Stiffness Mode Research Articles

Experimental and Design Parametric Study of a Novel Grouted Square Steel Tube Upper Chord–Concrete Composite Floor

A novel composite floor consisting of an upper chord of a grouted square steel tube truss and a lower chord of a reinforced concrete slab is proposed to address existing problems with precast composite floors, such as a complex construction process, high cost, significant resource consumption, and severe environmental pollution. Sixteen-point loading tests were carried out on five simply supported one-way slabs to simulate the stress state of the floor under a uniform load and to investigate the stiffness, cracking moment, load-carrying capacity, and failure mode of the composite floor system. The results showed that the composite floor system exhibited ductile failure, with cracks uniformly distributed at the bottom of the slab; additionally, the grouted steel tube did not experience uplift or out-of-plane buckling, and the truss welds did not fracture, indicating that the composite floor system still had further deformation capacity and load-carrying capacity with satisfactory stress conditions. After unloading, the residual deformation of the composite floor accounted for 28% to 36% of the maximum deflection, demonstrating good deformation recovery ability. The bottom slab thickness and truss height are key parameters that influence the new composite floor, and increasing the two parameters enhances the cracking load, deformation resistance, and flexural capacity of the composite floor, with a significant improvement achieved by increasing the truss height. The floor slab should have a minimum thickness of 60 mm, as required by China’s Code for Design of Concrete Structures, and the truss height-to-span ratio should be 1:30, which meets the deformation and load-carrying requirements of the floor and conforms to the design concept of green buildings.

Test and design of built-up cold-formed steel-lightweight concrete (CFS-LWC) composite beams

Cold-formed steel (CFS) products are extremely versatile and can be used in multiple applications. To further extend their applicability, this investigation assesses the structural behaviour of innovative built-up CFS-lightweight concrete (LWC) composite beams through experimental testing and numerical modelling. First, four full-scale CFS-LWC simply supported composite beams were tested. The specimens included two distinct configurations of the innovative CFS built-up composite beams. The tested CFS-LWC composite beams comprised a lightweight concrete slab supported by profiled metal decking, connected to a CFS built-up beam using bolted shear studs. Following EN 1994-1-1, the degree of shear connection attained in the specimens ranged from 21 % to 57 %. The test procedure, setup, and collected results are described in detail, including load-bearing capacity, deformation, flexural stiffness, and failure modes. Numerical simulations and calibration against the experimental data were then performed, and the CFS-LWC composite beams with different degrees of shear connections were investigated using the calibrated finite element model. The validity of the available design code for the innovative CFS-LWC composite beams was assessed by comparing experimental findings and analytical predictions using EN1994-1-1 (equilibrium and simplified methods) and AISC 360 methodologies. The results showed that the maximum bending resistance of the composite beam obtained from the equilibrium method of EN1994-1-1 and AISC 360 was unconservative due to the overprediction of shear resistance for the bolted shear connectors. Consequently, a calibrated finite element modelling technique determined the bolted shear connectors’ shear resistance. Finally, the bending resistance design prediction of CFS-LWC composite beams following EN1994-1-1 and AISC 360 methodologies, incorporating the shear connector resistance obtained from simulations, was compared with experimental and numerical results from the parametric study. An excellent agreement was observed between experimental and numerical results with the design predictions according to EN1994-1-1.

Experimental study of innovative bolted shear connectors in demountable cold-formed steel–concrete composite beams

