Frequency Sweep Experiments Research Articles

Chemically Degradable Vitrimers Based on Divanillin Imine Diepoxy Monomer and Aliphatic Diamines for Enhanced Carbon Fiber Composite Applications

This study presents the development of a diglycidyl monomer containing two imine groups that can act as dynamic and reversible bonds. During the curing of the monomer with two different amine hardeners, we confirmed the formation of new imine groups due to the transamination reaction between the imine groups of the diepoxy monomer with the amine groups of the hardener. The effect of this structural change was observed in the stress relaxation behavior, resulting in the overlapping of two different relaxation modes. The analytical modelling was able to extract two distinct characteristic relaxation times using a double-element Maxwell model. A second characterization of the stress relaxation process by frequency sweep experiments was performed to corroborate the results obtained, confirming speedy stress relaxation. Acid-catalyzed hydrolysis was performed on the studied materials, demonstrating the complete degradation of the network. We finally confirmed that the synthesized diepoxy compound is suitable for preparing carbon-fiber-reinforced composite materials, demonstrating easy fiber impregnation, fast reshaping, and especially a total degradation of the polymer matrix that allows for the recovery of the carbon fibers in mild conditions. This epoxy–amine system is an excellent candidate for overcoming the traditional limits of thermosets in preparing fiber-reinforced composites.

High strength induced wide band gap formations in additively manufactured cubic metamaterial

Strength and band gap are the two basic physical features of the cubic metamaterial. How to design band gap characteristics with high strength of structure is the key for the further industrial application in vibration control of the cubic metamaterial. Here a cubic metamaterial is designed by optimal selection of crystal orientation angle to obtain wide band gaps with high strength. The prototype samples were fabricated using advanced additive manufacturing technology to tensile-pressure experiments and sine frequency sweep experiment, thereby demonstrating the validity of the obtained results. Results indicated that the normalized bandwidth of simple cubic (SC) metamaterials is 0.47 and the ultimate strength is 25.99 MPa. The normalized bandwidth is increased by 3.1 times and 47 times higher than that of the metamaterials of face-centered cubic (FCC) and body-centered cubic (BCC). Its ultimate strength is increased by 3.5 times and 6.7 times. The static simulation results revealed that the maximum mises stress values of SC, FCC, and BCC metamaterials were 1.71, 10.49, and 31.40 MPa respectively. The attenuation amplitude of the elastic wave measured by experiment is 80 dB, which is consistent with the simulation results. The bandwidths of cubic metamaterials exhibit a positive correlation with their strength. The variation in crystal orientation angles plays a crucial role in elucidating the underlying mechanism behind the positive correlation between the strength and the band gap. The further buckling analysis of SC metamaterial with high strength and wide bandgap characteristics reveals that the negative Poisson’s ratio structure experiences a reduction in bandwidth and strength as buckling deformation intensifies.

Augmenting the quality and storage stability of soymilk by incorporation of untreated and ozonated oat 1,4-β-D-glucan

The study aimed to improve the quality and storage stability of novel plant-based soymilk with the incorporation of untreated (UtβG) and modified oat derived 1,4-β-D-glucan (OzβG) at varying concentrations (0, 1, and 2 % labelled as S0, S1 and S2). The treated soymilk was characterized for physical, chemical, nutritional, rheological, particle size, zeta potential, sensory and storage stability characteristics. The results revealed that 1, 4-β-D-glucan incorporation increased the acidity (0.67 to 0.73 %), viscosity (3.4 to 4.7 Cp) and ash content (0.74 to 0.92 %), however color remains natural. The frequency sweep and shear experiments showed that the 1,4-β-D-glucan modified the rheological parameters of the soymilk. The sensory analysis (n = 30) indicated that texture, mouthfeel and overall acceptability (8.38). Compared to OzβG-treated soymilk, UtβG soymilk, especially S2, exhibited superior thickening and rheological properties. The storage study indicated minimal phase separation in 1,4-β-D-glucan-incorporated samples, maintaining stability for 15 days under refrigerated conditions without compromising overall quality. Thus, this study provides valuable insights into the potential application of 1,4-β-D-glucan for improving the technological quality of soymilk that highlights possible implications for its commercialization potential.

