Snap-through Mechanism Research Articles

Snap-through mechanism of an elastic bimetal ring for rotary actuation

Smart materials responsive to stimuli and new rotary mechanisms have drawn much attention in recent years. This paper investigates a novel snap-through rotation mechanism of a thermal responsive bimetal ring confined inside the annular region constraint when subjected to the local curvature induced by thermal loading. The buckled ring has movable boundaries and keeps an invariant shape during the snap-through rotation so that it requires small energy input to drive the bistable transition. To uncover the elusive mechanism behind this intriguing snap-through phenomenon, a theory based on the bifurcation and energy principle is developed to predict the critical loading curvature and the critical loading temperature, which agrees with the experimental results. The effects of loading positions and the constraint width are studied and a map predicting the critical loading curvature with different geometrical parameters is demonstrated. The rotary buckled ring not only enables the bistable actuation, but also triggers various actuation applications such as the motor actuation and the pump actuation. Due to great advantages in terms of energy supply and actuation loading, this study exhibits great potential of the proposed snap-through mechanism in bistable structures and rotary actuation.

Out-of-plane non-intrusive seismic retrofitting of in-plane damaged masonry infill through 3D printed recycled plastic devices

The current techniques used for strengthening existing buildings made out of masonry infilled frames are characterized by high impact on the daily internal activities, as people living or working in the building need to move out while the retrofitting activities take place.The seismic vulnerability of infilled concrete frames is related to their high stiffness that, being due to the presence of masonry infills, induces a very brittle failure mechanism. The available retrofitting techniques do not allow to decrease the frames stiffness without making demolitions, especially for what concerns the double layer infills, a typology that characterizes a large part of the Italian buildings built from 1970s. For such infills only traditional techniques are available, mostly lime/cement-based mortars strengthened with various fibers that increase the frame stiffness. This study proposes an innovative technique for the out-of-plane strengthening of masonry double layer infills. The system is based on a 3D printed system made of recycled plastic, placed between the two masonry layers composing the infill and acts: i) triggering preferential in-plane sliding surfaces for reducing stiffness and therefore damaging, ii) warranting out-of-plane stability avoiding the snap-through mechanism, iii) ensuring low environmental impact by using recycled materials and, iv) no disrupting the daily internal activities. The full scale out of plane test is carried out on an infill specimen previously damaged in the in plane direction, by using a hydraulic jack placed at mid-height of the infill. The experimental results are compared with analytical models available in literature combined with a model specifically developed for the resistant mechanism provided by the plastic devices. The obtained results suggest that plastic joints are effective in preventing instability and can be used as effective strengthening system for concrete frames with masonry infills.

A Triboelectric-Piezoelectric-Electromagnetic hybrid wind energy harvester based on a snap-through bistable mechanism

Real-time monitoring is essential for safeguarding infrastructure in extreme environments, particularly in remote areas with abundant wind resources where sustained strong winds are common. A triboelectric-piezoelectric-electromagnetic hybrid wind energy harvester based on a snap-through bistable mechanism (ST-HWEH) is proposed, which can be used as a sustainable power source and a self-powered wind speed sensor. The piezoelectric energy harvester (PEH), triboelectric nanogenerator (TENG), and electromagnetic energy harvester (EMH) are placed at the maximum strain location, maximum contact area, and maximum displacement position of the bistable buckled beam, respectively, fully utilizing its characteristics in the structural space. The bistable buckled beam undergoes convex and concave snap-through transformations under the magnetic excitation, working synergistically with the buckled boundary to simultaneously achieve the three electromechanical energy conversions, thereby enhancing the electrical output performance of the system. Moreover, both the starting wind speed and resistance torque of the system are effectively reduced through the arrangement of symmetrical excited magnets with antimagnetic poles on the buckled beam. A prototype is prepared and subsequently experimentally verified. The results show that the overall output power density in a composite power generation unit for the ST-HWEH is 83.97 W/m3 at a wind speed of 11 m/s, which is 6.88 times higher than that of the ST-HWEH that only contains TENG. The prototype can easily light up 1000 LED lights at a mild wind speed of 5 m/s and demonstrate self-powered environmental monitoring (taking temperature and wind speed as examples), providing a potential solution for self-powered environmental monitoring and status monitoring of the infrastructure.

