Stiffness Configuration Research Articles

Hygrothermal effect of bio-inspired helicoid laminate plate for strengthening damaged RC beam

This study investigates the influence of moisture and Thermo-hygroscopic influence on bio-inspired helicoidally laminated composite plates (BHLC) for strengthening damaged reinforced concrete (RC) beams. Borrowing inspiration from nature’s design principles, BHLC plates represent a novel approach to rehabilitation. An analytical approach based on deformation compatibility is employed, assuming constant shear and normal stresses across the adhesive layer within the framework of linear elasticity. BHLC plates are implemented to restrict crack propagation within the RC beam. The analytical methodology considers equilibrium and deformation compatibility across all components: the concrete beam, BHLC plate, and adhesive layer. A parametric study explores the sensitivity of interfacial stresses to parameters such as laminate and adhesive stiffness, plate thickness, and helicoidal layup configuration. The results reveal a significant influence of these factors on the magnitude of maximum interfacial shear and normal stresses. This study establishes a foundation for future analyses of damaged RC beams strengthened with BHLC plates, potentially leading to advancements in rehabilitation methodologies and improved crack propagation modeling in RC beams.

Role of inertial nonlinearity and coupling stiffness on a series of coupled harvesters

This study investigates the effect of coupling configuration on a state-of-the-art harvesting system consisting of a series of spring-coupled piezoelectric nonlinear monostable tip-loaded cantilever-based vibration energy harvesters (VEHs). The investigation finds favorable coupling configurations that exploit complex nonlinear interaction for improving system performance. The system model incorporates the beam's geometric nonlinearities in the governing equation using inertial and stiffness nonlinear terms. The study first analytically investigates the effect of grounding stiffness on a single harvester's performance. Then, it numerically investigates the effect of increasing array size and changing stiffness distribution. The study discovers a novel hardening/softening transition phenomenon at a critical grounding stiffness value induced by the inertial nonlinear effect. This critical event also maximizes peak output power near the largest eigenfrequency at an optimum grounding stiffness value. At larger arrays, the peak power shows a complex bifurcation behavior, hinting at the presence of multiple attractors. The numerical study of different stiffness configurations and linearized eigenanalysis reveals the necessity of a multilevel optimization strategy focused on both the stiffness ratio between the coupling springs' and their overall stiffness level. This study bridges the gap between previous studies on spring-coupled linear and bistable arrays, offering new insights into coupled nonlinear interactions and proposing novel optimization strategies.

Large-ratio stiffness switching via harnessing the in-plane buckling and bi-stability of high-load capacity composite laminates

Stiffness tuning is crucial in developing many advanced engineering systems such as morphing aircraft, soft robotics, etc. Achieving a significant stiffness change while maintaining a high load-bearing capacity in compression is a challenging task. This study addressed this challenge by exploiting the in-plane buckling and bistability of asymmetric carbon fiber composite laminates. Due to the inverted curvatures of these laminates at their two stable states, they showed two distinct in-plane compression behaviors—stiff response where the laminate behaved like a thin curved column (with a stiffness coefficient near 68 N mm−1), and compliant response where the laminate acted like a soft non-linear spring (with stiffness near 0.7 N mm−1). As a result, a simple snap-though between these two stable states offered a significant stiffness change, with a high/low stiffness ratio of ≈97. By using extensive finite element simulations, experimentation, and a semi-empirical curved plate buckling model, this study examined the mechanics underpinning the laminate’s 3-step buckling and the influences of asymmetric boundary contacts with friction. Additionally, three strategies to enhance the laminate’s performance—adjusting the aspect ratio, applying lateral loads, and parallel stacking were examined. It also demonstrated cellular structure concepts using these bistable laminates. Remarkably, by simply adding a small lateral in-plane force (e.g., a rubber band), one could substantially increase the stiffness ratio of the composite laminates to ≈200 and load-bearing capacity at the stiff configuration, outperforming the state-of-art variable stiffness methods.

