Uniform Stiffness Research Articles

Field and numerical investigation of full depth reclamation with Portland cement in Georgia

The Full Depth Reclamation (FDR) technique has recently become a popular rehabilitation method to reconstruct old flexible pavement. Pulverized existing roadway materials are blended with stabilizing agents such as foamed asphalt and Portland cement to form a stiff base course for the new pavement. In Georgia, Portland cement is commonly used as a stabilizing agent for FDR stabilization. One of the challenges in FDR stabilization projects is to achieve uniform stiffness throughout the stabilized layer. FDR mix design often specifies a fixed rate of stabilizing agent, which could generate a range of FDR strengths due to the inherent variability in the existing pavement. Stiffness variation in the stabilized base should be minimized for optimum long-term pavement performance. This study investigates the influence of the variability in the FDR base layer stiffness on pavement performance. A series of field and laboratory tests were performed on a pavement reconstruction project in Georgia to assess variability. Tests included Unconfined Compressive Strength (UCS) tests and deflection tests with a Light Weight Deflectometer (LWD) and Falling Weight Deflectometer (FWD). Mechanistic sensitivity analyses were performed based on the field and laboratory test results to investigate the effect of variability in FDR properties on pavement performance. The results indicate both the FDR base thickness and strength have a significant influence on the predicted pavement responses. Findings presented here will be of interest to pavement engineers involved in the design and performance modeling of FDR pavement.

The effect of variable stiffness of tuna-like fish body and fin on swimming performance

The work in this paper focuses on the examination of the effect of variable stiffness distributions on the kinematics and propulsion performance of a tuna-like swimmer. This is performed with the use of a recently developed fully coupled fluid-structure interaction solver. The two different scenarios considered in the present study are the stiffness varied along the fish body and the caudal fin, respectively. Our results show that it is feasible to replicate the similar kinematics and propulsive capability to that of the real fish via purely passive structural deformations. In addition, propulsion performance improvement is mainly dependent on the better orientation of the force near the posterior part of swimmers towards the thrust direction. Specifically, when a variable body stiffness scenario is considered, the bionic body stiffness profile results in better performance in most cases studied herein compared with a uniform stiffness commonly investigated in previous studies. Given the second scenario, where the stiffness is varied only in the spanwise direction of the tail, similar tail kinematics to that of the live scombrid fish only occurs in association with the heterocercal flexural rigidity profile. The resulting asymmetric tail conformation also yields performance improvement at intermediate stiffness in comparison to the cupping and uniform stiffness.

Vibration and stability analysis of a spinning thin-walled composite beam carrying a rigid body

In this paper, we presented the formulation of dynamic stability of spinning thin-walled composite beams carrying rigid bodies. First, we used a circumferentially uniform stiffness lay-up configuration to generate the coupled motion of bending–bending-shear and to solve the eigenvalue problem using the extended Galerkin method. The dynamic stability analysis was carried out for the beam carrying a single rigid body. The divergence and flutter instabilities were found by addressing the effect of the mass ratio and its location along the span of the beam. Along with these, the combined effects of several parameters such as spin speed, axial load, and ply angle were examined to study how they affect the stability of the beam.

Free vibration analysis of functionally graded double-beam system using Haar wavelet discretization method

In this paper, the free vibration characteristics of functionally graded double-beam system (FGDBS) elastically connected by an elastic layer with uniform elastic stiffness under arbitrary boundary conditions are investigated. It is assumption that the material distribution properties of the individual beams of FGDBS depends on different four-parameters. Timoshenko beam theory in which the effects of shear deformation and rotational inertia are considered is utilized to model the free vibration of FGDBS and displacement components can be obtained from the Haar wavelet series and their integral. Hamilton’s principle is applied to construct coupled governing equations and the virtual spring boundary technique is applied to generalize boundary conditions at four ends of FGDBS. The convergency and accuracy of the proposed method can be confirmed through the comparison results with previous literature and finite element method (FEM). A lot of new results have been proposed, such as the frequency characteristics of FGDBS under arbitrary boundary conditions which can be provided as reference data for future research.

