This study investigates the effects of loading rates on the stick–slip behavior of polymethyl methacrylate material.A series of friction experiments were conducted using a direct shear apparatus to systematically ass...This study investigates the effects of loading rates on the stick–slip behavior of polymethyl methacrylate material.A series of friction experiments were conducted using a direct shear apparatus to systematically assess how loading rates influences stick–slip behavior.Three loading rates were adopted:0.1,1,and 5 mm/min,all under a constant normal stress of 2.5 MPa.The experimental results indicate that loading rates significantly influence the mechanical behavior of stick–slip.The recurrence intervals,shear force drops,and fracture energy decrease as loading rates increase.By monitoring changes in the interface contact area using the total internal reflection method,we observed that the reduction in interface contact area diminishes with increasing loading rates.At lower loading rates,micro-asperities have sufficient time to reform,resulting in stronger interaction forces and fracture dissipated energy;conversely,at higher loading rates,limited recovery of contact area results in reduced fracture dissipated energy.These findings highlight the close relationship between loading rates and interface contact behavior,providing new experimental data and insights for analyzing and understanding fault slip and rupture processes.展开更多
Many experiments have supported the contact models,such as the GW and MB models,but the majority of previous validations have been performed under light loads,resulting in a linear relationship between normal force an...Many experiments have supported the contact models,such as the GW and MB models,but the majority of previous validations have been performed under light loads,resulting in a linear relationship between normal force and contact area.However,the real contact area fraction should never equal one;there must be a limit smaller than the apparent area,implying that the real contact area cannot increase linearly indefinitely.In this paper,the real contact area between two polymethylmethacrylate(PMMA)blocks under heavy load is measured using the total reflection method,and the contact area is analyzed using the image processing method.The results show that the real contact area increases with normal load linearly in light loads but non-linearly in heavy loads;the number of contact spots increases with load linearly in light loads but also non-linearly in heavy loads,synchronous with the change in the real contact area.The GW,MB,and Zhao,Maietta,and Chang(ZMC)models were used to predict the experiment results,but none of them predicted the non-linear stage.A revised GW model based on the bulk deformation hypothesis performs better in predicting the non-linear stage.The study’s findings can be applied to PMMA or other similar materials,and they can serve as a useful reference for future research on the contact mechanisms of other materials.展开更多
In spline rolling process, the contact area between roller and workpiece plays an important role in calculating rolling-force and rolling-moment. For the purpose of studying the contact area, contact state between rol...In spline rolling process, the contact area between roller and workpiece plays an important role in calculating rolling-force and rolling-moment. For the purpose of studying the contact area, contact state between roller and workpiece in process of spline cold rolling based upon cross rolling is analyzed. According to the suitable hypothesis, the mathematic model of roller-tooth-curve in optional position of rolling process is established. Combing the theory of conjugate curves with the theory of envelope curve, the corresponding mathematic model of workpiece-tooth-curve is established. By utilizing establishing mathematic models, the algorithm of entire contact area in rolling process is created. On the basis of the algorithm, calculation-program is compiled under MATLAB program language environment. The calculation-program actualizes quantitative analysis and quantitative calculation of contact areas. Utilizing the calculation-program, the influence of parameters on contact area is analyzed, and the tendency is consistent with the manufacturing experience. In consideration of rolling-force optimization, the primary process parameters may be selected according to results of calculation. The result of the present study may provide basis for research on rolling-force and rolling-moment.展开更多
BACKGROUND In rotator cuff repair surgery,the double-row technique is widely performed.Studies have shown that with increased contact area and pressure between tendon and bone interface,better healing is promoted.AIM ...BACKGROUND In rotator cuff repair surgery,the double-row technique is widely performed.Studies have shown that with increased contact area and pressure between tendon and bone interface,better healing is promoted.AIM To assess the different suture configurations with the double-row technique and how this influences the contact area of the rotator cuff tendon to bone.METHODS This was a controlled laboratory study where identical tears were created in 24 fresh porcine shoulders over a 1.5 cm×2.5 cm infraspinatus insertion footprint.Double-row repair techniques,with 3 to 4-suture anchors in different configurations(2 medial,2 lateral vs 2 medial,1 lateral vs 1 medial,2 lateral),were employed for three control groups.Each