Mechanics of the artillery fungus

Species of artillery fungus, Sphaerobolus, use a unique, snap-through buckling mechanism to discharge their spores over a longer distance than any other known fungi. Here, we provide new information on biomechanics of glebal discharge by capturing the launch using high-speed video, measuring the force generated by the inner cup that expels the gleba, and modeling the relationship between the force and speed of the gleba to its trajectory. Associated calculations reveal that patches of the artillery fungus consume 80 times more energy than an individual gilled mushroom to release the same number of spores. The evolution of this costly mechanism may be counterbalanced by the relatively low wastage of spores carried in its sporangia compared with the greater losses of spores released from conventional mushrooms.

3D printed negative stiffness meta-structures with superior energy absorption and super-elastic shape-recovery features

The aim of this paper is to create novel 3D cubic negative stiffness (NS) structures (NSSs) with superior mechanical performances such as high energy absorption, shape recovery, super-elasticity, and reversibility. The conceptual design is based on an understanding of geometrical influences, non-linear buckling-type instability, snap-through mechanism, elasto-plastic deformation growth and plastic hinges. A finite element (FE) based computational model with an elasto-plastic material behavior is developed to design and analyze NSSs, saving time, material, and energy consumption. Material samples and meta-structures are 3D printed by selective laser sintering printing method. Material properties are determined via mechanical testing revealing that the printing process does not introduce much anisotropy into the fabricated parts. Experimental tests are then conducted to study the behavior of novel designs under loading–unloading cycles verifying the accuracy of the computational model. A good correlation is observed between experimental and numerical data revealing the high accuracy of the FE modeling. The structural model is then implemented to digitally design and test NSSs. Effects of the geometrical parameters of the negative stiffness members under three cyclic loading are investigated, and their implications on the non-linear mechanical behavior of NSSs under cyclic loading are put into evidence, and pertinent conclusions are outlined. In addition, the dissipated energy and loss factor values of the designed structures are studied and the proposed unit cell is presented for the energy absorbing systems. The results show that the structural and geometry of energy absorbers are key parameters to improve the energy absorption capability of the designed structures. This paper is likely to fill a gap in the state-of-the-art NS meta-structures and provide guidelines that would be instrumental in the design of NSS with superior energy absorption, super-elasticity and reversibility features.

Stick-Slip Dynamics of Moiré Superstructures in Polycrystalline 2D Material Interfaces.

A new frictional mechanism, based on collective stick-slip motion of moiré superstructures across polycrystalline two-dimensional material interfaces, is predicted. The dissipative stick-slip behavior originates from an energetic bistability between low- and high-commensurability configurations of large-scale moiré superstructures. When the grain boundary separates between grains of small and large interfacial twist angle, the corresponding moiré periods are significantly different, resulting in forbidden grain boundary crossing of the moiré superstructures during shear induced motion. For small twist angle grains, where the moiré periods are much larger than the lattice constant, this results in multiple reflections of collective surface waves between the surrounding grain boundaries. In combination with the individual grain boundary dislocation snap-through buckling mechanism dominating at the low normal load regime, the friction exhibits nonmonotonic behavior with the normal load. While the discovered phenomenon is demonstrated for h-BN/graphene polycrystalline junctions, it is expected to be of general nature and occur in many other large-scale layered material interfaces.

GSG: A Granary-Shaped Soft Gripper With Mechanical Sensing via Snap-Through Structure

Soft robotic grippers have attracted considerable attention in terms of the advantages of the high compliance and robustness to variance in object geometry; however, they are still limited by the corresponding sensing capabilities. We propose a novel soft gripper that looks like a ‘granary’ in the geometrical shape with a snap-through bistable mechanism fabricated by an ordered mold technology, which consists of a palm chamber, shell, cap and three fingers. It can achieve ‘sensing’ mechanically and perform pinching, enveloping and caging grasps for objects with various profiles. In particular, the snap-through bistable mechanism in the proposed gripper allows us to reduce the complexity of the mechanism, control, and sensing designs. The grasping behavior is activated once the gripper's deformation or perceived pressure arrives at a certain value. First, after the theoretical model for snap-through behavior is constructed, the modularized design of the gripper is described in detail. Then, the ordered molding method is employed to fabricate the proposed gripper. Finally, the finite element (FE) simulations are conducted to verify the built theoretical model. Further, a series of grasping experiments are carried out to assess the performance of the proposed gripper on grasping and sensing. The experimental results illustrate that the proposed gripper can manipulate a variety of soft and rigid objects and remain stable even though it undergoes external disturbances.

