Limiting Shear Stress Research Articles

Measurement of EHD Friction at Very High Contact Pressures

EHD friction curves have been measured up to very high pressure (pmean = 5 GPa, pmax = 7.5 GPa) using a newly developed, rolling-sliding, ball on disc machine, the ETM. Six base fluids have been studied, spanning the API base oil categories Group I to Group V. At high pressures, thermal effects become substantial even at quite modest slide-roll ratios, and these must be considered when analysing friction measurements in terms of the underlying rheological properties of the oils. By comparing measurements from steel/steel and WC/WC ball and disc combinations with very different thermal conductivities, the use of thermal correction to derive isothermal friction curves has been validated. At relatively low pressures (mean pressure = 1 GPa), there are substantial differences between the EHD friction properties of the various API Group base oils, but as pressure is raised these diminish and the EHD friction coefficients of all the Groups approach a similar maximum value at a given temperature. EHD friction continues to be quite strongly temperature dependent even at very high pressure. As pressure is increased, EHD friction curves become progressively steeper, so that friction coefficients at very low slide-roll ratios (1 to 2% SRR) become several times greater at high than at low pressure. This has important practical implications for the efficiency of rolling element bearings at high pressures since these components normally operate in this SRR range. There is no evidence of any of the base oils reaching a limiting shear stress over the whole pressure and temperature range studied. Instead, shear stress continues to increase with log(strain rate) in accord with the Eyring-activated flow model up to very high pressures.

Delamination and current-carrying degradation behavior of epoxy-impregnated superconducting coil winding with 2G HTS tape caused by thermal stress

A thermo-mechanical-electromagnetic model is developed for estimating the delamination and current-carrying degradation of epoxy-impregnated pancake coils. The mixed-mode traction–separation law and the Weibull distribution of delamination strength are considered in simulating the onset and extension of the delamination caused by thermal stress. Because of the considerable differences in thermal shrinkage between the epoxy resin, insulation tape, and second-generation high temperature superconducting (2G HTS) tape, the accumulated radial thermal stress locally exceeds the electro-mechanical delamination strength and even the mechanical delamination strength. The electro-mechanical delamination strength of the 2G HTS tape is the transverse tensile or shear stress level corresponding to an abrupt irreversible critical current degradation. The mechanical delamination strength is the transverse tensile or shear stress limit corresponding to a structural failure. After injecting current into the coil, we obtained the E–I curve of the coil. The critical current and n-value of the superconducting coil indicate a strong degradation after epoxy-impregnation. The current-carrying degradation precedes delamination because the electro-mechanical delamination strength is smaller than the mechanical delamination strength. The degradation is more obvious in large than in small superconducting coils because the radial thermal stress is larger. The onset of degradation depends on the minimum delamination strength, suggesting that caution is required in screening the 2G HTS tape before winding the coil. The simulation results indicate that reducing the thickness of the insulation tape and the amount of epoxy resin effectively reduces the degradation of epoxy-impregnated pancake coils.

ИССЛЕДОВАНИЕ ПРЕДЕЛЬНОГО НАПРЯЖЕНИЯ СДВИГА СИСТЕМЫ ИЗВЕСТНЯКОВАЯ МУКА ─ МЕЖСЛАНЦЕВАЯ ГЛИНА ─ ВОДА ПРИ ПОВЫШЕННЫХ ТЕМПЕРАТУРАХ

Ceramic brick based on interlayer clay and lime flour is obtained without using natural traditional materials. The process of drying ceramic bricks should be carried out according to the optimal mode, which means a combination of its possibly short duration, minimum expenses of energy and quality semi-finished product ─ lack of buckling, Cracks and hidden stresses, which contribute to the formation of cracks in the burning When making ceramic brick only from interlayer clay, Used as clay binder, without the use of repellents, there is a sharp drop in the limit shear stress in the temperature range of 20-80 °C, obviously due to the appearance of cracks and deformation curves. The use of lime flour in ceramic masses in the production of ceramic brick slows down the drop of the limit shear stress in the temperature range 20-80 °C. Researches showed that for improvement of drying properties of ceramic weight the "limy flour  interslate clay water" system is more uniform, in relation to the "interslate clay water" system. Analysis of the shear stress limit under hot molding conditions has shown that the function of plastic strength versus molding humidity Pm = f (W, t) is of particular importance in this case than the function of plastic strength versus temperature Pm = f (t).