Although cold-formed steel (CFS) members have been widely used in structures, their application is limited due to the buckling and instability of these members. To enhance their strength and stiffness, they are often combined with other materials like concrete or timber. One of the main challenges in composite structures is ensuring proper connection between different components to facilitate safe shear load transfer. This is achieved through various types of shear connectors. Bolted shear connections are preferred over headed shear studs because they offer greater strength and can be easily dismantled. Headed shear studs welded onto a steel section are unsuitable for CFS members due to their low thickness.In this paper, we propose a novel demountable composite connection specifically designed for prefabricated CFS structures. The behavior of this connection was evaluated through fifteen push-out test specimens, with key variables including CFS section thickness and bolt diameter/strength. The ultimate load, stiffness, ductility, and failure modes of all specimens were determined. The results revealed that specimens with a higher thickness of CFS members experienced bolt fracture, exhibiting superior effective stiffness, strength, and ductility. Conversely, specimens with a lower thickness of CFS members failed due to distortional buckling of the CFS members. The test results underscore the importance of carefully designing and providing a proportionate CFS section thickness to ensure shear connectors fracture before the CFS members. This significantly improves the shear strength capacity of the composite connection with CFS sections and precast concrete slabs. Furthermore, the deconstructability of the connection at the end of its service life was investigated. The steel–concrete composite connection with precast concrete slabs and demountable bolted shear connections was also modeled using a 3D finite element method. The numerical model’s accuracy in simulating the observed structural behavior was confirmed against experimental data. Finally, appropriate and straightforward formulations for predicting the ultimate load of the CFS-concrete composite connection were proposed. It was concluded that the predictions of ultimate shear load capacity values by EC4, AISC, and AISI provisions for this composite connection are significantly less accurate.

Modeling and characteristic analysis of mid-rear axle hydraulically interconnected suspensions for tri-axle heavy trucks

Truck suspension is a vital component supporting the weight of the vehicle body and plays an important role in transferring wheel load and isolating external vibration. The uneven load distribution between the middle and rear wheels of a tri-axle heavy truck will directly affect the driving stability and safety of the vehicle. To enhance the truck’s wheel load distribution performance and ride comfort, this paper proposes a new coupling suspension system, including an anti-synchronous middle and rear axle hydraulically interconnected suspension (MR-HIS) and the front axle conventional independent suspension (CIS). Considering the load-bearing characteristics of the middle and rear wheels of the tri-axle heavy truck, a five-degree-of-freedom (5-DOF) half-vehicle model of the truck equipped with MR-HIS is constructed. Further, the dynamics equations of the mechanical-hydraulic coupling system are established according to the impedance transfer matrix method and mechanical-hydraulic boundary conditions, and the truck body motion modes are completely decoupled. The modal analysis demonstrates that compared with CIS the proposed MR-HIS system can reduce the body bounce modal stiffness, increase the modal damping ratio, and enhance the dynamic coupling performance of middle and rear wheels. Subsequently, the frequency response analysis is adopted to evaluate the load distribution and vibration isolation performance of the MR-HIS. The comparison analysis results indicate that the proposed MR-HIS system significantly improves the load balance distribution between the middle and rear wheels and suppresses the load transfer to the front wheel, thus the handling stability of the truck is improved. Meanwhile, the body vibration frequency response analysis shows that the MR-HIS can effectively isolate external vibration and improve driving comfort of the tri-axle heavy truck.

Nanohardness Measurements of CdSiP2 and ZnGeP2 Chalcopyrite-Type Nonlinear Optical Crystals

We study the nanohardness and Young’s modulus of randomly oriented CdSiP2 (CSP) and ZnGeP2 (ZGP) single crystals, grown via the horizontal Bridgman method. CSP and ZGP are the only two pnictide chalcopyrites widely used as nonlinear optical crystals in the mid-IR part of the spectrum. Nanoindentation is employed in the continuous stiffness mode (45 Hz, 2 nm) using a Berkovich tip. Nanohardness values of 9.9 ± 0.2 GPa (Knoop hardness of 905 kg/mm2) for CSP and 11.5 ± 0.1 GPa (993 kg/mm2) for ZGP are derived. For Young’s modulus, we obtain 136 ± 2 GPa (CSP) and 150 ± 2 GPa (ZGP). The trend of increasing hardness with bandgap and melting point of the isostructural CSP and ZGP, as deduced from previous measurements, is not confirmed. The results for ZGP are compared to 2GaP, its binary isoelectronic analog, and the values obtained, 11.0 ± 0.3 GPa for the nanohardness and 154 ± 2 GPa for Young’s modulus, indicate good matching within the accuracy limits.

