ABSTRACT Mechanical metamaterials with tunable multistability and vibration isolation are highly desirable for impact protection and vibration suppression in complex service environments. However, in most existing systems, geometric parameters are fixed during fabrication, resulting in predetermined mechanical responses that cannot be reconfigured without remanufacturing. This study propose a modular metamaterial design enabled by thermally programmed shape memory polymers. Thermal-triggered geometric programming reconstructs unit cell end constraints, allowing programmable regulation of stability type and nonlinear force – displacement behavior while preserving structural topology. A programmable stability window is established through parametric analysis, enabling controllable stability transition and load-level tuning. At the metamaterial level, interlayer differentiated programming achieves layered steady-state configurations and designed collapse propagation paths, producing hierarchical peak-sequence responses. Vibration experiments further verify that the isolation onset frequency can be tuned to match different applied masses, with programming shifting the isolation region toward lower frequencies. This work extends thermal shape memory programming from a material response to a structural regulation strategy, providing a post-fabrication route for reconfigurable mechanical performance in multistable metamaterials.

ABSTRACT We study thin viscoelastic layers with different constraints in steady-state adhesive sliding contact against wavy indenters. This is crucial for many applications involving compliant films and coatings, where adhesion, friction, and confinement may strongly interact, leading to distinct results compared to half-space geometries. Both a confined layer bonded to a rigid substrate and a free layer subjected to uniform pressure are considered. The problem is formulated within a rigorous energy- based framework for non-conservative viscoelastic materials. The results show that varying the thickness induces distinct responses on the two configurations for pull-off force, friction, and contact stability, with specific dependence on sliding speed due to different scale-dependent dissipation mechanisms. At low sliding speeds, small-scale dissipation localized at contact edges enhances effective adhesion. For thinner layers, this effect is suppressed by confinement and strongly amplified in free layers. Conversely, pull-off forces increase (decrease) in confined (free) layers. The friction coefficient under load control is independent of layer thickness and boundary conditions. At higher speeds, bulk viscoelasticity dominates, and friction decreases (increases) for confined (free) layers due to different bulk deformation mechanisms. A full-contact instability at intermediate velocities leads to discontinuous friction – velocity curves, a distinctive feature of adhesive viscoelastic contacts.

ABSTRACT 5G communication technology has spurred piezoelectric film bulk acoustic resonators (FBARs) to higher frequencies and thinner structures, highlighting the surface effect’s significance for their dynamic performance. This study systematically investigates the influence of surface effect on the Lamb wave propagation characteristics in ZnO-based FBARs. The state-space formalism and a Taylor series expansion are employed to formulate the surface piezoelectricity theory, which provides effective boundary conditions to characterize the role of surface effect in FBARs. Based on the displacement method, the dispersion equation is derived by combining the governing equations with the effective boundary conditions. Numerical results show that surface effects play a significant role in the propagation of Lamb waves in FBARs, and that the dispersion characteristics depend strongly on the film thickness and surface material parameters of FBARs. Although the FBAR’s thickness is on the order of micrometers, surface effects introduce a deviation of approximately 50 ppm in the cutoff frequency of the thickness-extensional (TE) mode. Furthermore, as the thickness of the ZnO-based FBAR approaches the nanoscale, the TE mode and the second thickness-shear (TSh2) mode exhibit a mode flip. Therefore, the Lamb wave propagation characteristics in FBARs can be effectively modulated through surface engineering.

ABSTRACT This study presents a 4D-printed metamaterial with thermally programmable negative stiffness, addressing the critical need for adaptive materials in dynamic environments. Conventional mechanical metamaterials suffer from fixed functionalities and limited adaptability, restricting their use in applications requiring responsive behavior. By leveraging the thermomechanical properties of shape memory polylactic acid, we developed a distributed thermal control system capable of tuning the nonlinear force – displacement response. Through systematic material modeling and structural optimization, the metamaterial achieves controlled buckling and sequential collapse under compressive loads, significantly enhancing adaptability. In the vibration tests of the metamaterial base, different programming states markedly modify the vibration-isolation response and enable a tunable isolation region, while under impact loading the structure exhibits effective energy dissipation and impact-mitigation capability. Experimental validation demonstrates its potential in aerospace applications, where it attenuates shocks, suppresses vibrations, and dissipates energy across the spacecraft’s lifecycle. This work pioneers a systematic framework for programmable metamaterial development, offering a versatile solution for advanced engineering challenges.

ABSTRACT Although research on triboelectric nanogenerators (TENGs) has evolved rapidly, comprehensive data-driven studies remain limited. This study addresses this gap by conducting a scientometric analysis integrated with unsupervised topic modeling. Utilizing 8556 Scopus documents (1999–2024), we present a quantitative overview of the TENG research field. The analysis revealed exponential growth in publications, particularly after 2018. The TENG research landscape has been geographically led by China, the U.S.A. and South Korea, featuring a collaborative network centered on influential authors such as Wang, Z.L. and characterized by a highly modular author collaboration structure with comparatively weaker cross-cluster integration at the researcher level. Separate co-occurrence network analyses of commonly used materials and application terms confirmed that TENG studies primarily emphasize energy harvesting and self-powered sensors, with polydimethylsiloxane (PDMS) emerging as the most dominant material. Unsupervised topic modeling further identified six core thematic structures, including wearable electronics, sensing technologies, low-power energy harvesting, output enhancement, material innovation, and large-scale power generation. The findings of this study indicate a clear shift in research emphasis from low-power devices to large-scale energy production applications. Furthermore, the growing interest in biomedical applications and data-driven signal processing highlights important directions for future TENG development and deployment.

