Mechanical Contact Conditions Research Articles

The Effect of the Chemical Structures of Synthetic Hydrocarbon Oils on Their Tribochemical Decomposition

Under boundary lubrication conditions, the formation of a highly active nascent surface can catalyze the decomposition of lubricant oils. Extreme pressure additives can reduce this decomposition but have undesirable environmental impacts, and so it is necessary to develop eco-friendly lubricants. The following study investigated the effects of the chemical structures of hydrocarbon oils on their tribochemical decomposition by comparing an alkyldiphenylether (ADE) to a multialkylated cyclopentane. The analysis of gaseous decomposition products via mass spectrometry during the application of friction showed that the multialkylated cyclopentane decomposed after an induction period of 1.95 km while the ADE did not degrade, even at a distance of 57.5 km, under the same mechanical contact conditions, and exhibited superior anti-wear properties. However, a high concentration of the ADE was necessary to extend the induction period for lubricant decomposition when it was used as an additive. Although the ADE showed high stability in response to friction, it was prone to decomposition once a nascent steel surface was formed. These decomposition characteristics of the ADE are closely related to its chemical structure, because the ether group in the ADE molecule preferentially adsorbs on metal oxide surfaces, forming a dense and robust film. In contrast, the weak bonding between adjacent alkyl groups reduces the shear strength at the interface. Both the benzene and alkyl groups of ADE molecules were found to detach, followed by the further rupture of C–C, C–H, and C–O bonds, catalyzed by active sites on the nascent surface such as surface defects.

Constraining the shear strain in viscoelastic materials and utlization of the “incompressible” properties for damping treatment in hybrid joint interface module to improve their effect for vibration control in machining

A hybrid joint interface module (HJIM) was developed using viscoelastic materials’ (VEM) “incompressible” property. The HJIM composes VEM layers compressed by screws. Its static stiffness and damping had been characterized by inverse receptance method. The analysis result showed that its static stiffness increases by nearly 50 % with increasing compression preload without compromising its loss factor. A comparison study of HJIM with a viscoelastic material joint interface module (VJIM) revealed that the change of the screws mechanical contact conditions affected the HJIM’s stiffness. Compression preload by fastening the screws, however, did not significantly affect the damping property of the HJIM. On the contrary to shear pre-strain, compression preload did not affect the VEM’s properties shown by studying the VJIM case. A workpiece was studied while fixed on the HJIM. Varying compression preload affected the stiffness of HJIM and that resulted in increased shear strain in VEM for certain modes while decreased shear strain in VEM for other modes. The affected shear strain in VEM altered the vibrational strain energy distribution and changed the receptance amplitude of different modes. In addition to apply the VEM where it is significantly strained, the analysis revealed that constraining the shear strain in VEM resulted in reduced receptance amplitude for different modes. The changes of receptance will further affect the vibration conditions in machining.

Detection of damage in multiwire cables based on wavelet entropy evolution

Multiwire cables are widely used in important engineering structures. Since they are exposed to several dynamic and static loads as well as detrimental environmental conditions, their structural health can be compromised. Due to the critical role played by multiwire cables, it is necessary to develop a non-destructive health monitoring method to maintain their structure and proper performance. Ultrasonic inspection using guided waves is a promising non-destructive damage monitoring technique for rods, single and multiwire cables. However, the propagated guided waves are composed of an infinite number of dispersive vibrational modes making their analysis difficult. In this work, an entropy-based method to identify small changes in non-stationary signals is proposed. An experimental system to capture and post-process acoustic signals is implemented. The discrete wavelet transform is computed in order to obtain the reconstructed wavelet coefficients of the signals and to analyze the energy at different scales. The use of the concept of entropy evolution of non-stationary signals to detect damage in multiwire cables is evaluated. The results show that there is a correlation between the entropy value and level of damage of the cable including breaking of single wires and change in the mechanical contact conditions among the wires. It is found that the studied method has low sensitivity to signal noise and can reduce the computational complexity encountered in a typical time–frequency analysis.

