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Piezoelectret metamaterials are a kind of piezoelectrets with distinctive properties such as negative Poisson's ratio and positive piezoelectric d31 coefficient. These materials hold significant promise for applications in sensing and mechanical energy harvesting. In this article, flexible piezoelectret metamaterials with a simple double-V cell structure are designed, and their properties are theoretically analyzed, simulated, and experimentally tested. The results indicate that the effective piezoelectric d33 and d31 coefficients can be tuned over a wide range by adjusting the geometric parameters of the cell. The experimental results are consistent with the theoretical prediction and simulation calculation. Differing from the piezoelectric d33 coefficient, which always has positive polarity, the sign of the piezoelectric d31 coefficient depends on Poisson's ratio of the material. In particular, d31 coefficients are negative for the materials with positive Poisson's ratio, while they have a positive sign when Poisson's ratio is negative. For a piezoelectret metamaterial sample prepared in this study, a large effective piezoelectric d31 coefficient up to 1200 pC/N is achieved experimentally. The significant piezoelectric effects in the piezoelectret metamaterials as well as their excellent mechanical adaptivity to irregular surface structures, such as the curved surfaces of humanoid robots and aircraft wings, introduce a novel solution for diverse applications. [ABSTRACT FROM AUTHOR]

Diffusion of electrons over distances on the order of 100 μm has been observed in crystals of a small tetraheme cytochrome (STC) from Shewanella oneidensis [J. Huang et al. J. Am. Chem. Soc. 142, 10459–10467 (2020)]. Electron transfer between hemes in adjacent subunits of the crystal is slower and more strongly dependent on temperature than had been expected based on semiclassical electron-transfer theory. We here explore explanations for these findings by molecular-dynamics simulations of crystalline and monomeric STC. New procedures are developed for including time-dependent quantum mechanical energy differences in the gap between the energies of the reactant and product states and for evaluating fluctuations of the electronic-interaction matrix element that couples the two hemes. Rate constants for electron transfer are calculated from the time- and temperature-dependent energy gaps, coupling factors, and Franck–Condon-weighted densities of states using an expression with no freely adjustable parameters. Back reactions are considered, as are the effects of various protonation states of the carboxyl groups on the heme side chains. Interactions with water are found to dominate the fluctuations of the energy gap between the reactant and product states. The calculated rate constant for electron transfer from heme IV to heme Ib in a neighboring subunit at 300 K agrees well with the measured value. However, the calculated activation energy of the reaction in the crystal is considerably smaller than observed. We suggest two possible explanations for this discrepancy. The calculated rate constant for transfer from heme I to II within the same subunit of the crystal is about one-third that for monomeric STC in solution. [ABSTRACT FROM AUTHOR]

The purpose of this work was to provide a fundamental, in-depth analysis of kinematics and kinetics of the bobsleigh push phase to establish a basis for performance analysis and enhancement. Fifteen elite male athletes performed maximal effort push starts, while ground reaction forces (GRF) and 3D marker trajectories were simultaneously recorded for ground contacts of different sub-sections of the push phase (start acceleration phase: first and second ground contact after the initial push-off from the start block, acceleration phase: 10 m and high-velocity phase: 30 m). To obtain a comprehensive view of the push phase, whole-body kinematics as well as joint kinetics were analysed and compared across the push phase. The results showed that propulsion during the start acceleration was hip extensor dominant. With increasing running speed, the contribution to propulsion increased at the ankle and decreased at the knee. In contrast to unresisted sprinting, bobsleigh athletes relied more on mechanical energy generation at the hip than at the ankle, especially during start acceleration. These findings should be considered for the strength and conditioning of bobsleigh athletes and further investigated in relation to a suitable performance measure. [ABSTRACT FROM AUTHOR]

As a novel environmental treatment technology, piezo-photocatalysis, which can simultaneously harvest mechanical and solar energy, and convert it into chemical energy to drive redox reactions, is currently receiving widespread attention. In this study, Bi 4 Ti 3 O 12 irregular particles and Bi 4- x La x Ti 3 O 12 (x = 0, 0.25, 0.75, 1) sheets were synthesized by solid-phase and one-step molten-salt methods respectively, and the effect of different La doping levels on piezo-photocatalytic activity of Bi 4- x La x Ti 3 O 12 sheets was investigated. Among all samples, Bi 3 La 1 Ti 3 O 12 exhibited the highest piezo-photocatalytic efficiency, achieving complete methyl orange (MO) degradation in just 60 min, with an apparent rate constant of 0.0705 min⁻1. This activity was approximately 2.78 and 1.95 times greater than that of Bi 4 Ti 3 O 12 irregular particles and Bi 4 Ti 3 O 12 sheets respectively, also surpassing the performance of most piezo-photocatalysts reported. The mechanism of the enhanced piezo-photocatalytic activity of Bi 4- x La x Ti 3 O 12 is deeply analyzed by combining finite element (FEM) simulation analysis with piezo-response force microscopy (PFM). The study demonstrates that the utilization of the molten salt method and appropriate amount of La doping increases the specific surface area, optimizes energy band structure, as well as promotes the synergy between the piezoelectric and photocatalytic effects, which contribute to the excellent piezo-photocatalytic activity of Bi 3 La 1 Ti 3 O 12. In this work, it is shown that the combination of the preparation method and doping strategy has enabled the successful enhancement of the piezo-photocatalytic performance of the piezoelectric material, thereby greatly expanding its potential for practical applications. [Display omitted] [ABSTRACT FROM AUTHOR]

While piezoelectric sensing and energy-harvesting devices still largely rely on inorganic components, biocompatible and biodegradable piezoelectric materials, such as cellulose nanocrystals, might constitute optimal and sustainable building blocks for a variety of applications in electronics and transient implants. To this aim, however, effective methods are needed to position cellulose nanocrystals in large and high-performance architectures. Here, we report on scalable assemblies of cellulose nanocrystals in multilayered piezoelectric systems with exceptional response, for various application scopes. The submicrometer patterning with effective-flow topography and multilayer stacking promote piezoelectric performance. Record output power and pressure sensitivity in the gentle touch range are obtained in flexible, fully biodegradable systems with stable piezoelectric properties and demonstrated compatibility with different cell lines and implanted devices. These architectures offer new design principles for piezoelectric sustainable materials and for realizing an innovative class of practical components for mechanical energy harvesting and biologically relevant wearables and implants. [ABSTRACT FROM AUTHOR]

