Different Coefficients Of Friction Research Articles

A passive dynamic walking model with Coulomb friction at the hip joint

SUMMARYThis paper presents a modified passive dynamic walking model with hip friction. We add Coulomb friction to the hip joint of a two-dimensional straight-legged passive dynamic walker. The walking map is divided into two parts – the swing phase and the impact phase. Coulomb friction and impact make the model's dynamic equations nonlinear and non-smooth, and a numerical algorithm is given to deal with this model. We study the effects of hip friction on gait and obtain basins of attraction of different coefficients of friction.

Simplified Anchor System for CFRP Rods

AbstractThe increased use of carbon fiber–reinforced polymer (CFRP) rods in prestressed concrete applications has been challenged by identifying a suitable anchor system. To overcome such a challenge, the design of a simplified anchor system composed of three wedges and a barrel, without a soft sleeve, is presented to duplicate the simplicity of the widely utilized anchor systems in steel strands. A numerical study and experimental verification of the simplified anchor system for a CFRP rod are presented. Three-dimensional (3D) finite-element modeling has been conducted on the anchor system consisting of isotropic steel wedges and barrel, in addition to an orthotropic CFRP rod. The wedges and barrel are modeled as elastoplastic materials of different hardness. The rod/wedge and wedge/barrel interfaces are simulated using surface-based contact models having different coefficients of friction. Two hardness levels [171 and 319 Vickers hardness numbers (VPN)] are considered for the wedges. When the softer wed...

A Method for Determination of Friction Coefficient in Sheet Metal Forming of Magnesium Alloy

A new method for determination of friction coefficient in sheet metal forming of Mg alloy AZ31B is presented in this paper. The method is based on the bulging test of sheet metal in which the specimen is processed with a hole at the center. The diameter of the hole will increase along the stroke of the punch and the dimensional change of the hole has a certain relationship with the friction coefficient at the punch/specimen interface. Thus, the friction coefficient can be determined indirectly according to the dimension of the hole in the bulging process. The bulging process of the sheet is simulated using FE code DEFORM-2D. The analysis model is set up according to the experiment of the bulging and the friction calibration curves are generated from the simulations of the bulging by setting different coefficients of friction. The friction coefficient can be determined by measuring the hole dimension during the bulging process and comparing it with the friction calibration curves.

Manufacturing of inchworm robot using shape memory alloy (SMA) embedded composite structure

To design effective movement of robots, various locomotive mechanisms have been investigated. In this study, an inchworm robot was manufactured using shape memory alloy (SMA) which was embedded in composite materials. A Ni-Ti SMA wire was pre-strained and embedded in the glass fiber reinforced polymer (GFRP) strip laid on an ∩-shape mold. Then SMA embedded composite structure was cured at room temperature for 72 hours. Controlling DC current through the SMA wire, the SMA-composite structure, body, could be actuated by changing the radius of curvature. Two legs were attached to the end of body and the leg has two edges which have different coefficients of friction to provide directional movement. One stroke of inchworm provided 4.0 mm translational movement. Repeating on and off of DC current, the inchworm robot gives continuous movement. This mechanism can be applied to the soft morphing robotics, bio medical devices, airplane inlet, etc. instead of using traditional components for their movement.

Effect of spring stiffness and coefficient of friction on the stresses distribution and die angle in extrusion process