Influence of flex-sub on mechanical properties of rotary steerable drilling system

Abstract. In this paper we analysis of the dynamic model of the Rotary Steerable System (RSS) with a single stabilizer and flex-sub. The drill collar and joint are simplified to obtain suitable bottom-hole assembly (BHA) model of mechanical analysis and finite element analysis. Using the BHA uniform stiffness assumption (ignoring the influence of variable section and variable stiffness of drill collar in BHA), the paper takes a method of the longitudinal and transverse bending continuous beam method to analyse the stress of BHA with RSS with the flex-sub under different inclination angles and curvature conditions. In addition, a dynamic analysis model is proposed, and the dynamic stress of drilling tools with RSS is analysed. Through the software of ANSYS, the finite element analysis of the RSS with the flex-sub is carried out to verify the theoretical results. The influence of flex-sub on the steering drilling performance of RSS is studied, and the optimized parameters of flex-sub design are obtained, which provides useful suggestions for the design of RSS.

A new micro/nano-touch-trigger probe using an optoelectronic sensor with a wedge prism.

In this paper, we propose a new touch-trigger probe with high precision and a large permissible measurement range. A wedge prism was used in the sensing unit to achieve 3D detection using only one optoelectronic sensor. The measurement range was expanded from ±8 µm to ±14 µm through the new optical structure. The probe has uniform stiffness and uniform sensitivity. Some experiments were performed to investigate the performance of the probe. It was found that the probe has a resolution of 10 nm and a repeatability of less than 9.1 nm. The applicability of the probe was also verified.

Natural frequency calculation of elastically connected double-beam system with arbitrary boundary condition

In this paper, the free vibration characteristics of an elastically connected double-beam system (ECDBS) connected by an elastic layer with uniform elastic stiffness under arbitrary boundary conditions have been investigated. Timoshenko beam theory, in which the effects of shear deformation and rotational inertia are considered, is utilized to model the free vibration of the ECDBS, and displacement components can be obtained from the Haar wavelet series and their integral. Hamilton’s principle is applied to construct coupled governing equations, and the virtual spring boundary technique is applied to generalize boundary conditions at four ends of the ECDBS. The convergence and accuracy of the proposed method can be confirmed by comparing the results with those of previous literature. A lot of new features have been identified, such as the frequency characteristics of the ECDBS with different cross sections under arbitrary boundary conditions, which can be provided as reference data for future research.

The Impact of Elastic Deformations of the Extracellular Matrix on Cell Migration

The mechanical properties of the extracellular matrix, in particular its stiffness, are known to impact cell migration. In this paper, we develop a mathematical model of a single cell migrating on an elastic matrix, which accounts for the deformation of the matrix induced by forces exerted by the cell, and investigate how the stiffness impacts the direction and speed of migration. We model a cell in 1D as a nucleus connected to a number of adhesion sites through elastic springs. The cell migrates by randomly updating the position of its adhesion sites. We start by investigating the case where the cell springs are constant, and then go on to assuming that they depend on the matrix stiffness, on matrices of both uniform stiffness as well as those with a stiffness gradient. We find that the assumption that cell springs depend on the substrate stiffness is necessary and sufficient for an efficient durotactic response. We compare simulations to recent experimental observations of human cancer cells exhibiting durotaxis, which show good qualitative agreement.

SMART coils for intracranial aneurysm repair \u2013 a single center experience

BackgroundDue to uniform stiffness of standard platinum coils, dense packing of intracranial aneurysms can be difficult to achieve, since stiffer coils can cause microcatheter prolapse or coil migration. SMART coils have a varying softness along the length of the coils to improve deliverability. We report our initial 2 year experience with the SMART coil system, including direct and follow-up results.MethodsWe performed a retrospective study of all patients who underwent coil embolization of an intracranial aneurysm with SMART coils between July 2016 and August 2018 at our institution. We analyzed clinical and angiographic data before and directly after treatment as well as at 6 months follow-up.ResultsA total of 49 patients harboring 49 aneurysms were treated; 23 (47%) were ruptured aneurysms. Most aneurysms (57%) were located in the anterior circulation. Median patient age was 55 (31–88), 63% were female. Mean aneurysm size was: neck 3.4 (±1.5), height 6.3 (±2.9) and width 5.2 (±2.3) mm. SMART coils were solely used in 96% of cases. Initial favorable angiographic results were achieved in 45 (92%) of 49 cases, which were stable at 6 months in 26/29 (90%). Thromboembolic complications occurred in 4 (8%) cases without clinical sequelae; microcatheter prolapse occurred in 1 case. No aneurysm rupture or device malfunction was observed.ConclusionThe treatment of ruptured and unruptured intracranial aneurysms with SMART Coils was safe and efficacious in our cohort.