This work aims to determine the mechanism of the photomechanical response of poly(Methyl methacrylate) polymer doped with the photo-isomerizable dye Disperse Red 1 using the non-isomerizable dye Disperse Orange 11 as a control to isolate photoisomerization. Samples are free-standing thin films with thickness that is small compared with the optical skin depth to assure uniform illumination and photomechanical response throughout their volume, which differentiates these studies from most others. Polarization-dependent measurements of the photomechanical stress response are used to deconvolute the contributions of angular hole burning, molecular reorientation and photothermal heating. While photo-isomerization of dopant molecules is commonly observed in dye-doped polymers, the shape changes of a molecule might not couple strongly to the host polymer through steric mechanical interactions, thus not contributing substantially to a macroscopic shape change. To gain insights into the effectiveness of such mechanical coupling, we directly probe the dopant molecules using dichroism measurements simultaneously while measuring the photomechanical response and find mechanical coupling to be small enough to make photothermal heating—mediated by the transfer of optical energy as heat to the polymer—the dominant mechanism. We also predict the fraction of light energy converted to mechanical energy using a model whose parameters are thermodynamic material properties that are measured with independent experiments. We find that in the thin-film geometry, these dye-doped glassy polymers are as efficient as any other material but their large Young's modulus relative to other organic materials, such as liquid crystal elastomers, makes them suitable in applications that require mechanically strong materials. The mechanical properties and the photomechanical response of thin films are observed to be significantly different than in fibers, suggesting that the geometry of the material and surface effects might play an important role. [ABSTRACT FROM AUTHOR]

This study focuses on the development of a novel 0–3 bio-piezoelectric composite utilizing bismuth titanate (Bi 4 Ti 3 O 12) as the piezoelectric ceramic, chitosan as the polymer matrix, and graphene as a doping element. The primary objective of this study was to study the relationship between the mechanical, piezoelectric, and electrical properties within the developed 0–3 bio-piezoelectric composite. By systematically varying the weight percentages of bismuth titanate ceramic and graphene, our aim was to gain a comprehensive understanding of how these variations influence key properties such as young's modulus for flexibility, the piezoelectric coefficient d 33 , electrical conductivity, and voltage output. This exploration was driven by the recognition that these properties are interconnected and crucial for the performance of piezoelectric materials in practical applications. Response surface methodology (RSM) was specifically utilized to model the composite response, optimizing process variables through statistical and mathematical analyses. Through rigorous experimentation and modeling the optimal balance that maximizes conductivity, enhances piezoelectric performance, and ensures reliable voltage generation was determined. The optimum formula selected is the one prepared using a piezoelectric ceramic weight percentage of 35,07 %, and graphene weight percentage of 0.9 % for a harvested voltage equal to 12,05 V. The Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) Images of the optimized composite system revealed the uniform distribution of the fillers in the polymer matrix. The optimized composite design resulting from this systematic investigation holds significant promise for practical applications in energy harvesting and sensor technologies, aligning with the demands of Industry 4.0 and IoT technologies. [ABSTRACT FROM AUTHOR]

Fuel cell heavy-duty trucks (FCHDTs) are pivotal for achieving carbon emission reductions in the transportation sector, with energy flow distribution and transient characteristics critical to their overall efficiency. This study presents comprehensive energy flow tests on an FCHDT under various operating conditions, providing a detailed analysis of energy distribution and subcomponent behavior. Results demonstrate significant variations in hydrogen consumption, battery charging/discharging and mechanical energy recovery patterns across different driving cycles. During transient conditions, hydrogen consumption frequently fluctuates with changes in vehicle speed and power demand, while battery state of charge (SOC) is affected by multiple factors and shows no direct correlation with hydrogen consumption. Proton Exchange Membrane Fuel Cell (PEMFC) efficiency declines with increasing power output due to reduced electrochemical reaction efficiency from intensified mass and heat transfer limitations, and brake energy recovery is found to be crucial for capturing and converting kinetic energy into electrical energy, particularly on complex terrains or under heavy loads. These findings provide valuable insights and guidance for the development of high-performance FCHDTs. [Display omitted] • Energy flow tests of FCHDT were conducted across different driving cycles. • Energy flow distribution and transient characteristics were thoroughly analyzed. • FCS efficiency declines with higher output power due to lower electrochemical efficiency. • Brake energy recovery plays a dominant role in vehicle operation under heavy loads. [ABSTRACT FROM AUTHOR]

Mechanoluminescence (ML) is a unique luminescent phenomenon that converts mechanical energy into light energy. Recently, it has attracted the attention of many researchers due to its potential applications in visible stress sensing, wearable luminescent devices, self-powered electronic skin, and luminescent anti-counterfeiting. However, the absence of standardized testing methods and metrics for evaluating the performance of ML materials poses a hinderance to the development of ML. In this review, we focus on the ML testing methods, with an emphasis on the testing specimen, force application equipment and optical signal acquisition systems. Finally, we conclude with discussions on the future necessity and existing problems regarding the standardization of characterization methods for ML properties, aiming to provide a reference idea for the standardization of ML. [ABSTRACT FROM AUTHOR]

The structural and mechanical properties of dickite and nacrite are important in numerous scientific and industrial applications, but currently lacking in experimental data. To address this issue, numerical methods have been employed to quantify these properties. First, the interaction potential parameters for dickite and nacrite were determined using a relaxed fitting approach. Then, the properties were calculated with energy minimization. The results revealed that the interaction potential parameters and properties exhibit polytypic-specific dependence. When compared to available experimental and simulation data at zero pressure, such as lattice parameters, elastic constants, bulk modulus, and shear modulus, the calculated results are in good agreement, confirming the reliability of the numerical methods. In addition, properties challenging to determine experimentally, such as transverse and longitudinal wave velocities, dielectric properties, and piezoelectric constants, were also investigated, offering valuable contributions to the understanding of these kaolin polytypes. [ABSTRACT FROM AUTHOR]

