Function Of Contact Pressure Research Articles

Thermal contact resistance on cylindrical flux channel with high contact ratio

This paper considers the resistance to the heat flow between two thick solids with a high contact ratio in a vacuum. Based on the least-action principle, we derive closed-form mathematical expressions for the temperature distribution and non-dimensional thermal contact resistance, both expressed as functions of the radii ratio. Furthermore, the thermal contact resistance is investigated as a function of contact pressure and microhardness. A comparison of the conventional and proposed methods reveals that the proposed method is more accurate for calculating the thermal contact resistance with a high contact ratio. In equipment with high contact pressure, such as electromagnetic launch, we compare armature melting models using different calculation methods for contact resistance. Small and all contact models were established based on the traditional and proposed methods, respectively. The melting morphology of the armature obtained from the all contact model is highly consistent with the experimental results. During the experiment, in areas where the armature did not melt, the small contact model incorrectly calculated the melting of the armature. The all contact model can describe the strong cooling effect of the rail on the armature, preventing the armature from melting. The all contact model obtained higher heat sources, contact thermal conductivity, and contact pressure in the melting region. Under the combined effect of the three factors, a deeper and more concentrated melting morphology was obtained. This morphology is more consistent with the experimental results.

On the Use of Advanced Friction Models for the Simulation of an Industrial Stamping Process including the Analysis of Material and Lubricant Fluctuations

The use of numerical simulations for tool tryout and process control is becoming increasingly prevalent. In this work, the deep drawing process of a car inner door panel of DC06 mild steel is numerically analyzed and compared with industrial process results. Five batches of DC06 material were analyzed mechanically and tribologically. Diverse tribological models were developed based on experimental strip drawing tests, where a Coefficient of Friction (CoF) was obtained as a function of contact pressure, sliding velocity, and amount of lubricant. A topography analysis was defined to compare material batches and to replicate industrial tool conditions. The simulation was fed with three tribological models: constant (CoF 0.15), Filzek pressure and velocity dependent, and TriboForm with lubrication zones. Thinning, Forming Limit Diagram (FLD) and draw-in were used as indicators for the comparison. Using the industrial tool, both FLD and draw-in were measured and compared with the numerical models. The constant model predicted the most conservative strain state and also differed most from the experimental results. The P-v-dependent and TriboForm models more accurately predicted the experimental results. This work highlights the importance of considering more complex tribological models to feed numerical simulations to yield results closer to real process conditions.

Torque capacity of multidisc wet clutch with reference to friction occurrence on its spline connections

In this article developed mathematical model that includes friction occurrence on spline connections is presented. The work also contains results of experimental research on torque capacity of multidisc wet clutch. These results are expressed as a function of contact pressure for different number of friction surfaces. Due to increased interest in research concerning multidisc wet clutches it is essential to determine impact of friction on fit connections on transmitted torque. Analytical calculations that include both known loss coefficient and assumed lack of friction on fit connections are compared to results of experiments. The paper contains detailed description of test stand and methodology of experiment. As a result of conducted tests it was found that correction coefficients known from literature are highly inaccurate. Measured values of torque indicate that transmitted torque reach significantly higher values. It was also revealed that after slippage appeared, the pressure plate usually moved in the direction of exerted clamping force, but movement in reversed direction also took place for some experiments. While movement corresponding to clamping force reached ca. 0.08 mm, in opposite direction amounted to 0.02 mm. Furthermore, studies presented that lapping of adjacent friction surfaces greatly affects differences between respective results obtained for a specific experiment.

Enhanced computational modelling of UHMWPE wear in total hip joint replacements: The role of frictional work and contact pressure

Hip joint replacements using highly cross-linked ultra-high-molecular-weight polyethylene (UHMWPE) as a bearing material offer an effective treatment to arthritis. However, the bearing wear may affect the longevity of implants which requires more rigorous investigations. UHMWPE wear is highly dependent on cross-shear (CS) as a result of the multidirectional motion of bearings. Previously, a wear model has been developed with the potential to provide accurate simulation, of which the wear was determined as an exponential function of sliding distance with CS at the crossing angle of 90°, and the effect of CS for an arbitrary angle was quantified to be proportional to the perpendicular component of sliding distance. In the present study, the wear was proposed as the exponential function of frictional work. In particular, contact pressure was incorporated into the calculation of frictional work by considering that the friction coefficient is a function of contact pressure. In addition, the CS quantification for arbitrary crossing angles was considered by coupling the sliding distance and frictional force. The computational formulations of the present and previous models were compared by applying the models to a 36 mm diameter metal-on-polyethylene total hip prosthesis with highly cross-linked UHMWPE at 5 Mrad irradiation under simulated walking conditions. The simulation was also validated against the published results of hip simulator tests with respect to the influence of motion/loading condition on wear. The study showed markedly improved computational predictions with wear rate discrepancies in the range of 8–25% for two kinematic conditions either with the fixed or with varying direction of load. The results show the important role of both the frictional work and contact pressure, and they should be both incorporated appropriately to enhance the computational modelling.