Evaluating the thermal effect on the rheological properties of waste vegetable oil biodiesel modified bentonite drilling muds using Vipulanandan model

In this investigation, the effects of the temperature on the flow characteristics of waste vegetable oil biodiesel modified bentonite drilling mud were quantified. The biodiesel contents in the drilling muds are varied from 0 to 9% of the volume of the water in order to reduce the yield point and upper shear stress limit produced by the mud during drilling activities under simulated thermal condition ranging from 28°C to 180°C. The results revealed that the bentonite drilling mud loaded with 6 vol.% waste vegetable oil biodiesel demonstrates fairly favorable rheological properties compared to others when exposed to similar heat treatment. Moreover, the rheological behavior of biodiesel-free bentonite mud and samples loaded with biodiesel were quantified using the Vipulanandan model and compared with others existing models used in the industry such as: Bingham plastic model and Herschel-Bulkley model. It was observed that the Vipulanandan model predicts satisfactorily the shear thinning relationship between the shear stress and shear strain rate of the modified bentonite drilling muds. Furthermore, by using the Vipulanandan model, the maximum shear stress values for 0%, 3%, 6% and 9% biodiesel content at 28°C are found to be 63.67 Pa, 84.54 Pa, 84.64 Pa and 85.02 Pa, respectively. When exposed to heat treatment at 180°C, the maximum shear stress values recorded for 0%, 3%, 6% and 9% are 53.18 Pa, 67.39 Pa, 64.52 Pa and 72.70 Pa, respectively, which represent a reduction of 14% to 16% in the upper shear stress limit.

Evaluation of thixotropy of clay-gypsum based plaster

Introduction. Requirements for mineral based plasters are divided into two groups: technological and performance properties. Plasticity and ease of application are attributed to the technological properties. This determines the requirements for rheological characteristics and, first of all, for the thixotropy of mixtures. The technique is developed and tested on plaster compositions based on clay-gypsum binder which belongs to the category of local binders. Russia has significant reserves of this raw material, but at present it is not used industrially. The main reason is the lack of a scientifically based theory of hardening of the clay-gypsum binder and the formation of its properties without and with modifying additives. The purpose of this research is to develop the composition and technology of application of interior plaster compositions based on stabilized binder. The methodology of studying the rheological properties of dry plaster mixtures based on clay gypsum for interior work and wet rooms is presented. The study of the rheology of clay gypsum mixtures is carried out for the first time, it determines scientific novelty. Recommendations for the use of clay gypsum plaster mixtures determine the practical significance of the research. Materials and methods. Methods of evaluation of thixotropic properties of plaster mixtures based on clay-gypsum binder in laboratory environment are realized. Results. The valid intervals of the water-cement ratio are established which ranged from 0.46 to 0.50, determined by the limiting shear stress and the actual sliding from the vertical wall. Conclusions. The possibility of using a clay-gypsum binder in dry plaster mixtures for interior decorative works in the premises, in particular, confirmed by research. It allows us to assess the actual adhesion of plaster coatings to the concrete based surfaces and to brickwork.

Slip status in lubricated point-contact based on layered oil slip lubrication model

Abstract Obvious decrease of film thickness is found in recent experimental tests of high-speed elastohydrodynamic lubrication (EHL) point contact, which shows a close correlation with interface slip. This work investigates the effects of operation conditions and rheological properties of lubricant on slip behaviours based on a layered oil slip model. The results show that slip extent generally increases with the aggravation of sliding, the increase of applied load and the decrease of limiting shear stress; furthermore, the slip extent will also be affected by the contacting surface properties. Essentially, as internal factors, the rheological properties of lubricant may determine the slip behaviour under various operation conditions.

