Self-sustaining systems can generate and maintain periodic or chaotic motion under constant external stimulation, and has potential applications in fields such as soft robotics, energy harvesting, and active machinery. However, self-sustaining systems often come with excessive oscillations and increased friction, which limit their applications. Unlike oscillatory self-sustaining systems, we have developed a novel steadily self-sustaining system, which is composed of a liquid crystal elastomer rod and a support sleeve. Experiments demonstrate that the liquid crystal elastomer rod on a support sleeve can self-rotate steadily and continuously under constant small area illumination. During the self-rotation, the shape of the liquid crystal elastomer rod remains unchanged, which avoids excessive oscillations and decreases friction. Based on a photothermal-responsive liquid crystal elastomer model, we derived the lateral curvature and the actuating rotation moment. Numerical simulations reveal that the liquid crystal elastomer system balances damping dissipation during motion by absorbing heat converted from constant illumination. The angular velocity of the self-rotation is influenced by parameters such as heat flux, heat transfer coefficient, length of the rod, and damping rotation moment. The theoretical predictions match the experimental results. The novel steadily self-rotating system not only has the advantages of maintaining shape and reducing friction, but also offers benefits such as structural simplicity, small illumination area, and higher energy efficiency compared to other steadily self-sustaining systems driven by large illumination areas or hot surfaces. This research is anticipated to offer valuable insights for applications in soft robotics, energy harvesting, and active machinery.
Various design strategies have been explored to achieve wide local resonance (LR) bandgaps in acoustic metamaterials (AMMs), which have applications in vibration absorption and low-frequency noise mitigation. Conventionally, most methodologies model AMMs as periodic systems. Additionally, maintaining a reasonable resonator mass is desirable for many engineering applications. These factors restrict their possible design space and effectiveness. Such periodic structures are also sensitive to imperfections or manufacturing variabilities. To overcome these issues, we propose a novel methodology for optimal design of robust aperiodic AMMs. First, through a detailed parametric study, we establish a relationship among degree of aperiodicity, bandgap width, and its robustness. A robustness measure is defined to quantify the sensitivity of the bandgap with respect to manufacturing defects. We report two key observations: (i) aperiodicity helps in enhancing the bandgap and robustness, and (ii) the bandgap is not monotonically related to the robustness. These observations suggest the need for a multi-objective optimization in the aperiodic regime. Subsequently, all resonators’ mass, stiffness, and position are treated as design variables in a global optimization problem, which is solved using the genetic algorithm. This methodology offers users complete flexibility in imposing various design constraints.
Numerically, an AMM beam or metabeam is considered, comprising equally spaced double-cantilever-like resonators on a homogeneous host beam, producing an LR bandgap spanning 750–1000 Hz. Through multi-objective optimization, aperiodic designs with enhanced performance are achieved, with significantly wider and more robust bandgaps than periodic systems with similar mass. Interestingly, the global optima resides in the vicinity of the periodic configuration, as shown by parametric studies. The optimized aperiodic designs are validated through physical experiments on a vibrating beam. These findings open a new avenue for designing metamaterials.
This manuscript provides a new idea to solve the problems related to measurement of impact force wave especially in the situation with a strong environment noise by utilizing the polyvinylidene fluoride (PVDF) film instead of strain gauge. According to our experimental results, an extremely high ratio between output signal and noise in environment (SN ratio) is realized even the infrequently employed in-plane piezoelectricity of film is utilized. Instead of piezoelectricity in thickness direction, attachment of film on Hopkinson pressure bar based on in-plane piezoelectricity could help us to avoid the influence from separation of Hopkinson pressure bar on measurement result. We could conclude that a more precise measurement could be realized by a short film rather than long one even though the short film shows a relative lower output signal. However, the output signal could be amplified by increasing its width. Besides, comparing with the use of strain gauge, broader bandwidth of film measurement is discovered. In the situation where the duration of impact force wave is extremely short, high accuracy measurement should be realized by PVDF film rather than strain gauge.
In this paper, a fast Chebyshev-Ritz method for vibro-acoustic analytical modeling of plate-open cavity coupled systems is developed for the first time. Based on the Chebyshev spectral method and the Rayleigh-Ritz solution procedure, the vibro-acoustic model of the open cavity coupled with a rectangular plate is established. The exterior acoustic field of the open cavity is expressed by the Rayleigh integral. Additionally, the Rayleigh integral is divided into a frequency-independent singular integral and a frequency-dependent non-singular integral, accelerating the calculation process. Furthermore, the Gauss-Chebyshev-Lobato sampling method is first developed for the plate-open cavity coupling model. By converting the integrals into tensor products, the method avoids complex quadruple integrals, increasing the efficiency of the entire integral operation. The vibration and acoustic responses from the proposed method agree well with existing literature and FEM analysis results, demonstrating the convergence and correctness of the current methodology. The mechanism of cavity depth on vibro-acoustic features of plate-open cavity systems is studied, which is less focused in the published literature. Other factors governing the plate-open cavity coupled model encompassed boundary conditions, fluid mediums, and plate thickness are fully examined. The results provide a theoretical foundation for the design and future research of plate-open cavity structures.