Friction coefficient of solid lubricating coating as a function of contact pressure: experimental results and microscale modeling

The paper presents experimental analysis of relation between friction coefficient and contact pressure of hbox {MoS}_2 film deposited on hbox {Ti}_6hbox {Al}_4hbox {V} substrate in contact with sapphire ball during reciprocating sliding motion. It is shown that the value of friction coefficient decreases with increasing contact pressure. A microscale modeling approach is next developed to mimic the experimental observations. Representative volume element is defined based on the actual topography of outer surface of hbox {MoS}_2 film. Assuming thermo-elastic material properties, the calculations on the asperity level are performed in two steps. Firstly, the mechanical contact between two surfaces is calculated. As a result, the relation between the global load and micro-stress distribution is obtained. Secondly, for a given stress load, thermal analysis is performed providing temperature fluctuation within simplified conical asperity. By assuming relation between friction coefficient and temperature on the microscale, it is possible to obtain macroscopic friction coefficient as a function of contact pressure. In the end, model results are compared with experimental data. The novel aspects of presented approach lie in the selection of three main factors on a micro-level defining macroscopic friction. They are actual surface topography, microscopic temperature and microscopic friction-temperature relation.

On a static solution to the contact interaction of a flexible rectangular in plane of a microshell with a rectangular in plane of microplate

The mathematical model of the contact interaction of a flexible rectangular in plane of a spherical microshell and a flexible rectangular microplate is constructed. The material of the shell and plate is elastic isotropic and homogeneous. Kirchhoff hypotheses are taken into account. A system of nonlinear partial differential equations is derived from the Hamiltonian energy principle on the basis of a modified couple stress theory. The contact pressure function was determined according to the theory of B.Ya. Cantor. Contact interaction of sensitive elements of micromechanical sensors of inertial information in the form of a micro-shell and micro-plate is investigated. A micro-shell is affected by a static, uniformly distributed load. The static solution is determined by the establishment method. A new phenomenon related to the loss of stability is obtained.

The Effect of Applied Pressure Function on Thermo-elastic Problem in the Dry Friction Clutches

The main purpose of this paper is to investigate deeply the effect the contact pressure function on the thermo-mechanical behavior of the friction clutch system during the slipping time (heating stage). The other purpose is to explore theoretically the complex interaction among the contact pressure, sliding speed and frictional characteristics of frictional facings to specify the magnitude and distribution of the frictional heating generation on the contact surfaces of the dry friction clutch under different applied pressure. It was developed a numerical code based on finite element method (ANSYS/APDL 2019) to determine accurately the contact pressure, temperature and frictional heat generated on contact surfaces of the friction clutch disc that has two effective frictional faces at any instant during the slipping period. It was found a significant effect of the magnitude and variation of applied pressure during the heating phase on the surface temperatures, contact pressure and frictional heat generated. Where, the hot spot can be appeared when the applied pressure is constant. Under such circumstances, high amount of temperature and contact pressure focused on a small zone of the nominal contact area. This phenomenon is considered one of the main reasons for the early failure of the contacting surfaces of friction clutch.

