Stiffness Device Research Articles

Apparent-weakening by adaptive passive stiffness shaping along the height of multistory building using negative stiffness devices and dampers for seismic protection

Apparent weakening by adaptive stiffness shaping of structures using a negative stiffness device (NSD) is a new passive structural control approach to mitigate the response of structures during severe ground motions. Experimental studies confirm significant reductions in acceleration, inter-story drift and base shear experienced by a structure when a NSD is deployed in conjunction with a viscous damper. So far, the focus and application of NSDs has been restricted to response reduction in the story where a NSD is installed and its effect on stories above that. In this paper, we demonstrate the efficacy of deploying NSDs accompanied by dampers at multiple locations along the height of an example nine story structure. Typically, the NSDs in the lower floors undergo large deformations since the energy from a ground motion is transmitted from the bottom to top. To prevent this, we propose a generalized scheme to incorporate NSDs at all floors with different apparent-yield displacements. We incorporate the variations with a view towards minimizing localized inter-story deformations (ISD) and distribute the effects of strong ground motions evenly along the height of the structure. In addition to the variation of apparent-yield of NSDs, we study the effects of supplementing the NSD with linear and non-linear dampers. We compare the response of the nine story structure without passive control devices (BS) to cases where linear dampers (LD) are attached, nonlinear dampers are attached (ND), NSDs in conjuction with linear dampers are attached (NSD + LD) and NSDs in conjunction with nonlinear dampers are attached (NSD + ND). Our results show that if the distribution of apparent yield is properly designed for a structure, NSDs with nonlinear dampers results in a superior performance compared to the other cases considered in this paper.

A Layer Jamming Actuator for Tunable Stiffness and Shape-Changing Devices.

Changing the shape and the stiffness of a device in a dynamic and controlled way enables important advancements in the field of robotics and wearable robotics. Variable stiffness materials and technologies can be used to address this challenge. In particular, layer jamming actuation is a very promising technology, featured by high efficiency and low cost. In this article, a stiffness- and shape-changing device based on a novel mechanism including a multiple-chamber structure is proposed. It allows to effectively modulate the shape and stiffness of a device, by activating two jamming chambers while pressurizing/depressurizing one or more interposed inflatable chambers. Prototypes with a size of 45 × 270 mm2 and an average thickness ranging from 4.4 to 13 mm were developed and their ability to undergo a stiffness change over two orders of magnitude was demonstrated. The prototypes were also able to change their shape according to the position and inflation level of the interposed inflatable chambers, thus resulting in an overall deflection >10 mm. The possibility to wear the system as an orthotic brace was also demonstrated: this technology increased the patient comfort in static positions, yet keeping a supportive function when needed (e.g., in dynamic conditions). The device working principle highlighted in this article could also be exploited in other domains, for example, to build walking soft robots, prostheses, or grippers, as demonstrated through additional tests.

An Electromagnetic Variable Stiffness Device for Semiactive Seat Suspension Vibration Control

This article introduces a rotary semiactive variable stiffness (VS) device, including its system design and model validation. A variable damping (VD) device and springs can form mechanical networks with specific topologies, and the VD device can control the equivalent stiffness of the network by varying its damping. The proposed rotary electromagnetic VS device consists of an electromagnetic VD device and a torsion spring, which are in series, while a planetary gear reducer is used to amplify the torque output of the VS device. The models of the electromagnetic VD and VS devices are both built and validated with tests. Then, a seat suspension applies the VS device by installing the device in the center of its scissors structure. Bump and random vibration experiments are implemented to validate the effectiveness of the proposed VS system for seat suspension vibration control. The new system has merits in controllability and energy efficiency. It is easier to design the controller than the conventional one with magnetorheological damper due to the linear damping of the electromagnetic VD device. Besides, the system energy consumption is low and has energy harvesting capability. The proposed semiactive electromagnetic VS device also has potential in other practical applicationss.