A magnetically assembled electrode (MAE) is a modular electrode format in electrochemical oxidation wastewater treatment. MAE utilizes magnetic forces to attract the magnetic catalytic auxiliary electrodes (AEs) on the main electrode (ME), which has the advantages of high efficiency and flexible adjustability. However, the issue of the insufficient polarization of the AEs leaves the potential of this electrode underutilized. In this study, natural tourmaline (Tml) particles with pyroelectric and piezoelectric properties were utilized to solve the above issue by harvesting and converting the waste energy (i.e., the joule heating energy and the bubble striking mechanical energy) from the electrolysis environment into additional electrical energy applied on the AEs. Different contents of Tml particles were composited with Fe3O4/Sb-SnO2 particles as novel AEs, and the structure–activity relationship of the novel MAE was investigated by various electrochemical measurements and orthogonal tests of dye wastewater treatment. The results showed that Tml could effectively enhance all electrochemical properties of the electrode. The optimal dye removal rate was obtained by loading the AEs with 0.2 g·cm−2 when the Tml content was 4.5 wt%. The interaction of current density and Tml content had a significant effect on the COD removal rate, and the mineralization capacity of the electrode was significantly enhanced. The findings of this study have unveiled the potential application of minerals and energy conversion materials in the realm of electrochemical oxidation wastewater treatment. [ABSTRACT FROM AUTHOR]

To investigate the mechanical and energy evolution characteristics of fractured rock under true triaxial stresses, true triaxial strength compression experiments on fractured sandstone were conducted with varying crack lengths and widths. The results indicate that under true triaxial stresses, the peak stress of the rock exhibits a gradual decline with an increase in crack length and width. Meanwhile, crack initiation stress and crack damage stress of fractured sandstone also demonstrate a declining trend overall, and the influence of crack length on the characteristic stress (crack initiation stress and crack damage stress) of sandstone is more pronounced than that of crack width. According to the energy analysis results, the total strain energy of fractured sandstone gradually decreases with an increase in crack length and width. The results offer a theoretical foundation for the strength assessment and stability management of fractured rock materials during deep coal mining operations. [ABSTRACT FROM AUTHOR]

We propose a new kinetic BGK-type model for a mixture of four monatomic gases, undergoing a bimolecular and reversible chemical reaction. The elastic and reactive interactions are described separately by distinct relaxation terms and the mechanical operator is the sum of binary BGK contributions, one for each pair of interacting species. In this way, our model separately incorporates the effects of mechanical processes and chemical reactions. Additionally, it retains the effects of inter-species interactions which are proper of the mixture. The dependence of Maxwellian attractors on the main macroscopic fields is explicitly expressed by assuming that the exchange rates for momentum and energy of mechanical and chemical operators coincide with the ones of the corresponding Boltzmann terms. Under suitable hypotheses, the relaxation of the distribution functions to equilibrium is shown through entropy dissipation. Some numerical simulations are included to investigate the trend to equilibrium. [ABSTRACT FROM AUTHOR]

Utilizing lead-free dielectric materials for mechanical energy harvesting and energy storage is an inevitable choice for environmental friendly energy storage devices. In this paper, (1-x) [(K0.5Na0.5)(Nb0.9Sr0.05Ta0.08)O3]-x (CaZrO3) (KNN-CZ, x=0, 0.05, 0.07, 0.09) ceramics were prepared by solid-state reaction method, and their phase structure, microscopic morphology, dielectric and energy-storage properties were systematically studied. The results show that the KNN-CZ ceramics exhibit good relaxation and the P-E hysteresis lines are “thin”, which is due to the long-range ordered structure of KNN is destroyed by doping. Among them, 0.93KNN-0.07CZ has the best energy storage performance with effective energy storage density of 1.98 J/cm~3, which is due to the higher energy storage efficiency (83.19%) and breakdown field strength (250 kV/cm) at x=0.07. The above results provide an environmentally friendly and promising material for ceramic capacitors.  [ABSTRACT FROM AUTHOR]

In this work, we investigate a dynamic internal dissipation mechanism in the context of cement-based materials by introducing a 1D-enhanced micromorphic beam model with a dynamic internal friction term. Here, we consider an inherent feature in concrete-like materials arising from the multi-scale structure, namely, microcracks. Thus, we assume that the internal dissipation of the energy depends on the overall relative sliding displacement of the opposite faces in the microcracks under the effects of an applied cyclic load whenever no significant phenomena related to damage occur at the macroscopic level. The dynamic friction term is based on a well-known model for dry friction in solids due to P. R. Dahl, where the friction force depends only on the sliding displacement and evolves in time, reproducing an elastoplastic behavior. The model proposed in this paper takes into account a mechanical energy interchange between both bending and shear distortion in the beam with the sliding occurring at the microcracks, a storage of mechanical energy because of the asperities inside the faces of the microcracks, and the dissipation of the energy that follows from the interaction between the bending and the microcracks. Numerical simulations of the kinematic descriptors and the dissipative cycles are also provided by using the Finite Element Method and the commercial software COMSOL Multiphysics®. [ABSTRACT FROM AUTHOR]

Flexible electromechanical sensors frequently suffer from unexpected impact loadings caused by slipping, collisions and falling objects, to name a few. Without sufficient protection, these undesired impacts would lead to critical mechanical instability even damage to flexible sensors, resulting in restricted measurement range and imprecise sensing. Thus, it is of significance, but still is a fresh challenge to enhance the mechanical stability and energy‐absorption capacity of flexible sensors under impacts. Here, a multi‐design strategy is proposed to construct an interpenetrating‐phase cellulose‐acetate composite (IPC2) architecture for flexible sensors in impact‐intensive sensing applications. The external structure mimics bellows‐morphology of beverage‐straws that deform in programmed loading direction to enhance the mechanical stability, while the internal conductive core has a co‐continuous interpenetrating‐phase architecture that can efficiently absorb impact energy. Systematic numerical analysis and experimental tests demonstrate that IPC2 architecture presents excellent structural stability, cyclic performance and a unique combination of exceptional specific energy absorption (SEA = 2.66±1.2 kJ kg−1), low density (ρ = 720±10 kg m−3), electromechanical properties (GF≈39.6). Remarkably, the recovery behaviors in terms of shape and electrical signals show good repeatability and reliability. This study offers a new composite framework to exploit the potentialities of flexible sensors with protective functions and commercial values. [ABSTRACT FROM AUTHOR]

Perovskite La0.6Sr0.4Fe1−xCuxO3−δ (x = 0, 0.05, 0.1, and 0.2) polycrystalline samples have been synthesized in air and investigated by X‐Ray diffraction, scanning electron microscope, magnetization, and mechanical spectroscopy. An antiferromagnetic transition is observed around 300 K, while no corresponding anomaly is observed in the mechanical spectrum, indicating the absence of conventional magnetoelastic coupling. For La0.6Sr0.4FeO3, an internal friction peak (P1) presents around 140 K and shifts to lower temperatures with increasing Cu‐doping content. Meanwhile, a magnetic anomaly is also observed around P1 peak temperature. As explained, the P1 peak is related to the freezing of the ferroelastic domain walls, and the mechanical energy dissipation is induced by the lagging variation of the octahedral tilting under the alternating stress. This work suggests a peculiar magnetic property of the octahedra within ferroelastic domain walls. [ABSTRACT FROM AUTHOR]

