Large Displacement Range Research Articles

Constraint loss correction: Enhancing transferability of fracture toughness of Eurofer97 compact tension specimens between specimen sizes

This paper proposes and implements a constraint loss correction as a supplement to the ASTM correction factor for the master curve method (ASTM-E1921) for Eurofer97 reduced activation steel in order to enhance the transferability of results between compact tension (CT) specimen sizes. To facilitate the acquisition and scalability of reliable fracture toughness data from one specimen size to another and to apply the Master-Curve method with confidence when using small specimens, the difference in the structure of the crack tip stress field between large and small specimens was modeled. The stress intensity factors and critical volumes/stresses of two specimen sizes (1 T and 0.18 T) were calculated with 3D finite element simulations run over a large displacement range and at a temperature range of −120 to −80 °C and the results were compared with real data from a database of toughness tests of Eurofer97 steel. The K values from a miniaturized specimen without or with ASTM correction and with the proposed combined correction with constraint loss factor (which is variable over the entire loading range) were compared with simulated CT-1 T stress intensity values. It was found that this new correction factor was accurate up to a target toughness of 200 MPa.m1/2, which represents a major improvement on the current state of the art.

Design of Novel Artificial Anal Sphincter

ObjectivesArtificial anal sphincter is considered to be a good method for the treatment of severe fecal incontinence, but there are problems of low biomechanical compatibility in clinical application. The purpose of this study is to design a novel artificial anal sphincter that can output stable loading force to solve the mechanical mismatch between artificial anal sphincter and intestinal tissue caused by excessive or too small local pressure on the intestine. Material and methodsAiming at the shortcomings of the existing artificial anal sphincter, a novel artificial anal sphincter with constant force mechanism is designed based on the C-shaped SMA sheet of Ti-55.9at%Ni and combined with the normal defecation mechanism of human body. In this paper, the chord length l of C-shaped SMA sheet is used as the design variable, and the artificial anal sphincter is optimized and verified initially by using finite element analysis method. ResultsThe results show that the artificial anal sphincter can achieve a constant force to clamp the intestine in a large displacement range when the chord length l of the C-shaped SMA is 13 mm, the chord height h is 1.5 mm, the width w is 2 mm, and the thickness δ is 0.2 mm. ConclusionThe design of artificial anal sphincter with constant force loading mechanism has a good effect in solving the biomechanical compatibility problem between the implant device and the intestinal tissue. In addition, the designed artificial anal sphincter occupies a small space, which provides a new idea for future clinical application.

Drain-to-Drain Abutment-Aware Detailed Placement Refinement for Power Staple Insertion Optimization

Power staple insertion is an effective means to mitigate IR drop in the advanced technology process. Previous works have shown that detailed placement refinement can greatly enhance the power staple insertion rate. However, with FinFET, it is necessary to consider the drain-to-drain abutment (DDA) constraints. We note that extra source node insertion for resolving DDA violations can interfere with power staple insertion. To handle DDA constraints and optimize power staple insertion at the same time, we formulate and solve a new DDA-aware placement refinement problem in this work. Given an initial nonoverlapping placement optimized for other conventional objectives, we compute a refined placement with cell shifting subject to DDA constraints such that the number of staple insertion slots can be maximized. An effective approach is proposed that supports a nonrestricted cell displacement range during placement refinement. It provides the flexibility to adjust the displacement bound of each cell dynamically in order to resolve all DDA violations. As a result, our algorithm guarantees that a placement solution with no DDA violation can always be computed without using a large displacement range for each cell which would require much longer runtime and larger average cell displacement. In addition to cell shifting, we incorporate concurrent cell flipping to reduce the required cell displacement to satisfy the DDA constraints and to facilitate power staple insertion. The experimental results showed the effectiveness of the proposed approach.