A Novel Design Concept of a Magnetorheological Fluid-Based Damper Utilizing the Porous Medium for Implementation in Small-Scale Applications

Magnetorheological (MR) dampers have a virtue over conventional dampers, where their damping properties can be adjusted using a magnetic field. However, MR dampers have been barely implemented in small vibratory systems, in which the modal mass and stiffness are relatively small. This is due to two major reasons, namely its high parasitic damping force and big moving mass. When such an MR damper is installed in a small vibratory system, the system‘s default damping ratio is increased and therefore its dynamic is reduced. Here, a new concept of an MR damper utilizing the porous medium and shear operating mode together with an external non-moving electromagnet is proposed. This combination results in an MR damper with a low parasitic damping force and a small moving mass. For comparison purposes, a benchmark MR damper with comparable geometry is constructed. The proposed MR damper possesses a passive friction force that is 8× smaller and OFF-state passive viscous damping that is 10–20× smaller than the benchmark MR damper. An investigation of the proposed MR damper performance in a test vibratory system shows almost no reduction of the system dynamic. Therefore, this proposed MR damper configuration can be suitable for applications in small vibratory systems.

An investigation of the flexural behaviour of large-span prestressed and steel-reinforced concrete slabs

The prestressed and steel-reinforced concrete slab (PSRCS) is an innovative composite structural member offering high load capacity and stiffness and exceptional anti-crack performance, making it a leading trend in composite structures. This paper presents the derived calculation formulas for bearing capacity, section stiffness, mid-span deflection of PSRCS. Additionally, a numerical analysis of PSRCS is conducted using ABAQUS software, with several models created to systematically investigate bearing capacity, section stiffness, anti-crack performance, and failure mode. Concurrently, PSRCS member parameters are analyzed for optimal design, and the results of finite element (FE) calculations are compared with theoretical formula calculations. The results demonstrate that PSRCS exhibits superior load capacity, section stiffness, and anti-crack performance comparing to conventional slabs. The parametric analysis offers optimal design for each parameter and presents the corresponding recommended span-to-depth ratios for various spans in PSRCS applications.

Development and Characterization of Polymeric Composites Reinforced with Lignocellulosic Wastes for Packaging Applications

In this work, the effects of different fiber loadings on the mechanical properties of the composites at the sub-micron scale were studied through nanoindentation followed by physical characterization. The composites were prepared by incorporating different loadings of wheat straw, corn stalk, and rice husk in polypropylene copolymer using a melt processing method followed by thermal–hydraulic compression technique. Nanoindentation experiments in quasi-continuous stiffness mode were performed on the surfaces of produced composites to study the composites’ elastic modulus, hardness, and creep properties. The obtained results expressed the in-depth study of the micro- and macro-level structure and behavior of particle interactions. The findings demonstrated that observable shifts in composites’ hardness, elastic modulus, and creep rate had occurred. The WS-reinforced biocomposite sheet showed the highest elastic modulus of 1.09 and hardness of 0.11 GPa at 40 wt% loading in comparison to other loadings. An impact strength of 7.55 kJ/m2 was noted for the biocomposite at 40 wt% RH loading. In addition, optical microscopy, Fourier transform infrared spectroscopy, water absorption, thickness swelling, and Vicat softening point studies were conducted on biocomposite sheets to evaluate differences in physical, mechanical, and thermal properties. The outstanding mechanical performance of the newly developed composites makes them suitable for use as a biodegradable packaging material.

A Variable Stiffness Sole for Biped Robot and Its Experimental Verification

Abstract The foot sole of the biped robot is an important factor for stable walking. In this study, the limitations of existing bipedal robot soles are introduced and the necessity for the development of a new sole mechanism is presented. Inspired by a robot sole based on the granular jamming effect, we have developed a variable stiffness sole (VSS), which adapts to the shape of obstacles on the ground in compliant mode and provides robust support in stiff mode. Finally, the performance of the VSS is verified by several experiments integrating the VSS with the real humanoid robot platform RoK-3. The experimental results verified that the VSS is advantageous in uneven terrain walking.

Structural modal identification and damage detection with incomplete data utilized by genetic algorithm optimization

This research presented a procedure to automatically identify the modal characteristics of structures to detect and quantify any localized damage of structures in frequency domain. This process uses the acceleration frequency response of the selected degree of freedoms, some natural frequency of the damaged structure and mode shapes of the undamaged Finite Element (FE) model to identify the structural stiffness of the damaged structure containing the damping ratio. The incomplete data in the unmeasured degree of freedom will be updated using a genetic optimization algorithm. The accuracy and efficiency of the proposed method have been tested on a 25 elements truss subjected to a harmonic load with sweep frequency. Optimization results demonstrated that the proposed method is capable of precisely identifying the modal characteristics and stiffness reduction factor of the damaged structure using genetic optimization algorithm along with 5% noise in the input force and response of the system in all studied cases.