ABSTRACT This study investigates the static behavior of single-walled carbon nanotubes (SWCNTs) with helical axes using Eringen’s nonlocal elasticity theory. The differential form of the nonlocal theory is employed to establish the relationship between local and nonlocal field variables within beam theory. These relationships are formulated in Frenet coordinates for a spatially curved beam model, yielding governing equations for helical SWCNTs with constant radius and uniform cross-section. Exact analytical solutions for the equations are obtained using the method of initial values, yielding closed-form solutions. Explicit expressions are provided for closed-coiled helical SWCNTs, representing the first exact analytical solution of Eringen’s differential nonlocal elasticity theory applied to helical nanostructures. A comprehensive parametric study is conducted to systematically analyze the effects of helix geometry (pitch angle, aspect ratio, winding angle) and nonlocal parameters on the static response. The results reveal that pitch angle and helix geometry strongly influence the coupling between normal, binormal, and tangential displacements. For small pitch angles, binormal displacement dominates, while larger pitch angles substantially increase normal and tangential displacements. The parametric studies establish clear relationships between geometric configuration, nonlocal length scale, and mechanical response, providing essential design guidelines for helical nanostructures in engineering applications.

ABSTRACT Piezoelectric energy harvesting is vital for powering microelectronic systems, with great potential for dual-scale structures. However, implementing such designs presents challenges, such as multiphysics coupling, performance measurement, and design complexity. A concurrent topology optimization (TO) method was proposed herein. This method maximized the energy output by optimizing the macrostructure, periodic microstructure, and polarization direction. An objective function was defined as a weighted function of mechanical energy and electrical energy. In addition, a concurrent TO framework was developed based on the penalized polarized piezoelectric material (PEMAP-P) model by incorporating both macro and micro density design variables as well as polarization direction design variables. This framework optimized the sum of mechanical energy and electrical energy, thereby improving the energy harvesting performance. An explicit sensitivity analysis relative to macro and micro design variables as well as the polarization direction design variables was derived. The optimization problem was solved using mathematical programming algorithms. Additionally, numerical simulations corroborate the effectiveness of the proposed design, demonstrating enhanced energy-harvesting performance with minimal loss of structural stiffness. The optimized design also exhibited excellent static and dynamic properties in finite element simulations, offering valuable insights into the design of microelectronic structures and self-powered devices.

ABSTRACT Soft actuators that combine compactness, rapid response, and high output power are critical for advancing high-performance soft robotic systems. Although bistable architectures can amplify speed through elastic instability, their combination with functional materials often sacrifices compactness or lacks tunability. Here, we report a Venus flytrap-inspired shape memory alloy – embedded snapping actuator (SMA-ESA) that achieves compact and tunable actuation by structurally integrating pre-shaped SMA wires within a double-tilted elastomeric matrix. The SMA skeleton not only triggers snap-through via its thermal shape memory effect but also regulates performance. Combined experimental and finite element studies reveal that the actuation performance – characterized by energy storage capacity as well as the rate and efficiency of energy release – is tunable through both geometric parameters and input power. As a proof of concept, the SMA-ESA is demonstrated in a flytrap-inspired capture device that selectively responds to external stimuli. These results establish a generalizable strategy for embedding high-power-density materials into bistable soft structures, offering new opportunities for compact, responsive, and bio-inspired soft robotics.

ABSTRACT With the growing expansion of the Internet of Things (IoT), demand is increasing for self-powered sensors that operate without external energy sources. Piezoelectric materials, which convert mechanical energy into electrical energy, offer a promising solution, especially in vibration-rich environments such as aerospace and transportation systems. In this study, a piezoelectric composite was fabricated by laminating carbon fiber-reinforced polymer (CFRP) onto a potassium sodium niobate – epoxy (KNN – epoxy) layer. The CFRP/KNN – epoxy composite serves as both a structural material and an effective energy harvester. The crack-free laminate produced a high piezoelectric voltage of 13.6 V under bending vibration at 262 Hz, with a d 33 of 7.8 pC/N. To evaluate damage sensing, interlaminar cracks of varying lengths were introduced. Experiments and finite element analysis revealed a clear quantitative relationship between crack length, electrical output, and energy harvesting capability. The harvested energy was sufficient to power light-emitting diodes (LEDs) and wireless IoT modules. When subjected to vibration, the composite enabled self-powered sensing and wireless data transmission. Furthermore, the charging behavior and transmission interval were influenced by crack length, allowing estimation of the structural health state without external power. These findings demonstrate the potential of CFRP/KNN – epoxy composites as multifunctional materials for self-powered sensing and structural health monitoring.

ABSTRACT Beams subjected to axial and spinning motions exhibit complex vibration characteristics that can be utilized for piezoelectric energy harvesting. This study investigates the harvesting performance of an axially compressed piezoelectric beam under coupled axial-spinning motions by establishing a continuous electromechanical model that, for the first time, incorporates both translational and rotational effects. Based on Euler–Bernoulli beam theory, the extended Hamilton principle, and PZT-5A constitutive relations, closed-form solutions of forced vibrations are derived using the Green’s function method and Laplace transform, and validated against experimental and benchmark results to ensure accuracy. The results reveal a remarkable synergistic effect: coupled axial-spinning motions significantly amplify voltage output by orders of magnitude and broaden the effective frequency bandwidth compared with single-motion cases, thereby overcoming the limitations of low amplitude and narrowband response in conventional harvesters. Parametric analyses further demonstrate the decisive influence of load resistance, axial velocity, spinning speed, and piezoelectric constants on system behavior and harvesting efficiency. This work not only develops a closed-form analytical framework for piezoelectric harvesters under dual-motion excitations but also establishes clear physical insights and design guidelines, providing a solid theoretical foundation for the development of high-performance energy harvesting devices in drilling, aerospace, precision machinery, and rotating engineering applications.