Novel Combination 3DOF Radial/Axial Eddy-Current Position Sensor

A novel noncontact three-degree-of-freedom radial/axial eddy-current position sensor for applications in active magnetic bearings (AMBs) is developed. The sensor meets the most demanding industrial requirements, including ability to work in dusty environments and in many working fluids, low sensitivity to temperature changes, as well as external electrical and magnetic fields, excellent linearity over a large displacement range, large intrinsic gain, easy integration into a machine, robustness to variations in parameters of the external wiring, simplicity of the signal processing electronics, and large frequency bandwidth. The sensor is inexpensive in volume and well suited for high-volume production. It has been successfully integrated into a commercial 175-kW air compressor, which was spun to the full operation speed of 50 kRPM. Operation with a 4-m twisted-pair sensor cable has been demonstrated and using much longer (tens of meters) cables would be possible with design modifications. Even though the sensor was developed for AMBs, it can find other uses in rotating machinery and beyond.

Nonlinear Reduced Order Modeling of Structures Near Buckling and Application to an Energy Harvester

Abstract The focus of this investigation is first on assessing the validity to structures under in-plane forces, in particular near buckling, of the reduced order modeling approach for nonlinear geometric response that has been extensively developed in the last two decades. This focus is motivated by a class of piezoelectric energy harvesters that rely on strongly nonlinear behavior, such as large amplitude responses, to achieve broadband energy harvesting. A simple, two-rigid bars linkage that approximates a buckling beam is first considered to discover the features of the nonlinear force–displacement relationship induced by an in-plane loading. It is observed that the corresponding form of this relationship is not consistent with the one derived from a reduced order model (ROM) but can be closely approximated by it over a large displacement range. This analysis emphasizes in particular the role of a group of ROM coefficients that are usually considered unimportant. A similar study is performed next for the buckled harvester modeled within nastran and it is again found that a close match of the force–displacement relationship can be achieved. Based on that positive outlook, a six basis functions ROM of this beam harvester that includes piezoelectric effects is built and identified. It is found to provide a close match of nastran nonlinear predictions over a broad range of transverse and in-plane loadings in static and dynamic conditions. The ROM usefulness in predicting the open-circuit voltage is demonstrated.

Ultimate behaviour of bolted T-stubs under large displacements: A mechanical model

To provide a tool able to deliver an accurate prediction of the response of bolted T-stubs, a novel mechanical model based on a step-by-step integration algorithm is presented. It explicitly considers some complexities occurring in the large displacement range, with detailed non-linear modelling of the sources of strength and deformability. As shown by the comparison of model and test data, all the complexities considered are necessary to predict T-stubs' resistance and their ultimate displacement capacity accurately. Indeed, the accuracy of the proposed model, both in terms of ultimate displacement (δv) and maximum strength (F), is evaluated considering a set of recent test data and the predictions of a previous mechanical model, which is based on a first-order approximation of the T-stub response. Concerning the prediction of δv, the mean value of the ratio δv, th/δv, exp is equal to 0.87, with a CoV equal to 0.14. Similarly, the mean value of the ratio Fth/Fexp is equal to 0.96, with a CoV equal to 0.17.

Vibration isolation properties of the nonlinear X-combined structure with a high-static and low-dynamic stiffness: Theory and experiment

A nonlinear X-combined structure is proposed and designed using an X-shaped structure (positive/negative stiffness mechanism) and two inclined springs (positive stiffness mechanism) to avoid existing negative stiffness and unstable factors and to improve vibration isolation characteristics. The mechanism possesses high-static and low-dynamic stiffness in a larger displacement range. In order to identify the coupling mechanism of the combination of two nonlinear mechanisms, its static and dynamic properties are investigated. The displacement transmissibility with multi-valued solutions is used to evaluate the vibration isolation of the combined structure. A new mathematical method is used to obtain a multi-valued analytical solution to the nonlinear equation of motion, which is then validated using the ADAMS software and the least square method (LSM). The novelty of this study is (a) the system has a very useful nonlinear stiffness with positive/quasi-zero/zero stiffness over the entire working range, and the quasi-zero stiffness (QZS) range is greater than that of an X-shaped structure; (b) compared to the existing nonlinear vibration isolator, such as three-spring systems and X-shaped structures, the proposed structure has the lower equivalent stiffness in the vibration isolation range, and the vibration isolation performance is better when each system has the same loading capacity; (c) the completed and accurate nonlinear solutions can more scientifically reflect the vibration isolation characteristics of the system; (d) the introduction of oblique springs into the X-shaped structure can make it easier to adjust the stiffness of the structure and use the tensile spring as the oblique spring, and the X-combined structure is easier to be designed and installed. This study shows that the proposed structure can provide excellent vibration isolation by combining two nonlinear mechanisms. These advantages are also confirmed by experiments and comparison with existing QZS isolators. This vibration isolation system will provide a new analytic method and innovative approach to the QZS isolator.