Axisymmetric Contact Problem of Thermoelasticity for a Three-Layer Elastic Cylinder with Rigid Nonuniformly Heated Core

We consider an axisymmetric contact problem for a finite three-layer elastic cylinder with rigid nonuniformly heated core under the conditions of external convective heat transfer. The conditions of perfect unilateral mechanical contact and imperfect thermal contact are given on the surfaces of possible contact of the cylinder with the core and between the layers. The variational formulation of the problem is obtained. Finally, we propose an iterative numerical algorithm in each iteration of which it is necessary to solve the uncoupled elasticity and heat-conduction problems and the finite-element method is used for their discretization.

Axisymmetric Stress-Strain State of a Body with Thin Rigid Disk-Shaped Heat-Resistant Inclusion

UDC 539.3 We construct a solution of the problem of thermoelasticity for a body with thin rigid heat-resistant inclusion in the class of functions specifying the stress-strain state with constant displacements normal to the plane of inclusion at infinity. The inclusion is modeled by a boundary layer corresponding, from the mathematical viewpoint, to a sheet of moment dipoles and forces and the jump of radial displacements and stresses normal to the plane of inclusion serves as its mechanical manifestation. The solution of the heat-conduction and thermoelasticity equations with satisfying the requirement of continuous dependence of the solutions on boundary conditions is reduced to integral equations of the first kind and realized by the method of Neumann generalized series. We also determine the jump of normal stresses on the surfaces of the inclusion, which guarantees the realization of perfect mechanical contact between the rigid inclusion and the elastic matrix. The plane problems of thermoelasticity for bodies with rigid striplike inclusions were investigated in numerous works by the methods of the theory of functions of complex variables. A survey of these works can be found in the monograph [11]. However, there are only few works devoted to the study of three-dimensional problems of thermoelasticity for bodies with thin rigid plane inclusions. The problems of thermoelasticity for bodies with inclusions in the case of known temperature displacements and stresses in the domain of location of the inclusions are reduced to the problems of elasticity with the corresponding conditions of contact interaction between thin rigid diskshaped inclusions and the elastic matrix under force loading [3, 9, 12, 13]. These problems, in turn, can be reduced by the methods of potential theory to the solution of integral equations of the first kind. Their solution specifies the singular distribution of the characteristics of the stress-strain state at the edge of the inclusion. The problems of thermoelasticity for a body with heat-generating thin rigid disk-shaped inclusion were solved in [4, 6]. In the presence of a heat-resistant inclusion, the temperature jump and tangential stresses is formed on its surfaces. This is explained by the unequal heating of the surfaces. Under the conditions of perfect mechanical contact between the rigid inclusion and the elastic matrix, a jump of normal stresses (which should be determined) is formed on the surfaces of he inclusions [5]. In [1], we proposed a model of heat-resistant inclusion as a sheet of heat dipoles distributed in its plane with a certain density and determined the temperature displacements and stresses induced by these dipoles. In the present work, we apply this model for the solution of the problem of interaction of a thin rigid diskshaped inclusion under the conditions of perfect mechanical contact with the elastic matrix in the class of functions specifying the component of the vector of heat flux normal to the plane of inclusion and its normal displacements (with constant value at infinity).

TRANSIENT PLANE WAVES IN MULTILAYERED HALF-SPACE

Abstract Considered the dynamic problem of the theory of elasticity for multilayered half-space. Boundary surface of inhomogeneous half-space loaded with normal load, and the boundaries of separation layers are in conditions of ideal mechanical contact. The formulation involves non-classical separation of equations of motion using two functions with a particular mechanical meaning volumetric expansion and function of acceleration of the shift. In terms of these functions obtained two wave equation, written boundary conditions and the conditions of ideal mechanical contact of layers. Using the Laguerre and Fourier integral transformations was obtained the solution of the formulated problem. The results of the calculation of the stress-strain state in the half-space with a coating for a local impact loading are presented.