Optimization of elastic spring supports for cantilever beams

In this study,a new approach of optimization algorithm is developed. The optimum distribution of elastic springs on which a cantilever Timoshenko beam is seated and minimization of the shear force on the support of the beam is investigated.The Fourier transform is applied to the beam vibration equation in the time domain and transfer function, independent from the external influence, is used to define the structural response. For all translational modes of the beam, the optimum locations and amounts of the springs are investigated so that the transfer function amplitude of the support shear force is minimized. The stiffness coefficients of the springs placed on the nodes of the beam divided into finite elements are considered as design variables. There is an active constraint on the sum of the spring coefficients taken as design variables and passive constraints on each of them as the upper and lower bounds. Optimality criteria are derived using the Lagrange Multipliers method. The gradient information required for solving the optimization problem is analytically derived. Verification of the new approach optimization algorithm was carried out by comparing the results presented in this paper with those ones from analysis of the model of the beam without springs, with springs with uniform stiffness and with optimal distribution of springs which support a cantilever beam to minimize the tip deflection of the beam found in the literature. The numerical results show that the presented method is effective in finding the optimum spring stiffness coefficients and location of springs for all translational modes.The proposed method can give designers an idea of how to support the cantilever beams under different harmonic vibrations.

Theoretical modeling and vibration control for pre-twisted composite blade based on LLI controller

Theoretical modeling and vibration control for divergent motion of thin-walled pre-twisted wind turbine blade have been investigated based on “linear quadratic Gaussian (LQG) controller using loop transfer recovery (LTR) at plant input” (LLI). The blade section is a single-celled composite structure with symmetric layup configuration of circumferentially uniform stiffness (CUS), exhibiting displacements of vertical/lateral bending coupling. Flutter suppression for divergent instability is investigated, with blade driven by nonlinear aerodynamic forces. Theoretical modeling of CUS-based structure is implemented based on Hamilton variational principle of elasticity theory. The discretization of aeroelastic equations is solved by Galerkin method, with blade tip responses demonstrated. The LLI controller is characterized by LTR at the plant input. The effects of LLI controller are achieved and illustrated by displacement responses, controller responses and frequency spectrum analysis, respectively.

Nonlinear vibrations of a rotating thin-walled composite piezo-beam with circumferentially uniform stiffness (CUS)

A rotating system comprising a hub and a thin-walled laminate cantilever beam with embedded nonlinear piezoelectric layers is analysed in the paper. The reinforcing fibres set-up in composite material conforms to circumferentially uniform stiffness lamination scheme. This configuration exhibits the mutual bending couplings in two orthogonal planes. Nonlinear analytical model of a piezoelectric material embedded onto the beam walls is postulated by considering the higher-order constitutive relations with respect to electric field variable. Moreover, to properly model electromechanical structural behaviour, the full two-way coupling piezoelectric effect is considered. To this aim, the assumption of a spanwise electric field variation is postulated in the mathematical model of the structure. Based on previous authors’ research, the system of mutually coupled nonlinear equations of motion is formulated. In the numerical analysis the forced response of the system under zero and nonzero mean value harmonic torque excitation is considered. In particular, the influence of hub inertia, excitation amplitude and mean rotating speed on system dynamics is investigated. The results are presented in the form of appropriate frequency response plots and bifurcation diagrams.