Shear stress limit, rheological properties and compressive strength of cement-based grout modified with polymers

Grouting is a comprehensive technology used in the construction projects due to the rapid development of sub-surface urban infrastructures, the main reasons for grouting soils are strengthened the cohesion-less soils and increasing the shear stress (pure shear) of the grouted soils. Providing high flowability with high viscosity for the cement-based grout in the liquid stage (slurry) and high compressive strength of the cement-based grout in the hardened stage are significant challenges. In this study, the impact of two types of water reducer [polycarboxylate (PCE)] polymer on the rheological properties with the ultimate shear strength and compressive strength of cement-based grout with water-cement ratios (w/c) of 0.6 and 1.0 at two different temperatures 25 °C and 50 °C were studied. XRD and TGA were used to analysis the cement, polymers, and cement modified with polymers. The behavior of cement-based grout in the liquid phase (slurry) and hardened phase modified with different percentages of polymer up to 0.16% (by dry weight of cement) were investigated. The compressive strength of cement-based grout modified with polymer was tested from the young age up to 28 days of curing. Vipulanandan rheological model was used to predict the shear stress-shear strain behavior of cement-based grout slurry and compared to the Herschel–Bulkley (HB) model. The rheological and the compressive strength are increased with increasing the of PCE content. The polymer modification increased the yield stress, apparent viscosity and plastic viscosity of the cement grout by 19–136%, 32–319% and 58–367%, respectively based on the types of polymer, polymer content, w/c, and temperature. The compressive strength of the cement-based grout increased by 94–786% based on the types of polymer, polymer content, w/c and curing time. Increasing the temperature of cement-based grout slurry to 50 °C increased the maximum shear stresses by 110% and 107%, respectively. Effects of polymer content, w/c, curing time and the temperature of the plastic and hardened properties of cement-grout were modeled using a multiple nonlinear regression analysis.

TGA, rheological properties with maximum shear stress and compressive strength of cement-based grout modified with polycarboxylate polymers

Grouting is a comprehensive technology used in the construction projects due to the rapid development of sub-surface urban infrastructures, the main reasons for grouting soils are strengthened the cohesion less soils and increasing the shear stress (pure shear) of the grouted soils. Providing high flowability with high viscosity for the cement-based grout in the liquid stage (slurry) and high compressive strength of the cement-based grout in the hardened stage are significant challenges. In this study, the impact of two types of polycarboxylate (PCE) polymer on the rheological properties with the ultimate shear strength and compressive strength of cement-based grout with water-cement ratios (w/c) of 0.6 and 1.0 at two different temperatures 25 °C and 50 °C were studied. XRD and TGA were used to analysis the cement, polymers, and cement modified with polymers. The behavior of cement-based grout in the liquid phase (slurry) and hardened phase modified with different percentages of polymer up to 0.16% (by dry weight of cement) were investigated. The compressive strength of cement-based grout modified with polymer was tested from the young age up to 28 days of curing. Vipulanandan rheological model was used to predict the shear stress-shear strain behavior of cement-based grout slurry and compared to the Herschel-Bulkley (HB) model. The rheological and the compressive strength are increased with increasing the of PCE content. The polymer modification increased the yield stress, apparent viscosity, and plastic viscosity by 19–136%, 32–319% and 58–367%, respectively based on the types of polymer, polymer content, w/c, and temperature. The compressive strength of the cement-based grout increased by 94–786% based on the types of polymer, polymer content, w/c, and curing time. Based on the Vipulanandan rheological model, increasing the temperature of cement-based grout slurry to 50 °C increased the maximum shear stresses by 110% and 107% respectively. The unprecedented conclusions can be drawn in this study, which is the effect of the polymer content, water-cement-ratio and temperature effect on the plastic viscosity, yield points and shear stress limit in the fresh stage (slurry) of the cement-based grout and also, the impact of the polymer content, water-cement-ratio and curing time on the flexural with compressive strengths in the hardened stages of the cement modified with polymers were modeled.