Interaction of several bodies as applied to solving tribo-fatigue problems

Mechanical systems that work in conditions of both mechanical fatigue and contact with friction are studied by tribo-fatigue. Such systems (wheel/rail, gearings, pipe-viscous fluid flow) are of significant practical importance. They require a special statement of the problem for contact interaction of several bodies with the account of the set of non-contact boundary conditions. Equations for determining the three-dimensional stress-strain state for a multi-component system of solids are constructed and analyzed. Their solution using the boundary element method is considered. The example of calculation of contact pressure change under the influence of non-contact loading in a roller/shaft tribo-fatigue system is given.

Remote determination of mechanical parameters of thin flat inclusions of low contrast

We propose an algorithm for the remote determination of physicomechanical parameters of thin flat spa- tial elastic inclusions of variable thickness. We also establish the orientation of inclusions in an elastic matrix. The difference between the mechanical properties of the inhomogeneity and the matrix is insig- nificant as compared with the small thickness of an inclusion. The defect is sounded by elastic longitu- dinal waves, and the required parameters are determined from the amplitudes of scattering of these waves to the Fraunhofer zone. Earlier studies in inverse problems of the dynamic theory of elasticity (the determination of mechanical and geometric parameters of thin-walled elastic inclusions from the data of wave scattering to a far-field zone) have mainly been carried out in a two-dimensional setting. In these studies, inhomogeneities of high or low stiffness were considered under the conditions of their perfect mechanical contact with the matrix (1, 2). Algorithms for the solution of related inverse problems of the remote determination of geometric parameters of cracks and the stress-strain state in their neighborhood were proposed in (1, 3-5). A necessary component of approaches to the solution of inverse scattering problems is the analysis of the corresponding direct problems. In this connection, we should mention the works (6-11), in which methods for studying problems of scattering of elastic waves by cracks and thin elastic inclusions of different stiffness are presented. For engineering applications, in particular, for the development of new methods of ultrasound diagnostics and nondestructive testing, of special importance are thin inhomogeneities of low contrast for which the differ- ence between the elastic properties of the inclusion and the matrix is insignificant as compared with a small pa- rameter that characterizes the relative thickness of the defect. The appearance of such defects in an elastic mate- rial may be a sign of its prefracture state. In the present work, we propose an algorithm for the identification of the material of a thin-walled elastic low-contrast flat spatial inhomogeneity of variable thickness, which is lo- cated in an elastic matrix under conditions of perfect mechanical contact, according to the amplitudes of longitu- dinal waves scattered to the Fraunhofer zone. Under similar assumptions, the inverse scattering problem was considered in (12) in the case of a longitudinal shear of an elastic system.

Finite-Element Analysis of Current-Induced Thermal Stress in a Conducting Sphere

Understanding the electrothermal-mechanical behavior of electronic interconnects is of practical importance in improving the structural reliability of electronic devices. In this work, we use the finite-element method to analyze the Joule-heating-induced thermomechanical deformation of a metallic sphere that is sandwiched between two rigid plates. The deformation behavior of the sphere is elastic–perfectly plastic with Young’s modulus and yield stress decreasing with temperature. The mechanical stresses created by Joule heating are found to depend on the thermal and mechanical contact conditions between the sphere and the plates. The temperature rise in the sphere for the diathermal condition between the sphere and the plates deviates from the square relation between Joule heat and electric current, due to the temperature dependence of the electrothermal properties of the material. For large electric currents, the simulations reveal the decrease of von Mises stress near the contact interfaces, which suggests that current-induced structural damage will likely occur near the contact interfaces.

Effect of heat-conductive filler of interface gap on thermoelastic contact of solids

The thermoelastic contact of two isotropic solids separated by an interface gap is considered. The gap is formed due to an initial sloping smooth groove on the boundary of one of solids and filled with heat-conductive substance (gas or liquid). The heat is supposed to flow from the material with the higher thermal distortivity into the material with the smaller one. The gap filler influence on heat transfer between two solids is modeled by thermal resistance linearly dependent on the height of the gap. The conditions of perfect thermal contact and nonfrictional mechanical contact are assumed to be satisfied at the interface outside the gap. The contact problem is reduced to a set of two nonlinear singular integrodifferential equations, which is solved numerically. The effect of the imposed heat flow and heat conduction of the gap filler on the length and height of the gap, temperature jump at the interface, distribution of heat flows along and through the interface and distribution of contact stresses is analyzed.

