Constant Stiffness Research Articles

Experimental study of the direct shear characteristics of cement grout under constant normal loading and stiffness boundary conditions

Grouting cable is one of the most critical technologies in mine support. However, there are few studies on the effect of post-peak residual strength of cement grout under direct shear testing, which is of great significance for evaluating the bearing capacity of grouting cables in the whole life cycle. Hence, in this study, we carried out a systematic direct shear test to reveal the shear characteristics of cement grout under both constant normal loading (CNL) and constant normal stiffness (CNS) boundary conditions. The test results show that, under the same initial normal stress, the peak shear strength of the cement grout obtained under the CNS boundary conditions is greater than that under the CNL boundary conditions. The high initial normal force reduces the effect of boundary conditions on the post-peak mechanical shear properties of the cement specimens. Moreover, under CNS boundary conditions, the acoustic emission activity of the sample has an obvious signal before the peak shear strength, accompanied by multiple large ringing counts, indicating that the internal damage of the cement grout starts before the peak intensity. This work enhances the understanding of cement grouting fracture characteristics in mining support engineering.

Numerical and experimental study of viscoelastic properties and energy absorption of functionally graded negative stiffness honeycomb made of Polyamide 12

ABSTRACT This study investigated the effect of a functionally graded negative stiffness (FGNS) honeycomb made from polyamide 12 (PA12) on increasing energy absorption. In addition, the viscoelastic model of PA12 was implemented in the finite element (FE) model to demonstrate the recoverability of these structures. First, a quasi-static compression test was performed on constant thickness negative stiffness (CTNS) honeycomb and FGNS honeycomb. Then, these two structures’ FE models were simulated. The stress relaxation test was utilised to derive Prony series coefficients for PA12 viscoelastic properties. The results of viscoelastic simulation for CTNS and FGNS honeycombs were then compared with experimental results. The comparison demonstrated a good agreement in both the loading and unloading stages. Furthermore, the critical parameters for evaluating the structural energy absorption of the FGNS honeycomb and the conventional ones were used in this study. According to the results, the FGNS honeycomb outperformed the CTNS experimentally and numerically.

Dynamical analysis of an inverted pendulum with positive position feedback controller approximate uniform solution

The inverted pendulum is controlled in this article by using the nonlinear control theory. From classical analytical mechanics, its substructure equation of motion is derived. Because of the inclusion of the restoring forces, the Taylor expansion is employed to facilitate the analysis. An estimated satisfactory periodic solution is obtained with the aid of the modified Homotopy perturbation method. A numerical technique based on the fourth-order Runge–Kutta method is employed to justify the previous solution. On the other hand, a positive position feedback control is developed to dampen the vibrations of an IP system subjected to multi-excitation forces. The multiple time scale perturbation technique of the second order is introduced as a mathematical method to solve a two-degree-of-freedom system that simulates the IP with the PPF at primary and 1:1 internal resonance. The stability of these solutions is checked with the aid of the Routh–Hurwitz criterion. A set of graphs, based on the frequency response equations resulting from the MSPT method, is incorporated. Additionally, a numerical simulation is set up with RK-4 to confirm the overall controlled performance of the studied model. The quality of the solution is confirmed by the match between the approximate solution and the numerical simulation. Numerous other nonlinear systems can be controlled using the provided control method. Illustrations are offered that pertain to implications in design and pedagogy. The linearized stability of IP near the fixed points as well as the phase portraits is depicted for the autonomous and non-autonomous cases. Because of the static stability of the IP, it is found that its instability can be suppressed by the increase of both the generalized force as well as the torsional constant stiffness of the spring. Additionally, the presence of the magnetic field enhances the stability of IP.

Tissue Dielectric Constant and Skin Stiffness Relationships in Lower Extremity Lymphedema.