Thermal interactions in rolling bearings

Rolling bearing dynamics model, based on classical differential equations of motion of bearing elements coupled with thermal interactions, is presented. While churning and drag effects are based on classical laminar and turbulent flow theories, independently measured lubricant rheology, including shear dependence of viscosity, is used to model lubricant traction. The energy equation is integrated through the lubricant film to first compute Newtonian traction with thermal effects. Viscosity dependence on shear stress is then applied to model “shear-thinning” effects. At very high contact pressure and very low slide-to-roll ratios material creep effects, where the behavior of lubricated and dry contacts is similar, are implemented, while a shear stress limit is applied at very high slide-to-roll ratios. Traction predictions for a typical contact in a traction rig show good agreement with experimental traction data. Transient heat generations are time-averaged over thermal time step to compute time-varying temperature fields in the bearing, which alter properties of bearing materials, operating bearing geometry, and rheology of the lubricant. As the transient solutions converge to stable operating temperatures, bearing heat generation approaches the expected steady-state value. Heat generation predictions for both ball and rolling bearings are in good agreement with measured experimental data.

Study of the Limiting Shear Stress of Pyrophyllite – Clay Component – Water Composition at Elevated Temperature

A sharp reduction in the ultimate shear stress in the range of 20 – 80°C is observed during preparation of specimens of a clay component without thinners that facilitates development of cracks and defects. Use of pyrophyllite in ceramic mixes during specimen preparation provides a gradual reduction in the ultimate shear stress Pm in the range of 20 – 80°C. It is shown that in order to improve specimen drying properties a pyrophyllite – clay – water composition is more homogeneous than a clay – water composition Analysis of specimen Pm under hot forming conditions shows that the relationship Pm = f(t) is not as important as the relationship Pm = f(W,t), where t is temperature, and W is humidity.

Delayed installation of piles in consolidating soil

The current general accepted view is that the delayed installation of piles after the commencement of consolidation leads to a reduction of drag load acting on piles. Despite the extensive studies reported in the literature, no work that focussed on the quantitative effect of delayed installation of piles after the commencement of the consolidation on pile behavior was reported. A study on the effect of the delayed pile installation after the commencement of the consolidation is undertaken using a load transfer theory. Pile-soil slip is accounted for by limiting shear stress at the pile-soil interface to the average undrained shear strength of the consolidating soil. The effect of pile-soil slip in limiting the drag load is accounted for once the location of the neutral plane resulting from the delayed pile installation after the commencement of the consolidation is known. Contrary to the current generally accepted understanding that delayed installation of piles after the commencement of the consolidation will lead to a reduction of drag load on such piles, the present study finds that the drag load on piles may not be reduced for consolidating soft clay with two ways drainage or one way bottom drainage conditions.

Modeling the rheological properties with shear stress limit and compressive strength of ordinary Portland cement modified with polymers

In this study, the effect of two water reducer polymers on the thermal stability, rheology, and compressive strength of Gasin Ordinary Cement (GC) was investigated. The behavior of cement in the liquid phase (slurry) and hardened phase modified with two types of polymer varied from 0 to 0.06% (%wt) were investigated. XRD and TGA tests were conducted to identify the effect of polymers on the behavior of GC. The compressive strength of cement mixed with polymers was tested from the early aged of 1 days to 28 days of curing. The shear stress versus shear strain rate of cement slurry was modeled and the results of prediction using Vipulanandan rheological model were compared to the Vocadlo model. High reduction in the total weight loss of the GC at 800 °C when the cement modified with 0.06% of polymers detected using TGA test. The rheological properties of the cement slurry such as viscosity, yield stress, and the compressive strength of GC were increased significantly with increasing the polymer contents. Addition of polymers reduced the required water to reach the desired fluidity by about 43% based on the type of polymer. The results showed that the Vipulanandan rheological model predicted the shear-thinning relationship between the shear stress and shear strain rate of the cement slurry modified polymers very well. Also, the Vipulanandan rheological model has a maximum shear stress limit were as the Vocadlo model did not have a limit on the maximum shear stress. Based on the Vipulanandan rheological model the maximum shear stress produced by the cement slurry modified with 0% and 0.06% of polymers at the temperature of 25 °C were increased from 38.7 to 171.3 Pa and to 184.8 Pa respectively due to the addition of polymers. Nonlinear models were used to separate the effect polymer quantity, water–cement ratio, and curing time on the rheological properties and compressive strength of cement.

