate the lateral drift capacities for the T‐shaped RC wall specimens under high axial force ratios. Finally, three conceptually different models, namely the MVLEM‐3D, SFI‐MVLEM‐3D, and multi‐layer shear element models were adopted for the simulation of T‐shaped RC walls. All three models reasonably predicted the flexural strength capacity (mostly less than 15% error) and the effective stiffness (13%–43% error) of the T‐shaped walls.