Influence of Specimen Geometries on the Viscoelastic Properties of Bituminous Mixtures in Torsion

A standard test method for determining the linear viscoelastic properties of bituminous mixtures in torsion prescribes the use of a dynamic shear rheometer. The method uses a prismoidal specimen (rectangular cross section) sliced from a Superpave gyratory compacted sample. However, the geometry utilized in the standard is a non-axisymmetric cross section, which leads to the development of warping stress when the specimen is tested in torsion using a dynamic shear rheometer. On the other hand, axisymmetric geometry, such as cylinder (circular cross section), does not warp when subjected to torsion. This study investigates the influence of the two specimen geometries, namely prismoidal and cylindrical, on the linear viscoelastic properties of bituminous mixtures in torsion. The specimens are obtained from a bituminous mixture with a nominal maximum aggregate size of 13.2 mm, and statistical analysis is carried out to determine whether the material constitution in the specimen geometries is identical. A frequency sweep experiment is conducted at 10, 20, and 30°C with a constant strain amplitude of 0.001% in the frequency range of 0.01 Hz to 20 Hz. The absolute shear modulus of the prismoidal specimens is found to be higher than that of cylindrical specimens because of warping stresses in prismoidal specimens. It was seen that the effect of warping stress is greater at higher temperatures and lower frequencies. Correction factors are used to correct the absolute shear modulus of prismoidal specimens. These findings highlight the importance of considering correction factors for warping when characterizing prismoidal bituminous mixtures in torsion.

Comparative outcomes of the voltage-dependent current density, charge transportation and rectification ratio of electronic devices fabricated using mechanically flexible supramolecular networks.

In this study, we report the synthetic method of two distinct supramolecular metallogels, namely Mn-BDA and Cd-BDA, using Mn(II) acetate tetrahydrate, Cd(II) acetate dihydrate and butane-1,4-dicarboxylic acid (BDA). DMF, a polar aprotic solvent, was immobilized in both metallogel-networks for their synthesis. The metallogelation of Mn-BDA was successfully attained through the instant mixing of a Mn(II)-source and BDA in DMF solvent media. By applying ultrasonication, a Cd-BDA metallogel was prepared. The stoichiometry of gel-forming components concerning metal salts and the LMWG are accountable to obtain respective stable metallogels. Rheological parameters such as storage modulus (G') and loss modulus (G'') explored the mechanical flexibility of the synthesized metallogels through amplitude and angular frequency sweep experiments. Both the metallogels possess significant mechanical stability, which was determined by monitoring diverse gel-to-sol transition shear strain values (γ%). Distinctive morphological visualizations of both of these metallogels (i.e., Mn-BDA and Cd-BDA) were made via field emission scanning electron microscopic (FESEM) studies, demonstrating a fibrous inter-connected network with a hierarchical self-assembled arrangement for Mn(II)-based metallogels and a typical stacked-flake-like association with hierarchical motifs for Cd(II)-based metallogels. EDAX elemental mapping substantiated the presence of metallogel-forming agents such as individual metal acetate salts, BDA acting as a low-molecular weight gelator, and gel-immobilized solvents such as DMF. Furthermore, Fourier transform infrared spectroscopy and ESI-mass spectroscopy were performed for both these supramolecular metallogels. FT-IR spectroscopic and ESI-mass spectroscopic results clearly substantiate the possible non-covalent supramolecular interactions among basic molecular repeating moieties, i.e., butane-1,4-dicarboxylic acid (the low-molecular weight gelator), individual metal salts and gel-immobilized polar aprotic solvent DMF for the construction of distinct stable supramolecular metallogel-systems. The semiconducting property of the fabricated metallogels was investigated. Two Schottky diodes (SDs) composed of ITO/Cd-BDA/Al and ITO/Mn-BDA/Al in a sandwich pattern with Al serving as the metal electrode were fabricated. Both these metallogel-based devices effectively offer significant semiconducting diode features with non-linear J-V characteristics. The non-ohmic conduction protocol of the fabricated metallogel-based devices was explored. Mn-BDA and Cd-BDA metallogel-based fabricated devices have rectification ratios of 6.67 and 23.50, respectively. The gel-based diode performances were examined by observing the voltage-dependent current density, charge transportation and rectification ratio.