group consisted of eight shoulders with identical repair configurations.Footprint contact areas of the repaired tendon against the tuberosity were determined using pressure sensitive Fujifilm placed between the tendon and tuberosity.RESULTS The mean contact area between tendon and insertion footprint from the imprinted Fujifilm was obtained using computer software.The contact area measured from a standard 4-suture anchor double row repair was 75.1±9.3 mm2,whereas areas obtained for the 2 lateral-1 medial and 2 medial-1 lateral anchor configurations were 72.9±5.2 mm2 and 75.0±4.9 mm2 respectively.No statistical significance was noted between the three groups.CONCLUSION In the technique of double-row repair,using a 3-suture anchor configuration may offer a non-inferior alternative to the standard 4-anchor construct in terms of efficacy.This may also result in overall cost reduction and shorter surgical time.展开更多
This study experimentally analyzes the nonlinear flow characteristics and channelization of fluid through rough-walled fractures during the shear process using a shear-flow-visualization apparatus.A series of fluid fl...This study experimentally analyzes the nonlinear flow characteristics and channelization of fluid through rough-walled fractures during the shear process using a shear-flow-visualization apparatus.A series of fluid flow and visualization tests is performed on four transparent fracture specimens with various shear displacements of 1 mm,3 mm,5 mm,7 mm and 10 mm under a normal stress of 0.5 MPa.Four granite fractures with different roughnesses are selected and quantified using variogram fractal dimensions.The obtained results show that the critical Reynolds number tends to increase with increasing shear displacement but decrease with increasing roughness of fracture surface.The flow paths are more tortuous at the beginning of shear because of the wide distribution of small contact spots.As the shear displacement continues to increase,preferential flow paths are more distinctly observed due to the decrease in the number of contact spots caused by shear dilation;yet the area of single contacts in-creases.Based on the experimental results,an empirical mathematical equation is proposed to quantify the critical Reynolds number using the contact area ratio and fractal dimension.展开更多
An experimental method of measuring the real contact area of transparent blocks based on the principle of total internal reflection is presented, intending to support the investigation of friction characteristics, hea...An experimental method of measuring the real contact area of transparent blocks based on the principle of total internal reflection is presented, intending to support the investigation of friction characteristics, heat conduction, and energy dissipation at the contact interface. A laser sheet illuminates the contact interface, and the transmitted laser sheet is projected onto a screen. Then the contact information is acquired from the screen by a camera. An improved Otsu method is proposed to process the data of experimental images. It can compute the threshold of the overall image and filter out all the pixels one by one. Through analyzing the experimental results, we describe the relationship between the real contact area and the positive pressure during a continuous loading process, at different loading rates, with the polymethyl methacrylate（PMMA）material. A hysteresis phenomenon in the relationship between the real contact area and the positive pressure is found and explained.展开更多
The pre-sliding regime is typically neglected in the dynamic modelling of mechanical systems. However, the change in contact state caused by static friction may decrease positional accuracy and control precision. To i...The pre-sliding regime is typically neglected in the dynamic modelling of mechanical systems. However, the change in contact state caused by static friction may decrease positional accuracy and control precision. To investigate the relationship between contact status and contact force in pre-sliding friction, an optical experimental method is presented in this paper.With this method, the real contact state at the interface of a transparent material can be observed based on the total reflection principle of light by using an image processing technique. A novel setup, which includes a pair of rectangular trapezoidal blocks, is proposed to solve the challenging issue of accurately applying different tangential and normal forces to the contact interface. The improved Otsu＇s method is used for measurement. Through an experimental study performed on polymethyl methacrylate（PMMA）, the quantity of contact asperities is proven to be the dominant factor that affects the real contact area. The relationship between the real contact area and the contact force in the pre-sliding regime is studied, and the distribution of static friction at the contact interface is qualitatively discussed. New phenomena in which the real contact area expands along with increasing static friction are identified. The aforementioned relationship is approximately linear at the contact interface under a constant normal pressure, and the distribution of friction stress decreases from the leading edge to the trailing edge.展开更多
