Deterministic normal contact of rough surfaces with adhesion using a surface integral method

The fundamental problem of adhesion in the presence of surface roughness and its effect on the prediction of friction has been a hot topic for decades in numerous areas of science and engineering, attracting even more attention in recent years in areas such as geotechnics and tectonics, nanotechnology, high-value manufacturing and biomechanics. In this paper a new model for deterministic calculation of the contact mechanics for rough surfaces in the presence of adhesion is presented. The contact solver is an in-house boundary element method that incorporates fast Fourier transform for numerical efficiency. The adhesive contact model considers full Lennard-Jones potentials and surface integration at the asperity level and is validated against models in the literature. Finally, the effect of surface roughness on the adhesion between surfaces was studied, and it was shown that the root mean square gradient of surface roughness can change the adhesive pressures irrespective of the root mean square surface roughness. We have tested two adhesion parameters based on Johnson's modified criteria and Ciavarella's model. We showed that Civarella's model introduces the most reasonable criteria suggesting that the RMS roughness and large wavelength of surfaces roughness are the important parameters of adhesion between rough surfaces

Modelling tribochemistry in the mixed lubrication regime

Mixed lubrication is a contact condition when the total load is carried by both the fluid lubricant and the solid contacting asperities. The aim of this study is to couple tribochemistry with lubrication. A recent semi-deterministic tribochemical model of tribofilm growth is integrated in a deterministic mixed lubrication model. The model considers the variable hardness of the tribofilm and enables the study of lubrication and tribochemistry and their mutual interaction. Results from the current model are compared against the previously published results. The model can be easily adapted to actual experimental conditions and geometries. The model can be used beyond pure boundary lubrication conditions to monitor tribofilm growth under mixed lubrication conditions

A novel method for estimating the real-time dullness of tri-cone oil drill bits

It is vital to anticipate the durability of drill bits for all drilling operations to prolong the drilling processes and reducing economic losses. Many attempts have been made to handle this issue by assessing the parameter of specific energy of the bits generated during drilling along with well logs and geophysical analyses. Although these techniques could provide acceptable precautions of the dullness of bit cutters, lack of consideration of material properties of the rock and the bit is their main shortcoming which could result in misleading interpretations. The present study deals with the development of a wear model for Tri-cone bits based on the concept of three body abrasive wear taking into account two main factors, rock hardness and the hardness of the materials forming the Tri-cone drill bit. Other parameters that have significant effect are also considered, such as weight on bit, penetration rate, rotation speed and the required time to penetrate an interval depth. In this research, the bit wear quantified by the new developed model is compared with the in-situ dullness of the drill bit for a number of wells being excavated in southern Iraq, where a reasonable agreement was observed. The present work could be extended to other oil wells and applied on other types of drill bits to confirm the validation of the new model

Effect of particle roughness on the bulk deformation using coupled boundary element and discrete element methods

Particles slide and roll on each other when a granular medium is sheared. Consequently, the tribological properties, such as inter-particle friction and adhesion, play a major role in influencing their bulk failure and rheology. Although the influence of roughness on adhesion and friction of contacting surfaces is known, the incorporation of the surface roughness in the numerical modelling of granular materials has received little attention. In this study, the boundary element method (BEM), which is widely used for simulating the mechanics of interacting surfaces, is coupled with discrete element method (DEM) and the bulk deformation of granular materials is analysed. A BEM code, developed in-house, is employed to calculate the normal force–displacement behaviour for rough contact deformations, based on which a contact model is proposed. This is an efficient and relatively fast method of calculating the contact mechanics of rough surfaces. The resulting model is then implemented in the simulations by DEM to determine the effect of micro-scale surface roughness on the bulk compression of granular materials. This study highlights the importance of the effect of surface characteristics on contact behaviour of particles for the case of shallow footing and provides an efficient approach for modelling the flow behaviour of a large number of rough particles

A Review on Modelling of Viscoelastic Contact Problems

Approaches to solving viscoelastic problems have received extensive attention in recent decades as viscoelastic materials have been widely applied in various fields. An overview of relevant modelling approaches is provided in the paper. The review starts with a brief introduction of some basic terminologies and theories that are commonly used to describe the contact behaviour of viscoelastic materials. By building up the complexity of contact problems, including dry contact, lubricated contact, thermoviscoelastic contact and non-linear viscoelastic contact, tentative analytical solutions are first introduced as essential milestones. Afterwards, a series of numerical models for the various types of contact problems with and without surface roughness are presented and discussed. Examples, in which computational tools were employed to assist the analysis of viscoelastic components in different fields, are given as case studies to demonstrate that a comprehensive numerical framework is currently being developed to address complex viscoelastic contact problems that are prevalent in real life