Stressed Skin Design Versus Braced Frame Design Through Efficient Numerical Modelling

Steel frame structures are traditionally designed with bracings that stabilize the main bearing structure. Another approach is to apply the “stressed skin design” where the cladding structure takes the role of the bracings. In this research, the two approaches were analysed and compared in order to find advantages of any. A typical steel frame structure was chosen, and for the cladding, trapezoidal sheet metal was selected. Optional bracings were also considered. All static analyses were done using the finite element method (FEM) and ANSYS Workbench software, including geometric and material nonlinearity. The structure was loaded perpendicularly to its gable, thus simulating typical wind action. Number of fasteners connecting the cladding and the frame is crucial for the “stressed skin design” concept, so this parameter was varied through the analysis. The stiffness of the fastener devices was simulated in two ways: (1) simplified, by merging of the adjacent nodes of the cladding and the frame, and (2) more accurately, using special joint elements with prescribed stiffness. Obtained stresses and deformations were compared, and they showed obvious advantages of the “stressed skin design” over braced frame design, both in structural, and in economical aspect. In addition, important practical guidelines for the “stressed skin design” using FEM were proposed.

Test and evaluation of modified TADAS devices with different grades of steel

Triangular-plate added damping and stiffness (TADAS) devices are reliable metallic energy dissipaters for seismic upgrading used in design and retrofitting of civil structures. Conventional TADAS devices are designed with closed-ended slots. In this study, a modified form of the TADAS device is proposed with open-ended slots in order to reduce the manufacture cost, facilitate the assembling and avoid abrupt stiffness increase. Cyclic and monotonic loading tests are then conducted to investigate the mechanical characteristics of the modified TADAS devices with regular Q345 steel and low-yield point LY160 steel triangular plates. The test results show that although the hysteresis performances are stable, the cyclic hardening behavior is different between the TADAS specimens with different grades of steel. The TADAS specimen with LY160 triangular plates exhibits more significant overstrength behavior than the one with Q345 triangular plates in cyclic loading, which is unsuitable to be described by the classic Bouc-Wen model. Therefore, a modified Bouc-Wen model is proposed to describe such overstrength behavior. It is shown that the modified model is able to simulate different extent of overstrength behavior in cyclic loading, based on which the cyclic hardening behavior of the TADAS specimen with LY160 triangular plates can be well described.

Manufacturing of Hybrid Yarn Thermoplastic Composites by the Method of Filament Winding

Filament winding is a manufacturing method to produce composite materials with a thermosetting or thermoplastic matrix impregnated continuous fibers. Due to the high melt viscosity of thermoplastics, some problems such as inadequate fiber dispersion in the structure, low wetting ability, and interface quality problems between matrix and fiber might emerge. Preforms can be used in order to solve these problems in the continuous fiber reinforced composite manufacturing; division of the polymer can be made using either powder or fiber form. In this study, polyethylene/E-glass fiber and polypropylene/E-glass fiber hybrid yarns were used as materials to produce thermoplastic composite structures. Firstly, plate samples were produced at various temperatures, periods of time and different thicknesses of ply. After the preparation of samples tensile test, 3 point bending flexural test, and calcination test were applied and Scanning Electron Microscopy (SEM) observations were also performed to these samples, and then the production parameters were optimized. After that, cylindrical shaped samples were produced by a laboratory-type manual filament winding device and ring stiffness tests were performed. According to the test results of the plate samples, the optimum production conditions were determined as 200 °C for 5 minutes for polyethylene/E-glass fiber composite structure and 220 °C for 5 minutes for polypropylene/E-glass fiber composite structure. Mechanical test results of plate samples revealed that polypropylene composites presented slightly better results than polyethylene ones. Additionally, in the cylindrical specimens, close results to the ring stiffness values determined by the standards were obtained.

