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Contact Pressure (contact + pressure)
Selected AbstractsMechanotransduction of bovine articular cartilage superficial zone protein by transforming growth factor , signalingARTHRITIS & RHEUMATISM, Issue 11 2007Corey P. Neu Objective Mechanical signals are key determinants in tissue morphogenesis, maintenance, and restoration strategies in regenerative medicine, although molecular mechanisms of mechanotransduction remain to be elucidated. This study was undertaken to investigate the mechanotransduction process of expression of superficial zone protein (SZP), a critical joint lubricant. Methods Regional expression of SZP was first quantified in cartilage obtained from the femoral condyles of immature bovines, using immunoblotting, and visualized by immunohistochemistry. Contact pressure mapping in whole joints was accomplished using pressure-sensitive film and a load application system for joint testing. Friction measurements on cartilage plugs were acquired under boundary lubrication conditions using a pin-on-disk tribometer modified for reciprocating sliding. Direct mechanical stimulation by shear loading of articular cartilage explants was performed with and without inhibition of transforming growth factor , (TGF,) signaling, and SZP content in media was quantified by enzyme-linked immunosorbent assay. Results An unexpected pattern of SZP localization in knee cartilage was initially identified, with anterior regions exhibiting high levels of SZP expression. Regional SZP patterns were regulated by mechanical signals and correlated with tribological behavior. Direct relationships were demonstrated between high levels of SZP expression, maximum contact pressures, and low friction coefficients. Levels of SZP expression and accumulation were increased by applying shear stress, depending on location within the knee, and were decreased to control levels with the use of a specific inhibitor of TGF, receptor type I kinase and subsequent phospho-Smad2/3 activity. Conclusion These findings indicate a new role for TGF, signaling in the mechanism of cellular mechanotransduction that is especially significant for joint lubrication. [source] Rolling contact fatigue mechanism of a plasma-sprayed and laser-remelted Ni alloy coatingFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 2 2009X. C. ZHANG ABSTRACT The rolling contact fatigue behaviour of the plasma-sprayed and laser-remelted Ni-Cr-B-Si alloy coatings under two different tribological conditions of contact pressure was investigated. Two sets of fatigue-life data of coatings were characterized by Weibull distributions. The failure mode of the coatings was identified on the basis of worn morphologies as observed at the surfaces of the failed coatings. The tribological mechanism leading to the formation of the fatigue spall was discussed on the basis of the subsurface morphologies observed in the failed coating. Experimental results showed that, the mean life and characteristic life of the coating decreased with increasing the contact pressure. The failure of the coatings can be termed as spalling-type failure. A refined ,ring-crack model' was proposed to explain the formation of the fatigue spall. In the refined model, it was postulated that the joining of the ring-type cracks and subsurface branched cracks was directly responsible for the spall formation. [source] Fretting fatigue limit as a short crack problem at the edge of contactFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 5 2004Y. KONDO ABSTRACT This paper proposes a local stress concept to evaluate the fretting fatigue limit for contact edge cracks. A unique S,N curve based on the local stress could be obtained for a contact edge crack irrespective of mechanical factors such as contact pressure, relative slip, contact length, specimen size and loading type. The analytical background for the local stress concept was studied using FEM analysis. It was shown that the local stress uniquely determined the ,K change due to crack growth as well as the stress distribution near the contact edge. The condition that determined the fretting fatigue limit was predicted by combining the ,K change due to crack growth and the ,Kth for a short crack. The formation of a non-propagating crack at the fatigue limit was predicted by the model and it was experimentally confirmed by a long-life fretting fatigue test. [source] Fretting fatigue crack nucleation in Ti,6Al,4VFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 3 2003J. M. WALLACE ABSTRACT Fretting fatigue crack nucleation in Ti,6Al,4V when fretted against itself is investigated to determine the influence of contact pressure, stress amplitude, stress ratio, and contact geometry on the degradation process. For the test parameters considered in this investigation, a partial slip condition generally prevails. The resulting fatigue modifying factors are 0.53 or less. Cycles to crack nucleation, frictional force evolution, crack orientations and their relationship to the microstructure are reported. The crack nucleation process volume is of the same order as the microstructural length scales with several non-dominant cracks penetrating 50 ,m or less. The effective coefficient of friction increases during early part of fretting. Observations suggest that cyclic plastic deformation is extensive in the surface layers and that cyclic ratchetting of plastic strain may play a key role in nucleation of the fretting cracks. A Kitagawa,Takahashi diagram is used to relate the depth of fretting damage