Geometric effects on impact mitigation in architected auxetic metamaterials

Lightweight materials used for impact mitigation must be able to resist impact and absorb the maximum amount of energy from the impactor. Auxetic materials have the potential to achieve high resistance by drawing material into the impact zone and providing higher indentation and shear resistance. However, these materials must be artificially designed, and the large deformation dynamic effects of the created structures must be taken into consideration when deciding on a protection concept. Despite their promise, little attention has been given to understanding the working mechanisms of high-rate and finite deformation effects of architected auxetic lattice structures. This study compares the static and dynamic elastic properties of different auxetic structures with a honeycomb structure, a typical non-auxetic lattice, at equivalent mass and stiffness levels. In this study, we limit the investigation to elastic material behavior and do not consider contact between the beams of the lattices. It is demonstrated that the equivalent static and dynamic properties of individual lattices at an undeformed state are insufficient to explain the variations observed in impact situations. In particular, the initial Poisson’s ratio does not determine the ability of a structure to resist impact. To gain a thorough comprehension of the overall behavior of these structures during localized, high rate compression, the evolution of the elastic tangent properties under compression and shear deformation was monitored, leading to a more profound understanding. Observations made in one configuration of stiffness and mass are replicated and analyzed in related configurations.

Design and Simulation of End Effector for Young-Pear-Bagging Robot

In order to address the time-consuming and labor-intensive challenges as well as the suboptimal operational quality encountered in the conventional processes of fruit bagging within expansive orchards, an innovative end-of-bagging actuator is proposed, which can be installed on a fruit-production robot. Due to the excessive power sources required to complete the bagging operation, while also taking into account the quality and cost of the end effector, we have implemented a clutch transmission system to control individual motors, thereby achieving efficient bag-opening and collection actions. Through kinematic analysis of the bagging end effector, the optimal bag opening size is determined to be 40.3372 mm, with a deviation of 0.1428 mm from the design target and an error rate of 0.35%. This ensures the desired bag size for bagging juvenile fruits. Moreover, a dynamic simulation model comprising rigid drive components and a flexible clutch was developed. The simulation results demonstrate the system’s stable performance. However, it is evident that the gear speed falls below that of the flexible clutch, resulting in insufficient bag opening and bag gathering compared to the intended design target. The observed phenomenon is a result of the characteristics exhibited by the flexible clutch. Specifically, the demands for bagging and stretching can be accommodated by modifying the stiffness and geometric configuration of the flexible clutch, alongside the level of operational force. To conclude, the suggested end effector can successfully simulate the implementation of the manual bagging process. By taking into account the quality and cost of the end effector, a clutch drive system was utilized to regulate a single motor, resulting in efficient bag-opening and collection actions. This approach offers a more integrated and efficient solution compared to manual bagging and semi-automatic mechanically assisted bagging methods.

Switchable origami adhesives.

Creating a reusable adhesive that can hold objects on a wall and can yet be easily removed al for researchers in the adhesives community for many years. Geckos and other climbing organisms demonstrate just this ability: to hold large loads (on-state) due to specialized digits and microstructures, yet they are also able to quickly peel their feet from a surface while climbing (off-state). Inspired by the simplicity of the gecko's geometric switching mechanism, we have investigated the use of origami design methods to create geometries that can transition from a stiff configuration to a more flexible and easily peeled configuration. Specifically, we examined three different origami designs (Kresling, Waterbomb and Ron Resch) fabricated in polycarbonate and supplemented with 3D printed structures. Although the polycarbonate could be coated with a commercial adhesive, we investigated the devices in contact with polydimethylsiloxane adhesive pads in order to chemically control interfaces and create a range of differing adhesion levels. We show that the devices are capable of moderate switching ratios (Fon/Foff up to ∼50). We give a simple model to aid design and provide many options for scaling design performance through size, adhesive strength or through repetition of the pattern beyond a single unit cell.

Flexible furniture structure based on braiding technology and fiber jamming

Inspired by the fields of composites and soft robotics, this paper designs a textile-based structure which could realize variable stiffness and variable configurations, and they thus make it present high adaptability as a flexible furniture structure. The mechanisms of the variable properties realized by the braiding and jamming, together with the fabrication considerations and analytical methods are introduced in detail. Down-scale beam elements with different profiles were physically and numerically established, based on which the distinct foldability, variable stiffness and reconfigurability are quantitated and analyzed. This study brings a new idea for the development of the indoor furniture structure.

