Sharif Digital Repository

The wear and friction properties of G-Cu (Sn) composite coatings containing different amounts of graphite particles were examined. The presence of graphite particles improved antifriction properties of composite coatings. The effects of graphite percent in composite coating, tin content of matrix, load, speed and distance on the coefficient of friction and wear resistance of bronze-graphite composite coatings were studied. The results of the present studies revealed that hardness, wear resistance, coefficient of friction are dependent on volume percentage of graphite in composite coating. It was shown that the composite coating containing approximately 5.4 (vol.%) graphite and matrix with... 

In this paper, the two-dimensional tractive rolling contact problem between a rigid cylinder and an orthotropic half plane is considered. A pair of singular integral equations is derived for the contact tractions using the Fourier transform technique. The governing integral equations are decoupled using the Goodman approximation and then solved numerically. Finally, the distribution of surface tractions is achieved in the contact region. This study emphasizes the significant effect of the orthotropic material parameters and the coefficient of friction on the contact tractions as well as the rolling contact characteristics for an orthotropic medium. © 2016, Springer-Verlag Wien  

Plane strain compression (PSC) test was used to study the flow stress of ultrafine grained commercially pure aluminum at large strains. AA 1050 sheets were processed by various Accumulative Roll-Bonding (ARB) cycles up to 10 cycles as the initial specimens for the test. An approach was developed to measure the coefficient of friction and to suppress its effect on the results. It is shown that as a result of having an anisotropy parameter (R-value) of less than one, Von-Mises tensile strengths are significantly higher than PSC strengths. Comparing these strengths, the R-value as an average anisotropy parameter of rolling and transverse directions is estimated for the ARBed sheets, where it is... 

Large-scale molecular dynamics simulations are carried out to investigate the friction behavior of nanocrystalline nickel (Ni) at low temperature. Grain sizes ranging from 4–11 nm are used to explore the behavior of nanocrystalline Ni near the Hall-Petch breakdown. Effects of grain size, grain morphology and scratch depth on coefficient of friction (COF) and deformation mechanism of nanocrystalline Ni are investigated. Reduction of the COF with the refinement of grain size in nanocrystalline nickel is observed, however, for a grain size of 5 nm this trend is observed to be reversed. It is also found that scratch depth can influence the estimation of friction behavior of nanocrystalline... 

In this paper, an algorithm is developed for more realistic investigation of rotor-to-stator rubbing vibration, based on finite element theory with unilateral contact and friction conditions. To model the rotor, cross sections are assumed to be radially rigid. A finite element discretization based on traditional beam theories which sufficiently accounts for axial and transversal flexibility of the rotor is used. A general finite element discretization model considering inertial and viscoelastic characteristics of the stator is used for modeling the stator. Therefore, for contact analysis, only the boundary of the stator is discretized. The contact problem is defined as the contact between... 

The analytical formulation of the fully coupled and the uncoupled rolling contact mechanics problems for all possible stick/slip regimes are derived in Part I (Alinia et al., 2013). In this part, we focus on the numerical algorithm, the iteration procedure and the numerical results. The coupled and the uncoupled solutions corresponding to each of the assumed stick/slip regimes are provided. The uncoupled solution provides an acceptable approximation to the problem for small coefficient of friction values. However, for high values of the coefficient of friction the problem should be solved in fully coupled form. In addition, the effect of several parameters such as the stiffness ratio, the... 

In this paper, sliding friction and wear behaviors of gray cast iron A35 and white cast iron manufactured by quenching from the same cast iron in water were studied and compared by employing pin-on-disk wear tests. Microstructure of the worn surfaces before and after the wear tests were investigated by optical microscope observations. These images show that flakes separated from the surface in gray cast iron due to delamination process, while in white cast iron, the separation of materials from its surface is in the form of powder. In addition, the gray cast iron had higher graphite volume fraction with Type-A graphite flake morphology. The results show that white cast iron has less rate of... 

In this work, improvement in fretting fatigue life of AL7075-T6 has been investigated by titanium surface coating using ion-beam-enhanced deposition (IBED) technique and shot peening. From the experiments, the following conclusions were derived: (i) Shot peening increased the fretting fatigue life up to 350%. (ii) Titanium coating increased the fatigue life up to 100% with respect to virgin specimens for low working stresses, while it reduced the fatigue life at higher working stresses significantly. (iii) Titanium coating+shot peening increased the fatigue life up to 130% with respect to the virgin specimens for low working stresses, while it reduced the fatigue life at higher working... 

AA7075 alloy is exposed to severe shot peening (SSP) with an Almen intensity of 20A. The microstructure is analyzed via FEGSEM and EBSD microscopy. X-ray diffraction (XRD) analysis is used to evaluate the crystallite size in terms of FWHM measurements. The residual stress measurements show a thicker compressed layer (600 µm) formed by SSP. Microhardness improvements are observed as 30% on topmost surface and releases substantially after 600 µm. Coefficient of friction (COF) of treated material remains higher throughout the sliding distance, however, is reduced particularly in the early stage of sliding distance for the wear load of 20 N. The effect of SSP is vanished particularly at lower... 

The seismic performance of steel-plate concrete (SC) shear walls is numerically investigated for various SC wall geometrical shapes. Steel faceplates and infill concrete are two main components of SC walls; a full connection between steel and concrete is required for composite action. A comprehensive set of SC wall specimens with different cross-sectional shapes and geometrical properties were tested in the literature for evaluating their seismic behavior. The numerical responses of 17 tested SC walls, including rectangular, T-shaped, and flanged walls, isolated and coupled walls, with and without boundary elements and endplates, subjected to cyclic lateral loading and a wide range of axial... 