Effect of nonlinear elastic foundations on dynamic behavior of FG plates using four-unknown plate theory

This present paper concerned with the analytic modelling for vibration of the functionally graded (FG) plates resting on non-variable and variable two parameter elastic foundation, based on two-dimensional elasticity using higher shear deformation theory. Our present theory has four unknown, which mean that have less than other higher order and lower theory, and we denote do not require the factor of correction like the first shear deformation theory. The indeterminate integral are introduced in the fields of displacement, it is allowed to reduce the number from five unknown to only four variables. The elastic foundations are assumed a classical model of Winkler-Pasternak with uniform distribution stiffness of the Winkler coefficient (kw), or it is with variables distribution coefficient (kw). The variable\'s stiffness of elastic foundation is supposed linear, parabolic and trigonometry along the length of functionally plate. The properties of the FG plates vary according to the thickness, following a simple distribution of the power law in terms of volume fractions of the constituents of the material. The equations of motions for natural frequency of the functionally graded plates resting on variables elastic foundation are derived using Hamilton principal. The government equations are resolved, with respect boundary condition for simply supported FG plate, employing Navier series solution. The extensive validation with other works found in the literature and our results are present in this work to demonstrate the efficient and accuracy of this analytic model to predict free vibration of FG plates, with and without the effect of variables elastic foundations.

Numerical simulation of wave interaction with deformable plates

The two-dimension problems of the interaction between waves and one, two and a series of deformable plates are discussed, respectively, in this study. The fluid motion is described by the linear wave theory. The deformable plates are simplified as the one-dimensional beams with uniform stiffness and mass distribution. A boundary element method (BEM) is used to calculate the wave field. The Euler–Bernoulli beam theory is used to describe the motion of the deformable plate, which is calculated on the basis of a finite element method (FEM) in combination with the BEM model. The numerical solutions are derived for the velocity potentials together with the displacements of the deformable plates. After the energy conservation is calculated to verity the numerical models, the effects of structural flexibilities and interval between deformable plates on the reflecting waves are investigated.

An efficient model for vehicle-slab track coupled dynamic analysis considering multiple slab cracks

Abstract This paper presents an efficient model for vehicle-slab track coupled dynamic analysis addressing the challenge problem of multiple slab cracks in high-speed railways. The multi-cracked track slabs are described as free Euler-Bernoulli beams supported on the cement asphalt (CA) mortar. The concentrated cracks are assumed to be in the through-transverse form, which are considered to locally affect the vertical bending stiffness of the beam and treated by applying the uniform flexural stiffness with Dirac’s delta singularities. The introduced damage parameters are expressed by incorporating the rotational springs using the linear elastic fracture mechanics. The vibration equations of slab track subsystem involving multiple slab cracks are finally set up by adopting the modal superposition approach, and are coupled with vehicle subsystem motions through nonlinear wheel-rail contact forces. An explicit integration method is applied to solve the whole coupled system excited by the random track irregularities. The developed model can also be expediently degenerated to the traditional model with intact slabs by setting the damage parameters to zero. Time-frequency analysis is conducted through fully investigating the effect of the crack depth, the crack numbers with different distribution properties and the CA mortar stiffness on the system dynamic responses. Some practical conclusions are drawn and may provide potent information for the maintenance in railway engineering and vibration based damage identification.

Engineering microenvironments towards harnessing pro-angiogenic potential of mesenchymal stem cells

Mesenchymal stem cell (MSC)-based therapy for promoting vascular regeneration is a promising strategy for treating ischemic diseases. However, low engraftment and retention rate of MSCs at the target site highlights the importance of paracrine signaling of MSCs in the reparative process. Thus, harnessing MSC-secretome is essential for rational design of MSC-based therapies. The role of microenvironment in regulating the paracrine signaling of MSCs is not well known. In this study, human bone marrow-derived MSCs were seeded on matrices with varying stiffness or cell adhesive sites, and conditioned media was collected. The concentrations of angiogenic molecules in the media was measured via ELISA. In addition, the bioactivity of the released molecules was investigated via assessing the proliferation and capillary morphogenesis of human umbilical vein endothelial cells (HUVECs) incubated with conditioned media. Our study revealed that secretion of vascular endothelial growth factor (VEGF) is dependent on substrate stiffness. Maximal secretion was observed when MSCs were seeded on hydrogel matrices of 5.0 kPa stiffness. Proliferation and tubulogenesis of HUVECs supported ELISA data. On the other hand, variation of cell adhesive sites while maintaining a uniform optimal stiffness, did not influence the pro-angiogenic activity of MSCs.