Background: Lower extremity lymphedema or edema (LELE) may progressively transition from a state of excess tissue fluid to increased fat accumulation and collagen deposition, with tissue fibrosis and hardening. Such changes may lead to altered tissue water holding and thereby impact tissue dielectric constant (TDC). This study seeks to evaluate the relationship between TDC and tissue indentation force (TIF) in patients with LELE and assess the utility of the leg/arm TDC ratio (LAR) as an indicator of LELE. Methods and Results: Thirty females (49-91 years) with previously diagnosed LELE were evaluated during a scheduled session. TDC and TIF were measured 8 cm proximal to the medial malleolus on the medial and lateral aspects of both legs and on one forearm 8 cm distal to the antecubital fossa. The TDC-TIC relationship and the LAR were subsequently determined. Main results showed an absence of a significant correlation between TDC and TIF on medial or lateral leg sites but a positive correlation on the normal forearm site. Further, LAR values exceeded the published proposed threshold of 1.35 for 29/30 patients when using medial-side TDC values and 28/30 patients when using lateral-side TDC values. Conclusions: Findings suggest that for patients with LELE, TDC values are significantly elevated on medial and lateral standardized sites. The LAR determined using either medial or lateral sites that are similar to each other and have values consistent with a lymphedema threshold of 1.35. In edematous legs of the type evaluated herein, there is no apparent relationship between TDC values and indentation force.

Experimentally investigating the influence of changing payload stiffness on outer loop iterative learning control strategies with shaking table tests

Shaking tables are widely used across numerous engineering research and industrial sectors, including mechanical (e.g. automotive and aerospace testing), electrical (e.g. instrumentation testing) and civil (e.g. structural and geotechnical testing) engineering. It is commonly required to replicate the shake table motions accurately and precisely. Iterative learning control algorithms can be used to complement traditional proportional–integral–differential feedback control algorithms to optimize drive signals using a test payload prior to the real experiment. Historically, the design of these test payloads has focused on matching the mass of the actual payload and neglected its dynamic response. In this study, experimental results from shake table tests using multiple geotechnical containers with dry and saturated beds that exhibit a range of stiffnesses and material damping when shaken are presented. Errors between the demanded and achieved motions are explored and compared to the changing secant stiffness abstracted from the dynamic shear stress–strain loops of the payload. A clear trend emerges that demonstrates increased errors as the payload stiffness deviates from the constant stiffness test payload originally used with the open loop iterative learning control, and further the errors are not necessarily bounded by test payloads significantly softer or stiffer than the actual specimen. The findings support that in cases where repeatable, accurate and precise shake table motions are required for payloads that exhibit a complex material response that is not readily modelled mathematically, it may be necessary to reproduce the specimen’s overall dynamic response during the iterative learning control process.

Sliding modes of fault activation under constant normal stiffness conditions

Fault activation has been the focus of research community for years. However, the studies of fault activation remain immature, such as the fault activation mode and its major factors under constant normal stiffness (CNS) conditions associated with large thickness of fault surrounding rock mass. In this study, the rock friction experiments were conducted to understand the fault activation modes under the CNS conditions. Two major parameters, i.e. the initial normal stress and loading rate, were considered and calibrated in the tests. To reveal the response mechanism of fault activation, the local strains near the fault plane were recorded, and the macroscopic stresses and displacements were analyzed. The testing results show that the effect of displacement-controlled loading rate is more pronounced under the CNS conditions than that under constant normal load (CNL) conditions. Both the normal and shear stresses drop suddenly when the stick-slip occurs. The decrease and increase of the normal stress are synchronous with the shear stress in the regular stick-slip scenario, but mismatch with the shear stress during the chaotic stick-slip process. The results are helpful for understanding the fault sliding mode and the prediction and prevention of fault slip.