A Soft Multi-Axis High Force Range Magnetic Tactile Sensor for Force Feedback in Robotic Surgical Systems.

This paper presents a multi-axis low-cost soft magnetic tactile sensor with a high force range for force feedback in robotic surgical systems. The proposed sensor is designed to fully decouple the output response for normal, shear and angular forces. The proposed sensor is fabricated using rapid prototyping techniques and utilizes Neodymium magnets embedded in an elastomer over Hall sensors such that their displacement produces a voltage change that can be used to calculate the applied force. The initial spacing between the magnets and the Hall sensors is optimized to achieve a large displacement range using finite element method (FEM) simulations. The experimental characterization of the proposed sensor is performed for applied force in normal, shear and 45° angular direction. The force sensitivity of the proposed sensor in normal, shear and angular directions is 16 mV/N, 30 mV/N and 81 mV/N, respectively, with minimum mechanical crosstalk. The force range for the normal, shear and angular direction is obtained as 0–20 N, 0–3.5 N and 0–1.5 N, respectively. The proposed sensor shows a perfectly linear behavior and a low hysteresis error of 8.3%, making it suitable for tactile sensing and biomedical applications. The effect of the material properties of the elastomer on force ranges and sensitivity values of the proposed sensor is also discussed.

Retrieving the GBT modal decomposition from large displacement shell finite element results

This paper proposes an efficient procedure to post-process non-linear shell finite element analysis results, in order to recover the unique modal decomposition features of Generalised Beam Theory (GBT). This procedure extends, to the large displacement range, a previous one developed for first-order elastoplastic, bifurcation and vibration analyses of thin-walled members (Manta et al., 2021). This means that the structural insight typically obtained through GBT analyses can be quite straightforwardly recovered from shell finite element collapse analysis results. The proposed procedure retrieves modal strains rather than modal displacements, since the GBT kinematic description is not suited to describe cross-section finite rotations. To show the capabilities and potential of the proposed procedure, two validation examples, in which a comparison with analytical solutions is made, and two application examples, involving large displacements and plasticity, are presented and discussed.

Self-Actuating Isothermal Nanomechanical Test Platform for Tensile Creep Measurement of Freestanding Thin Films

Microelectromechanical systems (MEMS) enable potent methods for nanomechanical testing. In many MEMS-based test platforms, high force polycrystalline silicon chevron-type thermal actuators are used as on chip actuators. Here, we implement thermal actuators using the refractory metal tantalum (Ta) as a new micromachined structural material in such a way that it can be integrated to test mechanical properties of a wide variety of metals. Because Ta’s coefficient of thermal expansion is more than double that of silicon, actuation is accomplished simply by raising the temperature. An isothermal condition is attained, while no force is lost to a heat sink. The platform is batch fabricated by conventional surface micromachining processes and is especially well suited to measure creep of thin films because the load spring is soft compared to the specimen uniaxially loaded in tension. This in turn gives rise to a slow stress relaxation and a large displacement range. Specimen alignment is excellent and handling procedures for thin films are simple and robust. As a validation of the apparatus, uniaxial Au tensile specimens of 110 nm thickness are fabricated and tested. Young’s modulus is measured and compares well with literature values, while creep rate is lower than other reported values for nanocrystalline Au, indicating that steady state creep is approached. This work demonstrates a powerful drift-free nanomechanical test platform for mechanical property measurement of freestanding thin films subjected to uniaxial tension. [2021-0171]