Fluid-structure interaction modeling on a 3D ray-strengthened caudal fin

In this paper, we present a numerical model capable of solving the fluid-structure interaction problems involved in the dynamics of skeleton-reinforced fish fins. In this model, the fluid dynamics is simulated by solving the Navier–Stokes equations using a finite-volume method based on an overset, multi-block structured grid system. The bony rays embedded in the fin are modeled as nonlinear Euler–Bernoulli beams. To demonstrate the capability of this model, we numerically investigate the effect of various ray stiffness distributions on the deformation and propulsion performance of a 3D caudal fin. Our numerical results show that with specific ray stiffness distributions, certain caudal fin deformation patterns observed in real fish (e.g. the cupping deformation) can be reproduced through passive structural deformations. Among the four different stiffness distributions (uniform, cupping, W-shape and heterocercal) considered here, we find that the cupping distribution requires the least power expenditure. The uniform distribution, on the other hand, performs the best in terms of thrust generation and efficiency. The uniform stiffness distribution, per se, also leads to ‘cupping’ deformation patterns with relatively smaller phase differences between various rays. The present model paves the way for future work on dynamics of skeleton-reinforced membranes.

Generalized Differential Quadrature Method for Free Vibration Analysis of a Rotating Composite Thin-Walled Shaft

A refined variational asymptotic method (VAM) and Hamilton’s principle were used to establish the free vibration differential equations of a rotating composite thin-walled shaft with circumferential uniform stiffness (CUS) configuration. The generalized differential quadrature method (GDQM) was adopted to discretize and solve the governing equations. The accuracy and efficiency of the GDQM were validated in analyzing the frequency of a rotating composite shaft. Compared to the available results in literature, the computational results by the GDQM are accurate. In addition, effects of boundary conditions, rotating speed, ply angle, ratio of radius over thickness, and ratio of length over radius on the frequency characteristics were also investigated.

Free Vibration of Composite Thin-Walled Beams with Chordwise Asymmetric Closed Cross-Sections

In this paper, some analytical results via extended Galerkin method on free vibration characteristics of an anisotropic composite beam, which is modeled as a nonuniform thin-walled structure with a chordwise asymmetric closed cross-section and corrected the warping functions, are newly presented. For this study, nonclassical parameters such as warping restraint, transverse shear flexibility, and structural couplings induced by two special configurations, such as circumferentially uniform stiffness (CUS) and circumferentially asymmetric stiffness (CAS), are incorporated. And also, design parameters of the beam associated with preset angles, pretwist angles, taper ratios, and section ratios are additionally investigated. The results of this study could play an important role in more efficient designs of composite thin-walled beams.

Designing a hydraulic support system for large monolithic mirror's precise in-situ testing-polishing iteration.

In order to improve the fabrication efficiency and testing accuracy of meter-scale, large monolithic mirrors, hydraulic support units (HSUs) are commonly used. However, the challenges to reduce the disparity of the HSUs' stiffness and keep the stability of the mirrors' altitude are hard to resolve, especially for large-scale mirrors. In this paper, we found the air ratio of the working fluid for HSUs is a key factor for designing the HSUs to resolve the challenges from the analytical solution that we derived for supporting large mirrors. Here we designed, tested and fabricated dozens of HSUs and used a four-meter SiC mirror, the world's largest monolithic SiC reported in public, as a study case. It shows that the stiffness values of grouped HSUs vary within ± 3%, and the mirror's reference surface PV is less than 20 μm in 10 days, producing a mirror tip/tilt angle less than 1.5″. The surface error of the supported mirror is about 20 nm, which is very close to the ideal case where uniform stiffness exists, for randomly distributed stiffness values. The repeatability of the in-situ interferometric test with 0.019 λ RMS of the mirror surface demonstrates the supporting system in a high precision. With such a supporting system, the fabrication process of this mirror was estimated to be sped up by 47% compared to the typical fabricating iteration. With minor modifications and easy extensions, such a novel supporting system could be used widely for many in-situ, high-quality fabricating-testing processes.