New equations are proposed to describe the shapes and sizes of the contact areas between tire and ground(the contact areas)under the conditions of static load,slip,camber,and rolling.The equations are super ellipse an...New equations are proposed to describe the shapes and sizes of the contact areas between tire and ground(the contact areas)under the conditions of static load,slip,camber,and rolling.The equations are super ellipse and trigonometric function with four parameters.Results indicate that the contact areas under the conditions of static load,slip,camber can be well described by super ellipse,in the most complicated case,fourteen parameters are used.The rolling contact area can be reconstructed by the trigonometric function completely.Symmetrical and asymmetrical shapes can both be described perfectly.The suggested curves will make calculate size of the areas easily.The described shapes and sizes are in good agreement with the measured,the maximum deviation is only 2.88%.This work will provide an instrument for understanding the contact area and be helpful for analyzing tire and road.展开更多
Bacterial adhesion is a critical process in many fields,such as implant infections,microbiologically influenced corrosion and bioelectricity generation in microbial fuel cells.During bacterial adhesion,the contact are...Bacterial adhesion is a critical process in many fields,such as implant infections,microbiologically influenced corrosion and bioelectricity generation in microbial fuel cells.During bacterial adhesion,the contact area between the attached bacteria and the patterned surface plays an important role.In this study,different surface topographies and treatments were employed to simulate three circumstances with different contact areas.A nanostripe structure with a period of 576.9 nm and a height of 203.5 nm was fabricated on pure titanium by femtosecond laser ablation.Bacteria in liquid attached to the peaks of the nanostripe structure and were stretched on the two adjacent nanostripes.Compared with the polished surface,the contact area between bacteria and the nanostripe surface was reduced to 50%,resulting in a reduction(about 50%)in the coverage rate of attached bacteria.In addition,the nanostripe surface was a hydrophobic surface with a water contact angle(WCA)of 112.1°,and the surface potential of the nanostripe surface was higher than that of the polished surface.However,the surface potential and wettability of the nanostripe surface played a minor role in the bacterial adhesion due to the reduced contact area.Upon drying,the attached bacteria on the nanostripe surface sank into the valley region and the contact area was about 40%larger than that on the polished surface.The lateral strength of bacterial adhesion on nanostripe surfaces was higher than that on polished surfaces,due to the larger contact area.Upon applying a lateral force of 10.0 nN,the percentage of bacteria remaining on the nanostripe surface(31.1%)was higher than that on the polished surface(11.9%).Hence,the bacterial adhesion on the nanopatterned surface was mainly determined by the contact area.The in-depth exploration of the relation between bacterial adhesion on the nanopatterned surface and the contact area enables the rational surface designs of biomaterials to regulate bacterial adhesion.展开更多
The simulation of real contact area between materials is foundationally important for the contact mechanics of mechanical structures. The Greenwood and Williamson(GW) model and the Majumdar(MB) model are the basic mod...The simulation of real contact area between materials is foundationally important for the contact mechanics of mechanical structures. The Greenwood and Williamson(GW) model and the Majumdar(MB) model are the basic models in this field, which are widely accepted and proven to be valid in many experiments and engineering. Although the contact models have evolved considerably in recent years, the verifications of the models are most based on the indirect methods such as electrical conductivity and contact stiffness, because of the lack of effective methods to directly measure the variation of contact surface. In this paper, the total reflection(TR) method is introduced into the verification of contact models.An experiment system based on TR method is constructed to measure the real contact area of two PMMA specimens. The comparison analysis between the results of experiment and models suggests that the experiment result has the same trend with simulation, the MB model has better agreement with the experimental result because this method can take into account the variation of radius and the merging of asperities, while the GW model has a huge deviation because of the dependence on resolution and the lack of considering the variation of radius and asperity's merging process. Taking the interaction of asperities into account could give a better result that is closer to the experiment. Our results and analysis prove that the experimental methods in this paper could be used as a more direct and valid method to quantitatively measure the real contact area and to verify the contact models.展开更多