Bionic paw-inspired structure for vibration isolation with novel nonlinear compensation mechanism

Inspired by the compensation effect of fat pad on toes in a paw of digitigrade, a unique paw-inspired structure (PIS) is proposed and systematically investigated to explore its advantages in passive vibration isolation. The PIS consists of two key parts, one is the toe-like structure (TLS) simulated by two rods and a spring, and the other is fat pad mimicked by a pair of repulsive magnets. Based on the principle of virtual work, the static stiffness characteristics of the TLS are firstly analyzed. Then, the stiffness compensation mechanism of fat pad to toe is comprehensively studied. The hardening stiffness of the fat pad can compensate the negative stiffness of the TLS so that the PIS can realize quasi-zero stiffness (QZS) over large displacement range. It is for the first time to reveal this nonlinear stiffness compensation mechanism, which is essentially different from the conventional realization of QZS (connecting negative stiffness mechanism and linear spring in parallel). A dynamic model is established to estimate the vibration isolation performance. The displacement transmissibility derived by harmonic balance method (HBM) indicates that the PIS can achieve low resonance frequency and wide effective vibration isolation frequency band. Static tests are completed to verify effectiveness of the stiffness compensation mechanism. Tests under different external excitations (including periodic, sweep and random) show that the PIS can effectively suppress frequency components above 4 Hz. The PIS provides a new low-frequency vibration isolation method with high application prospects. And the proposed nonlinear stiffness mechanism can also be extended as a guideline to design nonlinear vibration isolators.

Detecting subtle yet fast skeletal muscle contractions with ultrasoft and durable graphene-based cellular materials.

Human bodily movements are primarily controlled by the contractions of skeletal muscles. Unlike joint or skeletal movements that are generally performed in the large displacement range, the contractions of the skeletal muscles that underpin these movements are subtle in intensity yet high in frequency. This subtlety of movement makes it a formidable challenge to develop wearable and durable soft materials to electrically monitor such motions with high fidelity for the purpose of, for example, muscle/neuromuscular disease diagnosis. Here we report that an intrinsically fragile ultralow-density graphene-based cellular monolith sandwiched between silicone rubbers can exhibit a highly effective stress and strain transfer mechanism at its interface with the rubber, with a remarkable improvement in stretchability (>100%). In particular, this hybrid also exhibits a highly sensitive, broadband-frequency electrical response (up to 180 Hz) for a wide range of strains. By correlating the mechanical signal of muscle movements obtained from this hybrid material with electromyography, we demonstrate that the strain sensor based on this hybrid material may provide a new, soft and wearable mechanomyography approach for real-time monitoring of complex neuromuscular–skeletal interactions in a broad range of healthcare and human–machine interface applications. This work also provides a new architecture-enabled functional soft material platform for wearable electronics.

Piezo stepping actuator design with macrometer range and sub micrometer precision

A piezoelectric actuator is a device that provides a large displacement with micro-meter accuracy and nanometer resolution. The displacement accomplished by the piezo stepping actuator was in steps. This mechanism provides high precision, accuracy, and resolution with low power consumption. A stepping linear motion of the actuator based on the piezo stepping actuator was proposed in this work. The stepping motion of the piezo stepping actuator was attained with the help of four push–pull actuators, four lock-release actuators, and two flexural amplified piezoelectric (FAP) actuators. The designed actuator provided a large range of linear displacements with high accuracy. The designed configuration and working principle of the piezo stepping actuator are discussed. Finite element analysis was carried out on the final assembly of the piezo stepping actuator for the maximum displacement obtained from the supporting block. A suitable stepping frequency for the smooth operation of the piezo stepping actuator was adopted. The push-pull actuator, lock-release actuator, FAP actuator, and base assembly were fabricated and tested for displacement values.