The triboelectric nanogenerator (TENG) has emerged as a promising technology for efficiently converting ambient mechanical energy into electrical energy. Among various designs, the disk-based rotational TENG has demonstrated significant potential, as it can continuously harvest energy in a sliding mode via a grating mechanism. However, horizontal mechanical energy is more common than rotational energy in many practical applications. Herein, the present study introduces a novel device: the double horizontal linear-to-rotational triboelectric nanogenerator (DHLR-TENG). This innovative approach utilizes a gear system to convert horizontal linear mechanical energy into electrical energy. The experimental results revealed that the DHLR-TENG produces a full cycle of alternating current (AC) when integrated into an electrical circuit. It consistently delivers robust performance with an open-circuit voltage of 544 V, a short-circuit current of 61.16 µA, and a maximum power output of 33.27 mW. Additionally, the device durability, capable of withstanding over 1,000,000 cycles, makes it highly effective for powering small electronic devices, such as charging capacitors and illuminating commercial LEDs. The DHLR-TENG's versatility and efficiency mark it as a major advancement in energy harvesting, with broad implications for powering portable electronic devices in a wide range of environments. [ABSTRACT FROM AUTHOR]

In this work, we focused on the characterization of closed-cell Al foams and aluminum honeycomb panels, in particular their energy absorption capacity under conditions of static compressive stress. Through experimental tests, the specific energy absorbed by different samples was evaluated: in the honeycomb panels the mechanical behavior was analyzed both for large assemblies and for structures with a reduced number of cells, and the effect of the number of cells was studied too. Furthermore, for larger structures, the specific energy absorbed was calculated from stress–strain compressive graphs. For the closed-cell Al foams, manufactured in the laboratory using the powder compaction method with different percentages of SiC and TiH2 and characterized by different relative densities, the specific energy absorbed was evaluated too. The experimental results showed that the specific energy absorbed by the Al honeycomb was always higher than that of the different types of closed-cell foams. However, when selecting the material for each specific application, it is necessary to take into account numerous parameters such as the relative density, absorbed energy, peak stress, plateau stress, plateau extension, densification strain and so on. Consequently, the overall performance must be evaluated from time to time based on the type of application in which the best compromise between strength, stiffness and lightness can be achieved. [ABSTRACT FROM AUTHOR]

Eco‐friendly and antimicrobial globular protein lysozyme is widely produced for several commercial applications. Interestingly, it can also be able to convert mechanical and thermal energy into electricity due to its piezo‐ and pyroelectric nature. Here, we demonstrate engineering of lysozyme into piezoelectric devices that can exploit the potential of lysozyme as environmentally friendly, biocompatible material for mechanical energy harvesting and sensorics, especially in micropowered electronic applications. Noteworthy that this flexible, shape adaptive devices made of crystalline lysozyme obtained from hen egg white exhibited a longitudinal piezoelectric charge coefficient (d ~ 2.7 pC N−1) and piezoelectric voltage coefficient (g ~ 76.24 mV m N−1) which are comparable to those of quartz (~2.3 pC N−1 and 50 mV m N−1). Simple finger tapping on bio‐organic energy harvester (BEH) made of lysozyme produced up to 350 mV peak‐to‐peak voltage, and a maximum instantaneous power output of 2.2 nW cm−2. We also demonstrated that the BEH could be used for self‐powered motion sensing for real‐time monitoring of different body functions. These results pave the way toward self‐powered, autonomous, environmental‐friendly bio‐organic devices for flexible energy harvesting, storage, and in wearable healthcare monitoring. [ABSTRACT FROM AUTHOR]

For several decades, energy regeneration has been attempting to fulfill the growing demand for green and sustainable energy. Various devices have been designed and developed to capture energy and convert it into useful forms. Piezoelectric nanogenerators (PNGs) have been seen as a promising option for traditional rechargeable batteries because they directly scavenge a wide spectrum of unlimited mechanical energy. Piezoelectric materials exhibit extraordinary electrical properties, great adaptability, superior maneuverability, and durability. Among the various materials used for developing piezoelectric materials, polyvinylidene fluoride (PVDF) and its derivatives have been known to be the best options for fabricating nano-piezoelectric producers. Nevertheless, the nanogenerator's piezo response generation is too small and insufficient to run thermionic equipment. Extensive efforts have been made to improve and reinforce PVDF-derived nano-piezoelectric devices. Considering the key aspects of materials and production technologies, this review focuses on carbon-based nanocomposite materials, their manufacturing methods, and performance indicators. In addition, the corresponding cutting-edge methods, alternative models, and beneficial substances are highlighted to improve the piezoelectric structure, arrangement of electric doublets, charge carriers, etc. Consequently, productivity-based materials can transform mechanical energy into electricity, opening the door for PVDF-based nanogenerators to eventually become practical energy sources. [ABSTRACT FROM AUTHOR]

Perovskite La0.6Sr0.4Fe1−xCuxO3−δ (x = 0, 0.05, 0.1, and 0.2) polycrystalline samples have been synthesized in air and investigated by X‐Ray diffraction, scanning electron microscope, magnetization, and mechanical spectroscopy. An antiferromagnetic transition is observed around 300 K, while no corresponding anomaly is observed in the mechanical spectrum, indicating the absence of conventional magnetoelastic coupling. For La0.6Sr0.4FeO3, an internal friction peak (P1) presents around 140 K and shifts to lower temperatures with increasing Cu‐doping content. Meanwhile, a magnetic anomaly is also observed around P1 peak temperature. As explained, the P1 peak is related to the freezing of the ferroelastic domain walls, and the mechanical energy dissipation is induced by the lagging variation of the octahedral tilting under the alternating stress. This work suggests a peculiar magnetic property of the octahedra within ferroelastic domain walls. [ABSTRACT FROM AUTHOR]