We build an experiment system based on total reflection(TR) method to observe the evolution of real contact area of polymethyl methacrylate(PMMA) in the continual stick-slip movement. The bilateral friction is adopted...We build an experiment system based on total reflection(TR) method to observe the evolution of real contact area of polymethyl methacrylate(PMMA) in the continual stick-slip movement. The bilateral friction is adopted to overcome the bending moment in the lateral friction movement. Besides some classical phenomena of stick-slip movement such as periodical slow increase of frictional force in sticking phase and a sudden drop when slipping, a special phenomenon that the contact area increases with the tangential force is observed, which was called junction growth by Tabor in 1959.Image processing methods are developed to observe the variation of the junction area. The results show that the center of the strongest contact region will keep sticking under the tangential force until the whole slipping, the strongest point undergoes three stages in one cycle, which are named as sticking stage, fretting stage, and cracking stage, respectively. The combined analysis reveals a physical process of stick-slip movement: the tangential force causes the increase of the real contact area, which reduces the pressure between the contact spots and finally leads to the slipping. Once slipping occurs,the real contact area drops to the original level resulting in the pressure increase to the original level, which makes the sticking happen again.展开更多
Friction is a complex phenomenon that depends on many parameters.Despite this,we still rely on and describe friction in the vast majority of cases with a single value,namely,the coefficient of friction(µ),as firs...Friction is a complex phenomenon that depends on many parameters.Despite this,we still rely on and describe friction in the vast majority of cases with a single value,namely,the coefficient of friction(µ),as first proposed by Amontons in 1699.Later,Coulomb introduced a two-parameter description by separating the adhesive and load-dependent terms.However,experimental evidence that determines under what conditions either of the two historical models is more appropriate has not been investigated in detail.In particular,to take full advantage and achieve better accuracy with the two-parameter equation,the real contact area must be well characterized to determine the constant adhesive component sufficiently and accurately.In this study,we performed sliding experiments and measured friction,but at the same time,we also measured the real contact area with sub-micron lateral resolution,which allowed us to design a two-parameter(Coulomb-type)friction equation.We compared these results with the historical friction models of Amontons and Coulomb to better understand the actual differences between them and how this corresponds to the linearity between the friction force and the normal force,which is a key assumption in the more common and simpler Amontons relation.A strong scaling effect of roughness was observed,as well as the related nonlinearity between the normal load,friction,and real contact area.Under high loads and roughnesses,the one-or two-parameter friction descriptive models differed from experiments by only a few percent,and the variation among them was small in the same range.However,for very low roughnesses and loads(close to the nanoscale region),the two-parameter Coulomb model was required for any relevant friction prediction due to the strong adhesive contribution,while the one-parameter description was not appropriate.展开更多
Simulation model optimization plays a crucial role in the accurate prediction of material removal function in bonnet polishing processes,but model complexity often poses challenges to the practical implementation and ...Simulation model optimization plays a crucial role in the accurate prediction of material removal function in bonnet polishing processes,but model complexity often poses challenges to the practical implementation and efficiency of these processes.This paper presents an innovative method for optimizing simulation model parameters,focusing on achieving consistent contact area and the accurate prediction of the material removal function while preventing increase in model complexity.First,controllable and uncontrollable factors in bonnet simulations are analyzed,and then a simplified contact model is developed and applied under constant force conditions.To characterize the bonnet's contact performance,a contact area response curve is introduced,which can be obtained through a series of single spot contact experiments.Furthermore,a rubber hyperelastic parameter optimization model based on a neural network is proposed to achieve optimal matching of the contact area between simulation and experiment.The average deviation of the contact area under different conditions was reduced from 22.78%before optimization to 3.43%after optimization,preliminarily proving the effectiveness of the proposed simulation optimization model.Additionally,orthogonal experiments are further conducted to validate the proposed approach.The comparison between the experimental and predicted material removal functions reveals a high consistency,validating the accuracy and effectiveness of the proposed optimization method based on consistent contact response.This research provides valuable insights into enhancing the reliability and effectiveness of bonnet polishing simulations with a simple and practical approach while mitigating the complexity of the model.展开更多
