Interlayer Friction Coefficient Research Articles

Parameter sensitivity analysis of the axial stability for a marine flexible pipe

Marine unbonded flexible pipes serve as the most essential equipment in offshore oil and gas exploration and exploitation. Axial compressive loads during installation or in service in the complex marine environment usually lead to buckling failure. A flexible pipe is a composite structure with multiple functional layers, of which the tensile armor layer plays a key role with regard to the response of the pipe subjected to axial loads. In this paper, a simplified three-dimensional finite element model is developed, focusing on the tensile layer and replacing the carcass layer, pressure sheath layer, and pressure armor layer by a cylindrical rigid body to reduce computational expense. By using this model, the buckling failure modes of the tensile armor layer (in particular the birdcaging phenomenon) are analyzed. Several key parameters that affect the stability of the flexible pipe under axial compression and torsion are emphasized, and their effects on its axial and torsional stiffness are compared and discussed. The results show that both the lay angle of the steel wires and the interlayer friction coefficient have a significant influence on the axial and torsional stiffness of the pipe, whereas the damaged length of the outer sheath has virtually no effect.

Study on the influence of bending curvature on the bending characteristics of unbonded flexible pipes

Unbonded flexible pipes are widely used in offshore oil and gas development as key equipment for offshore oil and gas transportation. Their biggest advantages are their flexibility and easy bending, making them more adaptable to the complex marine environment and the movement of offshore floating oil and gas platforms. These platforms also bring challenges to the structural design and analysis of flexible pipes. The mechanical response of each spiral strip layer used to construct the flexible pipe is the most complex response in the bending process. To analyze the bending performance of flexible pipes, this study focuses on the derivation of the explicit relationship between the bending curvature and the minimum critical curvature of a flexible pipe in the partial slip process of a spiral strip layer. An interlayer friction coefficient is introduced in the theoretical derivation, and complete expressions of the bending moment and bending stiffness for the non-slip, partial slip, and complete slip stages of the spiral strip layer are established. The expressions explain the hysteresis phenomenon of the bending characteristics of flexible pipes under a bending load. The results show that the hysteresis present when analyzing the bending moment-curvature relationship and the bending stiffness-curvature relationship of flexible pipes is mainly caused by the nonlinearity of the bending moment-curvature relationship of the tensile armor layer, while the bending moment-curvature relationship of all other layers is linear and the bending stiffness remains constant. The contribution of the carcass layer, pressure armor layer and anti-wear layer to the bending moment and bending stiffness is very small. The minimum critical curvature is the key variable affecting the slippage of the spiral strips of the tensile armor layer. The minimum critical curvature has a linear relationship with the interlayer friction coefficient and a nonlinear relationship with the helix angle.

Atomic-scale interlayer friction of gibbsite is lower than brucite due to interactions of hydroxyls

AbstractTo investigate the role of atomic-scale structure on the frictional properties of gibbsite, a dioctahedral-type aluminum hydroxide, we calculated the atomic-scale interlayer shear properties using the first-principles method based on density functional theory. We found that the presence of vacant sites within the octahedral sheet of gibbsite enables hydroxyls to move to more stable positions and reduce the repulsive force, leading to a lower atomic-scale shear stress of gibbsite compared with brucite, a trioctahedral-type magnesium hydroxide. We also estimated the macroscopic single-crystal friction coefficient of gibbsite with the assumption that only the atomic-scale interlayer friction controls macroscopic friction. The estimated single-crystal friction coefficient for gibbsite is 0.36(6), which is clearly lower than the experimentally obtained friction coefficient of the powdered gouge of gibbsite (0.74). This difference between the interlayer friction coefficient and gouge friction coefficient suggests the presence of additional mechanisms that affect the frictional strength, such as microstructures within a fault gouge.