Understanding the gradual performance degradation of carbon fiber reinforced polymers (CFRP) is critical for the design of engineering structures that are expected to be affected by hygrothermal environments. This study aims to investigate the effects of hygrothermal aging on the degradation mechanisms of the mechanical properties and energy absorption of CFRP structures. An experimental database comprising tensile, compressive, and shear tests for CFRP composite laminates (in this study) and axial crushing tests for energy-absorbing structures (from the literature) was constructed, in which all CFRP samples were immersed in deionized water to achieve a saturated water-absorption state. A material constitutive model considering the effects of water absorption and temperature was developed and implemented via the user subroutine VUMAT of the ABAQUS software. The simulated results correlated well with the experimental measurements. Simulation results of axial crushing indicated that the degradation of the inter-layer properties tends to worsen the mismatch between the intra-layer and inter-layer properties, thus significantly degrading the load-carrying capability. Owing to degradation in compressive fracture toughness, the simulated results showed reduced post-crushing integrity, thus indicating a favorable effect on the load-carrying capability. [ABSTRACT FROM AUTHOR]

The behavior of PET foam under quasi-static and dynamic compressive loads is analyzed. An experimental study is performed to evaluate its mechanical behavior and energy absorption characteristics at different strain rates. Also, a numerical model is developed to reproduce the dynamic compression behavior of the PET foam. Differences between the properties obtained in the quasi-static and dynamic tests show that the strength, stiffness and energy absorption efficiency are dependent on strain rate. The evolution of the Poisson ratio with strain and strain ratio is studied. A decrease in the Poisson ratio is observed as the strain rate increases. [ABSTRACT FROM AUTHOR]

The mechanical energy transfer between the source structure and active material in piezoelectric energy harvesters is investigated here. First, an analytical prediction to improve mechanical energy transfer into the piezoelectric material is proposed and validated by tailoring the stiffness of the beam to off-the-shelf piezocomposites through numerical models and experiments. An experimental study is then presented whereby the mechanical properties of freeze cast piezocomposites are controlled by adjusting the volume fraction of piezoceramic and epoxy filler phases. This offers a new approach for tailoring the mechanical properties of piezocomposites to maximize mechanical energy transfer into the material via stiffness matching. [ABSTRACT FROM AUTHOR]

This study investigates the axial compression mechanics and energy absorption characteristics of aluminum foam-filled carbon fiber-reinforced polymer (CFRP) tubes from both experimental and simulation perspectives. Quasi-static compression experiments of aluminum foam specimens, single CFRP tubes and aluminum foam-filled CFRP tubes are carried out. Based on the experimental results, compression simulations are conducted to investigate the structural deformation and the energy absorption characteristics of aluminum foam-filled CFRP tubes at the mesoscale. The effects of structural parameters on the compression mechanics and energy absorption characteristics of aluminum foam-filled CFRP tubes are explored. [ABSTRACT FROM AUTHOR]

Triboelectric nanogenerators (TENGs) represent an effective approach for transforming mechanical energy into electrical power, making them suitable for wearable electronic applications. Hydrogels as TENGs electrodes are common, but their use as direct triboelectric layers remains insufficiently explored. Here, a novel zwitterionic monomer 3‐{1‐[6‐(hydroxymethyl)‐2‐methyl‐3,8‐dioxo‐9‐aza‐4,7‐dioxadodec‐1‐en‐12‐yl]imidazol‐3‐ium‐3‐yl}propane‐1‐sulfonate (VNIPS) is synthesized in combination with acrylic acid (AA) and zwitterionic sulfobetaine methacrylate (SBMA) to create a double‐network zwitterionic hydrogel. The hydrogel is developed using a solvent‐exchange process that facilitated the creation of microphase‐separated domains, notablely increasing its mechanical strength (211.9 kPa, 472.3%), conductivity (0.6 mS cm−1), and anti‐freezing capability (−18.3 °C). In addition, the hydrogel's hydrophilic groups interacted with water molecules, reducing charge loss in humid conditions. When employed as the triboelectric positive layer, the hydrogel‐based TENGs achieved a substantial charge density of 456 µC m−2 and an output power density of 464 mW m−2, while maintaining a steady open‐circuit voltage (Voc) of 97 V, with 92% retention under 80% relative humidity. Moreover, the hydrogel's strong adhesion and biocompatibility make it suitable for wearable applications, such as motion sensing and Morse code communication. This work demonstrates the feasibility of zwitterionic hydrogels as triboelectric materials, providing a new strategy for creating efficient, humidity‐resistant energy harvesters. [ABSTRACT FROM AUTHOR]

Initiating or sustaining physical and chemical transformations with mechanical force – mechanochemistry – provides an opportunity for more sustainable chemical processes, and access to new chemical reactivity. These transformations, however, do not always adhere to 'conventional' chemical wisdom, making them difficult to design and rationalise. This challenge is exacerbated by the fact that not all mechanochemical transformations are equal, with mechanical force playing a different role in different types of processes. In this review we discuss some of the different roles mechanical force can play in mechanochemical transformations, set primarily against the author's own research. We classify mechanochemical reactions broadly as those (1) where mechanical energy is for mixing, (2) where mechanical energy alters the stability of the reagent, and (3) where mechanical energy directly excites the solid. Finally, we demonstrate how – while useful – these classifications have fuzzy boundaries and concepts from across them are needed to understand many mechanochemical reactions. [ABSTRACT FROM AUTHOR]

Mechanical metamaterials as a category of lightweight materials have exhibited superior specific mechanical properties and energy absorption. However, lack of reasonable structural design always results in weakening or failure to achieve the optimal mechanical performance of metamaterials. Here, we firstly adopted bio-inspired and structural optimization methods to guideline the design of microlattice structures. Inspired by the strong and ductile 'Moso' bamboo, three bamboo-inspired hierarchical microlattice structures (BHMSs) were initially designed, and the bamboo's features, viz., hollow, gradient distribution, and cellular, were imitated by designed BHMSs. Optimization design with the objective of maximum ratio of strain energy and reaction force was carried out to furthermore improve the mechanical properties of BHMSs. Afterwards, the optimal BHMSs were printed via Stereolithography. The compressive responses of BHMSs were elaborated by experiments and simulation. Results show that these designed biomimetic structures can be easily tailored via tuning the geometric sizes of unit cell, achieving high compressive specific modulus and strength 48.16 kPa m3 kg−1 and 1389.23 Pa m3 kg−1 and the energy-absorbing efficient 85.43% in the octet bamboo-inspired hierarchical microlattice structures (OCT BHMSs). The design strategies and findings shed light on the realization of advanced metamaterials with tailored mechanical properties. How to design ordered cellular structures for lightweight structures with high mechanical strength and energy absorption efficiency remains a crucial challenge in automation, aerospace and lightweight construction fields. Inspired by the lightweight and high load-bearing capacity of nature bamboos and metamaterials in the previous researches (Song et al., Materials & Design, 2019, 173: 107773; Song et al., International Journal of Fatigue, 2017, 100: 126), here a comprehensive approach of bionics and optimization was proposed to design hierarchical microlattice structures with lightweight, good mechanical properties and high energy absorption efficiency. The bamboo's characteristics, including hollow, gradient enhancement, and cellular, were completely copied to reasonably produce hierarchical microlattice configurations, while the optimal sizes were obtained though the optimization design. Highlights: A combination strategy of bamboo-inspired and optimization design to realize light-weight and high mechanical metamaterials was provided. Mechanical properties of biomimetic structures can be tailored by tuning dimensions based on optimization. A high energy absorption efficiency in octet-truss bamboo-inspired hierarchical microlattice structures. [ABSTRACT FROM AUTHOR]