Effect of preloading on vibration and buckling responses of variable stiffness composite cylindrical shells

Composite cylindrical shell has been utilized in marine shipping industry as a reliable auxiliary wind propulsion device that can reduce fuel consumption. However, due to the special boundary constraints and complex loading cases, the vibration and buckling responses of such devices differ significantly from traditional cylindrical shells. The present research deals with the vibration and buckling behavior of composite cylindrical shells utilizing the promising variable stiffness (VS) concept. The effects of the preloading states and the design constraints were analyzed to explore the effectiveness of VS concept in cylindrical shell structures. Compared to constant stiffness (CS) composite cylindrical shells, VS cylindrical shells are less sensitive to variation in preloading states and geometric imperfections, and the buckling strength can be improved under some combination of loading sates. The effect of boundary constraints on stiffness and bucking strength is less prominent own to VS properties, in that stress concentrations near the end of the cylindrical shell structures can be smoothed out, and the strain distributions are spaced more evenly.

Nonlocal higher-order finite element modeling for vibration analysis of viscoelastic orthotropic nanoplates resting on variable viscoelastic foundation

This article develops a novel finite element formulation based on nonlocal theory to analyze the vibration of viscoelastic orthotropic nanoplates resting on the variable viscoelastic foundation (VEF). The mechanical properties of nanoplate are assumed to be viscoelastic orthotropic according to Kelvin’s model. The variable VEF consists of two layers: a shear layer with constant stiffness, the other layer is described as a system composed of alternating damping and springs, in which the spring stiffness and the damping coefficient vary x-axis only. Based on Hamilton’s principle, a refined higher-order shear deformation plate theory (HSDT) and nonlocal theory are used to establish motion equations of the nanoplates. A four-node quadrilateral finite element is created by combining two interpolation functions including Lagrangian and Hermitian interpolation functions to describe the primary variables corresponding to the in-plane displacements and transverse displacement. The obtained results in our work are verified through reliable publications. A series of factors influencing the vibration of orthotropic nanoplates resting on the variable VEF is discovered and discussed.

Optimization design of variable stiffness composite cylinders based on non-uniform rational B-spline curve/surface

Compared with a constant stiffness (CS) composite structure of traditional straight fiber path, a variable stiffness (VS) composite structure of curvilinear fiber path has a broader design space. Taking the performance of a CS cylinder as the baseline, two curved fiber formats of VS cylinders are investigated. One is that the fiber angle varies in circumferential direction (i.e., circumferential stiffness variation, VS-C), the other is that the fiber angle varies in both circumferential and axial directions (i.e., circumferential and axial stiffness variation,VS-CA). Non-uniform rational B-spline curve/surface is used to define the continuously varying fiber orientation angle, making the VS design come true. This paper focuses on the maximum buckling capacity of VS composite cylinders under two kinds of loading cases (pure bending moment at both ends, concentrated force at the middle of the axis), and the optimal fiber angles of VS-C and VS-CA cylinders are obtained. In order to improve the computational efficiency, the adaptive Kriging surrogate model is integrated with genetic algorithm to solve the above optimization problems. The results show that the anti-buckling performance of VS-CA structure is always better or close to VS-C structure under different loading conditions, and all are better than that of CS structure.

HHV-7 Encephalitis: The Rare Infection and Its Outcome on a Child (P11-10.006)