The main objective of this study is to find the best angle for the extrusion die by linking the die with springs having different stiffness and the die having different coefficient of friction. The Model of this study Consider the visco-plastic case for the material and the coefficient of friction between the die and the metal flow where the coefficient of friction value are (0.005, 0.008, 0.015). A numerical solution considered the element (visco106) for the metal, the element (beam3) for the die and the element (combin14) for the spring stiffness. The spring stiffness' vary (100 N/mm, 1000 N/mm and 10000 N/mm). The model considered in this work is the axi-symmtric model where this model considered the non-linear method of three elements to solve this problem and the effect of the friction in the solution which explain it by friction surface to surface by using the element (contac171 and targ169) and Newton–Raphson method using ANSYS code was adopted to solve this model. The thesis focuses on two parts, theoretical part and experimental part .In the theoretical part the extrusion process is investigated by using ANSYS software to demonstrate the stress distributions in the billet with several cases of stiffness and coefficient of friction, this program depends on the finite element method in the analysis. On the other hand the apparatus of the extrusion process is made to study this process and to find the best angle for the extrusion die. The material of the billet that was used in the extrusion process was wax. The selection of Paraffin wax is not randomly, this because it has the behaviors of the metal. The experimental part is done depending on the theoretical part and prepared the samples according to the standard methods with choosing slow and fixed velocity of extrusion; the assembly of extrusion is designed by using Acrylic material it is also used in order that the process can be photo and bearing the high pressure of extrusion process. In this study, casting the Paraffin wax in especial die, and manufacture the extrusion instrument containing upper and lower plate and the metal spring fixing at the beginning of the die and it has the stiffness (100 N/mm, 1000 N/mm, 10000 N/mm). The results show good agreement between the theoretical and experimental results for distributing of the strain and stress in metal at different point in the die under different boundary conditions. The reduction of area considered to be variable in two cases with lubricate (16.6, 22.2 and 27.7) and with out lubricate (22.2, 24.4 and 26.6) and it was found that the force–displacement curve at the case of variable coefficient of friction effect on the die angle, where the best die angle is 8.7° when the stiffness of the spring is K= 10000 N/mm and coefficient of friction is μ =0.005.

Crystal plasticity investigation of friction effect on texture evolution of Al single crystal during ECAP

A crystal plasticity finite element model has been developed to study the effect of friction between the die wall and the billet on texture evolution during equal channel angular pressing of an aluminum single crystal. Four cases with different coefficients of friction μ = 0, 0.05, 0.1, and 0.15 have been simulated. It has been found that the friction of μ = 0.05 and 0.1 can capture the major texture features shown in the experimental results, and μ = 0.05 predicts a slightly better texture than μ = 0.1. The frictional condition significantly affects texture evolution in the region between 1/2 and 3/4 of the billet thickness from the top surface. It can be attributed to the effect of friction on the corner gap and the distribution of stresses in the die corner.

Response of the Double Variable Frequency Pendulum Isolator under Triaxial Ground Excitations

Modeling techniques of double variable frequency pendulum isolator (DVFPI) are presented. Seismic responses of the three-dimensional single-story building isolated by DVFPI with different coefficient of friction and initial time period of top and bottom sliding surfaces are investigated under triaxial ground motions and compared with unilateral and bilateral ground excitations. Furthermore, the influence of vertical flexibility on horizontal response is also investigated. It is observed that the triaxial ground motion has noticeable effect on response of the building relative to unilateral ground motion. The error in peak resultant base shear, occurring by neglecting vertical ground motion component, could be significant.

Friction of soft elastomeric wrinkled surfaces

We evaluate the sliding of a rigid spherical lens over a surface-wrinkled, elastomeric substrate. Sliding is conducted both parallel and perpendicular to the aligned surface wrinkles, and the sliding force is compared to the required sliding forces on nonwrinkled surfaces. We evaluate the effects of wrinkle dimensions and applied normal force on the sliding resistance. A simple Bowden–Tabor friction model can describe the dependence of the sliding force on normal load, with different coefficients of friction associated with the nonwrinkled and wrinkled surfaces both perpendicular and parallel. The aspect ratio of the wrinkles has a secondary effect on the sliding force. We associate the changes in friction to changes in the tangential stiffness and fracture angle caused by the surface wrinkles.

Improvement of formability in T-shape hydroforming of tubes by pulsating pressure

The effect of oscillations in the internal pressure on tube wall thickness and die corner filling in the pulsating hydroforming of T-shape tubes is examined using finite element simulation and experiments. The thinning behaviour of the tube during the pulsating hydroforming is studied for different coefficients of friction to investigate the mechanism of the improvement in formability. It is shown that the thinning behaviour is similar for different friction conditions. It is also illustrated that the pulsating pressure improves the formability, even for frictionless tube hydroforming. Thus, the reason for the improvement in the formability may not be due to a reduction in the friction level. In addition, the effects of the amplitude and frequency of the pulsating pressure on the thinning behaviour of tubes and die corner filling are examined. It is shown that, while a small number of cycles and a large amplitude of the pulsating pressure decrease the filling ratio of the die corner, they are more effective in improving the formability.