The preparation of functionalised graphene often involves consuming significant amounts of organic solvents, complicated steps, and expensive equipment. This study presented a simple, low‐cost, and efficient method for preparing well‐dispersed functionalised graphene. This method involved the microwave heating of commercial graphene precursors and ball milling of grafted expanded graphite, resulting in a short and straightforward preparation process without requiring large amounts of organic solvents. This process enabled the preparation of few‐layer graphene with a thickness of only 3.5 ± 0.5 nm. During this process, the majority of the surface oxygen‐containing groups were replaced by polyetheramine (D2000) at a grafting rate of up to 5.14%, which improved the interface adhesion strength between the graphene and the epoxy resin. The fabricated altered graphene notably enhanced the mechanical characteristics of the epoxy resin., that is, the toughening effect reached up to 171% with a graphene content of only 0.3 wt%, while the Young's modulus and tensile strength values increased by 54% and 39%, respectively. This process is cost‐effective, easy to operate, and highly efficient, making it suitable for the large‐scale production of well‐dispersed functionalised graphene. Highlights: Pioneers mechanical chemical energy in graphene, a new materials science direction.First ball milling on microwave graphene, merging milling benefits with graphene.Ball milling cuts D2000 grafting time on graphene, boosting efficiency.Reduces organic solvent use, cutting costs and environmental effects.Ball milling lowers costs and impacts, aiding graphene material commercia‐lization. [ABSTRACT FROM AUTHOR]

The development of materials with excellent impact resistance is essential for safety protection. Polyurethane elastomer (PUE) is widely used in impact protection; however, it has low dynamic compressive strength and poor energy absorption efficiency under high‐speed impact, limiting its in‐depth application in the protection field. Herein, an organic montmorillonite (MMT) with interlayer anchored polymer chains, named M@NH3+, was synthesized via the dual modification of MMT involving surface grafting and cation intercalation by macromolecular amino silane, which was used to enhance the impact resistance of PUE. Based on the uniform dispersion of M@NH3+ within the PUE matrix, the strong bonding at the polymer‐filler interface and the synergistic action between M@NH3+ lamellae, PUE‐M@NH3+ composite exhibited excellent mechanical properties and energy absorption capacity. Compared with pure PUE, the elastic modulus (+98.3%), static compressive strength (+37.0%), dynamic compressive strength (+35.4%) and impact energy absorption (+50.8%) of PUE‐2wt%M@NH3+ were significantly improved, which shows that it has potential applications in the field of high‐speed impact protection. Highlights: Organic MMT with interlayer anchored polymer chains was prepared.The large interlayer spacing of M@NH3+ facilitates its dispersion within the matrix.The interconnected M@NH3+ lamellae can collaboratively disperse impact stress.The impact energy absorption of PUE composite has been remarkably improved. [ABSTRACT FROM AUTHOR]

Yarn artificial muscles offer an exciting avenue to replicate the extraordinary efficiency of biological muscles, converting chemical energy directly into mechanical work. Nevertheless, realizing the chemical‐mechanical energy conversion has posed significant challenges. In this study, a novel approach for harnessing direct catalysis to power yarn artificial muscles within a one‐compartment aqueous system is introduced. This research distinguishes itself through an innovative actuation mechanism using nanoscale catalytic particles. These nanoparticles synthesized and integrated onto the yarn surface act as a chemical trigger for muscle actuation. Notably, the resulting yarn muscle demonstrates a reversible tensile stroke of nearly 4% in ≈20 s. By bridging the gap between chemical catalysis and mechanical performance, this study paves the way for innovative applications in fields ranging from robotics to biomedical devices. [ABSTRACT FROM AUTHOR]

Hybrid nanogenerators are advanced mechanical energy harvesters capable of simultaneously scavenging multiple types of energy. Additionally, thermochromic materials provide a practical and visually assessable method for real‐time temperature monitoring. In this report, a novel energy harvester and sensing patch (EHSP) is introduced, that utilizes combined piezoelectric and triboelectric effects to harvest mechanical energy efficiently. To optimize the EHSP, various energy harvester configurations are fabricated and tested, and the dielectric properties of triboelectric films are systematically investigated. These improvements are implemented to augment the overall energy harvesting capability. The thermochromic properties of the EHSP are also explored to enhance both the electrical performance and thermal responsiveness. The EHSP demonstrates the ability to generate maximum voltage and current outputs of 350 V and 20.4 µA, respectively. Moreover, it can detect temperature changes within seconds, making it suitable for both energy harvesting and sensing applications. The EHSP is tested in practical scenarios, proving its efficiency as an energy harvester and sensor for everyday human activities. Furthermore, its integration with multiple hybrid nanogenerators showcases its potential for industrial and wearable sensing applications. [ABSTRACT FROM AUTHOR]