The numerical prediction of aerodynamic characteristics for vehicles is crucial to both industry and academia, with various numerical approaches playing a critical role in accurately resolving flow fields. This study aims to evaluate the effectiveness of three typical numerical approaches, including RANS, IDDES, and LES in predicting the afterbody vortex flows of a generic model, specifically a slanted-base cylinder. This study involved analyzing aerodynamic coefficients, time-averaged surface flow, time-averaged surrounding flow and transient flow, revealing the capabilities of each approach. RANS offers acceptable accuracy in predicting time-averaged aerodynamic coefficients and surface flow patterns, though it falls short in capturing time-varying physical quantities. LES, despite its higher computational cost, provides a more accurate prediction for both time-averaged and transient flow behaviors, particularly in capturing flow instabilities and multi-scale fluctuations. IDDES can be prioritized when a rough understanding of transient characteristics is sufficient. This study highlights the unique strengths and limitations of three typical numerical approaches in predicting vehicle-like afterbody vortex flows, guiding the selection of appropriate methods based on specific research needs.
Due to the structural information submerged into the meta-stable disordered long-range structure, quantitative prediction of the time-dependent deformation of metallic glasses under mechanical stimuli is a challenging task. Specifically, the present understanding of relaxation behavior, particularly in relation to dynamic heterogeneity and the memory effect in metallic glasses during thermo-mechanical treatment, is yet to be totally understood. Here we study the correlation between the relaxation decoupling and mechanical memory effect in metallic glasses, manifested by non-monotonic variation of activation energy and dynamic heterogeneity during creep and stress relaxation. The strain evolution and energetic state show a memory effect in an aging (recovery)-and-creep procedure. The relaxation decoupling and mechanical memory effect originate from the competition between the formation of fast defects by stress and the transition towards slow defects and their annihilation. The strain evolution is dependent on total loading time and total recovery/aging time rather than their orders. Our results shed light on the deformation and history-dependent behaviors of metallic glasses.
Snap-through instability can occur after a significant time delay for some viscoelastic structures under certain loading history; the mechanisms of this phenomenon in viscoelastic metamaterials are still unrevealed. This work uses a combined method of experiments, finite element analysis (FEA), and analytical modeling to investigate the rate-dependent and delayed snap-through behavior of viscoelastic metamaterials. The load-displacement responses under different loading-rates and viscoelastic parameters are illustrated with an emphasis on the programable load capacity and stability via FEA. Experimentally, a viscoelastic metamaterial made of silicone rubber is fabricated through 3D printed molds, and demonstrated for delayed snap-through after creeping under a constant force. The sensitivity of the delayed time to the applied force is presented. A phase diagram with respect to the applied force and material viscoelasticity is constructed to demonstrate different snapping behaviors, including near-instantaneous snapping, delayed snapping at finite time, and no snapping. A discrete model that can capture different snapping modes is developed to provide straightforward understanding of the underlying mechanisms. This work can open up potential novel applications of the tunable delayed snap-through behavior of viscoelastic metamaterials.
Sound transmission loss (STL), an essential index for assessing the sound insulation performance of composite laminated structures, typically relies on experimental methods to measure. The soundbox method (SBM), a straightforward technique for measuring the STL, is sensitive to microphones’ positions. Within the framework of the Chebyshev-Ritz method, a semi-analytical vibro-acoustic model extended to composite laminated panels with viscoelastic damping (VED) is proposed for the first time. Based on the developed simplified layer-wise theory, the panel is modeled using three layers: the top face layer, the VED layer, and the bottom face layer. A closed cavity is added to the model as the soundbox enclosure used in actual measurements. By employing the Hamilton's principle, the governing equation for the coupling system is derived, and the vibration and internal acoustic responses of the coupling system are calculated. A discretization strategy is introduced to address the frequency-dependent properties of the VED layer, avoiding the need to reconstruct the stiffness matrix at each frequency. To obtain the STL of the panel, sound pressures at external measurement points are calculated based on the Rayleigh integral. The proposed model is validated against numerical results from finite element analyses. The influences of the microphone position inside and outside the cavity on the measured STL are studied. Furthermore, parametric studies over the microphones' positions are performed to enhance the SBM-based evaluation of the composite panel's sound insulation performance. The optimal locations for two internal microphones and one external microphone are recommended. Finally, experimental studies are carried out to guide the implementation of the SBM.
The strength criterion of rock is essential for stability control and safety design of geotechnical engineering constructions. Due to its widespread adoption, the Mohr–Coulomb (M-C) criterion is prominent among strength criteria. However, the M-C criterion is constrained by three significant limitations: it fails to capture the nonlinear strength response, overlooks the critical state, and disregards σ2. This study introduces a novel Stress-dependent Instantaneous Friction angle and Cohesion (SIFC) model for the M-C criterion to represent the convex strength envelope of intact rock, covering the spectrum from non-critical to critical states. In pursuit of this objective, an innovative approach for calculating these instantaneous shear parameters at each corresponding σ3 is initially introduced. By examining the confining pressure dependency of the instantaneous friction angle and cohesion, the SIFC model is derived and introduced to the M-C criterion. The SIFC-enhanced M-C criterion, utilizing parameters obtained from triaxial tests under lower σ3, delineates the complete non-linear strength envelope in (σ1, σ3) space, covering brittle to ductile behavior. This criterion is then extended to polyaxial stress conditions. Validation through triaxial test data confirms that the SIFC-enhanced M-C criterion accurately reflects the strength characteristics of the tested rocks.
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