<h3>Objective:</h3> The primary aims of this study were to relate HHV-7 infection with clinical manifestations of encephalitis, transient hip synovitis and hepatic changes in childhood and to evidence the effectiveness of early diagnosis and treatment in patients with encephalitis. <h3>Background:</h3> A 2-year-old male patient reported persistent high fever, coughing and rhinorrhea starting 1 week ago. They sought outpatient pediatric care and was prescribed ceftriaxone and prednisolone, with no satisfactory response. The symptoms evolved to vomiting and dehydration. During the physical exam, he showed irritation, with constant cervical stiffness and hyperextension, photophobia, and convergent strabismus. On arrival, he had been taking ceftriaxone, acyclovir and desloratadine. Laboratory tests were very altered in leukocytes, platelets, PCR, AST, ALT and GGT. During the ophthalmologic examination, a paresis of cranial nerve VI on the right was identified as secondary to a probable meningitis. After 21 days of treatment with acyclovir, the patient had a good prognosis, however, he had claudication upon movement of the left hip. <h3>Design/Methods:</h3> NA <h3>Results:</h3> The CSF results were colorless, cloudy, pH 8, Glucose 58, Leukocytes 40, Proteins 81. To confirm this observation, the molecular panel detected the presence of HHV-7. The imaging tests helped and added to the results of the blood count and CSF. Cranial MRI showed a hyperintensity in the FLAIR sequence of the perimesencephalic subarachnoid space and small accumulation of liquid. In addition, the abdomen CT found a bit of fluid in the hip region. <h3>Conclusions:</h3> Encephalitis is an inflammation of the brain parenchyma that leads to neurological dysfunction caused by infection or autoimmunity. In this occasion, the patient had a rare case of HHV-7 encephalitis. Basically, the treatment was carried out with acyclovir and ketoprofen, obtaining a progressive improvement of the clinical picture. Consequently, further research on the complications of HHV-7 is essential to develop a good characterization of this disease. <b>Disclosure:</b> Mr. Andrade has nothing to disclose. Miss MARQUES has nothing to disclose. Dr. Mota has nothing to disclose.

Modeling of granular soil-structure interface under monotonic and cyclic loading with the nonlinear approach

A constitutive model that can accurately predict the behavior of the soil-structure interface is beneficial to the analysis and design for foundation engineering performance. This paper develops a new model with the nonlinear approach for simulating the monotonic and cyclic behaviors of the granular soil-structure interface. Based on the hyperbolic function, the concept of normalized shear displacement is proposed, and the basic model for the effects of the normal stress, the nonlinear shear modulus, and the friction angle on shear strength during shearing is deduced. On this basis, the Pyke criterion is modified to describe the cyclic loading responses. The relation between the friction angle and the state parameter of the interface in different interface states is established under the framework of critical state soil mechanics (CSSM) theory to further reflect the interface behaviors corresponding to the changes of void. Taking the phase transformation state as a reference state of volume increment, the incremental equation of stress-dilatancy induced by shearing is developed. In addition, the basic model is extended to be able to describe the interface mechanical behaviors under constant normal load (CNL), constant stiffness (CNS), and constant volume (CV) conditions by considering the characteristics of three typical stress loading paths. Only 10 calibrated parameters are required to reproduce the complex properties in the monotonic and cyclic behaviors. The predictive performance of the proposed interface model is examined by the previous experimental results.

Numerical study on the shear behavior of concrete-rock joints with similar triangular asperities

The shear behavior of concrete-rock joints with similar triangular asperities (STAs) was investigated by numerical direct shear tests under constant normal stiffness (CNS) conditions based on particle flow code (PFC), and numerical models were validated by laboratory observations recently published by the authors. The failure patterns and shear load transfer of asperities on the joints were revealed in terms of shear micro-cracks and shear force chains on the micro-scale. For comparison, joints with conventional profile, regular triangular asperities (RTAs), were also examined. The comparisons showed that a prominent ductile behavior at post-peak most likely occurred for the STAs profile owing to asynchronous asperity failure rather than brittle shear caused by a synchronous failure of the RTAs profile. Results also indicated that there would exist a critical inclination to limit the further mobilization of joint dilation and it may be useful to guide the preliminary design of the rock-socketed pile foundation.

Numerical simulations on shear behaviour of rock joint network under constant normal stiffness conditions.

In this study, the numerical direct shear tests were conducted to investigate the shear mechanical properties of joint networks under constant normal stiffness (CNS) boundary conditions. The influence of random joint number on shear stress (τ), dilation (normal displacement, δv) and normal stress (σn) of rock mass were studied quantitatively with fixed main slip surface. At the same time, the internal stress evolution process and failure process were analyzed. The results reveal that the number of random joints (γ) has little effect on the shear and normal stresses. The normal displacement of the sample generally decreases as the number of random joints increases. In addition, the normal displacement of the specimen is absorbed by the random joints when the number of random joints in the specimen increases to a certain level: when γ is greater than 6 and the shear displacement (μ) reaching 10 mm, the specimen exhibits shear contraction. Therefore, the internal random joints mainly control the failure mode and dilatancy performance of the specimen, while the main joint of the rock controls the shear stress of the specimen.