The Scatter Search Based Algorithm for Beam Angle Optimization in Intensity-Modulated Radiation Therapy

This article introduces a new framework for beam angle optimization (BAO) in intensity-modulated radiation therapy (IMRT) using the Scatter Search Based Algorithm. The potential benefits of plans employing the coplanar optimized beam sets are also examined. In the proposed beam angle selection algorithm, the problem is solved in two steps. Initially, the gantry angles are selected using the Scatter Search Based Algorithm, which is a global optimization method. Then, for each beam configuration, the intensity profile is calculated by the conjugate gradient method to score each beam angle set chosen. A simulated phantom case with obvious optimal beam angles was used to benchmark the validity of the presented algorithm. Two clinical cases (TG-119 phantom and prostate cases) were examined to prepare a dose volume histogram (DVH) and determine the dose distribution to evaluate efficiency of the algorithm. A clinical plan with the optimized beam configuration was compared with an equiangular plan to determine the efficiency of the proposed algorithm. The BAO plans yielded significant improvements in the DVHs and dose distributions compared to the equispaced coplanar beams for each case. The proposed algorithm showed its potential to effectively select the beam direction for IMRT inverse planning at different tumor sites. © 2018 Ali Ghanbarzadeh et al

A Fully Coupled Normal and Tangential Contact Model to Investigate the Effect of Surface Roughness on the Partial Slip of Dissimilar Elastic Materials

By adopting a global search method for all shear tractions in the contacting area and using an alternative convergence criterion regarding load balance in the lateral direction, the newly developed algorithm provides stable solutions to partial-slip problems of elastically dissimilar materials. The model is validated via the comparison between the simulation and literature results for a sphere-on-flat problem under fully coupled conditions. It is then employed to investigate the influence of surface roughness parameters including the root mean square (RMS) roughness and RMS slope on partial-slip solutions under coupled conditions. Since the gross sliding condition is modified under coupling effects, the relationship between the ratio of the stick area to the contacting area (stick ratio) and tangential load in the coupled case, unlike that in the uncoupled one, becomes non-linear for all tested rough surfaces. Under low or medium tangential loads, the surface with a higher RMS gradient or a lower RMS roughness experiences more stick regions within the contacting area. This trend then becomes irregular at higher tangential loads

Effects of Task Demands on Postural Control in Children of Different Ages: A Cross-Sectional Study

This is the final version. Available on open access from MDPI via the DOI in this recordData Availability Statement: Data will be made available upon contacting the corresponding author.This study evaluates the effect of dual tasks on balance during manipulation of visual information, the base of support, and cognitive tasks in 7–12-year-old children and adults. An equal number of girls (210) and boys (210) were selected from the schools by the cluster random sampling method (within the 7–12 age groups). Twenty young adults were also selected. Postural sway (PS) was measured in the anterior–posterior (AP) and mediolateral (ML) directions using a force plate in eight standing positions, including standing with closed eyes versus standing with open eyes and standing with feet together versus on a single limb, under the interference of cognitive dual tasks (DT) or a single task (ST). PS was found to decrease with advancing age. PS was also greater in men than women in both AP and ML directions (p < 0.05). PS was greater in the 7–12-year-old participants compared to adults under the manipulation of the base of support (p < 0.05). However, PS increased in all participants regardless of their age while manipulating both the base of support and visual information (p < 0.05). Children (within the 9–12 years age range) and adults could keep their balance while performing basic tasks; however, children had difficulty in maintaining their balance with higher task demand

Experimental and numerical study on wear characteristics of steel surfaces involving the tribochemistry of a fully formulated oil. Part I: Experiments

This paper studied a fully formulated oil (FFO) in the hydraulic piston pump, containing mainly zinc dialkyldithiophosphate (ZDDP), detergents, and dispersants. The results of tribo-tests show that the antiwear characteristics are different when using FFO compared to using a single additive oil such as base oil + ZDDP. A two-phase temperature-dependent wear trend was found when FFO is used. Phase (I) is in the low-temperature range, where wear increases with increasing tribofilm thickness and temperature. Phase (II) is in the high-temperature range where the tribofilm thickness increases but wear decreases as the temperature increases. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) analysis of the tribofilm show that the most temperature-dependent compositions are phosphates and iron sulphides, where the signal intensity of phosphates increases, but that of sulphides decreases when the temperature increases. It indicates that the participation of detergent and ZDDP in the tribofilm formation increases at low and high temperatures, respectively. However, no signal shows the dispersant participating in the tribofilm formation. A hypothesis of mechanisms of reaction priority or enhanced competition among different additives at different temperatures is proposed from the iron loss perspective to explain the two-phase temperature-dependent wear mechanism. This study also provides the experimental basis for the wear modelling involving the tribochemistry of FFO in Part (II) of the research series