Kinetic Gait Analysis of Healthy Dogs on Two Different Surfaces

To determine the effects of 2 different, commonly used surfaces with different coefficients of friction on ground reaction forces in normal dogs. Prospective, observational, single cross-over study. Dogs (n=10) with no gait abnormalities. Dogs were acclimated to the force plate and 5 valid trials for each dog and each limb were recorded. Velocity and acceleration were tightly controlled. Each dog was tested on both surfaces sequentially in different sequences. Data analysis was done on peak vertical force, peak impulse, breaking and propulsion peak forces and impulses. Three-way repeated measures analysis of variance was used to separately evaluate the effect of floor type on force plate measures in fore and hind limbs, while controlling for side (left versus right) and experimental replicate. P-values<.05 were considered significant. Mean force and 95% confidence interval for the 6 variables analyzed for all limbs on each surface were calculated. There were no significant differences in ground reaction forces between the linoleum and the carpet surface for thoracic or pelvic limbs for all gait variables measured. There were no significant differences between each individual gait trial per dog between the right and left thoracic limbs trials nor differences between the right and left pelvic limb trials. Normal dogs had no change in their ground reaction forces on linoleum and carpet surfaces. Kinetic results from multi-center or comparative trials will not be affected by use of either linoleum or carpet surfaces.

Experimental and analytical validation of a modular acetabular prosthesis in total hip arthroplasty

A finite element model has been developed to predict in vivo micro motion between a modular acetabular cup and liner after cement less total hip arthroplasty. The purpose of this study is to experimentally validate the model. Six LVDT sensors were used to monitor the micromotion of the liner when subjected to loading conditions ranging from 250 N to 5000 N. Deformations at points of interest for both the experiment and FEM were compared. Results of the FEM with different coefficient of friction between the liner and the cup were investigated to correlate with the experimental results.

Modelling hot plane strain compression test Part 3 – Effect of asymmetric conditions

Finite element analysis has been used to study possible causes of asymmetrical deformation in plane strain compression tests and their consequences. It is shown that U shaped or Z shaped distortion of the specimens can arise from temperature gradients, lateral offset of the tools and non-uniform lubrication, which causes different coefficients of friction at the top and bottom tools, or a changing coefficient of friction across the width of the tools. When the tools are rigid, the stress–strain curves are the same as for specimens deformed symmetrically under the same mean, but uniform conditions. With changing friction conditions across the width of the tools, the lateral load generated can be sufficiently large to cause lateral displacement of the tools in experimental practice. Computation with the top tool free to rotate leads to severe asymmetry and a small reduction in the level of the stress–strain curve.

Self-excited oscillations of a two-mass oscillator with dry “stick-slip” friction

A system of two masses, moving along a single straight line, is considered. The first is connected by a spring to a fixed point, while the second is connected by a spring to the first and is in contact with a belt with dry friction moving with constant velocity. A piecewise-constant model of dry friction with different coefficients of friction, sliding and at rest, is used. The limit “stick-slip” type cycles are investigated analytically. It is shown numerically that in the case of equal masses there are forward and reverse limit cycles. The period of the oscillations of the forward and reverse cycles increases as the ratio of the stick and slip coefficients of friction increases, and decreases when the velocity of the belt increases. The reverse cycle exists for all values of the parameters of the problem, while the forward cycle exists up to a certain critical value of the ratio of the stick and slip coefficients of friction, and this critical value increases when the velocity of the belt increases.

Biomechanics of a Micro-Slip

A study was conducted to investigate the differences in common biomechanical measures for slips not leading to falls, particularly micro-slips (heel displacement less than 30 mm). Thirty-one participants ranging in age from 18-67 years old performed walking trials at three gait speeds over three floor surfaces with substantially different coefficients of friction. The magnitude of heel displacement was significantly affected by floor surface and gait velocity. Trials were categorized by heel displacement (non-slip, micro-slip, slide) and then biomechanical measures were calculated. Findings of the present study indicated that biomechanical measures differed significantly for all three of the slip categories. Although micro-slips are not generally perceived by the individual, biomechanically they differ from normal walking and should be investigated further to understand the conditions that cause a fall to occur after a slip.

Predictions of neck load due to combined compression and lateral bending

This study is a finite element investigation of neck load during a lateral impact to the head. A previously developed detailed model of the human neck was modified to accomplish this study. The neck model was integrated with a head model to simulate drops on a laterally inclined plane using different coefficients of friction between head and plate. The results appear to confirm the hypothesis that high friction between the head and impact surface increases the risk of injury to the neck for lateral impact. Furthermore, it was predicted that the upper cervical spine was at higher risk for injury than the lower neck, due to a lateral impact. It was also found that in a drop test on a laterally inclined plane the loads borne by the intervertebral discs were lower than those borne by the facets.