Optimization of machining performance in deep hole boring: A study on cutting tool vibration and dynamic vibration absorber design

In the realm of precision engineering, particularly in deep hole boring processes, tool vibration emerges as a critical determinant of machining performance. This investigation elucidates the genesis of self-excited vibrations within deep hole boring operations and delineates the underlying mechanisms of cutting tool vibration. A focal point of this study is the optimal alignment of the boring bar to mitigate vibrational impacts, thereby enhancing surface finish quality and extending tool longevity. Central to this analysis is the employment of a Dynamic Vibration Absorber (DVA) aimed at attenuating cutting tool vibration. The deployment of DVA necessitates precise identification of modal parameters, namely the equivalent stiffness (K) and mass (M) of the cutting tool. This research juxtaposes various scholarly methodologies to amalgamate theoretical calculations with simulation approaches, thereby acquiring accurate modal parameters. Utilizing Matlab software, the vibration amplitude of the boring bar under varying spring stiffness scenarios was examined. Results indicate a direct correlation between increased stiffness and reduced amplitude, particularly when the frequency ratio 'g' ranges between 0.5 and 1.1. Consequently, a stiffer DVA configuration is posited as more effective in vibration reduction. Furthermore, the study conducted frequency sweep experiments on a damping boring bar, utilizing a vibration excitation platform. These experiments revealed the existence of an optimal stiffness value for the DVA, thereby underscoring the significance of stiffness matching in vibration mitigation strategies.

A Novel Electromagnetic Noise and Image Stripe Noise Suppression Method Between SMA-OIS Actuator and CMOS Image Sensor

In order to solve the electromagnetic compatibility (EMC) problem between shape memory alloy (SMA) optical image stabilization (OIS) actuator and CMOS image sensor (CIS), a novel electromagnetic noise and image stripe noise (ISN) suppression method is proposed in this paper. The electromagnetic interference mechanism of SMA-OIS actuator is analyzed by electromagnetic field simulation and magnetic dipole mathematical model. The CIS noise mechanism based on the readout circuit model and image signal processing (ISP) pipeline is analyzed and the ISN evaluation method of CIS is proposed. The drive frequency and ISN model is established by frequency sweep experiment through the joint experimental platform of SMA-OIS actuator and CIS. Furthermore, the random frequency modulation PWM method based on the spread spectrum principle is proposed to suppress the electromagnetic noise and ISN. The experimental results show that the state-of-the-art noise suppression method proposed in this paper can not only eliminate ISN, but also have good technical adaptability in practical applications.

Parametric modelling of vibration response for high-speed gear transmission system

In the process of dynamic modelling and vibration response simulation for the high-speed gear transmission system, complex structures with a high degree of freedom (DOF) often lead to very large calculation errors, especially under conditions with unpredictable internal and external excitations. In this paper, a novel parametric probabilistic regression (p-PR) model based on data-driven and parametric modelling theories are proposed for the vibration signals of the high-speed gear transmission system. Based on the proper orthogonal decomposition (POD) method, the dataset of vibration signals is constructed, and a new truncated parametric reduced-order model (PROM) of the dataset is given for the gear vibration, with the truncated modes and corresponding coefficient vectors obtained. Through training the PROM and operating parameters using probabilistic regression, maximum likelihood estimation, and the parametric mode reconstruction, the p-PR model of vibration data for the gear transmission system is established. To validate the parametric modelling theories and the p-PR model, a bevel gear transmission experimental rig and a vibration signal test system are built, and the constant-speed test (CST) experiment and speed-up frequency sweep (SUFS) experiment are conducted. Based on the results of the CST experiment, the p-PR model for the experimental system is established and compared with the SUFS results. Results show that the p-PR model can accurately predict the vibration signals in both the time-domain and the frequency-domain, and the maximum predictive error for the experiment is 7.8%, which validates the accuracy and feasibility of the proposed theories and methods.