A new class of “structure-by-design” polymer membranes for organic solvent nanofiltration with controllable selectivity

Membrane based separations offer an inexpensive and energy efficient pathway to purification. However, their use is often stifled during separation of organics due to low selectivity. This can be attributed to the inherent limitations of the synthesis methods employed, namely, interfacial polymerization and phase inversion. In this work, a new class of structure-by-design membranes are synthesized by surface modification of crosslinked polyimide using graft polymerization with a series of methacrylate monomers with differing chain lengths and chemistries. Graft polymerization of long chain monomers resulted in increased selectivity (α∼3.7 for 80-20 mol.% MeOH-toluene and α∼6 for 95-5 mol.% toluene-triisopropyl benzene (TIPB) for both test solutions) as a result of increased stiffness and order (using GIWAXS)). The observed selectivities renders these membranes competitive for organic solvent nanofiltration applications. In addition, pure solvent permeabilities were measured. The performance of these membranes was correlated with brush properties such as degree of grafting (using ATR-FTIR), surface polarity (contact angle measurements), stiffness (using QCM-D) and brush configuration in different solvent environments (using MD Simulations). MD simulations demonstrate that in mixtures containing hydrophobic species, polymers with longer hydrophobic side chains well more and form fuller matrices leading to increased solvent accessible surface area. GIWAXS results showed increased order with longer side chain length. These tunable membranes offer promise for organics separation.

Design and Mechanical Testing of a Novel Dual-Stiffness Ankle-Foot Orthosis

Abstract This study details the concept, design, and mechanical testing results of a novel dual-stiffness ankle-foot orthosis (DS-AFO). The DS-AFO utilizes two separate stiffness elements (rear struts) to yield an AFO with low stiffness properties about the ankle in the sagittal plane at small dorsiflexion angles, and higher stiffness at larger dorsiflexion angles. The motivation behind this DS-AFO follows from the existence of similar moment-angle (stiffness) properties of the healthy human ankle during walking, referred to as dual-stiffness natural ankle quasi-stiffness (DS-NAS). Crucial to the design of the DS-AFO is the ability to adjust both the stiffness and the dorsiflexion angle at which the net stiffness increases, referred to as the activation angle. Three different DS-AFO stiffness configurations were tested, each with three different activation angles, along with a standard single strut/stiffness AFO configuration. The DS-AFO was able to achieve distinct regions of low and high stiffness at every configuration. Additionally, altering the activation angle by ±1 deg generally did not result in different stiffness properties. This work is a step forward in AFOs with complex stiffness properties that can better approximate the mechanics of a healthy human ankle.

Study on a quasi-zero-stiffness isolator for variable mass load

Generally, the performance of the quasi-zero-stiffness (QZS) isolator is very sensitive to the deviation of mass load. Due to the ultra-low stiffness, even a slight deviation could lead to a large offset from the designed equilibrium. The performance degradation for low frequency vibration would be caused by the increase of the stiffness around the actual equilibrium. In this paper, a QZS isolator with the deviation of mass load is studied. The isolator comprises a nonlinear positive stiffness configuration with a pair of torsion springs, oblique bars and linear bearings, and a negative stiffness configuration with a pair of oblique bars connected to horizontal springs. The dynamic equation of the isolator with the deviation of mass load is derived. The amplitude-frequency property and force transmissibility are investigated. It is found that the deviation of mass load could significantly change the dynamic properties of the QZS isolator. To improve the isolation performance, the parameters of the positive stiffness configuration are considered to be adjustable. The results show that the adjustability can help remain the initial equilibrium, and reduce the response amplitude and force transmissibility in low frequency. The negative effect of the deviation of mass load on the QZS isolator can be decreased to a great extent.

Topological optimization method of locking unit layout for space manipulator based on plant root adaptive growth theory

The method of locking point layout optimization and stiffness configuration for multi-joint complex mechanisms of space manipulator is proposed based on the root adaptive growth theory. A simplified model of the space manipulator and 3D spring support model is established. According to the proposed layout optimization and stiffness configuration method, the parallel optimization method and finite element simulation software are utilized to simulate and analyze the differentiation process of space manipulator locking units and the manipulator systems with different stiffness configurations. The results of the layout optimization and stiffness configuration method are compared with the results of the conventional method. It is verified that the fundamental frequency of the optimization and configuration results is higher than that of the conventional method, and the effectiveness of the optimization and configuration method is verified.