Investigating the thermal effect on the flow characteristics of acrylamide-based polymer modified bentonite drilling mud using upper shear stress limit

ABSTRACT In this work, the effects of hot-aging on the rheological characteristics of acrylamide-based tetrapolymer modified bentonite drilling mud were examined. The dosage of the acrylamide-co-sodium 2-acrylamido-2-methylpropanesulfonate-co-N-vinyl pyrrolidone-co-N-butyl propenoate (ASAPB) in the drilling muds was varied from 0 to 1 wt.% under geothermal condition ranging from 25°C to 150°C. The rheological behaviour of ASAPB modified bentonite drilling mud were quantified using Vipulanandan model and compared with Herschel-Bulkley model. The Herschel-Bulkley model predicted satisfactorily the shear thinning relationship between the shear stress and shear strain rate. However, Vipulanandan has a maximum shear stress limit whereas the other did not.

Failure stress modification in fatigue life models for rolling bearings

The critical subsurface shear stress related to rolling contact fatigue is modified to model the effects of residual stress common in case hardened materials, such as M50-NiL. The role of hoop stress, generated due to race rotation and shrink fits, is also modeled. It is shown that even relatively low levels of compressive residual stress could contribute to notable increase in bearing life. An equivalent life modification factor is dependent on both residual stress and applied load. Model predictions are in agreement with available experimental life data obtained with a 40-mm angular contact ball bearing with M50-NiL races and silicon nitride balls. The stress modification approach is also applied to model the role of any fatigue limiting shear stress, such that the solutions converge to validated Lundberg–Palmgren solutions as limiting stress reduces to zero. However, bearing life predictions at light loads, under any reasonable limiting stress, are unreasonably high. As an alternate approach, the empirical constant in the limiting stress model, with a prescribed limiting stress, is determined by least-squared regression between model predictions and available experimental life data. With such an approach, the least-squared deviation between model predictions and experimental data shows a monotonic increase as a function of the limiting stress with a minimum at no limiting stress. This observation suggests that simple failure stress modification in the current subsurface stress-based models may not be suitable to implement any fatigue limiting stress for rolling contacts.

Boundary slippage modeling and optimization of hydrophobic tilting pad thrust bearing with elastic deformation

Abstract Boundary slippage modeling and optimization of the hydrophobic tilting pad thrust bearing with elastic deformation are conducted to obtain the optimal lubricating film geometry and power loss. The modified Reynolds equation combining with a nonlinear slip control equation of the limiting shear stress model is transformed to a series of linear complementary problems to be solved by the parametric quadratic programming technique. An optimization approach based on the Kriging surrogate model is developed to optimize the slip/no-slip configuration of the thrust pad. The effect of homogeneous slipping and heterogeneous slip/no-slipping coupled with elastic deformation on minimum film thickness and power loss is studied for the tilting pad thrust bearing with the flexible lining materials.