A Hybrid Energy Harvester Based on Piezoelectric and Electromagnetic mechanisms

This paper presents a hybridization energy harvester (HEH) based on a two degrees-of-freedom (2-DOF) cantilever beam, which combines the electromagnetic effect with the piezoelectric effect. Finite element simulations and some experiments were used to investigate the output of the HEH. For the electromagnetic energy harvester (EMEH), we used an alternating-arranged magnet array to get high electromotive forces. The frequency sweep experiment showed that the resonance frequency of the cantilever beam was about 26.7 Hz which is close to the simulation value, 28.7 Hz. With the increase of excitation acceleration, both piezoelectric and electromagnetic open-circuit voltages increased. The impedance matching experiments of the EMEH and piezoelectric energy harvester (PEH) displayed that the best matching resistors are 35 Ω and 8×105 Ω respectively. In capacitor charging experiments, the maximum voltage of the capacitor (220μF) increased from 0.95 V to 1.63 V by paralleling the EMEH and the PEH. But when the EMEH and the PEH were connected in series, the maximum voltage of the capacitor (220μF) was 4.4 V. Therefore, it is clear that the proposed hybridization energy harvester balances the weakness of electromagnetic and piezoelectric harvesters. Besides, it can also make higher output performance.

Hydrogel Formulations of Antibacterial Pyrazoles Using a Synthesized Polystyrene-Based Cationic Resin as a Gelling Agent.

Here, to develop new topical antibacterial formulations to treat staphylococcal infections, two pyrazoles (3c and 4b) previously reported as antibacterial agents, especially against staphylococci, were formulated as hydrogels (R1-HG-3c and R1HG-4b) using a cationic polystyrene-based resin (R1) and here synthetized and characterized as gelling agents. Thanks to the high hydrophilicity, high-level porosity, and excellent swelling capabilities of R1, R1HG-3c and R1HG-4b were achieved with an equilibrium degree of swelling (EDS) of 765% (R1HG-3c) and 675% (R1HG-4b) and equilibrium water content (EWC) of 88% and 87%, respectively. The chemical structure of soaked and dried gels was investigated by PCA-assisted attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectroscopy, while their morphology was investigated by optical microscopy. Weight loss studies were carried out with R1HG-3c and R1HG-4b to investigate their water release profiles and the related kinetics, while their stability was evaluated over time both by monitoring their inversion properties to detect possible impairments of the 3D network and by PCA-assisted ATR-FTIR spectroscopy to detect possible structural changes. The flow and dynamic rheological characterization of the gels was assessed by determining their viscosity vs. shear rate, applying the Cross rheological equation to achieve the curves of shear stress vs. shear rate, and carrying out amplitude and frequency sweep experiments. Finally, their content in NH3+ groups was determined by potentiometric titrations. Due to their favorable physicochemical characteristic and the antibacterial effects of 3c and 4b possibly improved by the cationic R1, the pyrazole-enriched gels reported here could represent new weapons to treat severe skin and wound infections sustained by MDR bacteria of staphylococcal species.