Novel open-section origami tubes with multiple programmable mechanical properties

The diverse folding motion of the origami structures induces a series of extraordinary properties, which enriches their application prospects in engineering. In this paper, we propose a class of open-section origami tubes with multiple extraordinary mechanical properties, and explore their potential applications. First, three types of origami tubes with one degree of freedom are developed. Their motion patterns are studied through the kinematic analysis, which induces several extraordinary mechanical properties. The Poisson’s ratio of the proposed structure undergoes multiple sign-flipping, which can be tuned by the geometric parameters of panels. Also, the origami tubes exhibit multiple mechanical properties under compression such as mono/bistable and zero stiffness mode, which can be tuned by the rotational stiffness assignment of the creases. The self-locking behavior of the tandem origami tubes is also discussed, which shows the remarkable programmability on the number and position of locking. Based on the proposed origami cells, the piecewise-stiffness structure and quasi-zero-stiffness structure are constructed, and their performance is verified through the prototype tests.

Performance of Beam-Column Connections with Mass Ply Lam and Steel Dowels under Cyclic Loads

A mass timber lateral force resisting system (LFRS) is proposed using a mass timber buckling restrained brace (TBRB) to improve the seismic resilience of mass timber buildings. To develop a resilient braced frame, it is essential to understand and quantify the behavior of the connections, including the possible failure modes and moment-rotation capacities. Cyclic and monotonic tests were conducted on six mass timber beam-column connections to study the response of a mass timber joint connected with slotted-in steel plates and mild steel dowels. The primary goal of the subassembly tests was to measure the maximum rotation, stiffness, ductility, and failure modes of such connections for both monotonic and cyclic loads. The tests showed that the connections with three steel dowel details reached a maximum rotation of 0.11 radians (6.3 degrees) with no loss of strength. In addition, a numerical model was developed to represent the moment-rotation relationship of the mass timber beam-column connections.

Incremental stiffening approach for CFS built-up-beams with large imperfections: Tests and flexural-behaviour

Cold-formed steel (CFS) members generally have slender cross-sections making them more prone to geometric imperfections. The presence of different geometric imperfections with varying magnitudes can significantly influence the structural behaviour of CFS members, which is well-reported in the literature. However, limited data is available on the strengthening of CFS beams with large imperfections. This paper reports the findings of the first experimental investigation that focuses on adopting incremental stiffening approach to strengthen CFS built-up beams with large geometric imperfections induced due to poor fabrication and subsequent testing. CFS channel sections were used to construct both open and closed built-up beams, which were evaluated under four-point loading with simply supported boundary conditions. The incremental adoption of simple, yet effective stiffening approaches helped in attaining considerable improvement in both capacity and stiffness characteristics. The peak load, flexural stiffness, and failure mode exhibited by each tested specimen were used as key performance indicators. The results, in general, confirmed the feasibility of adopting incremental stiffening for improving both the capacity and the stiffness in distressed CFS built-up beams by up to 145% and 125%, respectively. Also, it is shown that a yield capacity of up to 100% and a plastic capacity of up to 80% can be achieved through the proposed stiffening approach. The presented test results will supplement the current inadequate database on distressed CFS beams, and should prove useful for more efficient design of such systems.

Novel Concentric Magnetic Continuum Robot with Multiple Stiffness Modes for Potential Delivery of Nanomedicine

The utilisation of miniature robots has become essential in the domain of minimally invasive surgery and long-distance delivery of nanomedicine. Among these, the miniature magnetic continuum robot (MCR) stands out because of its simple structure and dexterity, which allow it to penetrate small cavities, transport specialised tools such as a laser, and deliver medications to support surgical treatment. Nevertheless, because of their soft bodies with a single stiffness, conventional MCRs have limited controllability when navigating through intricate cavities. To address this limitation, we propose a novel concentric magnetic continuum robot (C-MCR) comprising a concentric magnetic catheter with a guidewire having varying stiffness. The C-MCR allows substantial curvature bending owing to its difference in stiffness, and its detachable nature allows it to have four working modes to adapt to specific application requirements with improved stiffness controllability. Experiments demonstrate the ability of the C-MCR to navigate complex pathways and deliver nanomedicines over long distances to specific areas via its internal channels using a large homemade eight-coil electromagnetic system. The C-MCR offers promising application prospects for the long-distance delivery of tailored nanomedicines because of its simple operation, reduced risks, and larger attainable workspace.