A general IHTC model for hot/warm aluminium stamping

Different hot and warm stamping technologies with particular processing parameters were applied to deform aluminium alloy sheets to satisfy desired requirements, of which the post-form strength of formed components is one of the most important criteria. In order to save experimental efforts, the present research described an efficient method to determine the critical processing parameters, i.e. the integration of the finite element (FE) simulated temperature evolutions with the continuous cooling precipitation (CCP) diagrams of the aluminium alloys. Through the optimisation of the processing parameters, the temperature evolutions and CCP diagrams do not intersect, indicating that the post-form strength of the aluminium alloys could be fully retained after proper artificial ageing processes. Therefore, a precise FE simulation of the temperature evolution is of great importance to this method, which requires the implementation of an accurate interfacial heat transfer coefficient (IHTC) as a decisive boundary condition. A general aluminium alloy-independent model with one set of fixed model constants was therefore developed to predict the IHTC evolutions as a function of contact pressure, surface roughness, initial blank temperature, initial blank thickness, tool material, coating material and lubricant material. Subsequently, the predicted IHTCs for AA6082 and AA7075 aluminium alloys were used to simulate their temperature evolutions, which were then integrated with their CCP diagrams to identify the critical processing parameters in hot/warm stamping processes and thus meet the desired post-form strength of the AA6082 and AA7075. The developed IHTC model and determined critical processing parameters were then experimentally verified by the Fast light Alloy Stamping Technology (FAST) of the dissimilar aluminium alloy blanks joined by Friction Stir Welding (FSW).

On the reciprocating sliding wear of polypropylene against polyamide 6 in dry and aqueous environments

Polymer-polymer tribological components with sliding contacts usually are lighter, cheaper and more corrosion resistant in aqueous environment, presenting some advantages over the classic metal-polymer equivalents under certain conditions, such as in wear resistant external coatings for pipes in industry. Reciprocating sliding wear of polypropylene (PP) against polyamide 6 (PA6) in dry and aqueous environments (distilled water and synthetic sea water) was mapped as a function of contact pressure (2.4–7.2 MPa) and sliding speed (36–108 mm/s), aiming the identification of favorable operating conditions (low wear). Mild wear regime (k = 1.3–5.3 × 10−6 mm3/N m) was found for PP in all conditions, except for the highest sliding speed in dry environment, where there was a severe regime (k = 0.5–0.8 × 10−3 mm3/N m). Meanwhile, the PA6 counter-body always operated in the mild wear regime for all studied conditions. Wear mechanisms in the mild regime were: surface fatigue microcracks, film transfer and abrasion for both materials and, for PA6 only, also wear by roll formation. At higher speed, frictional heating (0.14 < μ < 0.25) and low thermal diffusivity of the materials resulted in high contact temperatures (up to 120 °C), which is enough to promote the removal of molten or softened material in the PP, typical of severe wear regime. The mild-to-severe wear regime transition observed for PP-PA6 tribopair occurs at a much lower sliding speed than in metal-polymer pairs, which may limit the applicability of these tribosystems. For PA6, the severity of surface damage as a function of the environment increased from dry to distilled water and sea water condition, as this material is susceptible to attack by polar solutions, which degrades its mechanical properties.

Characterization of heat transfer coefficient for non-isothermal elevated temperature forming of metal alloys

To accurately model warm and hot forming operations, correct values for heat transfer coefficient (HTC) are required, among other parameters. In this work, an experimental methodology is proposed to obtain HTC data for a high strength steel alloys (Usibor® 1500 A-S), seven aluminum alloys (AA5182-O, AA5754-O, AA6013-T6, AA6063-T6, AA6061-T6, AA7075-T6 and a developmental AA7xxx-T76) and two magnesium (AZ31B-O and ZEK100-O) alloys. The HTC was characterized as a function of contact pressure from 2 to 80 MPa. Overall, the HTC values exhibited significant variation (ranging from 355 to 1887 W/m2-K), but tended to correlate primarily with contact pressure and material strength/alloy system. Two lubricants were also considered (Fuchs Forge Ease 278 and PTFE spray) for some alloys, but had only a minor effect on HTC relative to “dry” tooling. Neither the initial temperature of the tooling nor the ramping up of the press tonnage had a noticeable effect on the HTC since the HTC values were identified based only upon the relative temperature difference and thermal properties of the materials. The experimentally determined HTC values were then applied in a series of non-isothermal cylindrical cup warm deep drawing experiments. The deep draws were modelled using finite element (FE) techniques, which utilized the measured HTC values. The temperature-time profiles from the deep draws were found to agree well with the predicted profiles, indicating that the methodology used to determine HTC and the resulting HTC values, are applicable for use in a metal forming simulation involving heat transfer.