Analytical and numerical methods of solution of three-dimensional problem of elasticity for functionally graded coated half-space

In the present paper, a three-dimensional problem of elasticity for homogeneous half-space with gradient coating is considered. Poisson's ratio of the layer is constant and its Young's modulus is a power function of the distance from the surface of the half-space. The surface of non-homogeneous half-space is under tangential loading applied in circular area. Analytical solution is obtained using Fourier integral transform technique. The analytical solution of the problem in continuous dependence of the Young modulus is compared with the solution of the problem in which the inhomogeneous layer is modeled by the package of homogeneous layers. In sub-layers of a package and in substrate is constructed an analytical solution satisfying the conditions of ideal mechanical contact at interlines. Good agreement between results is obtained.

Experimental study of thermal sliding contact parameters at interface seat of large strains

The study relates to the characterization of thermal sliding contact work piece-tool under great deformations, representative of the secondary shear zone of a High Speed Machining Process (HSMP). The literature shows that the authors consider a perfect thermal contact condition and suppose that the heat flux generated by friction at the sliding interface is divided in the ratio of the thermal effusivities of materials in contact. However the distribution of the normal stress at this interface is large and the thermal contact must become imperfect before the release of the chip. The improvement of HSMP simulation requires the checking of these two assumptions. The proposed experimental approach is founded on temperature field measurement by thermocouples in order to estimate thermal sliding contact parameters at the part/tool interface seat of heat dissipation in same condition as the HSMP: the thermal contact resistances RTC, the generated heat flux φ g and share ratio α of φ g . Experiments were carried out on friction device that reproduces mechanical and thermal contact conditions of a HSMP at the tip tool. The experimental plan allows to study the influence of sliding speed and materials in contact on the thermal parameters. Results show that heat flux is divided into the ratio of the thermal effusivities of materials in contact but the thermal contact is systematically imperfect.

柔軟材料の変形を利用した圧電振動型触覚センサによるすべり検出(機械力学,計測,自動制御)

This paper presents the detection of sliding with integrated piezoelectric vibration tactile sensors. A tactile sensor is important for robotics to find a mechanical pressure and contact condition between grippers and objects. We have proposed the piezoelectric vibration-type tactile sensors to realize a high sensitivity and a wide measurement range. This type of tactile sensor is proposed by embedding the piezoelectric vibration sensors in a PDMS basis for stable responses. The basic experiment of sliding detection is also presented using proposed tactile sensor between the aluminum protrusion and the sensor surface covered by soft-material to detect the sliding and its direction.

Simulation of interface temperature and control of lubrication in the study of friction and wear in cold rolling

A prototype testing device called upsetting rolling test (URT) was developed a few years ago and was recently optimised in our laboratory to simulate mechanical and thermal contact conditions in cold rolling. Moreover a new experimental protocol has been designed to reproduce the industrial quasi-boundary lubrication regime. Numerous experiments have been then carried out to study the influence of contact temperature and forward slip on friction, iron fines residues and microscopic surface aspects. A great influence of temperature on friction and wear has been put forward. An increase of the Coulomb friction coefficient associated with a decrease of the iron fines quantities has been shown when temperature increases which seems to indicate more adhesive wear.

A Study on Shock Absorbing Properties of Airbags

The promise of airbags for crew protection when automobiles collide has been realized. In previous airbag development, the prevailing purpose has been to protect the lives of the automobile's crew. It is necessary to understand in detail, as basic data, the impact of the airbag upon the crew, the crew's physique, and the specific accident situation, in order to minimize injuries due to airbag inflation for various accident situations and crews. There are two kinds of impact contact phenomena between the airbag and the crew. One occurs when the airbag is inflated then contacts the crew. The other occurs when the crew collides with the already inflated airbag. We focused our attention in the latter case in this research, designing an experiment apparatus based on a model of the airbag and the crew. The impact force is measured when the airbag contacts the crew under various conditions. The shock-absorbing characteristics of the airbag depend on the fabric's mechanical properties and contact conditions.