In this study, we conducted oxyacetylene ablation, followed by bending tests and comprehensive microstructural characterization, to investigate the mechanical behavior of C/HfC-SiC composites. The findings indicate a marked reduction in the flexural strength and fracture energy of ablated C/HfC-SiC composites, which decreased from 286.09 ± 17.56 MPa and 5.04 ± 0.27 mJ/mm2 to 190.97 ± 16.69 MPa and 3.80 ± 0.29 mJ/mm2, respectively. Notably, the fracture energy retention rate (75.40 %) of the composites exceeds the flexural strength retention rate (66.75 %). Based on the microstructure of cracks and energy release rate criterion, the failure process and crack propagation mechanism were analyzed. The ablation process was found to diminish the shear strength of the fiber-matrix interface, facilitating mechanical behaviors such as fiber debonding, slipping, and pull-out. Additionally, oxidative damage induces the simultaneous propagation of multiple cracks, thereby increasing energy consumption during the bending. These plastic mechanical behaviors not only increase the failure displacement but also enhance the plastic characteristics of the composites, allowing it to maintain higher flexural strength (from 69.3 MPa to 108.3 MPa) and fracture energy (from approximately 2.71 mJ/mm2 to 4.26 mJ/mm2) after failure (at the double failure displacement). [ABSTRACT FROM AUTHOR]

Triboelectric nanogenerators (TENGs) have attracted significant attention due to their capability to efficiently convert diverse forms of mechanical energy into electrical energy. However, maintaining operational efficiency and stability in the face of environmental temperature fluctuations is a crucial factor. In this study, microwave dielectric ceramics, specifically NaYW₂O₈ (NaYW) ceramic powder, were utilized for their excellent dielectric properties and high dielectric constant, and were seamlessly integrated with P(VDF-TrFE) to fabricate temperature-stable composite films as the triboelectric layer in the NaYW-TENG. The resulting NaYW-TENG exhibited outstanding electrical properties, achieving an open-circuit voltage (V OC) of 143.1 V and a short-circuit current (I SC) of 4.03 μA, which is 4.5 times higher than that of traditional ceramic-based P(VDF-TrFE) film TENGs. Notably, within the temperature range of - 10 °C–180 °C, the relative variations in V OC and I SC were 14.92 % and 22.33 %, respectively, meeting the requirements for wide-temperature range monitoring. Additionally, the NaYW-TENG demonstrated its versatility by simultaneously powering an LED display, while exhibiting high-precision human motion monitoring capabilities and excellent stability. This study not only advances the performance of TENGs from a material-centric perspective but also mitigates their sensitivity to environmental temperatures. [ABSTRACT FROM AUTHOR]

Ultrasonic vibration technology has significant potential for breaking hard rocks. Understanding the optimal frequency for rock breaking under ultrasonic vibration can significantly reduce the cost of rock breaking and extend the service life of polycrystalline diamond compact (PDC) cutters. This is important for practical engineering applications. This study presents a three-dimensional finite element model of rock breaking by a PDC cutter under ultrasonic vibration. The model was established using ABAQUS software and used to simulate the dynamic rock breaking process of the PDC cutter. A comparative analysis was performed between conventional rock breaking and rock breaking under ultrasonic vibration. According to the result, ultrasonic vibratory rock breaking is more likely to cause damage to the rock when a PDC cutter is used, particularly at a vibration frequency of 40 kHz. As the ultrasonic vibration frequency (20–40kHz) increases, the mechanical specific energy (MSE) initially decreases and then increases. The MSE reaches a minimum value at a frequency of 20–25 kHz, representing a decrease of 15.52%–22.24% compared with conventional rock breaking, which can significantly improve the rock breaking efficiency and reduce the drilling cost. The temperature of the PDC cutter increases significantly under ultrasonic vibration compared with conventional rock breaking. Additionally, the temperature of the PDC cutter increases gradually with an increase in the vibration frequency. These results provide theoretical support for the use of ultrasonic vibration technology. [ABSTRACT FROM AUTHOR]

In this paper, a particle method is used to approximate the solutions of a "fluid-like" macroscopic traffic flow model for automated vehicles. It is shown that this method preserves certain differential inequalities that hold for the macroscopic traffic model: mass is preserved, the mechanical energy is decaying and an energy functional is also decaying. To demonstrate the advantages of the particle method under consideration, a comparison with other numerical methods for viscous compressible fluid models is provided. Since the solutions of the macroscopic traffic model can be approximated by the solutions of a reduced model consisting of a single nonlinear heat-type partial differential equation, the numerical solutions produced by the particle method are also compared with the numerical solutions of the reduced model. Finally, a traffic simulation scenario and a comparison with the Aw-Rascle-Zhang (ARZ) model are provided, illustrating the advantages of the use of automated vehicles. [ABSTRACT FROM AUTHOR]

[Display omitted] • Recycled textiles are used to scavenge mechanical energy. • Silk-based TENG generated high output when contacting PVC film. • Devices are Integrated into shoe insoles to harvest energy from walking and jumping. • Rayon-based TENG generated the higher output while jumping on PVC coil mat. Textiles are an integral part of daily life globally, but their widespread use leads to significant waste generation. Repurposing these discarded fabrics for energy harvesting offers a sustainable solution to both energy demand and textile waste management. In this study, Textile-based Triboelectric Nanogenerators (T-TENGs) were developed using recycled cloth as tribopositive layers and polyvinyl chloride (PVC) film as the tribonegative layer, with aluminum foil tape serving as electrodes. Five different recycled textiles were evaluated, and Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDS) analysis revealed a correlation between yarn structure and carbon content, leading to enhanced triboelectric performance. Silk-based TENG (S-TENG) demonstrated the highest output, with 320.76 V and 8.73 µA, while exhibiting stable performance over 10,000 cycles. Practical applications were explored by integrating T-TENGs into shoe insoles for energy harvesting during walking and jumping, with rayon-based TENG generating up to 208.52 V on a PVC coil mat. This work highlights the dual benefits of waste reduction and sustainable energy applications, making a compelling case for advanced technologies where recycled textiles function as frictional materials to harvest mechanical energy from human motion and convert it into electrical energy for use in flexible sensors and wearable devices. [ABSTRACT FROM AUTHOR]

The expansion tube plays a crucial role in the field of train collision safety protection. In this study, we combine the expansion characteristics of negative Poisson's ratio (NPR) with the deformation mechanism of the expansion tube to propose a novel design for material reconstruction: embedding NPR structures into the conventional expansion tube wall. The typical specimens of NPR tube were fabricated using additive manufacturing technology, and the experiments and numerical simulations were conducted to investigate its expansion behavior by two expansion cones with different sizes. By employing a validated numerical simulation model, the effects of cone size, expansion angle, wall thickness, and material gradient design on the mechanical response of NPR tubes were investigated. It is demonstrated that the expansion behavior and energy absorption mechanism of NPR tubes differ from that of conventional circular tube. The material of wall generates radial expansion rather than compression due to a special auxeticity characteristics subjected to circumferential tensile load. The increased normal pressure on the inner surface from the expansion cone results in the resistance reinforcement of NPR tube. The proposed expansion tube achieves higher load capacity while utilizing less material, which provides a promising approach for designing lightweight and efficient expandable tubes. • A novel NPR circular tube for energy absorption was proposed. • Light-dark fringes related to the deformation was found on inner wall of NPR tube. • Layer ratio of NPR has a significant effect on the energy absorption and deformation. [ABSTRACT FROM AUTHOR]