to the modifying factor on fatigue life. [source] The effect of contact load reduction on the fatigue life of pearlitic rail steel in lubricated rolling,sliding contactFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 8 2000D. I. Fletcher Twin-disc contact simulation tests were carried out to investigate the influence of contact pressure variation on rail steel fatigue life. Both a colloidal suspension of molybdenum disulphide in an oil carrier fluid (similar to a commercial flange lubrication product) and water were used as lubricants. It was found that the reduction from 1500 to 900 MPa of the maximum Hertzian contact pressure (at which a molybdenum,disulphide-lubricated and previously worn rail sample was tested) extended the fatigue life of the rail steel by over five times. For water lubrication a similar reduction in contact pressure produced only a marginal increase in fatigue life. The results were found to be in qualitative agreement with the predictions of the newly developed Three Mechanism (TM) model of rolling contact fatigue, which is introduced here. This model combines the mechanisms of ratcheting and the fracture mechanics-based mechanisms of both shear stress- and tensile stress-driven, fluid-assisted, crack growth. [source] Fabrication of Micropatterned Stimulus-Responsive Polymer-Brush ,Anemone'ADVANCED MATERIALS, Issue 18 2009Tao Chen A simple strategy to fabricate stimulus-responsive patterned PNIPAAM-brush microstructures (,anemones') is presented. The size of the microstructures can be adjusted by setting the composition of thiol and the contact pressure. We demonstrate that the patterned PNIPAAM-brush microstructures have a triggerable and reversible conformation transition, and can potentially be used as microcontainers to reversibly dock and release microparticles. [source] Relations between load and settlement of circular foundations on or in a dense sand expressed by a function of diameter and depthINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 6 2005Hiroaki Nagaoka Abstract When load acts on a circular foundation on or in a dense sand, average contact pressure on the lower surface of the foundation is q and settlement of the foundation is s. Diameter and depth of the foundation are B and Df. When the sand, B and Df are given, we can know the relation between q and s/B by, e.g. a loading test, i.e. the relation is determined by B and Df for the sand. Using the results of numerical analyses, we express a relation between q and s/B up to s=0.1B by functions of a single variable which is a linear combination of B and Df. Consequently when two foundations have different B's and different Df's but have the same value of the variable, the relations are the same. Then we examine whether the functions can express the results of eleven tests of model foundations of wide range of B and/or Df. In all the tests, the relations are expressed with sufficient accuracy. Copyright © 2005 John Wiley & Sons, Ltd. [source] Computational modelling of the surface fatigue crack growth on gear teeth flanksINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN BIOMEDICAL ENGINEERING, Issue 8 2001S. Glode Abstract The paper describes a 2-dimensional computational model for simulation of the surface initiated fatigue crack growth in the contact area of gear teeth flanks that leads to surface pitting. The discretized model of a gear tooth with the assumed initial crack is subjected to normal contact pressure, which takes into account the EHD-lubrication conditions and tangential loading due to friction between gear teeth flanks. The model accounts also for the influence of a fluid driven into the crack by hydraulic mechanism on crack propagation. The J -integral method in the framework of the finite element analysis is used for simulation of the fatigue crack propagation from the initial to the critical crack length, when the surface material layer breaks away and pit appears on the surface. The model is applied to a real pitting problem of a gear and corresponding computational results in terms of pit sizes correlate well to the development of micropits observed in experimental testing. Copyright © 2001 John Wiley & Sons, Ltd. [source] A modified node-to-segment algorithm passing the contact patch testINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 4 2009Giorgio Zavarise Abstract Several investigations have shown that the classical one-pass node-to-segment (NTS) algorithms for the enforcement of contact constraints fail the contact patch test. This implies that the algorithms may introduce solution errors at the contacting surfaces, and these errors do not necessarily decrease with mesh refinement. The previous research has mainly focused on the Lagrange multiplier method to exactly enforce the contact geometry conditions. The situation is even worse with the penalty method, due to its inherent approximation that yields a solution affected by a non-zero penetration. The aim of this study is to analyze and improve the contact patch test behavior of the one-pass NTS algorithm used in conjunction with the penalty method for 2D frictionless contact. The paper deals with the case of linear elements. For this purpose, several sequential modifications of the basic formulation have been considered, which yield incremental improvements in results of the contact patch test. The final proposed formulation is a modified one-pass NTS algorithm which is able to pass the contact patch test also if used in conjunction with the penalty method. In other words, this algorithm is