Influence of a saltwater film on a radio-transparent dome on the amplitude-phase distribution of a millimeter-range antenna

Transition of communication systems installed on naval ships to the millimeter range requires an analysis of the effect of the saltwater film formed on the radio-transparent dome (RTD) on the radiotechnical characteristics of the mirror antenna. The latter are determined by the amplitude-phase distribution in the antenna aperture, so the topic of research is relevant. The objective of the article is to analyze the effect of the salt-water film on the surface of a marine mounted RTD, as well as the effect of millimeter wavelength antenna placement inside the RTD on the distortion of the field generated in the outer region. It has been shown that when installing millimeter-wave range antennas on a naval ship under a radio-transparent dome it is necessary to consider the distortions introduced by the water film on the dome surface into the generated amplitude-phase distribution when calculating the radio technical characteristics. We do so because most of the known methods for calculating the directivity characteristics of mirror antennas are based on the known amplitude-phase distribution in the antenna aperture. In the paper, the influence of the spherical dome wall and the saltwater film on the amplitude-phase distribution is taken into account as follows. Distortions in the amplitude distribution are caused by energy losses of the electromagnetic wave due to the passage of the curved dielectric wall at different angles. Phase distribution losses are due to the phase shift from the original phase as the dome wall is passed through at different angles. The angles of radio wave entry into the dome wall depend on the location of the antenna inside the radio-transparent dome. The electrical parameters of the saltwater film are determined using the well-known ITU methodology, while its thickness is found using the Gibbs formula, assuming that sea spray is similar to the intensity of drizzle rain. The studies have been conducted for the highest stiffness mode. Studies for a single-mirror antenna with a parabolic reflector 5.4 m in diameter and a radio-transparent dome 18 m in diameter showed that the water film influence is manifested in the phase front distortion, as well as a symmetry violation of the generated amplitude distribution. The degree of water salinity is manifested mainly in the distortion of the phase distribution. This is due to the fact that the electrical parameters of salt and fresh water do not differ much. The detected distortions are stronger for the horizontally polarized waves and in the case when the antenna swing center is offset with respect to the dome symmetry line. The detected patterns allow to give a scientific justification for the choice of the amplitude-phase distribution of the mirror antenna when installing it inside the RTD on the naval ship.

Design and Development of a Continuum Robot with Switching-Stiffness.

Continuum robots have the advantages of agility and adaptability. However, existing continuum robots have limitations of low stiffness and complex motion modes, and the existing variable stiffness methods cannot achieve a wide range of stiffness changes and fast switching stiffness simultaneously. A continuum robot structure, switching stiffness method, and motion principle are proposed in this article. The continuum robot is made up of three segments connected in series. Each segment comprises multiple spherical joints connected in series, and the joints can be locked by their respective airbag. A valve controls each airbag, quickly switching the segment between rigidity and flexibility. The motion of the segments is driven by three cables that run through the robot. The segment steers only when it is unlocked. When a segment becomes locked, it acts as a rigid body. As a result, by locking and unlocking each segment in sequence, the cables can alternately drive all the segments. The stiffness variation and movement of the continuum robot were tested. The segment's stiffness varies from 36.89 to 1300.95 N/m and the stiffness switching time is 0.25-0.48 s. The time-sharing control mode of segment stiffness and motion is validated by establishing a specific test platform and a mathematical model. The continuum robot's flexibility is demonstrated by controlling the fast bending of different segments sequentially.

Study on the flexural and ductility behaviour of plate reinforced concrete beam-A review

A comprehensive literature review on the basis of the flexural and ductility behaviour of the plate reinforced concrete beams are presented in this paper. The objective of this paper is to provide a review of the development and current progress in the flexural and ductile behaviour of plate-reinforced concrete beams. The composite beams are composed of plain concrete beams strengthened by steel plates, externally bonded to their tension faces. The effects of four parameters; namely, plate thickness, beam size, concrete strength, and adhesive type on the moment capacity, deflection, and failure mode of composite beams are studied. This paper also highlights the use of plate thickness and beam size, which has a significant impact on the flexural strength, stiffness, and failure mode of composite beams. Furthermore, this literature study also identifies areas, where further research is needed, such as, the effect of the adhesive type and concrete grade on the properties of composite beams.