The traction motor realizes the mutual conversion of electrical energy and mechanical energy during the train traction and braking process and is a key component of high-speed trains. The normal operation of the motor is directly related to the safety of high-speed train operation. Changes in temperature signals can reflect faults in the traction motor. By analyzing the internal and external influencing factors of temperature signals, a multi-factor prediction model for traction motors is established based on the maximal information coefficient and improved support vector regression. In this model, highly relevant features selections are performed based on time-delayed sequences and the maximal information coefficient. Using the adaptive particle swarm algorithm to optimize the improved support vector regression algorithm can enhance its accuracy and efficiency. Furthermore, using the K-nearest neighbor algorithm for error prediction will yield more accurate results. By comparing the R M S E , M B E , M A E , and other evaluation metrics of different algorithms under various working conditions, the results show that the prediction method proposed in this paper performs well across different working conditions. This method demonstrates greater adaptability to varying conditions and is more suitable for applications involving high-speed trains. [ABSTRACT FROM AUTHOR]

This study aims to develop flexible, biocompatible mechanical energy harvesters for self‐powered sensor applications, utilizing lead‐free potassium sodium niobate (K0.5, Na0.5) NbO3 (KNN) ceramic fillers embedded in natural rubber (NR) matrix. KNN ceramic fillers are synthesized using the conventional mixed oxide method and incorporated into the NR matrix to evaluate the effect of varying filler concentrations on the piezoelectric, dielectric, and mechanical properties of the composite. The findings suggest that as the KNN filler content increases, the electrical properties, including piezoelectric and dielectric performance, improve, whereas the mechanical properties, such as tensile strength and flexibility, decrease. This work offers a sustainable, lead‐free alternative for energy harvesting systems, with potential applications in flexible electronics, wearable devices, and biomedical sensors. [ABSTRACT FROM AUTHOR]

Carbon fiber-reinforced plastics (CFRPs) are known for their high strength and stiffness, essential in high-performance applications. However, in complex structures with fiber discontinuities, such as tapered sections and bolt holes, CFRP laminates are prone to interlaminar damage due to low interlaminar toughness. This study aimed to enhance CFRP interlaminar fracture toughness by inserting non-woven carbon-fiber-reinforced layers. Two non-woven tissues were used: resin-impregnated conventional carbon fiber and resin-free recycled carbon fiber. Evaluations of modes I and II interlaminar fracture toughness revealed that laminates with non-woven inserts had fracture toughness values two to three times higher than pristine CFRP. Non-woven tissues of varying lengths were embedded at ply discontinuities, and tensile tests were conducted to assess their effectiveness. Results confirmed that both non-woven tissues significantly improved interlaminar toughness and effectively suppressed delamination, attributed to high fiber content and complex crack paths, enhancing energy absorption and mechanical performance in CFRP laminates. [ABSTRACT FROM AUTHOR]

This article proposes a rotationally reinforced ribbed honeycomb structure by incorporating struts into the rotationally rigid honeycomb to enhance its stiffness. Finite element models of the honeycomb are developed using Abaqus/Explicit and validated through quasistatic experiments for accuracy. Based on the models, a series of studies on the honeycomb's structural behavior are conducted. Initially, the mechanical properties of the honeycomb under varying loading conditions and rib angles are analyzed. The results indicate that the RSH‐45 configuration exhibits the most favorable mechanical properties under both X‐direction and Y‐direction loading conditions. Specifically, under impact in the X‐direction, the RSH‐45 and RSH‐60 configurations demonstrate increases in energy absorption of 114.64% and 96.9%, respectively, compared to the RSH‐90 configuration. Subsequently, the mechanical properties of the RSH at different impact velocities are examined. The negative Poisson's effect of the RSH is most pronounced at low‐velocity, with the deformation modes changing as velocity increases. Under medium‐velocity impacts, RSH‐45 and RSH‐60 configurations exhibit a significant negative Poisson's ratio effect, while RSH‐75 and RSH‐90 configurations display a positive effect. In summary, reinforcing ribs produces a significant negative Poisson's ratio effect only at specific angles. [ABSTRACT FROM AUTHOR]

Architected cellular materials and structures provide the ability to tailor mechanical and functional properties based on design topological aspects. With the progressive advancement of additive manufacturing techniques, challenges and difficulties related to fabricating complex geometries are substantially reduced. Among different architected cellular materials, two types of closed-walls cellular materials, plate-lattices and triply periodic minimal surface (TPMS)–based lattices, provide outstanding mechanical properties. Plate-lattices are well-known for high stiffness, while TPMS lattices provide higher energy absorption capabilities. Herein, the mechanical behavior of the most two promising designs of both families is investigated experimentally and using finite-element analysis (FEA), namely sheet-based diamond TPMS and simple cubic–face-centered cubic–body-centered cubic (SC-FCC-BCC) plate-lattice. Fused deposition modeling (FDM) technology is utilized to fabricate the structures with acrylonitrile butadiene styrene (ABS) at several combinations of relative densities and unit cell sizes. Under quasi-static loading, diamond structures showed higher strength and energy absorption capabilities at various relative densities compared to plate-lattices. Based on experimental results, diamond is found to be 52% stiffer than the plate-lattice at low relative densities. These variations are diminished as relative density increased. ANOVA results, provided as main effects plots, show a significant dependence of mostly all mechanical properties on the three-dimensional (3D) topological design of the samples. Both structures presented outstanding mechanical energy absorption ability, suggesting their utilization in impact loading applications. [ABSTRACT FROM AUTHOR]

The long-term evolution of quasi-Keplerian systems is driven by a Hamiltonian that is independent of the fast angle. As this Hamiltonian may contain explicitly time-dependent parameters, the conservation of mechanical energy is not guaranteed in such systems. We derive how the semi-major axis evolves in these cases. We analyze two astrophysically interesting examples, those of the harmonic and quadrupole perturbations. [ABSTRACT FROM AUTHOR]