A BEM analysis for thermomechanical closure of interfacial cracks incorporating friction and thermal resistance

In this paper a sub-regional boundary element formulation is proposed for the treatment of general two-dimensional thermoelastic contact problems considering friction and thermal resistance along the contacting surfaces. These problems are solved by an incremental-iterative scheme since the extent and the status of the contact zone are not known in advance. The assumption of imperfect and pressure-dependent thermal contact increases the degree of non-linearity and couples the temperature and stress fields. Thus, both thermal and mechanical boundary integral equations are treated incrementally while iterative procedures are introduced to ensure equilibrium of mechanical and thermal contact conditions at each step of the process. The present work is focused on fracture problems of partially closed cracks situated on the interface of dissimilar isotropic solids under combined tensile loading and uniform transverse heat flow. Thermal stress intensity factors are evaluated through the use of quarter point elements. Numerical results are compared with solutions available in the literature, where possible. The good accuracy of the method is demonstrated.

Friction and Wear Phenomena in Cold Metal Forming: An Integrated Approach

For a given industrial forming process, the bulk behaviour laws in the near-contact surface of the tools and pieces are identified. Finite element models of the process are run to identify the mechanical contact conditions. The mechanical, physical, and chemical contact parameters are then reproduced in an upsetting sliding test. This test involves specimens and contactors taken from industrial workpieces and tools. These tests yield the friction coefficient and the surface quality of the specimens and contactor. A series of accurate correlations are shown between numerical results from finite element method (FEM) real process simulations and industrial data. The final goal of the paper is to apply the methodologies to the monitoring of wear.

Modeling and On-Line Estimation of Electrode Wear in Resistance Spot Welding

Electrode wear is inherent in the resistance spot welding process. It determines the electrical and mechanical contact condition and thus strongly affects the resistance spot weld quality. A practical approach to minimizing the electrode wear effect is to compensate the welding current as the electrodes wear. However, the existing methods for welding current compensation rely on either a predetermined stepper schedule or an expulsion detection algorithm. These methods are not reliable since the welding current is not determined based on the contact condition for each weld made in the welding process. This paper presents an on-line electrode wear estimation approach to determining the contact condition and the welding current needed to make every weld a good weld during the entire life of the electrodes. In the study, an incrementally coupled finite element simulation was first formulated to analyze the contact area behavior in the resistance spot welding process. A lumped parameter model was then developed to characterize the contact area change with the dynamic resistance measurement. A calibration and an estimation algorithm were subsequently devised for on-line applications. The proposed approach has been validated with experimental data. The results have shown that the estimation algorithm is robust under various process conditions including both welding current and electrode force.

Effective Elastic Characteristics of a Piecewise-Homogeneous Isotropic Body with a System of Rectilinear Thin Inclusions under Conditions of Longitudinal Shear

We study the problem of determination of effective elastic characteristics of a piecewise-homogeneous isotropic body containing a twice-periodic system of tunnel thin rectilinear inclusions. The body is subjected to antiplane strain, and the conditions of ideal mechanical contact between the matrix and inclusions are satisfied. We reduce the solution of the problem to a system of integro-differential equations which enable us to find the expressions for the effective elastic characteristics of the composite. We also present the results of numerical analysis of these characteristics for a rectangular grid of periods and for various geometric and mechanical parameters of the problem.

Comparison of fretting wear of uncoated and PVD TiN coated high-speed steel under different testing conditions

The fretting wear behaviour of materials in vibrating contacts is dependent on mechanical contact conditions, environmental conditions, and materials properties. These factors may interact with one another, thus making wear a complex system property. In this paper, the influence of relative humidity and normal load on the fretting wear behaviour of PVD TiN coated and uncoated ASP23 steel sliding against corundum is reported. The wear mechanism of TiN coatings is discussed for different fretting parameters. This study indicates that TiN coatings are more sensitive to relative humidity than ASP23 steel in fretting wear tests. The interaction between normal load and displacement determines the fretting regime.