Cyber-physical design and optimization of tall building dynamics using aeroelastic wind tunnel modeling

This study explores the use of an automated cyber-physical system (CPS) framework for the design and optimization of tall building dynamics through aeroelastic boundary layer wind tunnel (BLWT) testing. The framework is fully automated, integrating experimental BLWT modeling with numerical optimization algorithms to rapidly evaluate a wide range of candidate designs. In this study, candidate tall building designs are generated by physically adjusting the dynamic properties of a multi-degree-of-freedom (MDOF) aeroelastic model. The specimen is equipped with an actuation system consisting of a series of variable stiffness devices (VSD), which enable precise control of the lower natural frequencies. The specimen is also instrumented with accelerometers and displacement sensors to capture wind-induced building response. A stochastic optimization algorithm evaluates the fitness of each candidate design based on code-based and/or user-specified serviceability limit states related to occupant comfort and building drift (e.g., sway). Wind tunnel experiments were conducted at the University of Florida Natural Hazard Engineering Research Infrastructure (NHERI) Experimental Facility. Results show that the CPS framework can reliably drive the aeroelastic specimen to the optimal solution which minimizes stiffness (i.e., natural frequency) while satisfying multiple acceleration and drift constraints.

Negative stiffness devices for vibration isolation applications: A review

In recent years, negative stiffness vibration isolation device with nonlinear characteristic has become an emerging research area and attracted a significant amount of attentions in the community due to the promising potentials it brought into the field. Its high-static-low-dynamic stiffness property endows the capacity to realize effective vibration isolation and in the meantime to maintain the system stability. This article presents a comprehensive review of the recent research and developments on negative stiffness vibration isolation device. It begins with an introduction on the concept of negative stiffness and then provides a summary and discussion regarding the realization and characteristics of negative stiffness vibration isolation device. The article places its special interest on the principles, structure design, and device characterisation of different types of negative stiffness vibration isolation devices, including spring type, pre-bucked beam type, magnetism type, geometrically nonlinear structural type, and composite structural type. Besides, the applications of negative stiffness vibration isolation device, as well as negative stiffness damper, are summarized and discussed based on the current state-of-the-art. Finally, the conclusions and further discussion provide highlights of the investigation.

Use of Dynamic FEA for Design Modification and Energy Analysis of a Variable Stiffness Prosthetic Foot

Different tasks and conditions in gait call for different stiffness of prosthetic foot devices. The following work presents a case study on design modifications of a prosthetic foot, aimed at variable stiffness of the device. The objective is a proof-of-concept, achieved by simulating the modifications using finite element modeling. Design changes include the addition of a controlled damping element, connected both in parallel and series to a system of springs. The aim is to change the stiffness of the device under dynamic loading, by applying a high damping constant, approaching force coupling for the given boundary conditions. The dynamic modelling simulates mechanical test methods used to measure load response in full roll-over of prosthetic feet. Activation of the element during loading of the foot justifies the damped effect. As damping is in contrast to the main design objectives of energy return in prosthetic feet, it is considered important to quantify the dissipated energy in such an element. Our design case shows that the introduction of a damping element, with a high damping constant, can increase the overall rotational stiffness of the device by 50%. Given a large enough damping coefficient, the energy dissipation in the active element is about 20% of maximum strain energy.

Seismic Assessment of a Benchmark Highway Bridge Equipped with Optimized Shape Memory Alloy Wire-Based Isolators

In this paper, an evolutionary multi-objective optimization algorithm named NSGA-II was used to determine the optimum radius for shape memory alloy (SMA) wires employed in conjunction with the lead rubber bearing (LRB), referred to as an SMA-LRB isolator. This algorithm simultaneously minimizes the mid-span displacement and the base shear force. Then, the optimized SMA-LRBs were implemented in a benchmark bridge to reduce excessive displacements. The results obtained from the nonlinear dynamic analysis show that the implemented approach could effectively optimize the SMA-LRBs. These improved smart isolators can noticeably reduce the maximum displacements and residual deformations of the structure; meanwhile, the base shear and deck acceleration remain less than those of the non-isolated benchmark bridge. This isolator can reduce the maximum mid-span displacement of the bridge by up to 61%, and the mid-span residual deformations by up to 100%, compared to an uncontrolled isolated bridge under different ground motions. This optimized passive system was compared with nonlinear dampers, passive SMA dampers, and a negative stiffness device. The results indicate that the optimized SMA-LRB isolators are generally more successful in reducing and recovering displacements than the other controllers.