A Bistable Mechanical Metamaterial Based on Snap-Through Mechanism

A Bistable Mechanical Metamaterial Based on Snap-Through Mechanism

Experimental and numerical study on the constrained bending-induced collapse of hexagonal honeycomb

This study aims to analyze the deformation behavior of hexagonal honeycomb and classify the collapse modes during constrained bending for forming curved sandwich structures. To this end, the experiments with various bending radiuses and honeycomb thicknesses were carried out, and a finite element model (FEM) was developed to simulate the bending operation. To evaluate the honeycomb’s buckling level, a novel quantitative approach was proposed based on the extracted deformed configuration of the honeycomb top surface. The FEM model was validated by comparing predicted deformation modes and quantities with experimental results and then used to analyze the honeycomb deformation. The buckling evolution, as well as its dependence on the honeycomb thickness and imperfection, has been extensively explored. It was found that the cell wall thickness variance has the most significant influence on the bending behavior. Furthermore, this study revealed two distinct honeycomb collapse patterns during the constrained bending process along the transverse direction (WD). The likelihood of each collapse pattern varies with bending radius and sample thickness. Besides, a snap-through buckling mechanism at the cell scale was proposed for the collapse onset and evolution. These achievements pave the way for the development of curved honeycomb sandwich structures.

Crawling–jumping synergic bioinspired robots harnessing electroactive bistable actuators by adjusting mechanical responses and forces

Mechanical responses and forces are two urgently concerning factors in actuators. Actuation mechanisms inspired by natural systems are increasingly common, especially the snap-through mechanism in cascading power-induced bioinspired systems. Here, we propose an electroactive bistable actuator that can synergize the fast mechanical response and high mechanical forces. It is constructed by combining a single pretension spring (i.e., p-spring) bistable linkages as a “skeletal spine” and twinned SMA-springs (i.e., s-springs) as “skeletal muscle.” The p-spring connected to linkages plays a role in determining elastic instabilities threshold, while the coupled s-springs serve as actuation generators. The system shows a rapid and switchable snap-through between two stable states as each s-spring is individually actuated. The p-spring and the voltage exerted on s-springs are the two key parameters influencing response time (0.05 s) and mechanical forces (3.4 N). Additionally, synergic movements between crawling and jumping are enabled, which benefit from the ease of control of the exerted voltage and manual convenience in customized pretension. Our prototype can achieve a linear locomotion speed of 0.56 mm/s, a jump height of 95 mm, and a horizontal distance of 150 mm. We look forward to providing a new design paradigm in mechanical systems requiring fast dynamic properties and brilliant static capabilities.

Evaluation of the Double Snap-Through Mechanism on the Wave Energy Converter\u2019s Performance

Lower efficiencies induce higher energy costs and pose a barrier to wave energy devices’ commercial applications. Therefore, the efficiency enhancement of wave energy converters has received much attention in recent decades. The reported research presents the double snap-through mechanism applied to a hemispheric point absorber type wave energy converter (WEC) to improve the energy absorption performance. The double snap-through mechanism comprises four oblique springs mounted in an X-configuration. This provides the WEC with different dynamic stability behaviors depending on the particular geometric and physical parameters employed. The efficiency of these different WEC behaviors (linear, bistable, and tristable) was initially evaluated under the action of regular waves. The results for bistable or tristable responses indicated significant improvements in the WEC’s energy capture efficiency. Furthermore, the WEC frequency bandwidth was shown to be significantly enlarged when the tristable mode was in operation. However, the corresponding tristable trajectory showed intra-well behavior in the middle potential well, which induced a more severe low-energy absorption when a small wave amplitude acted on the WEC compared to when the bistable WEC was employed. Nevertheless, positive effects were observed when appropriate initial conditions were imposed. The results also showed that for bistable or tristable responses, a suitable spring stiffness may cause the buoy to oscillate in high energy modes.