Full-scale experimental and numerical analyses of a flexible riser under combined tension-bending loading

The flexible riser top connection with floating production units presents additional tensile armour integrity assessment uncertainties associated to the bend stiffener contact interaction that leads to a non-uniform curvature and contact pressure distribution in a region of large dynamic forces and moments. In this paper, a full-scale 6″ flexible riser and bend stiffener bending-tension experimental campaign is carried out in a horizontal rig. The external tensile armour wires are instrumented with strain gauges for axial strain measurement in several riser cross-sections inside and outside the bend stiffener region. The rig assembly allows the riser rotation in order to measure strains at different circumferential angular positions. The bend stiffener polyurethane hyperelastic mechanical response is obtained by uniaxial tensile tests performed at room temperature. A nonlinear finite element model (FEM), including the riser/bend stiffener contact interaction and interlayer friction mechanisms, is employed to numerically investigate the mechanical behaviour of the flexible riser subjected to the experimental loading conditions. The FEM axial strains on the tensile armour wires are compared with the measured results for pure tension and tension-bending loads. Under axisymmetric loading a relevant experimental axial strain dispersion is observed when compared to the average values. Under tension-bending loading, the interlayer friction coefficient influences on the contact pressure and curvature distribution are initially assessed with the numerical model. The strain gauges located in the cross-sections with highest values of contact pressure and curvature are selected for a detailed numerical-experimental strains comparison. In addition, a parametric study is conducted to calibrate the friction coefficient that yields the minimum mean squared error for all measured data. Generally, good correlations are found between the experimental and numerical results.

Computational Study of Low Interlayer Friction in Tin+1Cn (n = 1, 2, and 3) MXene.

The friction of adjacent Tin+1Cn (n = 1, 2, and 3) MXene layers is investigated using density functional theory (DFT) calculations and classical molecular dynamics simulations with ReaxFF potentials. The calculations reveal the sliding pathways in all three MXene systems with low energy barriers. The friction coefficients for interlayer sliding are evaluated using static calculations. Both DFT and ReaxFF methods predict friction coefficients between 0.24 and 0.27 for normal loads less than 1.2 GPa. The effect of titanium (Ti) vacancies in sublayers and terminal oxygen (O) vacancies at surfaces on the interlayer friction is further investigated using the ReaxFF potential. These defects are found to increase the friction coefficients by increasing surface roughness and creating additional attractive forces between adjacent layers. However, these defective MXenes still maintain friction coefficients below 0.31. We also consider functionalized Ti3C2 MXene terminated with -OH and -OCH3 and find that compared to the -O-terminated surface both groups further reduce the interlayer friction coefficient to 0.10-0.14.

Assessing pavement interfacial bonding condition

When analyzing the pavement structure, the interface condition is always supposed to be completely continuous, but it doesn’t correspond to the reality. To deal with this situation and provide suggestions on pavement construction, the asphalt pavement will be studied further. Taking a practical project as an example, the mechanical response of the structure layer of asphalt pavement has been analyzed by finite element software ABAQUS with different interlayer friction coefficients. It shows that the interlayer friction coefficient has great influence on the surface deflection, the tensile stress at the bottom layer and the stress differences among radial stresses of the interface bonding position. Moreover, the deflection basin index has a good relationship with the interlayer friction coefficient. The relation between deflection basin index and interlayer friction coefficients is established, and a method of assessing the pavement interfacial bonding condition is suggested.

Study on the Stability Influence Factors of Bedding Slope under the Action of Seismic Load

Taking Magunyan landslide located in Beichuan County as a typical example, the stability influence factors of bedding slopes under the action of seismic load were analyze by using the ANSYS software in this study. The influence factors include excavation, interlayer cohesion and friction coefficient, earthquake magnitude and vibration direction. The related analysis showed that significant influences of excavation existed on the deformation and failure of slopes. And the bedding slopes stability increasing with the interlayer cohesive, the interlayer friction coefficient and earthquake magnitude increases. The analysis also showed that horizontal vibration is the most detrimental on the stability of slopes.

The Quantum Casimir Effect May Be a Universal Force Organizing the Bilayer Structure of the Cell Membrane

A mathematic–physical model of the interaction between cell membrane bilayer leaflets is proposed based on the Casimir effect in dielectrics. This model explains why the layers of a lipid membrane gently slide one past another rather than penetrate each other. The presented model reveals the dependence of variations in the free energy of the system on the membrane thickness. This function is characterized by the two close minima corresponding to the different levels of interdigitation of the lipids from neighbor layers. The energy barrier of the compressing transition between the predicted minima is estimated to be 5.7 kT/lipid, and the return energy is estimated to be 3.1 kT/lipid. The proposed model enables estimation of the value of the membrane elastic thickness modulus of compressibility, which is 1.7 × 109 N/m2, and the value of the interlayer friction coefficient, which is 1.9 × 108 Ns/m3.