In this paper,an incremental contact model is developed for the elastic self-affine fractal rough surfaces under plane strain condition.The contact between a rough surface and a rigid plane is simplified by the accumu...In this paper,an incremental contact model is developed for the elastic self-affine fractal rough surfaces under plane strain condition.The contact between a rough surface and a rigid plane is simplified by the accumulation of identical line contacts with half-width given by the truncated area divided by the contact patch number at varying heights.Based on the contact stiffness of two-dimensional flat punch,the total stiffness of rough surface is estimated,and then the normal load is calculated by an incremental method.For various rough surfaces,the approximately linear load-area relationships predicted by the proposed model agree well with the results of finite element simulations.It is found that the real average contact pressure depends significantly on profile properties.展开更多
Molecular dynamics simulations of nanoindentation are performed on monocrystal copper. A new contact atoms method is presented for calculating the contact area. Compared with conventional methods, this method can prov...Molecular dynamics simulations of nanoindentation are performed on monocrystal copper. A new contact atoms method is presented for calculating the contact area. Compared with conventional methods, this method can provide the contact area more accurately not only for sink-in but also for pile-up situation. The effect of tip radius on indentation is investigated too. The results indicate that the measured hardness of the material will become higher as the tip radius increases.展开更多
A new expression for contact deformation is given, and the normal contact stiff- ness between single asperities is derived according to Hooke＇s law. A contact model between two ellipsoidal asperities is simulated by ...A new expression for contact deformation is given, and the normal contact stiff- ness between single asperities is derived according to Hooke＇s law. A contact model between two ellipsoidal asperities is simulated by the FE method, the result compared with the theoretical solution. It is found that the curves of the normal contact stiffness versus the included angle in the principal curvature direction show similar trends and evolve as a cosine feature. The effects of the parameters on normal contact stiffness are found to show that normal contact stiffness increases and reaches the upper limit gradually with an increase in these parameters.展开更多
Mechanism of frictional vibration and the difference of kinctic-static friction is an open question. Research shows that the change of real area of contact during transition process of friction results in the phenomen...Mechanism of frictional vibration and the difference of kinctic-static friction is an open question. Research shows that the change of real area of contact during transition process of friction results in the phenomena mentioned above. In substance, the F-Vr, descent and F-T ascent characters are caused by A-Vr, and A-T characters respectively. Based on ultrasonic experiment the mechanism of the phenomena is explained and the frictional formulas using the parameter-real area of contact are given in this article.展开更多
1.The problem The term'concussion'refers to a common form of traumatic brain injury,which typically occurs after a blow or injury to the head.It has been described as a'complex pathophysiological process a...1.The problem The term'concussion'refers to a common form of traumatic brain injury,which typically occurs after a blow or injury to the head.It has been described as a'complex pathophysiological process affecting the brain,induced by biomechanical factors',and shear forces induced by rotational acceleration are believed to be the primary mechanism of injury in concussion.~1The incidence of concussion in the UK has been shown to be展开更多
In non-conforming rolling contact, the contact stress is highly concentrated in the contact area. However, there are some limitations of the special contact model and stress model used for the theoretical study of the...In non-conforming rolling contact, the contact stress is highly concentrated in the contact area. However, there are some limitations of the special contact model and stress model used for the theoretical study of the phenomenon, and this has prevented in-depth analysis of the associated friction, wear, and failure. This paper is particularly aimed at investigating the area of rolling contact between a sphere and a cone, for which purpose the boundary is determined by the Hertz theory and the geometries of the non-conforming surfaces. The phenomenon of stick-slip contact is observed to occur in the contact area under the condition of no-full-slip（Q 〈 μ·P）. Using the two-dimensional rolling contact theory developed by CARTER, the relative positions of the stick and slip regions and the distribution of the tangential force over the contact area are analyzed. Furthermore, each stress component is calculated based on the Mc Ewen theory and the idea of narrow band. The stress equations for the three-dimensional rolling contact between the sphere and the cone are obtained by the principle of superposition, and are used to perform some numerical simulations. The results show that the stress components have a large gradient along the boundary between the stick and slip regions, and that the maximum stress is inversely proportional to the contact coefficient and proportional to the friction coefficient. A new method for investigating the stress during non-classical three-dimensional rolling contact is proposed as a theoretical foundation for the analysis of the associated friction, wear, and failure.展开更多
基金supported by the National Natural Science Foundation of China(12102044)The Beijing Institute of Technology Research Fund Program is also acknowledged.