Fluid-structure interaction analysis of nonlinear flapping dynamic behaviors of variable stiffness composite laminated plates in viscous flows

This paper is concerned with the development of a numerical model for the analysis of nonlinear fluid–structure interaction problems of large-deformable curvilinear fiber-reinforced composite laminated plates in viscous flows. A higher-order shear deformation zig-zag theory in conjunction with nonlinear von Kármán strains is employed to accommodate the large deformations of variable stiffness composite laminated plates. An implicit partitioned fluid–structure interaction method based on an arbitrary Lagrangian-Eulerian framework is adopted for dealing with the coupling of the large-deformable plate and the viscous flow. The validity of the proposed model and method is confirmed by comparing the computed results of a two-layered plate with those available solutions in the literature. Effects of material properties and fiber paths of composite plates on the periodic limit cycle oscillation and the wake-flow vortices are examined. It is found that symmetrical lay-ups of the constant stiffness composite laminated plates can produce asymmetrical flapping shapes, resulting in asymmetrical hairpin vortex structures in the wake flow. For a variable stiffness composite laminated plate, the flexibility of the leading or trailing sections can be adjusted by the curved fiber path, which in turn leads to changes in the plate flapping shapes.

Configurational dynamics of flexible filaments in bacterial active baths

Biopolymers with microscale length and nanoscale cross-sections subjected to active forces is a common non-equilibrium phenomenon in living creatures. It is therefore crucial to intuitively present and investigate the detailed dynamics of such flexible filaments in an active bath full of living matter. Hence, by introducing fluorescent actin filament into an active suspension of motile bacteria at different number densities in a quasi-two-dimensional chamber, we directly visualize the detailed interaction processes and find that bacteria deform a fluctuating filament by relative motion perpendicular to its principal axis of the filament and straighten a filament by parallel motions. We analyzed the evolution of bending energy in dilute and dense bacterial baths with gradual compact or coiled shapes. We successfully introduce a dimensionless number , named as active elasto-viscous number, which governs the generic deformation of filaments in the bacterial baths by comparing the viscous force generated in the bacterial active baths to the elastic restoring force of filaments. The persistence length measuring the tangential correlation of the flexible filament is found to be proportional to . Finally, an effective temperature of the bacterial bath is given through the relation between constant stiffness and loading forces instead of the popularly used Einstein relation. Our findings provide detailed information and specific scaling of flexible filaments interplay with active forces and in response to living and crowded environments.

The Effect of Nonlinear Feature of Tire on the Control of Active Rear Steering at High Speed

The two-degree-of-freedom model of the vehicle is more accurate with considering of the nonlinear feature of tire. Based on the model, the active rear steering control strategy for achieving zero sideslip angle is yielded. Simulation results show that both of the two cases, constant and variable cornering stiffness, are useful in the control of sideslip motion, and using the variable stiffness due to the effect of load transfer in control strategy is more effective.

Experimental realization of rate-independent linear damping using a semiactive damper to enhance the seismic performance of low-frequency structures