Geometrical nonlinearities and shape effects in electromechanical models of piezoelectric bridge structures

We consider nonlinear shape effects appearing in the lumped electromechanical model of a bimorph piezoelectric bridge structure due to the interaction between the electromechanical constitutive model and the geometry of the structure. At finite proof-mass displacement and electrode voltage, the shape of the beams is no longer given by Euler-Bernoulli theory which implies that shape effects enter in both the electrical and mechanical domains and in the coupling between them. Accounting for such effects is important for the accurate modelling of, e.g., piezoelectrical energy harvesters and actuators in the regime of large deflections and voltages. We present a general method, based on a variational approach minimizing the Gibbs enthalpy of the system, for computing corrections to the nominal shape function and the associated corrections to the lumped model. The lowest order correction is derived explicitly and is shown to produce significant improvements in model accuracy, both in terms of the Gibbs enthalpy and the shape function itself, over a large range of displacements and voltages. Furthermore, we validate the theoretical model using large deflection finite element simulations of the bridge structure and conclude that the lowest order correction substantially improve the model, obtaining a level of accuracy expected to be sufficient for most applications. Finally, we derive the equations of motion for the lowest order corrected model and show how the coupling between the electromechanical properties and the geometry of the bridge structure introduces nonlinear interaction terms.

System identification for a six-storey precast concrete frame building

This paper presents forced and ambient vibration tests on a six-storey precast concrete frame building, a structural system that has not often been the focus of such tests in spite of its widespread use. Precast buildings come with semi-rigid connections so their idealisation requires good knowledge of the construction practice for those connections and their details. Structural system dynamic properties are identified for the first three translational modes along each axis of the building and for the first three torsional modes. The study shows that the three-dimensional finite-element structural model for the building is capable of replicating the measured in situ dynamic properties. In the small displacement elastic range, the finite-element modelling of precast concrete frame systems is not very different from their cast-in-place counterparts but an extrapolation to the large displacement range requires empirical confirmation.

Conceptual mechanical design of antagonistic variable stiffness joint based on equivalent quadratic torsion spring.

The variable stiffness joint is a kind of flexible actuator with variable stiffness characteristics suitable for physical human-robot interaction applications. In the existing variable stiffness joints, the antagonistic variable stiffness joint has the advantages of simple implementation of variable stiffness mechanism and easy modular design of the nonlinear elastic element. The variable stiffness characteristics of antagonistic variable stiffness joints are realized by the antagonistic actuation of two nonlinear springs. A novel design scheme of the equivalent nonlinear torsion spring with compact structure, large angular displacement range, and desired stiffness characteristics is presented in this article. The design calculation for the equivalent quadratic torsion spring is given as an example, and the actuation characteristics of the antagonistic variable stiffness joint based on the equivalent quadratic torsion spring are illustrated. Based on the design idea of constructing the antagonistic variable stiffness joint with compact structure and high compliance, as well as the different design requirements of the joints at different positions of the multi-degrees of freedom robot arm, nine types of mechanical schemes of antagonistic variable stiffness joint with the open design concept are proposed in this article. Finally, the conceptual joint configuration schemes of the robot arm based on the antagonistic variable stiffness joint show the application scheme of the designed antagonistic variable stiffness joint in the multi-degrees of freedom robot.

Behaviour of steel sheeting connections with self-drilling screws under variable loading

Abstract Connections made by self-drilling screws are often used in steel cold-formed structures and sheeting. Their structural properties, such as resistance, flexibility and ductility are obtained by testing under monotonically increasing loading and can influence on overall behaviour of structures. This paper presents results of experimental study how variable load deteriorates those structural properties of lap shear connections with self-drilling screws. One monotonic and four variable loading histories were utilized, with displacement and force control. Applied loading histories were aimed to simulate persistent design situation, which referred to the conditions of normal use according to EN 1990. Deterioration of resistance and flexibility have been assessed for each variable loading history. It was observed that resistance deterioration is directly proportional to loading range. In case of flexibility, at small displacement range connectors become stiffer. Degradation of flexibility starts to increase at larger displacement range.