文摘This study investigates the effects of loading rates on the stick–slip behavior of polymethyl methacrylate material.A series of friction experiments were conducted using a direct shear apparatus to systematically assess how loading rates influences stick–slip behavior.Three loading rates were adopted:0.1,1,and 5 mm/min,all under a constant normal stress of 2.5 MPa.The experimental results indicate that loading rates significantly influence the mechanical behavior of stick–slip.The recurrence intervals,shear force drops,and fracture energy decrease as loading rates increase.By monitoring changes in the interface contact area using the total internal reflection method,we observed that the reduction in interface contact area diminishes with increasing loading rates.At lower loading rates,micro-asperities have sufficient time to reform,resulting in stronger interaction forces and fracture dissipated energy;conversely,at higher loading rates,limited recovery of contact area results in reduced fracture dissipated energy.These findings highlight the close relationship between loading rates and interface contact behavior,providing new experimental data and insights for analyzing and understanding fault slip and rupture processes.
基金Supported by National Natural Science Foundation of China(Grant No.11872033)Beijing Municipal Natural Science Foundation(Grant No.3172017)。
文摘Many experiments have supported the contact models,such as the GW and MB models,but the majority of previous validations have been performed under light loads,resulting in a linear relationship between normal force and contact area.However,the real contact area fraction should never equal one;there must be a limit smaller than the apparent area,implying that the real contact area cannot increase linearly indefinitely.In this paper,the real contact area between two polymethylmethacrylate(PMMA)blocks under heavy load is measured using the total reflection method,and the contact area is analyzed using the image processing method.The results show that the real contact area increases with normal load linearly in light loads but non-linearly in heavy loads;the number of contact spots increases with load linearly in light loads but also non-linearly in heavy loads,synchronous with the change in the real contact area.The GW,MB,and Zhao,Maietta,and Chang(ZMC)models were used to predict the experiment results,but none of them predicted the non-linear stage.A revised GW model based on the bulk deformation hypothesis performs better in predicting the non-linear stage.The study’s findings can be applied to PMMA or other similar materials,and they can serve as a useful reference for future research on the contact mechanisms of other materials.
基金supported by National Natural Science Foundation of China (No.50675145)Shanxi Province Key Project of Science and Technology of China (No.2006031147)+1 种基金Shanxi Province Innovation Project for Graduate Students of China (No.20061027)the Shanxi Province Key Project for Studied-abroad Returnee of China.
文摘In spline rolling process, the contact area between roller and workpiece plays an important role in calculating rolling-force and rolling-moment. For the purpose of studying the contact area, contact state between roller and workpiece in process of spline cold rolling based upon cross rolling is analyzed. According to the suitable hypothesis, the mathematic model of roller-tooth-curve in optional position of rolling process is established. Combing the theory of conjugate curves with the theory of envelope curve, the corresponding mathematic model of workpiece-tooth-curve is established. By utilizing establishing mathematic models, the algorithm of entire contact area in rolling process is created. On the basis of the algorithm, calculation-program is compiled under MATLAB program language environment. The calculation-program actualizes quantitative analysis and quantitative calculation of contact areas. Utilizing the calculation-program, the influence of parameters on contact area is analyzed, and the tendency is consistent with the manufacturing experience. In consideration of rolling-force optimization, the primary process parameters may be selected according to results of calculation. The result of the present study may provide basis for research on rolling-force and rolling-moment.
文摘BACKGROUND In rotator cuff repair surgery,the double-row technique is widely performed.Studies have shown that with increased contact area and pressure between tendon and bone interface,better healing is promoted.AIM To assess the different suture configurations with the double-row technique and how this influences the contact area of the rotator cuff tendon to bone.METHODS This was a controlled laboratory study where identical tears were created in 24 fresh porcine shoulders over a 1.5 cm×2.5 cm infraspinatus insertion footprint.Double-row repair techniques,with 3 to 4-suture anchors in different configurations(2 medial,2 lateral vs 2 medial,1 lateral vs 1 medial,2 lateral),were employed for three control groups.Each group consisted of eight shoulders with identical repair configurations.Footprint contact areas of the repaired tendon against the tuberosity were determined using pressure sensitive Fujifilm placed between the tendon and tuberosity.RESULTS The mean contact area between tendon and insertion footprint from the imprinted Fujifilm was obtained using computer software.The contact area measured from a standard 4-suture anchor double row repair was 75.1±9.3 mm2,whereas areas obtained for the 2 lateral-1 medial and 2 medial-1 lateral anchor configurations were 72.9±5.2 mm2 and 75.0±4.9 mm2 respectively.No statistical significance was noted between the three groups.CONCLUSION In the technique of double-row repair,using a 3-suture anchor configuration may offer a non-inferior alternative to the standard 4-anchor construct in terms of efficacy.This may also result in overall cost reduction and shorter surgical time.