A Novel Electrical Variable Stiffness Device for Vehicle Seat Suspension Control With Mismatched Disturbance Compensation

This paper presents a novel electrical variable stiffness device (EVSD) and its application in seat suspensions. The EVSD is inspired by the conventional mechanical variable stiffness device (VSD) and the force-current analogy, which applies an inductor and a variable resistor to simulate the functions of the spring and variable damper in the mechanical VSD. The electrical system has less physical limitation than the mechanical one in the practical application. Besides, the EVSD requires no power supply except for control signals at milliwatts level. The test result shows the stiffness controllability of the proposed EVSD and matches with the system model in the simulation. Seat suspensions have been extensively utilized in vehicles, especially heavy-duty ones, to protect drivers’ health and improve the ride comfort. An EVSD-based seat suspension is investigated in this paper. The seat suspension applies the suspension deflection, circuit current, and seat acceleration, which are all easy to measure with sensors, as feedback to estimate the mismatched disturbance in the system; and a ${\boldsymbol {H}_\infty }$ controller with compensation of the mismatched disturbance is designed for the seat suspension. The experiments validate the effectiveness of the proposed seat suspension and controller. This EVSD-based seat suspension consumes ignorable energy and has great potential in practical applications.

Study on Stiffness-Oriented Cable Tension Distribution for a Symmetrical Cable-Driven Mechanism

In this paper, we focus on the issues pertaining to stiffness-oriented cable tension distribution for a symmetrical 6-cable-driven spherical joint module (6-CSJM), which can be employed to construct modular cable-driven manipulators. Due to the redundant actuation of the 6-CSJM, three cables are employed for position regulation by adjusting the cable lengths, and the remaining three cables are utilized for stiffness regulation by adjusting the cable tensions, i.e., the position and stiffness can be regulated simultaneously. To increase the range of stiffness regulation, a variable stiffness device (VSD) is designed, which is serially connected to the driving cable. Since the stiffness model of the 6-CSJM with VSDs is very complicated, it is difficult to directly solve the cable tensions from the desired stiffness. The stiffness-oriented cable tension distribution issue is formulated as a nonlinear constrained optimization problem, and the Complex method is employed to obtain optimal tension distributions. Furthermore, to significantly improve the computation efficiency, a decision variable elimination technique is proposed to deal with the equality constraints, which reduces decision variables from 6 to 3. A comprehensive simulation study is conducted to verify the effectiveness of the proposed method, showing that the 6-CSJM can accurately achieve the desired stiffness through cable tension optimization.

Simplified optimal design of MDOF structures with negative stiffness amplifying dampers based on effective damping

SUMMARYThis paper presents a study on multi‐degree‐of‐freedom (MDOF) structures equipped with a negative stiffness amplifying damper (NSAD). The NSAD not only preserves the negative stiffness feature of negative stiffness devices (NSDs) but also achieves prominent damping magnification effect, substantially reducing a NSD's requirement for high additional damping, which is used to contain the increased displacements resulting from the reduction in overall stiffness of a system. The dynamic equations of MDOF systems with NSADs are described in state–space representation, and the effective damping and frequencies are parametrically studied. Then, a simple optimization method is proposed. A study of the 20‐storey benchmark building shows that NSADs are the most efficient of supplemental devices in reducing interstorey drifts compared with viscous dampers (VDs) and viscoelastic dampers (VEDs) with the same supplemental damping coefficient. For instance, when compared with that of VEDs, the maximal peak interstorey of NSADs can be further reduced by about 30%. In terms of reducing acceleration responses, NSADs perform much better than VDs and VEDs owing to their negative stiffness feature. Partially arranged, NSADs are best implemented at the storeys that have smaller interstorey drift responses. This is because the negative stiffness preserved by NSADs significantly reduces the interstorey drift of storeys without NSADs.