Miniaturized methane detection system based on photoacoustic spectroscopy

AbstractAs an important means of trace gas detection, photoacoustic spectroscopy mainly detects gas concentration indirectly through the thermal effect of light. Methane detection system based on photoacoustic spectrum has great application prospect in environmental pollution detection and industrial production. As a container for loading methane, the shape and quality of the photoacoustic cell will have a huge impact on the final detection results. To improve the operability of the photoacoustic spectrum technology in practical application, this system improves the traditional photoacoustic cell. The integrated micro photoacoustic cell designed with 3D printing technology has a volume of 9 cm3 and a theoretical resonance frequency of 11 000 Hz. Based on LabVIEW platform, a frequency sweep program and a concentration acquisition program were set up in the experiment. The frequency sweep experiment was carried out in an environment filled with methane in the photoacoustic cell. Based on the theoretical knowledge of photoacoustic spectroscopy, it can be known that the sound pressure detected by microphones is proportional to the concentration of methane in the photoacoustic cell. Methane with gradient concentration is passed into the photoacoustic cell for concentration calibration experiment, and the second harmonic peak detected at different concentrations is linearly fitted, and the line R2 = 0.99919. By calculating Allan variance, it can be obtained that the minimum detection of the system can reach 6.89 ppm when the integration time is 49 s.

Instantaneous Angular Speed Extraction Based on Nonuniform Local Polynomial Differentiator for the Stiffness Identification of the Robot Joint

In the identification of industrial robot joint system, in order to obtain the ideal frequency response of the system, additional excitation needs to be applied. The addition of excitation signals, such as chirp signals and pseudorandom binary signals, will make the instantaneous angular speed (IAS) obtained by the motor encoder present nonstationary characteristics. In order to solve this problem, this article proposes a method for system identification by IAS extraction under nonstationary conditions. Interferences, such as electrical perturbation, quantization error, and background noise, generally reduce the accuracy of extracted IAS. First, the influence of electrical perturbation on the accuracy of angle calculation is eliminated by utilizing the characteristics of the two-phase encoder signals. Second, considering the time-varying characteristics of the rotational speed in the identification process, a nonuniform local polynomial differentiator (NULPD) is proposed to optimize the estimation of IAS and reduce the influence of background noise and sampling error based on local polynomial differentiator (LPD). The IAS estimation under the condition of variable speed is simulated to verify the performance of NULPD. According to the transmission characteristics of the robot joint system, a two-mass dynamic model is established, and the transfer function between the torque and motor speed is obtained. The torque is calculated from the three-phase current signals, and the rotational speed is obtained by the encoder signal. The frequency sweep experiment is carried out on the single-degree-of-freedom articulated arm test bench, where the stiffness and inertia of the system are identified within a certain error range by the proposed method.

Flexibility Analysis and Experimental Design of Dual-Chamber Bellows Isolator for Low-Frequency Damping Performance

To achieve an effective and versatile solution for reducing vibrations and reducing maintenance costs, the dual-chamber flexible bellows isolator (D-CFBI) of rubber material is designed. The outer chamber of D-CFBI is used as an actuator to change its own variable stiffness and control the feedback force by filling negative pressure gas. The inner chamber is used as a rubber air spring to work as the main vibration isolation element. Under different loads and sinusoidal excitation forces, the damping characteristics and damping effects of the D-CFBI with different stiffness are investigated. By exciting the isolator, the frequency sweep experiment is carried out, the dynamic flexibility in the initial state is studied, and a preload of the D-CFBI is given to change the stiffness of the D-CFBI, and the changing trend between restoring force and amplitude and frequency is found out. Conclusions can be summarized that increasing the preload of the D-CFBI leads to increased stiffness and better damping effect. For compression amounts of 20% and frequencies higher than 7Hz or amplitudes lower than 2mm, the damping effect is also better. Consequently, this D-CFBI offers satisfactory damping and increased safety at low amplitudes and frequencies. To achieve optimal damping, it is recommended to adjust the stiffness of the driver.