基金This study has been partially funded by National Key Research and Development Program of China(Grant No.2020YFA0711800)the National Natural Science Foundation of China(Grant No.51979272)the Natural Science Foundation of Shandong Province,China(Grant No.ZR2021QE069).
文摘This study experimentally analyzes the nonlinear flow characteristics and channelization of fluid through rough-walled fractures during the shear process using a shear-flow-visualization apparatus.A series of fluid flow and visualization tests is performed on four transparent fracture specimens with various shear displacements of 1 mm,3 mm,5 mm,7 mm and 10 mm under a normal stress of 0.5 MPa.Four granite fractures with different roughnesses are selected and quantified using variogram fractal dimensions.The obtained results show that the critical Reynolds number tends to increase with increasing shear displacement but decrease with increasing roughness of fracture surface.The flow paths are more tortuous at the beginning of shear because of the wide distribution of small contact spots.As the shear displacement continues to increase,preferential flow paths are more distinctly observed due to the decrease in the number of contact spots caused by shear dilation;yet the area of single contacts in-creases.Based on the experimental results,an empirical mathematical equation is proposed to quantify the critical Reynolds number using the contact area ratio and fractal dimension.
基金Project supported by the National Natural Science Foundation of China(Grant No.11272171)the Beijing Natural Science Foundation,China(Grant No.3132030)the Education Ministry Doctoral Fund of China(Grant No.20120002110070)
文摘An experimental method of measuring the real contact area of transparent blocks based on the principle of total internal reflection is presented, intending to support the investigation of friction characteristics, heat conduction, and energy dissipation at the contact interface. A laser sheet illuminates the contact interface, and the transmitted laser sheet is projected onto a screen. Then the contact information is acquired from the screen by a camera. An improved Otsu method is proposed to process the data of experimental images. It can compute the threshold of the overall image and filter out all the pixels one by one. Through analyzing the experimental results, we describe the relationship between the real contact area and the positive pressure during a continuous loading process, at different loading rates, with the polymethyl methacrylate（PMMA）material. A hysteresis phenomenon in the relationship between the real contact area and the positive pressure is found and explained.
基金Project supported by the National Natural Science Foundation of China(Grant No.11272171)the Natural Science Foundation of Beijing City,China(Contract No.3172017)the Education Ministry Doctoral Fund of China(Grant No.20120002110070)
文摘The pre-sliding regime is typically neglected in the dynamic modelling of mechanical systems. However, the change in contact state caused by static friction may decrease positional accuracy and control precision. To investigate the relationship between contact status and contact force in pre-sliding friction, an optical experimental method is presented in this paper.With this method, the real contact state at the interface of a transparent material can be observed based on the total reflection principle of light by using an image processing technique. A novel setup, which includes a pair of rectangular trapezoidal blocks, is proposed to solve the challenging issue of accurately applying different tangential and normal forces to the contact interface. The improved Otsu＇s method is used for measurement. Through an experimental study performed on polymethyl methacrylate（PMMA）, the quantity of contact asperities is proven to be the dominant factor that affects the real contact area. The relationship between the real contact area and the contact force in the pre-sliding regime is studied, and the distribution of static friction at the contact interface is qualitatively discussed. New phenomena in which the real contact area expands along with increasing static friction are identified. The aforementioned relationship is approximately linear at the contact interface under a constant normal pressure, and the distribution of friction stress decreases from the leading edge to the trailing edge.
基金National Natural Science Foundation of China Project(51790502),Open Grant of National Key Laboratory of Science and Technology on Advanced Composites in Special Environments(JCKYS2019603C016)。
文摘New equations are proposed to describe the shapes and sizes of the contact areas between tire and ground(the contact areas)under the conditions of static load,slip,camber,and rolling.The equations are super ellipse and trigonometric function with four parameters.Results indicate that the contact areas under the conditions of static load,slip,camber can be well described by super ellipse,in the most complicated case,fourteen parameters are used.The rolling contact area can be reconstructed by the trigonometric function completely.Symmetrical and asymmetrical shapes can both be described perfectly.The suggested curves will make calculate size of the areas easily.The described shapes and sizes are in good agreement with the measured,the maximum deviation is only 2.88%.This work will provide an instrument for understanding the contact area and be helpful for analyzing tire and road.