Surgical simulation systems facilitate safe and efficient training processes of surgical trainees by providing a virtual environment in which the surgical procedure can be repeated unlimitedly in a wide variety of situations. The present study attempted to develop a real time simulation system for the grasping procedure of large internal organs during laparoscopic surgery. A mass-spring-damper model was developed to simulate the nonlinear viscoelastic large deformations of the spleen tissue while interacting with a triple-jaw grasper. A novel collision detection algorithm was designed and implemented to determine the contact points between the tissue and the grasper jaws. Force or... 

In this article, the sliding frictional contact problem for a half-plane which is graded in two dimensions is studied. The effect of medium properties gradient and coefficient of friction in contact mechanics of two dimensional (2D) graded materials which is loaded by a flat stamp have been investigated by developing two Finite Element (FE) models, in macro and micro scales. Discretizing the graded half-plane by quadrants for whose material properties are specified at the centroids by Mori-Tanaka method in both directions has been used to model the 2D FGM in macro scale. In micro scale, the ideal solid quadrant particles which are spatially distributed in a homogeneous matrix used to model... 

The present study was an attempt to examine the effects of carbon and cellulose fibers on the tribological characteristics of rubber-based friction materials (RBFMs). A fiber free RBFM as a reference material and a series of fiber included RBFMs at different volume fractions were prepared by two-roll mill. The friction tests were per-formed at different sliding velocities and various drum temperatures. The mechanical properties and surface microstructure of friction specimens were also examined. It was revealed that the carbon fiber infiuences slightly the coefficient of friction (COF) of RBFM but it improves the wear resistance and the fade behavior considerably. It reduces the drum... 

In this study, a gait optimization routine is developed to generate walking patterns which demand the lowest friction forces for implementation. The aim of this research is to fully address the question “which walking pattern demands the lowest coefficient of friction amongst all feasible patterns?”. To this end, first, the kinematic structure of the considered 31 DOF (Degrees of Freedom) humanoid robot is investigated and a closed-form dynamics model for its lower-body is developed. Then, the medium through which the walking pattern generation is conducted is presented. In this medium, after designing trajectories for the feet and the pelvis, the joint space variables are obtained, using... 

A genetic programming technique was employed to develop empirical models for predicting specific wear rate and coefficient of friction on brake friction materials. The models consist of independent variables representing the volume fraction of the ingredients. The average absolute relative error for specific wear rate and coefficient of friction were found to be 1.93% and 1.92%, respectively. The models were also verified by the experimental data used for further control of the formulation. Utilizing Tornado plots, the models were found to be able to properly demonstrate the role of the ingredients on overall tribological performance of the brake friction materials. Moreover, a non-dominated... 

An attempt was made to examine thermal effects as well as fade and wear characteristics of rubber-based friction materials (RBFMs). A series of RBFMs with and without fiber reinforcements were prepared. The fiber reinforcements used were carbon fiber, cellulose fiber and aramid pulp. A semi-empirical model describing the correlation of coefficient of friction (COF) and temperature was presented. The effectiveness of the model was evaluated using the experimental data. The results revealed that the model parameters for a given composite show a significant change above a critical sliding velocity, i.e. 300 rpm. This behavior was speculated to be due to the transition of rubbery state of the... 

In this study, a gait optimization routine is developed to generate walking patterns which demand the lowest friction forces for implementation. The aim of this research is to fully address the question which walking pattern demands the lowest coefficient of friction amongst all feasible patterns?. To this end, first, the kinematic structure of the considered 31 DOF (Degrees of Freedom) humanoid robot is investigated and a closed-form dynamics model for its lower-body is developed. Then, the medium through which the walking pattern generation is conducted is presented. In this medium, after designing trajectories for the feet and the pelvis, the joint space variables are obtained, using the... 

Ultrafine-grained Al-CNT (2 and 4 vol%) composites were successfully fabricated using the flake powder metallurgy and hot pressing route, and the microstructure and tribological properties of the produced composites were studied. The results showed that the coefficient of friction and the wear rate of Al decrease with the addition of the CNT reinforcement. A carbon-rich film formed on the worn surfaces during wear test, which prevented the Al oxidation and yielded the self-lubricating effect for the composites. This improvement in the wear behavior of the composites can be attributed to the simultaneous effects of the ultrafine-grained matrix and strengthening and self-lubricating properties... 

Medical engineering advances in total joint replacements and societies’ rising demand for long-lasting materials have proven it essential to manufacture materials that are more similar to the original tissue in the fields of mechanical, tribological, and biological properties. Ultra-high molecular weight polyethylene (UHMWPE) is a polymer widely used in arthroplasty applications due to its biocompatibility, chemical stability, and reasonable mechanical properties; however, it still fails to entirely meet the standards of the hip joint implant. In this study, different concentrations of nanosized zirconia were added to UHMWPE and HAp matrix with an intended application in arthroplasty.... 

Composite coatings containing self-lubricant particles like graphite, MoS2, PTFE are extensively used to decrease the coefficient of friction of tribo-surfaces. The aim of this study is to investigate the effect of pulse electrodeposition parameters on particle codeposition process and characteristics of bronze-graphite composite coatings electrodeposited from cyanide-stannate bronze electroplating bath. Results showed a slight increase in tin content and a promising increase in hardness of bronze coating by a decrease in duty cycle at constant average current density. Both tin content of the coating and grain refinement seem to participate in hardness enhancement. By increasing the...