Statistical linearization for random vibration energy harvesting with piezoelectric material nonlinearity

Due to piezoelectric softening and dissipative nonlinearities, the piezoelectric cantilever energy harvester exhibits nonlinear hysteresis when subjected to large excitation. These nonlinearities have brought significant challenges to the modeling and response prediction of randomly excited piezoelectric energy harvesting systems. In this study, the voltage responses of the nonlinear piezoelectric cantilever energy harvester under random excitation are initially assumed to follow the Gaussian distribution which is experimentally validated later. The equivalent linear transfer function are derived from the approximate linearization of the stiffness and damping using the statistical linearization (SL) technique. The mathematical expectations of the voltage responses are calculated from the multivariate normal distributions. Frequency sweep experiments are conducted to a cantilever energy harvester to identify the nonlinear piezoelectric material properties. The statistically linearized model was experimentally validated under random base acceleration excitation by comparing the probability density function of the predicted voltage responses and average power against the experimental measurements. The advantage of the SL technique lies in allowing one to use an iterative procedure to estimate the equivalent linear terms while the analytical expressions are unattainable because of the complex nonlinearity in the governing equations. The results show that the prediction of the SL model to the random base acceleration excitation agrees with experimental measurements with a broadband frequency range, although only the fundamental mode of the beam is considered.

Freezing-induced loss of wheat starch granule-associated proteins affected dough quality: From water distribution, rheological properties, microstructure, and gluten development

This study aimed to explore the effect of freezing/thawing (F/T) treatment on starch granule-associated proteins (SGAPs) and its mechanism for modifying dough quality. SDS-PAGE showed that F/T mainly induced a loss of proteins from the starch surface (SGSPs). It was found that the decrease in associated protein content increased the content of bound water from 27.04% and 18.94% to 32.40% and 27.78% in starch and reconstituted dough systems, respectively. The absence of SGSPs resulted in a rheological behavior with higher dynamic modulus and lower strain detected by frequency sweep and creep-recovery experiments. CLSM exhibited a local aggregation of the gluten network and more starch granules dissociated. The glutenin macropolymers (GMP) content increased from 76.45% to 83.31% while the values of fluorescence intensity and free sulfhydryl (SH) decreased to 2821 and 1.85 μmol/g, respectively. These results suggested that SGSPs acted as a contact site in the “starch-gluten” matrix, facilitating macromolecular adhesion.

Analysis and experimental study on vibration response characteristics of mechanical harvesting of jujube

To explore the vibration energy transmission characteristics of mechanical harvesting of jujube and improve the effectiveness of vibration harvesting, the vibration response characteristics of small and densely planted Jun jujube branches were experimentally studied and evaluated. According to the agronomic requirements of mechanical harvesting of jujube, first, this paper analyzes the vibration harvesting mechanism of jujube, establishes the transverse bending vibration mechanical model of jujube branches, and obtains the sixth-order natural frequency of jujube branches through vibration frequency sweep experiments. Based on the slope scoring principle and response surface method, the impact of different excitation parameters on the jujube branch vibration effect evaluation index Pz was characterized through field experiments, and an optimized vibration effect score was obtained. The results showed that the low-order natural frequencies of small and densely planted jujube branches were mainly concentrated in the range of 6 ∼ 22 Hz, and the vibration response was the most intense at 17.5, 18.0 and 22.5 Hz, which was more conducive to vibration harvesting. By optimizing the relationship between the excitation parameters and evaluation index Pz, a better jujube branch vibration effect score of Pz = 85.63 is obtained, which provides a reference for the design and development of high-efficiency jujube vibration harvesting devices.

Effect on the Thermo‐Gelation Process of the Degree and Molar Substitution of HPMC Polymer Hydrogels

AbstractHydroxypropyl methylcellulose (HPMC) is a polymer made of a cellulose backbone with etherified methoxy and hydroxypropyl substitutions that undergoes a reversible inverse thermo‐gelation upon heating. The thermo‐gelation follows a two‐step mechanism: first, phase separation occurs, then fibrils, formed in the polymer‐rich phase, entangle to form the three‐dimensional gel network. This work aims at understanding the influence of degree (DS) and molar substitution (MS) on the thermo‐gelation process. DS is the number of methyl groups, and the MS is the hydroxypropyl content per anhydroglucose unit. The thermo‐gelation process is here rheologically investigated for two HPMC samples, which have similar molecular weight but different MS and DS: thermal ramps are coupled with time sweep tests at selected temperatures and with frequency sweep experiments. Results show an important effect not only on the transition temperatures (beginning of the phase separation, beginning of the gelation) but also on the kinetics of the different phases of the process.