An electromagnetic variable inertance device for seat suspension vibration control

Abstract This paper designs and builds an electromagnetic variable inertance (VI) device for a heavy duty vehicle seat suspension to reduce the vibration transferred to driver bodies. The conventional semi-active vibration control mainly applies the variable damping (VD) and variable stiffness (VS) devices. The VI device is equivalent to an inerter with the capability to vary its inertance, which is a new semi-active system inspired by the VS device design. The VI device consists of a transmission device, a VD device and two flywheels. By varying the damping of the VD device, the equivalent inertance of the VI device is controllable in real time. The VI device prototype is built with an electromagnetic VD device which is controlled by the duty cycle of the pulse width modulation signal. The test results verify the proposed system model and identify the unknown model parameters. Then, a seat suspension applies the VI device prototype for improving the ride comfort. Based on the system model, an implementable controller is proposed and experimentally validated. The proposed seat suspension system shows excellent performance in a random vibration experiment. Compared with a conventional passive seat suspension, the frequency-weighted root mean square acceleration of the proposed one reduces 35.7%. The VI device has expanded the application of the inerter and the area of the semi-active research; it shows great potential in vibration control.

A comparative study of vibration isolation performance using negative stiffness and inerter dampers

Two types of passive devices, namely, negative stiffness damper (NSD) and inerter damper (ID), have been receiving growing interest in vibration isolation and suppression, because both can produce negative-slope force-displacement relationships that are similar to those associated with active control forces. Despite such a similarity, these two passive dampers possess obvious differences in their mechanical behaviors. This study aims to illustrate the similarity and difference between these two dampers in vibration isolation applications with respect to the H2 and H∞ performance. The comparative study indicates that both dampers can reduce the H∞ norm effectively; the negative stiffness devices can reduce the H2 norm as well, whereas the H2 norm cannot converge under the influence of inerter. This finding explains why a tuned-inerter damper, i.e., an inerter connected in series with a spring with proper frequency tuning, is more commonly adopted in vibration isolation. The pros and cons of both devices were further discussed.

Adaptive Negative Stiffness Device based nonconventional Tuned Mass Damper for seismic vibration control of tall buildings

Adaptive Negative Stiffness Device (ANSD) has been developed recently for passive vibration control of structures. ANSD emulates negative stiffness behavior (by exerting force toward the direction of motion) to a structure, so that, the combined structure-ANSD system shows smooth elasto-plastic force-deformation behavior with virtual yielding at target (“engaging”) displacement. The device has been studied analytically and its behavior has been demonstrated experimentally in simply supported bridge model and in Single Degree of Freedom (SDOF) structure considering limited input excitations. However, efficient and cost-effective exploitation of true potential of ANSD deserves further studies. With this being the eventual goal, in this study, we explore the performances of ANSD in seismic vibration control of a tall building by implementing a nonconventional Tuned Mass Damper (TMD) with drift control. In absence of a methodology for choice of ANSD parameters in literature, reported till date, we employ a tuning criteria for nonconventional TMD, on which the drift control is imposed as a constraint, given the special significance of drifts in tall buildings. The criteria of tuning and drift control are shown to be mutually conflicting and an algorithm is presented to address the same. Simplifying assumptions are made to develop the tuning criteria in presence of nonlinearity in ANSD and P−δ nonlinearity in tall building. A Reduced Order Modeling (ROM) approach for the building-ANSD system is developed to facilitate the algorithmic computations. Robustness of our design approach is shown under an acceptable level of detuning of TMD and near fault pulse type motions. Comparative assessment with a conventional design approach is presented to show the improvements. A step by step illustration of the developed methodology is briefly touched on in context of modern performance based approach. The study is a first step toward the possible utility of ANSD in controlling seismic vulnerability of tall buildings.