Interaction at the Membrane Midplane Mediates Interleaflet Coupling

Transmembrane signaling implies that peripheral protein binding to one leaflet be detected by the opposite leaflet. Even without involvement of raft lipids we showed (Horner et. al. Biophys. J. 2009) that, peripheral binding of charged molecules (poly-lysine, PLL) to planar lipid bilayers is detected at the other side. Addition of PLL to both sides of the membrane sandwiched lipids between two PLL molecules, indicating a formed nanodomain. As the 2dim surface tension between the monolayers is thought to be of comparable size as the line tension in one of the monolayers, interactions at the membrane midplane don't explain the existence of small nano domains. We tested this assumption by measuring the individual lipid mobility's in both leaflets of a free standing planar lipid bilayers and developed a model. The adsorption of a polypeptide decreased lipid mobility of the adjacent (lower) monolayer. This was revealed by fluorescence correlation spectroscopy (FCS). Depending on the size of the polypeptide, lipid diffusion was either slower or faster than that of the polypeptide. Although only one lipid type was used in both monolayers, lipid mobility's were not equal. The polypeptide decelerated lipid movement in the distant monolayer to a much smaller degree than it did affect lipid mobility in the adjacent monolayer. Based on these observations we propose a model which suggests that the coupling between monolayers is due to friction at the membrane midplane. The interlayer friction coefficient seems to be much larger than the in-layer friction coefficient which explains the existence of small nano-domains.

Treatment for Expansive Soil Channel Slope with Soilbags

AbstractExpansive soil is regarded as one of the trouble soils and is difficult to deal with in engineering because of its strong swelling–shrinkage behavior and well-developed fissures and overconsolidation. In this paper, a treatment method for an expansive soil channel slope with soilbags is proposed. The mechanism of the proposed treatment method is introduced. A number of the swell potential, swelling pressure, seepage, and interlayer friction tests were conducted on soilbags filled with expansive soils, which were taken from a construction site of the South-to-North Water Transfer Project in China. The test results indicate that soilbags can enhance the strength and restrict the swelling deformation of the expansive soil, and that the assembly of soilbags has a high permeability and interlayer friction coefficient. The stability of an expansive channel slope reinforced by soilbags is analyzed based on the limit-equilibrium theory.

Impact of Functional Layer for Thermal Stress of Concrete Pavement Structure

Flexible functional layer of asphalt sands setting between concrete pavement slab and base can change the contact state of different layers and reduce the friction coefficient between slab and base and the thermal stress of pavement slab can be decreased. This functional layer can also protect the base from void disease produced by infiltration and erosion of rain water and snow water. This paper mainly focuses on the impact of parameters variation of contact state of pavement structural layer, slab length and flexible asphalt functional layer thickness and modulus for concrete pavement thermal stress to provide some basis for the selection of flexible functional layer of concrete pavement structure. Calculation results show that interlayer friction coefficient has relatively obvious effect on thermal stress of pavement structure; setting a functional layer can reduce the slab thermal stress evidently. Slab thermal stress and base thermal stress increase non-linear with the growth of slab length. The thickness variation of the functional layer has a slight impact on the concrete slab thermal stress. The higher the modulus of the functional layer is, the greater the slab thermal stress is.

Exact finite element model for shear-deformable two-layer beams with discrete shear connection