Experimental research and theoretical analysis of novel polygon‐key assembly connections

AbstractTo improve the shear performance of assembly connections in precast RC shear wall structures, a novel polygon‐key assembly connection (PKA connection) was proposed. Shear tests were carried out to study the shear performance of the PKA connections, specimens' parameters considered in the tests consist of polygon‐key inclination angles, polygon‐key heights, and axial compression ratios. Shear capacity, stiffness, and failure modes of the PKA connections were investigated. Furthermore, theoretical analysis of the PKA connection was performed, equations of PKA connection's shear capacity were deduced, and the theoretical analysis results were verified by the test results. The results show that the polygon‐keys can effectively improve the shear capacity of assembly connection of precast RC shear wall structure. Compared with the parallel assembly connection, peak shear capacity of the PKA connection can be enhanced by more than 15%. When polygon‐key's inclination angle is within the range of 45–90°, the peak loads of PKA connections change slightly. When polygon‐key's height is within the range of 5–7 cm, the cracking load and peak load of the PKA connection change slightly. With an increase in axial compression ratio, the shear capacity of PKA connection gradually increases, and the peak displacement changes slightly. Based on the shear‐friction theory, the calculated PKA connection's peak loads are in good agreement with the test results.

A Novel Variable Stiffness Compliant Robotic Link Based on Discrete Variable Stiffness Units for Safe Human–Robot Interaction

AbstractVariable stiffness manipulators balance the trade-off between manipulation performance needing high stiffness and safe human–robot interaction desiring low stiffness. Variable stiffness links enable this flexible manipulation function during human–robot interaction. In this paper, we propose a novel variable stiffness link based on discrete variable stiffness units (DSUs). A DSU is a parallel guided beam that can adjust stiffness discretely by changing the cross-sectional area properties of the hollow beam segments. The variable stiffness link (Tri-DSU) consists of three tandem DSUs to achieve eight stiffness modes and a stiffness ratio of 31. To optimize the design, stiffness analysis of the DSU and Tri-DSU under various configurations and forces was performed by a derived linear analytical model which applies to small/intermediate deflections. The model is derived using the approach of serially connected beams and superposition combinations. 3D-Printed prototypes were built to verify the feature and performance of the Tri-DSU in comparison with the finite element analysis and analytical model results. It’s demonstrated that our model can accurately predict the stiffnesses of the DSU and Tri-DSU within a certain range of parameters. Impact tests were also conducted to validate the performance of the Tri-DSU. The developed method and analytical model are extendable to multiple DSUs with parameter configurations to achieve modularization and customization, and also provide a tool for the design of reconfigurable collaborative robot (cobot) manipulators.

A 2-Degree-of-Freedom Quasi-Passive Prosthetic Wrist With Two Levels of Compliance

Restoring the function of a missing hand is still a grand challenge for bioengineers. We witnessed significant recent advances in the development of myoelectric hand prostheses and their controllers. Conversely, the wrist joint is generally overlooked in prosthetics, despite playing a fundamental role in orienting the hand in space. Indeed, it may account for several degrees of freedom of the hand in reducing compensatory movements. We acknowledge that an active, three-degree-of-freedom prosthetic wrist is not a viable option for a self-contained prosthesis, therefore we merged in one design two opposed passive behaviors. The proposed wrist can automatically transition between a compliant mode, which exhibits relatively low stiffness allowing for passive motions around two rotational axes (wrist flexion/extension and radial/ulnar deviation), and a stiff mode, which grants stability during manipulation. To switch mode, no additional control input - hence cognitive burden - from the user is needed: it occurs synchronously with the prosthetic hand opening and closing motion, such that the wrist is compliant during reaching and stiff during manipulation. Our device proved reliable on the test bench and useful in a pilot test with an amputee volunteer, motivating further developments and more extensive testing to prove its effectiveness.