able to correctly reproduce the transfer of a constant contact pressure with a constant proportional penetration. Copyright © 2009 John Wiley & Sons, Ltd. [source] Numerical derivation of contact mechanics interface laws using a finite element approach for large 3D deformationINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 2 2004Alex Alves Bandeira Abstract In this work a homogenization method is presented to obtain by numerical simulation interface laws for normal contact pressure based on statistical surface models. For this purpose and assuming elastic behaviour of the asperities, the interface law of Kragelsky et al. (Friction and Wear,Calculation Methods, Pergamon, 1982) is chosen for comparison. The non-penetration condition and interface models for contact that take into account the surface micro-structure are investigated in detail. A theoretical basis for the three-dimensional contact problem with finite deformations is shortly presented. The augmented Lagrangian method is then used to solve the contact problem with friction. The algorithms for frictional contact are derived based on a slip rule using backward Euler integration like in plasticity. Special attention was dedicated to the consistent derivation of the contact equations between finite element surfaces. A matrix formulation for a node-to-surface contact element is derived consisting of a master surface segment with four nodes and a contacting slave node. It was also necessary to consider the special cases of node-to-edge contact and node-to-node contact in order to achieve the desired asymptotic quadratic convergence in the Newton method. A numerical example is selected to show the ability of the contact formulation and the algorithm to represent interface law for rough surfaces. Copyright © 2003 John Wiley & Sons, Ltd. [source] Effect of contact configuration on the durability and friction coefficient of pressure-sprayed MoS2 coatings under fretting conditions,LUBRICATION SCIENCE, Issue 5 2009D. B. Luo Abstract Fretting wear is often found at the contact surfaces of a tight assembly where small-amplitude oscillatory movement occurs, which can be the concealed origin of some enormous accidents. Employment of solid lubrication coatings, as one of effective measurements to palliate the fretting damage, has been widely acknowledged. The present work studied the fretting behaviour of a molybdenum disulphide coating on SUS 316 stainless steel substrate by a relatively cheap and easy-to-use process: pressure spraying. Two contact configurations (cylinder-on-flat and ball-on-flat) were used in the tests with different displacement amplitudes (from 5 to 75,µm) and normal loads (from 100 to 400,N for ball-on-flat and from 400 to 1000,N for cylinder-on-flat). The results showed that large displacement amplitude is adverse to friction coefficient and coating lifetime and that under a critical contact pressure, coating endurance is improved contact pressure increases. Contact configuration influences friction coefficient by changing contact area and distribution of contact pressure. One master curve of average dissipated energy per cycle in initial stable stage was obtained for two contact configurations, which can be employed to approximately predict coating lifetime. Copyright © 2009 John Wiley & Sons, Ltd. [source] Numerical evaluation of pressure from experimentally measured film thickness in EHL point contactLUBRICATION SCIENCE, Issue 1 2008Michal Vaverka Abstract This paper is concerned with elastohydrodynamic lubrication, especially the determination of lubricant film thickness and contact pressure within a point contact of friction surfaces of machine parts. A new solution technique for numerical determination of contact pressure is introduced. The direct measurement of contact pressure is very difficult. Hence, input data of lubricant film thickness obtained from the experiment based on colorimetric interferometry are used for the calculation of pressure using the inverse elasticity theory. The algorithm is enhanced by convolution in order to increase calculation speed. The approach described in this contribution gives reliable results on smooth contact and in the future, it will be extended to enable the study of contact of friction surfaces with asperities. Copyright © 2007 John Wiley & Sons, Ltd. [source] Fretting behaviour of low-friction coatingsLUBRICATION SCIENCE, Issue 3 2005C. Langlade Abstract The choice of an appropriate low-friction coating for an industrial application is generally a difficult task. As many parameters are involved and are often unknown, selection criteria are hard to define. In order to elucidate the case of fretting conditions, a simulator has been used to reproduce the degradation mechanisms observed for real situations. Numerous experiments performed with this machine permit one to define endurance life criteria. The evolution of the life observed has been analysed as a function of the contact pressure and the film thickness. Using a Wöhler approach, endurance curves have been plotted that give useful information for selecting appropriate coatings. As other parameters may be of some interest, performance indices have been associated with them, and the results can be presented on a multi-axis (polar) diagram. [source] Determinants of Lesion Sizes and Tissue Temperatures During Catheter CryoablationPACING AND CLINICAL ELECTROPHYSIOLOGY, Issue 5 2007MARK A. WOOD M.D. Background:Factors which influence lesion size from catheter-based cryoablation have not been well described. This