Steady-State and Dynamic Rheological Properties of a Mineral Oil-Based Ferrofluid

In this study, nanoparticles were suspended in L-AN32 total loss system oil. The thixotropic yield behavior and viscoelastic behavior of ferrofluid were analyzed by steady-state and dynamic methods and explained according to the microscopic mechanism of magneto-rheology. The Herschel–Bulkley (H–B) model was used to fit the ferrofluid flow curves, and the observed static yield stress was greater than the dynamic yield stress. Both the static and dynamic yield stress values increased as the magnetic field increased, and the corresponding shear thinning viscosity curve increased more significantly as the magnetic field strength increased. The amplitude scanning results show that the linear viscoelastic region (LVE) is reached when the shear stress is 10%. The frequency scanning results showed that the storage modulus increased with the increase of the frequency at first. The storage modulus increased steadily at a higher frequency range, while the loss modulus increased slowly at the initial stage and rapidly at the later stage. In the amplitude sweep and frequency sweep experiments, the energy storage modulus and loss modulus are enhanced with the decrease of temperature. These findings are helpful to better understand the microscopic mechanism of magneto-rheology of ferrofluids, and also provide guidance for many practical applications.

A dynamic stiffness improvement method for thin plate structures with laminated/embedded shape memory alloy actuators

Cantilever thin plates are commonly used as external structures in different aircraft, e.g., missile fins and aircraft rudders, which have important functions such as adjusting attitude and providing lift. However, when the aircraft is flying at a high speed, the elastic modulus of the material will be significantly reduced due to the thermal effect. This will decrease the dynamic stiffness of the aircraft and thereby lead to flutter and other serious problems. This study aims to investigate the dynamic stiffness improvement method for thin-walled structures at high temperatures with shape memory alloy (SMA) actuators. A theoretical model of an SMA laminated aluminum thin plate is established first. Then an improved stiffness enhancement method with distributed SMA actuators is proposed. The configuration of the actuator is designed and verified by the finite element method. A frequency prediction model of the thin plate structure is established with Back Propagation (BP) neural network and optimal placement of the actuator is achieved based on a genetic algorithm. The validity of the proposed method is verified through both numerical simulations and high-temperature frequency sweep experiments. The results show that the stiffness of the thin plate structure can be significantly improved by the SMA actuators. Specifically, the first-order bending frequency and first-order torsional frequency of the structure can be improved by nearly 10% at 200 °C.

Rheological Response of Polylactic Acid Dispersions in Water with Xanthan Gum.

In this work, the rheological behavior of stable poly(lactic acid) (PLA) dispersions in water, intended for coating applications, was investigated. The newly prepared dispersion consists of PLA particles with an average diameter of 222 ± 2 nm based on dynamic light scattering (DLS) and scanning electron microscopy (SEM) analyses, at concentrations varying in the 5–22 wt % range. Xanthan gum (XG), a bacterial polysaccharide, was used as a thickening agent to modulate the viscosity of the formulations. The rheological properties of the PLA dispersions with different XG and PLA contents were studied in steady shear, amplitude sweep, and frequency sweep experiments. Under steady shear conditions, the viscosity of all the formulations showed a shear-thinning behavior similar to XG solutions in the whole investigated 1–1000 s–1 range, with values dependent on both PLA particles and XG concentrations. Amplitude and frequency sweep data revealed a weak-gel behavior except in the case of the most diluted sample, with moduli dependent on both PLA and XG contents. A unified scaling parameter was identified in the volume fraction (ϕ) of the PLA particles, calculated by considering the dependence of the continuous phase density on the XG concentration. Accordingly, a master curve at different volume fractions was built using the time–concentration–superposition approach. The master curve describes the rheological response of the system over a wider frequency window than the experimentally accessible one and reveals the presence of a superimposed β relaxation process in the high-frequency region.