Nonlinear dynamics and parametric study of snap through electromagnetic vibration energy harvester using multi-term harmonic balance method

Abstract Vibration energy harvester (VEH) has proven to be a favorable potential technique to supply continuous energy from ambient vibrations and its performance is greatly influenced by the design of potential structures. A snap-through mechanism is used in an electromagnetic energy harvester to improve its effectiveness. It mainly comprises of three springs that are configured so that the potential energy of the system has two stable equilibrium points. In this work, a harmonically base excited snap-through electromagnetic vibration energy harvester is investigated by analytical and semi-analytical method. The approximate analytical outcomes are qualitatively and quantitatively supported by semi-analytical method using multi-term harmonic balance method (MHBM).The bifurcation diagram of response current shows that snap-through electromagnetic vibration energy harvesters exhibits periodic intrawell, interwell and chaotic motion when the system parameters are varied. The influence of system parameters on the response of snap-through electromagnetic vibration energy harvester are examined. Nonlinearity produced by the snap-through oscillator improves energy harvesting so that the snap-through electromagnetic energy harvester can outperform the linear energy harvester in the similar size under harmonic excitation. A fitness function was formulated and optimization of the selected parameters was done using genetic algorithm. The parametric optimization leads to a considerable improvement in the harvested current from the system.

Snap Pump: A Snap-Through Mechanism for a Pulsatile Pump

Pulsatile flow is widespread in nature, but replicating such impulsive periodic pumping routines with traditional rotary actuators is complex and energetically inefficient. We demonstrate the feasibility of an actuator capable of generating impulsive flow displacement by exploiting the bistable equilibrium of a simple mechanism. These kind of mechanisms, commonly known as snap-through mechanisms, offer the benefit of sudden release of elastic energy at the interface between two stable structural configurations. This property has been employed in certain actuators to drive abrupt motions of mechanical systems. Here, we use this principle to drive an inflation/deflation routine of a fluid-filled cavity, thus generating a peaked, pulsatile flow, comparable to that encountered in biological systems. Assessment of the various stages of actuation of this system shows that a sharp drop in the energy occurs from elastic to hydraulic work, highlighting the need for improved design elements involved in this stage of the actuation.

Durability of natural hydraulic lime (NHL) based TRM composites through hot water immersion method

The textile reinforced mortar (TRM) composites are widely utilized for the seismic strengthening of masonry walls subjected to in-plane and out-of-plane actions. The probability of out-of-plane failure via snap-through mechanism is dominated by the flexural characteristics of relatively slender masonry wall. In this study, a rapid and early decisive procedure is proposed to direct the design of TRM composite through the bending tests combining accelerated ageing. The durability of natural hydraulic lime based TRM composites, which is unknown in the current literature, is addressed for glass, basalt and carbon textiles. The interaction between fiber mesh and matrix is discussed regarding fiber type, the influence of impregnation, flexural performance and failure modes. For total five types of fabrics, the flexural capacities are compared at ambient conditions and then through hot water ageing test (immersion at 50 °C during 10 days). The experimental findings indicate to the importance of the interaction between fiber and matrix about the durability and ductility of TRM composites.

Force-sensorless human joint impedance estimation utilizing impulsive force

ABSTRACT The paper is concerned with force-sensorless estimation of human joint impedance utilizing an impulsive force. We analyze the dynamic response of a human joint induced by the impulsive perturbation and derive the relationship between the amplitude of the perturbation force and the initial value of the induced angular velocity. This relationship means that the force amplitude required by the estimation algorithm can be replaced with a specific value of the velocity, which leads to eliminating the need for force-sensing. In order to verify the force-sensorless impedance estimation experimentally, a wearable-type snap-through buckling mechanism is employed as the impulsive force generator. The experiments are carried out with a benchmark setup and a healthy male participant. The impedance estimation has been achieved without using any force sensor and performs good consistency with the specific mechanical impedance of the setup and with the ankle impedance reported by related works respectively. The obtained results thus indicate the feasibility of the proposed force-sensorless scheme.

Corotational 3D joint finite element tailored for the simulation of bistable deployable structures

The 3D deployable frames studied in this work are structures composed of elastic beam elements connected by complex joints. During transformation a controlled snap-through allows the instantaneous stabilization of the structure in an open and in a closed, compact configuration. The mechanics of the transformation is highly nonlinear, since it relies on finite rotations of the structural elements. It is also strongly influenced by geometrical features required for a manufacturing-ready design, such as the finite size of structural elements and sufficient spacing between the beams. These features are generally disregarded in the usual wireframe-based design, but they are taken into account in this work by applying a tailor-made corotational 3D joint finite element, developed to incorporate naturally finite joint size, finite nonlinear joint stiffness and friction effects. The formulation of the proposed joint FE is presented and the performance of the numerical implementation is verified using computational benchmarks. The joint FE is then applied to the numerical investigation of the transformation response of bistable deployable structures from a single module case to large, complex structures. Among other findings, it is shown that the incorporation of finite joint size and beam spacing in the numerical model leads to a different snap-through mechanism that significantly reduces the peak force required for transformation, which could be a basis for future design strategies. Additionally the performance of structures applying the bistable structural pattern on the whole structure or following an entirely modular design (interconnected single modules) is also compared, as a function of the structural size.