Metamaterial beam with dual-action absorbers for tunable and multi-band vibration absorption

This paper presents a new class of metamaterial beams of tunable and multi-band vibration absorption. The metamaterial beam is composed of uniform and periodic beam cells with locally resonant substructure called dual-action vibration absorber, DA. A DA vibration absorber comprising of three locally resonant subsystems, 3-DOF spring-mass-damper subsystems, is utilized to generate frequency stopbands to stop elastic wave propagation. The governing equations of motion for a periodic beam cell are derived. Several distinct mass and stiffness configurations for the metamaterial beam with DA vibration absorber are proposed. The dispersion relations and presence of three frequency stopbands are studied. A finite element method based on Timoshenko beam theory is used to model and analyze the introduced metamaterial beam with DA vibration absorber. The frequency response simulations agree well with the projected stopbands of the developed dispersion relations of the mass and stiffness configurations. The concept of the presented metamaterial beam with tunable and multi-stopbands is promising for wave propagation attenuation and control applications.

Application of the ps−Version of the Finite Element Method to the Analysis of Laminated Shells

The development of accurate and efficient numerical methods is of crucial importance for the analysis and design of composite structures. This is even more true in the presence of variable stiffness (VS) configurations, where intricate load paths can be responsible for complex and localized stress profiles. In this work, we present the ps-version of the finite elements method (ps-FEM), a novel FE approach which can perform global/local analysis through different refinement strategies efficiently and easily. Within this framework, the global behavior is captured through a p-refinement by increasing the polynomial order of the elements. For the local one, a mesh-superposition technique, called s-refinement, is used to improve locally the solution by defining a local/fine mesh overlaid to the global/coarse one. The combination of p- and s-refinements enables us to achieve excellent accuracy-to-cost ratios. This paper aims to present the numerical formulation and the implementation aspects of this novel approach to VS composite shell analysis. Numerical tests are reported to illustrate the potential of the method. The results provide a clear insight of its potential to guarantee fast convergence and easy mesh refinement where needed.

Simplified Approach to Nonlinear Vibration Analysis of Variable Stiffness Plates

A formulation for the analysis of the nonlinear vibrations of Variable Stiffness (VS) plates is presented. Third-order Shear Deformation Theory (TSDT) is employed in conjunction with a mixed variational formulation. The solution is sought via Ritz approximation for the spatial dependency, while time dependency is handled via Differential Quadrature (DQ) and Harmonic Balance (HB) methods. The main advantage of the framework is the reduced computational time, which is of particular interest to explore the large design space offered by variable stiffness configurations. The results are validated against reference solutions from the literature. Exemplary parametric studies are presented to demonstrate the potential of the approach as a powerful means for exploring the nonlinear vibration response of VS plates.

Stall nonlinear aeroelastic effects and active control of thin-walled composite blade with piezoelectric patches embedded

Stall nonlinear flutter behavior and active control of anisotropic composite blade have been investigated based on piezoelectric actuation. The blade body comprises an elliptical composite host structure and a lightweight honeycomb skin structure. The composite blade with elliptical section is modeled as a thin-walled beam structure with circumferential asymmetric stiffness (CAS) configuration, exhibiting transverse shear deformation, warping restraint effect, and vertical bending, with structural tailoring implemented. The stall-induced flutter suppression and nonlinear aeroelastic effects of linearized dynamic response characteristics of blade incorporating stall nonlinear aerodynamic model are investigated. The nonlinear aeroelastic coupled equations are reduced to ordinary equations by the Galerkin method, with the aerodynamic force decomposed by strip theory. Active feedback control based on boundary moment control strategy is developed to enhance the vibrational behavior and dynamic response to aerodynamic excitation and stabilize structures that might be damaged in the absence of control. Various active feedback control schemes tuned by optimal proportional-derivative (PD) controller are implemented. The excellent control effect is not only confirmed by the time response with its vibration amplitude greatly suppressed but also confirmed by the frequency change and comparison. The research provides a way for rare study of amplitude suppression of stall nonlinear aeroelastic effect of thin-walled composite blade with CAS configuration and piezoelectric patches embedded.