This paper presents an exact finite element model for the linear static analysis of shear-deformable two-layer beams with interlayer slip. The layers are connected in a discontinuous way and therefore the shear connection is modeled using concentrated spring elements at each connector location. The Timoshenko beam theory is adopted for each layer in order to take into account the transverse shear deformations. It is assumed that no uplift can occur. Both layers have, thus, the same transversal deflection but different rotations and curvatures. The effect of friction at the interface has been accounted for by assuming that the friction force is proportional to the normal force at the interface. Based on the above key assumptions, the governing equations of the problem are established and an original closed-form solution is derived. From the analytical expressions for the displacement and force fields, the exact stiffness matrix for a generic two-layer beam element is deduced, which can be incorporated in any displacement-based F.E. code for the linear static analysis of two-layer beams with interlayer slip and arbitrary loading and support conditions. Finally, a parametric study, dealing with a two-span layered composite beam subjected to the uniformly distributed load, is carried out to study the effects of varying material and geometric parameters, such as connector spacing, interlayer friction coefficient, the connector stiffness, flexural-to-shear moduli ratios and span-to-depth ratios. The results indicated that the deflection is a quasi-linear function of the flexural-to-shear ratio ( E/ G) and the shear deformation begins to affect significantly the deflection when the length-to-depth ratio ( L/ H) is below 8. Further, the effect of the interlayer friction on the deflection becomes significant if the connector is rigid. Furthermore, the shear deformation effect on the cross-section rotation is more pronounced in the layer with low flexural stiffness.

COMPUTATIONAL STUDY OF INTERFACE EFFECT ON IMPACT LOAD SPREADING IN SiC MULTI-LAYERED TARGETS

The effect of finite strength interface with friction on lateral load spreading and phenomena of interface crack initiation and propagation in bonded multi-layered targets under high velocity impact are presented through axisymmetric finite element simulations. The finite element code DYNA2D, developed at the Lawrence Livermore National Laboratory, was augmented with a phenomenological damage model for the silicon carbide ( SiC ) ceramic and contact/cohesive interface model. Simulations were carried out for four target configurations under varying interface strength (Tm), critical strain energy release rate (Gc), and inter-layer friction coefficient (μL). It is shown that the high wave speed SiC remains a potential material for enhanced load spreading if the confinement is ensured to reduce/delay its damage. The load spreading also increases with the increase in μL that comes into play after the interface failure. However, the μL of unity or more needed to approach the upper bound of load spreading found in the perfectly bonded target layers is not practical. It is shown that the resilience of the multi-layered targets depends on Tm as well as the Gc. But, the lateral load spreading depends dominantly on Tm and reaches the upper bound with its increase. It is further shown that the interface cracks initiate and propagate in shear, mode II. The crack speed is invariably the maximum at initiation and is of the order of the longitudinal wave speed of materials on either side. The maximum initiation speed of 11.29Cs and 7.79Cs are predicted at the SiC -aluminum and steel- SiC interfaces for the frictionless case, where Cs is the shear wave speed of the more compliant aluminum or steel. The crack speed reduces monotonously after initiation, but it remains in the intersonic region for more than 150 ns. Whether the propagating crack tip attains the steady state speed depends on the available driving energy. The steady state crack speed of 0.86Cs to 1.21Cs is predicted only at the steel- SiC interface at 2 mm depth from the impact surface, lasts for more than 3 μs, is shown to be independent of the interface strength upto 300 MPa, and is also independent of the friction coefficient.

Surface Viscosity, Diffusion, and Intermonolayer Friction: Simulating Sheared Amphiphilic Bilayers

The flow properties of an amphiphilic bilayer are studied in molecular dynamics simulations, by exposing a coarse grained model bilayer to two shear flows directed along the bilayer surface. The first field, with a vorticity perpendicular to the bilayer, induces a regular shear deformation, allowing a direct calculation of the surface viscosity. In experiments this property is measured indirectly, by relating it to the diffusion coefficient of a tracer particle through the Saffman-Einstein expression. The current calculations provide an independent test of this relation. The second flow field, with a vorticity parallel to the bilayer, causes the two constituent monolayers to slide past one another, yielding the interlayer friction coefficient.

Atomistic Simulations of Double-Walled Carbon Nanotubes (DWCNTs) as Rotational Bearings

Atomistic simulations of double-walled carbon nanotubes (DWCNTs) as rotational bearings were performed. Molecular mechanics (MM) calculations show that the interlayer energy surface of the bearings is nearly flat. Thermal effects on the bearings were studied with molecular dynamics (MD) simulations at finite temperature. These simulations show that the interlayer corrugation against rotation, and hence the interlayer friction coefficient, is extremely small, suggesting the possible application of DWCNTs as wearless bearings. Extreme operational conditions of the bearings for which the bearings disintegrate are also reported.