Effect of lubrication on the stress distribution in an extruded material

The visioplasticity method is used to find the complete strain, strain rate and stress distribution in the deformation zone, according to the deformation grid lines marked on the surface of the workpiece. From the experimental data (the values of the flow function for the extrusion), the velocity, strain rate and stress fields can be calculated by the finite-difference method from the stream function, equilibrium and plasticity equations. In this article, stress components distribution in forward extruded specimens of copper alloy are analysed using the visioplasticity method. Comparisons are made between stress distribution of the specimens extruded with three different coefficients of friction. The results are shown in the form of diagrams.

Does support surface coefficient of friction influence postural dynamics and isometric force productions of the upper extremities performed by seated subjects?

The purpose of this study is to determine if increasing the potential risk of slipping on support surface materials decreases postural dynamics and task performance. Seated subjects were asked to exert maximum horizontal two-handed isometric pushes as quickly as possible on a dynamometric bar. Reaction forces exerted at seat (Rxs and Rzs) and footrest (Rxf and Rzf) were measured using force sensors. A dynamometric bar measured horizontal push force (Fx). Global adherence ratio ( μ) was calculated. Three support surface materials, characterised by different coefficients of friction, were tested. The same surfaces were placed on the seat and the footrest during a series of pushes. It was shown that the different support surfaces significantly influence Fx, Rxs, Rzs, Rzf and μ maximum values obtained at the end of the push. Even when the instructions given to the subject are to produce a maximum push, the maximum horizontal force applied to the bar differs according to the support surface material. Dynamic postural adjustments are influenced by characteristics of slipperiness of support surfaces. The coefficient of friction contributes an essential element to the program of postural dynamics, which in turn modulates the task performance. During brief isometric pushes, seated subjects have the capacity to control the potential risk of slipping.

Estimate of consumed energy at backward extrusion process by means of modelling approach

In order to predict the consumed energy in backward extrusion process (on Al 99.5F7 specimens), the analytic (10 models), numerical, stochastic and experimental modelling of deformation work on the basis of multi-factorial experimental designs (by means of the rotatable design of experiments) was done. Thus, results of backward extrusion force versus punch motion with five different coefficients of friction and five different wall thicknesses were obtained. The most important factor contributing to the accuracy of modelling is the plastic curve of material, for that reason the experimental investigations (compression testing on specimens Ø20 mm×20mm) were performed and results in form of Hollomon–Ludwik’s power law were obtained. Investigations in this paper were supported with: data processing system, measuring sensors and Lab View software. Experimental research of deformation work was done for both the checking and the verification of obtained results.

Predicting stress distribution in cold-formed material with genetic programming

In this paper we propose a genetic programming approach to predict radial stress distribution in cold-formed material. As an example, cylindrical specimens of copper alloy were forward extruded and analysed by the visioplasticity method. They were extruded with different coefficients of friction. The values of three independent variables (i.e., radial and axial position of measured stress node, and coefficient of friction) were collected after each extrusion. These variables influence the value of the dependent variable, i.e., radial stress. On the basis of training data set, various different prediction models for radial stress distribution were developed during simulated evolution. Accuracy of the best models was proved with the testing data set. The research showed that by proposed approach the precise prediction models can be developed; therefore, it is widely used also in other areas in metal-forming industry, where the experimental data on the process are known.

Constitutive equations for the room temperature deformation of commercial purity aluminum

A series of compression tests were conducted using a servo-hydraulic, computer driven testing machine to deduce the constitutive equations that describe the behavior of commercial purity aluminum deformed at room temperature over a wide range of strain rates. The experimental samples were tested with and without the use of either mineral oil or polytetrafluoroethylene (PTFE) tape. All the tests were performed at constant strain rates up to an equivalent strain close to 1. The load–displacement curves were converted into stress–strain curves using different coefficients of friction. The constitutive equations deduced assuming that a steady state is achieved at large strains are considered to be good enough for using them to predict the strength of the material on a wide range of strains and strain rates. The friction coefficient found to yield the best results was 0.05 for PTFE tape, 0.10 for mineral oil and 0.20 when no lubricant was used.