Recurrent-Neural-Network-Based Predictive Control of Piezo Actuators for Trajectory Tracking

Precise trajectory tracking of piezo actuators (PEAs) in real time is essential to high-precision systems and applications. However, the real-time tracking accuracy is rather limited as the PEA cannot be accurately modeled over large bandwidth and displacement range due to its nonlinearities. In this article, we propose to use recurrent-neural-network (RNN) to model the PEA system and develop a nonlinear predictive controller for PEA trajectory tracking. Considering the computation efficiency, first, an RNN is trained to model the nonlinear dynamics of the PEA system at high-frequency range. Then, a second-order linear model is proposed to account for the PEA low-frequency dynamics. Therefore, the PEA dynamics is modeled by the nonlinear model consisting of the RNN and the linear model, which is further used for nonlinear predictive control of the displacement. To increase the prediction accuracy, an unscented Kalman filter is designed to estimate the states of the nonlinear model. The nonlinear predictive control problem is solved based on a gradient descent algorithm, in which a method for analytically calculating the gradient of the cost function is developed. The proposed technique was experimentally implemented on a nano piezo stage for demonstration and its performance was compared with that of a PID controller. The accuracy of an iterative learning control approach was used as a benchmark for comparison as well. The results showed that high precision trajectory tracking of PEAs in real time can be achieved using the proposed technique.

Nonlinear topology optimization of parallel-grasping microgripper

This paper presents a piezo-driven microgripper which can realize parallel grasping in a large displacement range (more than 140μm). The proposed microgripper is derived by a two-step nonlinear topology optimization method. A conventional linear topology optimization problem with loose boundary condition is solved first, and its solution is used as the initial design domain of the following nonlinear topology optimization. The comparison of the convergence results shows that this method can find a better structure of microgripper and save computation time simultaneously. Both the finite element analysis and experiment are used to verify the performance of the proposed microgripper. The experimental results show that the parasitic movement of the jaw is only 0.299% of the grasping movement and the average inclination angle of the jaw is 0.055mrad/μm. A pair of strain gages is integrated as a displacement sensor, and its performance is presented and verified by experiments.

Finite-energy Lévy-type motion through heterogeneous ensemble of Brownian particles

Complex systems are known to display anomalous diffusion, whose signature is a space/time scaling with in the probability density function (PDF). Anomalous diffusion can emerge jointly with both Gaussian, e.g. fractional Brownian motion, and power-law decaying distributions, e.g. Lévy Flights or Lévy Walks (LWs). Lévy flights get anomalous scaling, but, being jumps of any size allowed even at short times, have infinite position variance, infinite energy and discontinuous paths. LWs, which are based on random trapping events, overcome these limitations: they resemble a Lévy-type power-law distribution that is truncated in the large displacement range and have finite moments, finite energy and, even with discontinuous velocity, they are continuous. However, LWs do not take into account the role of strong heterogeneity in many complex systems, such as biological transport in the crowded cell environment. In this work we propose and discuss a model describing a heterogeneous ensemble of Brownian particles (HEBP). Velocity of each single particle obeys a standard underdamped Langevin equation for the velocity, with linear friction term and additive Gaussian noise. Each particle is characterized by its own relaxation time and velocity diffusivity. We show that, for proper distributions of relaxation time and velocity diffusivity, the HEBP resembles some LW statistical features, in particular power-law decaying PDF, long-range correlations and anomalous diffusion, at the same time keeping finite position moments and finite energy. The main differences between the HEBP model and two different LWs are investigated, finding that, even when both velocity and position PDFs are similar, they differ in four main aspects: (i) LWs are biscaling, while HEBP is monoscaling; (ii) a transition from anomalous () to normal () diffusion in the long-time regime is seen in the HEBP and not in LWs; (iii) the power-law index of the position PDF and the space/time diffusion scaling are independent in the HEBP, while they both depend on the scaling of the inter-event time PDF in LWs; (iv) at variance with LWs, our HEBP model obeys a fluctuation–dissipation theorem.