study describes factors which influence lesion size during catheter cryoablation. Methods and Results:Cryoablation was delivered to porcine left ventricular myocardium in a saline bath using 4- or 8-mm electrode catheters. Ablation was delivered with the electrodes either vertical or horizontal to the tissue and both with and without superfusate flow over the electrode. The effect of electrode contact pressure was tested. Lesion dimensions were measured. All experiments were duplicated to measure tissue temperatures at 1-, 2-, 3-, and 5-mm deep to the ablation electrode. The 8-mm electrode produced lower tissue temperatures and larger lesion volumes when compared with the 4-mm electrode (all P < 0.05). Superfusate flow slowed the rate of tissue cooling, markedly warmed tissue temperatures, and reduced lesion volume when compared with no flow conditions. By linear regression modeling, lesion sizes and tissue temperatures were related to the presence of superfusate flow, electrode orientation, contact pressure and electrode size, or catheter refrigerant flow rate (r2 for models = 0.90,0.96, all P < 0.001). Electrode temperature predicted lesion size or tissue temperatures only when analyzed independent of electrode size or refrigerant flow rate. Conclusions:Lesion sizes and tissue temperatures during catheter cryoablation are related to convective warming, electrode orientation, electrode contact pressure, and any of the following: electrode size, catheter refrigerant flow rate or electrode temperature. However, electrode temperature may be a poor predictor of lesion size and tissue temperature for a given catheter size. [source] Non-linear dynamic contact of thin-walled structuresPROCEEDINGS IN APPLIED MATHEMATICS & MECHANICS, Issue 1 2008Thomas Cichosz In many areas of mechanical engineering contact problems of thin,walled structures play a crucial role. Car crash tests and incremental sheet metal forming can be named as examples. But also in civil engineering, for instance when determining the moment,rotation characteristics of a bolted beam,column joint, contact occurs. Effective simulation of these and other contact problems, especially in three,dimensional non,linear implicit structural mechanic is still a challenging task. Modelling of those problems needs a robust method, which takes the thin,walled character and dynamic effects into account. We use a segment,to,segment approach for discretization of the contact and introduce Lagrange Multipliers, which physically represent the contact pressure. The geometric impenetrability condition is formulated in a weak, integral sense. Choosing dual shape functions for the interpolation of the Lagrange Multipliers, we obtain decoupled nodal constraint conditions. Combining this with an active set strategy, an elimination of the Lagrange multipliers is easily possible, so that the size of the resulting system of equations remains constant. Discretization in time is done with the implicit Generalized-, Method and the Generalized Energy,Momentum Method. Using the "Velocity,Update" Method, the total energy is conserved for frictionless contact. Various examples show the performance of the presented strategies. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Application of piece-wise linear weight functions for 2D 8-node quadrilateral element in contact problemsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 2 2004Chouping Luo Abstract The present study is a continuation of our previous work with the aim to reduce problems caused by standard higher order elements in contact problems. The difficulties can be attributed to the inherent property of the Galerkin method which gives uneven distributions of nodal forces resulting in oscillating contact pressures. The proposed remedy is use of piece-wise linear weight functions. The methods to establish stiffness and/or mass matrix for 8-node quadrilateral element in 2D are presented, i.e. the condensing and direct procedures. The energy and nodal displacement error norms are also checked to establish the convergence ratio. Interpretation of calculated contact pressures is discussed. Two new 2D 8-node quadrilateral elements, QUAD8C and QUAD8D, are derived and tested in many examples, which show their good performance in contact problems. Copyright © 2004 John Wiley & Sons, Ltd. [source] Influence of polyethylene creep behavior on wear in total hip arthroplastyJOURNAL OF ORTHOPAEDIC RESEARCH, Issue 3 2006Janaki R. Penmetsa Abstract After total hip arthroplasty (THA), polyethylene acetabular liner creep occurs quickly and serves to increases head,liner contact area and decrease contact pressures. What effect these early changes in contact mechanics will have on the wear behavior of the articulation remains unclear, and hence, selection or modification of polyethylene materials for optimal creep and wear performance is impossible. The objective of this study was to determine the influence of polyethylene creep behavior on volumetric wear and linear creep and wear penetration during simulated gait loading conditions. A finite element model of THA articulation was developed, and simultaneous numerical creep and wear simulation was performed to 10 million gait cycles with three levels of polyethylene creep behavior. Long-term volumetric wear and penetration were surprisingly unaffected by the polyethylene creep behavior due to the competing decrease in contact pressures coupled with increased contact area. In addition, variation in contact mechanics with the creep levels studied was only noteworthy in