Design of a new quasi-zero-stiffness isolator system with nonlinear positive stiffness configuration and its novel features

Design of a new quasi-zero-stiffness isolator system with nonlinear positive stiffness configuration and its novel features

Numerical and experimental buckling response of moderately thick and thick composite plates with various stiffener layouts

The buckling behavior of moderately thick and thick stiffened composite plates with different stiffener layouts is formulated based on the higher-order shear deformation theory. This investigation is performed under axial compression using the Finite Strip Method and experiment. The effects of dimensions, orientation, eccentricity, torsional stiffness, position and geometric configuration of stiffeners on the buckling load coefficient are achieved by employing the principle of minimum potential energy. Six grid-stiffened composite plate specimens with ortho-grid, angle-grid, and orthotropic-grid stiffeners are tested. The results of the presented method show good outcomes with those of experimental tests and the finite element ANSYS code.

Magnet based bi-stable nonlinear energy sink for torsional vibration suppression of rotor system

This paper investigates the application of a magnetic bi-stable nonlinear energy sink (MBNES) on torsional vibration suppression of the rotor system. Firstly, the structure of the proposed nonlinear energy sink is introduced, which simultaneously combines a set of positive stiffness connecting rods with the overlapping magnet negative stiffness configurations together to produce the bi-stable characteristic. Furthermore, the analytical expression of the nonlinear torsional stiffness of the MBNES is derived by applying the magnetic charge theorem, and the dynamic model of the rotor-MBNES system is established in turn. Next, the vibration attenuation performances of the MBNES under transient and steady-state excitations are evaluated, respectively, and the parameter affections are studied. Finally, experimental studies are carried out on the coupled system. The results demonstrate that adding MBNES to the main system can obtain 2.22 times dissipating speed in transient response, and the vibration suppression rates of 65.68% in simulation and 54.10% in experiment are achieved in steady-state response.

Comparative Study of Regular and Irregular RC Structure in Different Seismic Zones and Soil Types

The resistivity of lateral forces (Wind and Seismic loading) by any structure is very much challenging. It is also one of the reasons for the failure of the reinforced concrete (RC) structure due to its asymmetric distribution of mass, strength, stiffness, and non-uniform geometrical configurations. Knowing the effect of various soils in the construction of RC structures is essential to determine the structural performance in the presence of seismic load ahead of construction. This research aims to compare regular and irregular RC structures in different seismic zones, and soil types with the provisions suggested in IS code 1893(part1):2016 and IS 875(part3):2015 using STAAD Pro V8i. Research work calculated the critical design loading for multi-story buildings put through basic wind speeds of 50 m/s and seismic zones (II, III, IV, V). The response of a G+20 storeyed RC framed building to seismic loads is examined using Indian Standard code IS 1893(part1):2016 and wind loads using Indian Standard code IS 875(part3):2015.

Variable Stiffness Identification and Configuration Optimization of Industrial Robots for Machining Tasks

Industrial robots are increasingly being used in machining tasks because of their high flexibility and intelligence. However, the low structural stiffness of a robot significantly affects its positional accuracy and the machining quality of its operation equipment. Studying robot stiffness characteristics and optimization methods is an effective method of improving the stiffness performance of a robot. Accordingly, aiming at the poor accuracy of stiffness modeling caused by approximating the stiffness of each joint as a constant, a variable stiffness identification method is proposed based on space gridding. Subsequently, a task-oriented axial stiffness evaluation index is proposed to quantitatively assess the stiffness performance in the machining direction. In addition, by analyzing the redundant kinematic characteristics of the robot machining system, a configuration optimization method is further developed to maximize the index. For numerous points or trajectory-processing tasks, a configuration smoothing strategy is proposed to rapidly acquire optimized configurations. Finally, experiments on a KR500 robot were conducted to verify the feasibility and validity of the proposed stiffness identification and configuration optimization methods.