Stag Beetle Elytra: Localized Shape Retention and Puncture/Wear Resistance.

Beetles are by far one of the most successful groups of insects, with large diversity in terms of number of species. A part of this success is attributed to their elytra, which provide various functions such as protection to their bodies from mechanical forces. In this study, stag beetle (Lucanus cervus) elytra were first examined for their overall flexural properties and were observed to have a localized shape-retaining snap-through mechanism, which may play a possible role in partly absorbing impact energy, e.g., during battles and falls from heights. The snap-through mechanism was validated using theoretical calculations and also finite element simulations. Elytra were also characterized to examine their puncture and wear resistance. Our results show that elytra have a puncture resistance that is much higher than that of mandible bites. The measured values of modulus and hardness of elytra exocuticle were 10.3 ± 0.8 GPa and 0.7 ± 0.1 GPa, respectively. Using the hardness-to-modulus ratio as an indicator of wear resistance, the estimated value was observed to be in the range of wear-resistant biological material such as blood worms (Glyrcera dibranchiata). Thus, our study demonstrates different mechanical properties of the stag beetle elytra, which can be explored to design shape-retaining bio-inspired composites with enhanced puncture and wear resistance.

Simulation and Test of Discrete Mobile Surfaces for a RC-Aircraft

Morphing structures suitable for unmanned aerial vehicles (UAVs) have been investigated for several years. This paper presents a novel lightweight, morphing concept based on the exploitation of the “lever effect” of a bistable composite plate that can be integrated in an UAV horizontal tail. Flight dynamics equations are solved in Simulink environment, thus being able to simulate and compare different flight conditions with conventional and bistable command surfaces. Subsequently, bistable plates are built by using composite materials, paying particular attention to dimensions, asymmetric stacking sequence and total thickness needed to achieve bistability. NACA0011 airfoil is chosen for proving this concept. Wind tunnel tests demonstrate that the discrete surface is capable of withstanding the aerodynamic pressure. A remotely piloted vehicle is employed to test the discrete horizontal tail command during the take-off. The results show that, choosing a proper configuration of constraints, stacking sequence and aspect ratio for the bistable laminate, it is possible to tailor the snap-through mechanism. The proposed concept appears lighter and increases aerodynamic efficiency when compared to conventional UAV command surfaces.

Nonlinear energy harvesting based on a modified snap-through mechanism

A modified snap-through mechanism is used in an electromagnetic energy harvester to improve its effectiveness. It mainly comprises three springs that are configured so that the potential energy of the system has two stable equilibrium points. In particular, the small vibration behavior of the harvester around one of the equilibriums is of interest. A multi-scale method (MSM) is used to analyze the frequency response curve. Two snap-through mechanisms are considered. One has both horizontal and vertical springs. The other has only horizontal springs. The frequency response curves of these two classes are compared under the same excitation and electric loading conditions. The latter exhibits more bending of the frequency response curve than the former one. The results are also validated by some numerical work. The averaged power subject to the Gaussian white noise is calculated numerically, and the results demonstrate that bi-stable energy harvesting with only horizontal springs can outperform the mechanism with both horizontal and vertical springs for the same distance between two equilibriums.

Multistable kirigami for tunable architected materials

In nature, materials such as ferroelastics and multiferroics can switch their microstructure in response to external stimuli, and this reconfiguration causes a simultaneous modulation of its material properties. Rapid prototyping technologies have enabled origami and kirigami-inspired architected materials to provide a means for designing shape-shifting structures, and here we show how multistable structures inspired by kirigami provide novel design criteria for preparing mechanical metamaterials with tunable properties. By changing the geometry of kirigami unit cells, we obtain multistable kirigami lattice structures endowed with a bistable snap-through mechanism. We demonstrate the precise control of material stiffness, along with the ability to tune this property in situ by locally and reversibly switching the unit cell configurations. We anticipate these mechanical metamaterials will provide a platform to achieve in situ tunable electrical, optical, and mechanical properties for a variety of applications in multifunctional materials, two-dimensional materials, and soft robotics.