Rate-independent linear damping (RILD) has been shown to simultaneously reduce the displacement and acceleration responses of low-frequency structures. However, previous studies on RILD have focused on its theoretical aspects and have almost excluded experimental approaches. Among the existing causal RILD models, the tuned Maxwell–Wiechert model can provide constant loss stiffness in a specified frequency range, thus offering a viable option as a control device intended to be incorporated into a base-isolated structure. A challenge remains regarding its undesired storage stiffness that compromises the flexibility of the base-isolated structure. This study eliminates the undesired storage stiffness via a negative stiffness-tuned Maxwell-Wiechert (NSTMW) model and experimentally investigates its feasibility to enhance the seismic performance of low-frequency structures. However, it is impossible to realize the NSTMW model using mechanical devices owing to the need for the negative damping coefficient of the Maxwell elements in some special cases. Therefore, the NSTMW model's frequency-domain expressions were transformed into time-domain differential equations using a bilinear transformation, and a small-scale magnetorheological damper was designed and fabricated to realize the NSTMW model. A series of real-time hybrid simulations were then conducted to examine the effectiveness of the NSTMW model for the protection of low-frequency structures. The experimental results confirmed that the NSTMW model could effectively control the dynamic response of low-frequency structures, compared with ideal RILD and linear viscous damping.

Propulsive performance of oscillating plates with time-periodic flexibility

We use small-amplitude inviscid theory to study the swimming performance of a flexible flapping plate with time-varying flexibility. The stiffness of the plate oscillates at twice the frequency of the kinematics in order to maintain a symmetric motion. Plates with constant and time-periodic stiffness are compared over a range of mean plate stiffnesses, oscillating stiffness amplitudes and oscillating stiffness phases for isolated heaving, isolated pitching and combined leading-edge kinematics. We find that there is a profound impact of oscillating stiffness on the thrust, with a lesser impact on propulsive efficiency. Thrust improvements of up to 35 % relative to a constant-stiffness plate are observed. For large enough frequencies and amplitudes of the stiffness oscillation, instabilities emerge. The unstable regions may confer enhanced propulsive performance; this hypothesis must be verified via experiments or nonlinear simulations.

Analytical and numerical study of a vibrating magnetic inverted pendulum

The current study investigates the stability structure of the base periodic motion of an inverted pendulum (IP). A uniform magnetic field affects the motion in the direction of the plane configuration. Furthermore, a non-conservative force as one that dampens air is considered. Its underlying equation of motion is derived from traditional analytical mechanics. The mathematical analysis is made simpler by substituting the Taylor theory in order to expand the restoring forces. The modified Homotopy perturbation method (HPM) is employed to achieve a roughly adequate regular result. To support the prior result, a numerical method based on the fourth-order Runge-Kutta method (RK4) is employed. The graphs for both the analytic and numerical solutions are highly consistent with one another, which indicates that the perturbation strategy is accurate. The solution time history curve exhibits a decaying performance and indicates that it is steady and without chaos. The resonance and non-resonance cases are found through the stability study by using the time scale method. In all perturbation approaches, the methodology of multiple time scales is actually regarded as a further standard approach. The time history is used to create a collection of graphs. Some graphical representations are used to illustrate how the typical physical values affect the behavior of the discovered solution. It has been discovered that the statically unstable IP can have its instability reduced by raising the spring torsional constant stiffness as well as the damped coefficient. Moreover, the magnetic field has a significant role in the stability configuration, which explains that at higher values of this field, the decaying waves take much more time than the smaller values of this field. Accordingly, it can be employed in various engineering devices that need a certain period of time to be more stable.

Supersonic flutter of variable stiffness circular cylindrical shells

Curvilinear reinforcement fibres allow to tailor the elastic properties of composite laminates in space, leading to Variable Stiffness Composite Laminates (VSCL). In this work, the aeroelastic stability of VSCL cylindrical shells under a supersonic airflow along the axial direction is analysed in order to verify if curvilinear fibres can be used to increase the flutter speed. A linear mathematical model for thin circular cylindrical VSCL shells is developed for that purpose. The aerodynamic pressure caused by the supersonic airflow is modelled recurring to the linearised piston theory. The p-version of the finite element method is used to transform the differential equations of motion from partial to ordinary. Using the latter, eigenvalue problems are defined to obtain the natural frequencies and mode shapes of vibration, and the critical flutter conditions. Various types of curvilinear fibre paths are explored, to find paths that result in higher values of critical flutter free stream pressure. The values are compared with the ones of conventional, constant stiffness, composite laminated shells.