Performance analysis of vehicle suspension systems with negative stiffness

This work evaluates the influence of negative stiffness on the performances of various vehicle suspension systems, and proposes a re-centering negative stiffness device (NSD). The re-centering NSD consists of a passive magnetic negative stiffness spring and a positioning shaft with a re-centering function. The former produces negative stiffness control forces, and the latter prevents the amplification of static spring deflection. The numerical simulations reveal that negative stiffness can improve the ride comfort of a vehicle without affecting its road holding abilities for either passive or semi-active suspension systems. In general, the improvement degree of ride comfort increases as negative stiffness increases. For passive suspension system, negative stiffness brings in negative stiffness feature in the control forces, which is helpful for the ride comfort of a vehicle. For semi-active suspensions, negative stiffness can alleviate the impact of clipped damping in semi-active dampers, and thus the ride comfort of a vehicle can be improved.

Nuclear density gauge compaction testing alternatives: synthesis and critical analysis

ABSTRACTThe costs of training, licensing and security for agencies using the nuclear density gauge (NDG) have become increasingly onerous for public engineering agencies, leading them to seek less expensive alternatives to conduct compaction testing. The study focused on three comparators: difference in approaches between tested devices and methods, the setting of targets for modulus stiffness devices and the examination of reported accuracy and reliability. This paper’s objective is to summarize the reported results for each alternative and provide the details of the findings. The paper finds that previous research has been almost exclusively focused on laboratory testing and is nearly devoid of authoritative information on field test performance parameters such as repeatability and time to complete a field test. It concludes that research is needed that includes a broad set of testing methods and equipment on the same set of soil conditions. The paper’s primary contribution is to consolidate and critically analyze the results of the current compaction testing research and benchmark the state-of-the-practice in field compaction testing. This paper does provide comparisons without intent to identify a “best” alternative because each device is adapted to specific soil conditions. Testing agencies may require differing devices based on budget, manpower and training.

Unveiling the influence of device stiffness in single macromolecule unfolding

Single-molecule stretching experiments on DNA, RNA, and other biological macromolecules opened up the possibility of an impressive progress in many fields of life and medical sciences. The reliability of such experiments may be crucially limited by the possibility of determining the influence of the apparatus on the experimental outputs. Here we deduce a model that let us analytically evaluate such influence, fundamental for the interpretation of Single Molecule Force Spectroscopy experiments and intermolecular interactions phenomena. As we show, our model is coherent with previous numerical results and quantitively reproduce AFM experimental tests on titin macromolecules and P-selectin with variable probe stiffnesses.

Adjustable template stiffness device and SDOF nonlinear frequency response

Devices with displacement dependent stiffness have been explored for vibration isolation. In this paper, an adjustable template stiffness device (ATSD) is presented. The ATSD is made up of pre-compressed springs attached to rollers, rolling along curved templates that allow adjustment of stiffness as a predetermined function of displacement. The ATSD configuration is presented in detail, and an analytical model is developed and validated using test results. The ATSD is tested under harmonic motion to determine its force–displacement behavior; then, it is connected to a single degree of freedom system, with an additional linear elastic stiffness spring, and tested on a shaking table to determine its vibration characteristics. The combination of the ATSD and the additional linear spring results in a composite nonlinear elastic spring with an initial softening behavior followed by hardening behavior or vice versa, i.e., depending on the chosen curved template. Frequency response curves obtained from test results are presented to demonstrate the capability of the ATSD. The new ATSD can be used for vibration isolation studies in future studies.