Effect of surface roughness on film formation properties of screw tightening lubricant PIB

Abstract This paper describes the film formation properties of polyisobutylene (PIB) lubricant under various surface roughness conditions. PIB lubricant has been proposed as a new type of screw tightening lubricant, however the lubrication mechanism in the screw tightening process is yet to be understood. In this study, the in-situ observation tests were conducted under the starved lubrication condition, in which a thin film lubrication regime was simulated. The results showed that film thickness was greater than the surface roughness. Under the high roughness conditions, the time at which direct contact was prevented was shorter. In addition, the occurrence of wear was related to the lambda ratio Λ, and under the high roughness conditions, the friction coefficient increased with time owing to expansion of the wear area. It was also shown that under the fully starved condition, the shear stress did not depend on the shear rate, moreover, its value was constant under all roughness conditions. These results suggest that (i) solid-like PIB film was formed on the contact area under the starved condition and (ii) the formation of a film with a high limiting shear stress potentially inhibits wear. These results were consistent with the results of a tightening test.

Finite element models of friction behaviour in linear friction welding of a Ni-based superalloy

The accurate prediction of the thermo-mechanical fields such as temperature, contact stress, and strain fields is strongly related to the thermo-mechanical properties, material constitutive model, and particularly friction behaviour in simulations of friction welding. In this work, the influence of friction modelling on the interface temperature, burn-off rate, and other process variables in linear friction welding (LFW) is investigated using finite element (FE) simulations in ABAQUS. Sliding model and sliding-sticking model are respectively used to characterise the friction condition. Results show that a greater friction heat flux is predicted by the sliding model with an invariable friction coefficient and the sliding–sticking model with a fixed limited shear stress, resulting in an overestimation of the temperature and burn-off rate compared to the experimental results. Additionally, the average friction heat flux decreases with a decrease in friction coefficient and limited shear stress. Under the investigated welding parameters, the sliding model with the variable-dependent friction coefficient and sliding–sticking model limited by material shear flow stress provide more accuracy in simulations of LFW. The accuracy of friction models in predicting thermo-mechanical condition at the interface is further validated by comparing the plastic deformation zone morphology. The distribution of temperature and strain fields at the interface can be simulated more reasonably by using the sliding–sticking model limited by material shear flow stress, which suitably represents the friction of the sliding–sticking behaviour during LFW.

Mathematical Modeling of Swirling Herschel–Bulkley Pseudoplastic Fluid Flow in a Cylindrical Channel

Results of investigations into the swirling flow of a pseudoplastic fluid with a Herschel–Bulkley yield stress in a cylindrical channel have been presented. It has been established that a growth in the rates of shear strains in flows with a swirl causes the values of effective viscosity to decrease. It has been shown that at one and the same Rossby number, the recirculation strength is the greater the smaller the values of the limiting shear stress, the consistency, and the nonlinearity index.

Shear tests of interfaces between calcareous sand and steel

Various modified direct shear tests on the interface between calcareous sand and steel with different degrees of roughness were conducted in this article, and the different interface property results between calcareous sand-steel and siliceous sand-steel were compared. It was found that: (1) Under various normal stresses, the limit shear stress at the interface reaches the peak value when the groove depth is at the critical value Rcr. Further, when the flat width is at a critical value Gcr, the limit interface shear stress reaches to that of a grooved surface with a flat width of zero. (2) The magnitudes of Rcr and Gcr are related to the gradation and grain size of sand. For the calcareous sand tested, Rcr and Gcr are three times and seven times the mean grain size, respectively. (3) When the intersection angle between the symmetrical V-shaped grooves and the direct shear direction equals to 45°, the limit interface shear stress reaches the peak value and no significant increase is observed at the larger intersection angle. (4) Under the same interface testing conditions with siliceous and calcareous sands, the interface friction angle of the calcareous sand is ∼5°–6° greater than that of siliceous sand.

Layered oil slip model for investigation of film thickness behaviours at high speed conditions

Abstract A layered oil slip model considering the slip and thermal effect is proposed to explore the mechanism of film thickness behaviours at high speed. The modified Reynolds equation considering the interfacial slip is derived and the limiting shear stress of lubricant is utilised as the criteria for the occurrence of slip. The film thickness obtained from the proposed model matches the experimental results very well. The results indicate that both the interfacial slip and thermal effect contribute to the behaviours of film thickness at high speed. The interfacial slip may occur at both fast or/and slow surfaces depending on the speed conditions.