Three-dimensional graphene nanosheet films towards high performance solid lubricants

Graphene nanosheet films (GNSF) show great potential as solid lubricants. They, however, usually suffer from short lifetime and low load capability, limiting their further engineering application. In this work, we try to address these crucial challenges, for the first time, by inducing a unique three-dimensional microstructure into the GNSF. The as-prepared three-dimensional graphene nanosheet films (3D-GNSF) are composed of many crumpled and partially standing graphene nanosheets (GNS) that overlap with each other and form a three-dimensional interconnected network. The morphology, composition and structure of the 3D-GNSF are investigated in detail using scanning electron microscope, X-ray diffractometer, transmission electron microscope and X-ray photoelectron spectroscopy to deduce their possible formation mechanism. Tribological tests are conducted in air as a function of contact pressure that is up to about 1GPa. The results suggest it is the unique architecture of the 3D-GNSF that quickly produces a compact and stable sliding interface consisting of GNS against GNS, enabling 3D-GNSF as promising solid lubricants with low friction, excellent anti-wear ability, good durability and high load capability.

Friction Dependence on Surface Roughness for Castor Oil Lubricated NiTi Alloy Sliding on Steel

NiTi alloys are highly resistant to corrosion with hardness and elasticity properties suitable for bearing applications. However, lubrication of this material is still being explored since many commercial lubricants are designed to function through chemical and physical interactions with steel surfaces. Previous studies suggested castor oil as a natural lubricant for NiTi and reported friction coefficients less than 0.01, i.e. superlubricity, when sliding steel against 60NiTi (60wt.% Ni, 40 wt.% Ti). Here we explore the robustness of those findings by characterizing the change in friction coefficient as a function of contact pressure and surface roughness. It is found that friction is extremely sensitive to surface roughness, such that ultra-low friction could only be achieved with surfaces having roughness less than tens of nanometers, conditions which may not be achievable in some practical bearing applications.

Temperature dependent micromechanics-based friction model for cold stamping processes

Temperature rise in cold stamping processes due to frictional heating and plastic deformation of sheet metal alters the tool-sheet metal tribosystem. This is more prominent in forming advanced high strength steels and multi-stage forming operations where the temperature on the tool surface can rise significantly. The rise in temperature directly affects the friction due to break down of lubricant, change in physical properties of tribolayers and material behavior. This can result in formability issues such as workpiece-splitting, etc. Therefore, it is important to account for temperature effects on friction in sheet metal forming analyses. In this study, the temperature effect was included in a micromechanics-based friction model which allows calculation of local friction coefficients as a function of contact pressure, bulk strain and relative sliding velocity. The temperature influence on friction was introduced through material behavior of sheet metal, viscosity of lubricant and shear strength of boundary layer in the micromechanics-based model. The model validation has been done by comparing the calculated fractional real contact area with the experimental results. The model can be used in formability analyses and to predict optimum stamping press parameters such as the blank holder force and the press speed.

Effects of lubricant on the IHTC during the hot stamping of AA6082 aluminium alloy: Experimental and modelling studies

In this paper, the IHTC evolutions between a hot AA6082 specimen and cold P20 tools were quantitatively studied as a function of contact pressure, specimen thickness and lubricant, using the inverse FE simulation method. A novel interactive model was developed to predict IHTC evolutions with diminutions of different lubricants as a function of sliding distance (or time) at different contact pressures and sliding speeds. It was deduced that the interaction between the lubricant and the IHTC consists of three stages; namely stage I (excessive stage), in which the lubricant is applied excessively and the IHTC is plateaued, stage II (consumption stage), in which the lubricant diminishes during sliding and the IHTC decreases rapidly, and stage III (breakdown stage), in which lubricant breakdown occurs and the IHTC is equal to its values under dry conditions.

A new mathematical model of wrist pulse waveforms characterizes patients with cardiovascular disease – A pilot study

The purpose of this study was to analyze and compare a series of measured radial pulse waves as a function of contact pressure for young and old healthy volunteers, and old patients with cardiovascular disease. The radial pulse waves were detected with a pressure sensor and the contact pressure of the sensor was incremented by 20gf during the signal acquisition. A mathematical model of radial pulse waveform was developed by using two Gaussian functions modulated by radical functions and used to fit the pulse waveforms. Then, a ratio of area (rA) and a ratio of peak height (rPH) between percussion wave and dicrotic wave as a function of contact pressure were calculated based on fitted parameters. The results demonstrated that there was a maximum for waveform peak height, a minimum for rA (rAmin) and a minimum for rPH (rPHmin) appeared as contact pressure varied. On average, older patients had higher peak amplitude and a significantly smaller rAmin (p<0.001) and rPHmin (p<0.02) than the young and old volunteers. The rAmin and rPHmin calculated with the mathematical model had moderate to strong positive linear correlations (r=0.66 to 0.84, p<0.006) with those directly calculated without the model. The receiver operating characteristic (ROC) analysis showed that the rAmin calculated with the model and the contact pressure measured at the rAmin had good diagnostic accuracy to distinguish healthy volunteers vs. diseased patients. Therefore, using the mathematical model to quantitatively analyze the radial pulse waveforms as a function of contact pressure could be useful in the diagnosis of cardiovascular diseases.