the initial postoperative period; after 1 million gait cycles, peak contact pressures and areas were within 13% regardless of the creep material behavior selected. Femoral head size had considerable impact on wear and penetration, while liner thickness primarily affected only early penetration. These results suggest that polyethylene creep behavior plays a major role in early penetration, but has little influence on the more important long-term volumetric wear. © 2005 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 24:422,427, 2006 [source] Experimental evaluation of friction between contacting discs for the simulation of gear contactLUBRICATION SCIENCE, Issue 1 2007J. Kleemola Abstract Instant gear contact can be simulated with contacting discs, which provides steady operating conditions and eliminates most of the dynamics and manufacturing tolerances involved in real gears, resulting in an accurately controlled contact condition. A high-pressure twin-disc test device was developed, where loading and rolling velocity can be varied continuously. It is equipped with disc bulk temperature, mean contact resistance and friction moment measurements. The test discs were grinded transversal to the disc rolling direction with proper crowning corresponding to the real gear flank properties. The test device was applied by studying the friction behaviour against the slide-to-roll ratio at different contact pressures, rolling velocities and surface roughness. The measurements were performed using mineral base oil in the range of operation conditions often used in industrial gears. In general, the measured friction coefficient behaviour correlates with earlier published results and is logical with measured bulk temperature and mean contact resistance. The limiting shear stress of the lubricant has an essential role in friction behaviour. Copyright © 2006 John Wiley & Sons, Ltd. [source] Tribological behavior of pure and graphite-filled polyimides under atmospheric conditionsPOLYMER ENGINEERING & SCIENCE, Issue 8 2003P. Samyn As the use of common engineering plastics in tribological systems is limited to low sliding velocities and low loads because of creep and insufficient temperature resistance, there is increasing interest in application of high-performance polymers such as polyimides, characterized by their ability to maintain favorable mechanical properties up to their melting point. However, for practical design, tribotesting remains necessary for determination of the material's performance under a given contact situation. In this article, two commercially available polyimides are tested at relatively high sliding velocities and contact pressures under atmospheric conditions of temperature and humidity. A consistent overview of tendencies in friction and wear for pure polyimides as a function of applied normal loads and sliding velocities is given. Addition of 15% by weight graphite powder as internal solid lubricant strongly influences friction and wear. Its behavior is compared with pure polyimide grades and differences are discussed in relation with experimental measured bulk-temperatures. A linear temperature law is derived as a function of pv-levels and a steady-state condition is found at different temperature levels, in accordance with thermal conductivity of the polymer bulks. In case of graphite additives, a steady state in temperature coincides with the regime condition of wear rate. [source] Mechanotransduction of bovine articular cartilage superficial zone protein by transforming growth factor , signalingARTHRITIS & RHEUMATISM, Issue 11 2007Corey P. Neu Objective Mechanical signals are key determinants in tissue morphogenesis, maintenance, and restoration strategies in regenerative medicine, although molecular mechanisms of mechanotransduction remain to be elucidated. This study was undertaken to investigate the mechanotransduction process of expression of superficial zone protein (SZP), a critical joint lubricant. Methods Regional expression of SZP was first quantified in cartilage obtained from the femoral condyles of immature bovines, using immunoblotting, and visualized by immunohistochemistry. Contact pressure mapping in whole joints was accomplished using pressure-sensitive film and a load application system for joint testing. Friction measurements on cartilage plugs were acquired under boundary lubrication conditions using a pin-on-disk tribometer modified for reciprocating sliding. Direct mechanical stimulation by shear loading of articular cartilage explants was performed with and without inhibition of transforming growth factor , (TGF,) signaling, and SZP content in media was quantified by enzyme-linked immunosorbent assay. Results An unexpected pattern of SZP localization in knee cartilage was initially identified, with anterior regions exhibiting high levels of SZP expression. Regional SZP patterns were regulated by mechanical signals and correlated with tribological behavior. Direct relationships were demonstrated between high levels of SZP expression, maximum contact pressures, and low friction coefficients. Levels of SZP expression and accumulation were increased by applying shear stress, depending on location within the knee, and were decreased to control levels with the use of a specific inhibitor of TGF, receptor type I kinase and subsequent phospho-Smad2/3 activity. Conclusion These findings indicate a new role for TGF, signaling in the mechanism of cellular mechanotransduction that is especially significant for joint lubrication. [source] | ||||||||||||||||