Determination of the interfacial heat transfer coefficient for a hot aluminium stamping process

The interfacial heat transfer coefficient (IHTC) is an important thermophysical parameter in hot stamping processes and must be identified not only to retain the full mechanical strength of formed components, but also to optimise the production rate. In this work, a novel experimental facility was developed and applied to measure the temperature evolutions of the specimens and tools in stamping processes. Simulated temperature evolutions obtained using the FE software PAM-STAMP were then fit to this data. The IHTC values between AA7075 and three different tool materials were characterized at different contact pressures under both dry and lubricated conditions. In addition, a mechanism based IHTC model was developed and validated as a function of contact pressure, tool material and lubricant thickness to predict the IHTC values under different conditions.

Wear And Frictional Behaviours of Automotive Brake Pad Sliding Against GG 20 Cast Iron Counter Face

In the present study, wear and frictional behaviours of automotive brake pad sliding against GG 20 cast iron counter face have been investigated experimentally using a pin on disc type friction tester. Wear tests were carried out under dry conditions. A central composite design was used to describe response and to estimate the process parameters in the model. Empirical models have been developed to predict wear loss and friction coefficient as a function of contact pressure and sliding velocity using multiple regression. The statistical analysis show that the contact pressure is identified as the most dominant factor affecting wear loss, sliding velocity is identified is the most dominant factor affecting friction coefficients.

Evaluation of Contact Heat Transfer Coefficient and Phase Transformation during Hot Stamping of a Hat-Type Part.

Using an inverse analysis technique, the heat transfer coefficient on the die-workpiece contact surface of a hot stamping process was evaluated as a power law function of contact pressure. This evaluation was to determine whether the heat transfer coefficient on the contact surface could be used for finite element analysis of the entire hot stamping process. By comparing results of the finite element analysis and experimental measurements of the phase transformation, an evaluation was performed to determine whether the obtained heat transfer coefficient function could provide reasonable finite element prediction for workpiece properties affected by the hot stamping process.

Experimental determination of thermal contact conductance between pressure and calandria tubes of Indian pressurised heavy water reactors

Thermal contact conductance (TCC) is one of the most important parameters in determining the temperature distribution in contacting structures. Thermal contact conductance between the contacting structures depends on the mechanical properties of underlying materials, thermo-physical properties of the interstitial fluid and surface condition of the structures coming in contact. During a postulated accident scenario of loss of coolant with coincident loss of emergency core cooling system in a tube type heavy water nuclear reactor, the pressure tube is expected to sag/balloon and come in contact with outer cooler calandria tube to dissipate away the heat generated to the moderator. The amount of heat thus transferred is a function of thermal contact conductance and the nature of contact between the two tubes. An experimental facility was designed, fabricated and commissioned to measure thermal contact conductance between pressure tube and calandria tube specimens. Experiments were conducted on disc shaped specimens under axial contact pressure in between mandrels. Experimental results of TCC and a linear correlation as a function of contact pressure have been reported in this paper.

A Robust Numerical Procedure for the Thermomechanical Flow Simulation of Friction Stir Welding Process Using an Adaptive Element-Free Galerkin Method

A meshfree modeling technique of material flow in the three-dimensional multiphysics thermomechanical friction stir welding process is presented. In this numerical model, the discretization in space is derived by the Element-Free Galerkin method using a Lagrangian meshfree convex approximation. The discrete thermal and mechanical equations are weakly coupled as the time advances using a forward difference scheme. A mortar contact algorithm is employed to model the stirring effect and heat generation due to frictional contact. Heat conductance between contacting bodies is considered as a function of contact pressure. A two-way adaptive procedure is introduced to the coupled thermomechanical system to surpass potential numerical problems associated with the extensive material deformation and spatial discretization. In each adaptive phase, a consistent projection operation utilizing the first-order meshfree convex approximation is performed to remap the solution variables. Finally, a three-dimensional multiphysics thermomechanical coupled friction stir welding problem is analyzed to demonstrate the effectiveness of the present meshfree numerical procedure.