基金financial support from the National Nat-ural Science Foundation of China(Nos.52071028 and 52003028)the Beijing Nova Program(2022 Beijing Nova Program Cross Coop-eration Program No.20220484178).
文摘Bacterial adhesion is a critical process in many fields,such as implant infections,microbiologically influenced corrosion and bioelectricity generation in microbial fuel cells.During bacterial adhesion,the contact area between the attached bacteria and the patterned surface plays an important role.In this study,different surface topographies and treatments were employed to simulate three circumstances with different contact areas.A nanostripe structure with a period of 576.9 nm and a height of 203.5 nm was fabricated on pure titanium by femtosecond laser ablation.Bacteria in liquid attached to the peaks of the nanostripe structure and were stretched on the two adjacent nanostripes.Compared with the polished surface,the contact area between bacteria and the nanostripe surface was reduced to 50%,resulting in a reduction(about 50%)in the coverage rate of attached bacteria.In addition,the nanostripe surface was a hydrophobic surface with a water contact angle(WCA)of 112.1°,and the surface potential of the nanostripe surface was higher than that of the polished surface.However,the surface potential and wettability of the nanostripe surface played a minor role in the bacterial adhesion due to the reduced contact area.Upon drying,the attached bacteria on the nanostripe surface sank into the valley region and the contact area was about 40%larger than that on the polished surface.The lateral strength of bacterial adhesion on nanostripe surfaces was higher than that on polished surfaces,due to the larger contact area.Upon applying a lateral force of 10.0 nN,the percentage of bacteria remaining on the nanostripe surface(31.1%)was higher than that on the polished surface(11.9%).Hence,the bacterial adhesion on the nanopatterned surface was mainly determined by the contact area.The in-depth exploration of the relation between bacterial adhesion on the nanopatterned surface and the contact area enables the rational surface designs of biomaterials to regulate bacterial adhesion.
基金Project supported by the National Natural Science Foundation of China(Grant No.11872033)the Beijing Natural Science Foundation,China(Grant No.3172017)
文摘The simulation of real contact area between materials is foundationally important for the contact mechanics of mechanical structures. The Greenwood and Williamson(GW) model and the Majumdar(MB) model are the basic models in this field, which are widely accepted and proven to be valid in many experiments and engineering. Although the contact models have evolved considerably in recent years, the verifications of the models are most based on the indirect methods such as electrical conductivity and contact stiffness, because of the lack of effective methods to directly measure the variation of contact surface. In this paper, the total reflection(TR) method is introduced into the verification of contact models.An experiment system based on TR method is constructed to measure the real contact area of two PMMA specimens. The comparison analysis between the results of experiment and models suggests that the experiment result has the same trend with simulation, the MB model has better agreement with the experimental result because this method can take into account the variation of radius and the merging of asperities, while the GW model has a huge deviation because of the dependence on resolution and the lack of considering the variation of radius and asperity's merging process. Taking the interaction of asperities into account could give a better result that is closer to the experiment. Our results and analysis prove that the experimental methods in this paper could be used as a more direct and valid method to quantitatively measure the real contact area and to verify the contact models.
基金Project supported by the National Natural Science Foundation of China(Grant No.11872033)the Beijing Natural Science Foundation,China(Grant No.3172017)
文摘We build an experiment system based on total reflection(TR) method to observe the evolution of real contact area of polymethyl methacrylate(PMMA) in the continual stick-slip movement. The bilateral friction is adopted to overcome the bending moment in the lateral friction movement. Besides some classical phenomena of stick-slip movement such as periodical slow increase of frictional force in sticking phase and a sudden drop when slipping, a special phenomenon that the contact area increases with the tangential force is observed, which was called junction growth by Tabor in 1959.Image processing methods are developed to observe the variation of the junction area. The results show that the center of the strongest contact region will keep sticking under the tangential force until the whole slipping, the strongest point undergoes three stages in one cycle, which are named as sticking stage, fretting stage, and cracking stage, respectively. The combined analysis reveals a physical process of stick-slip movement: the tangential force causes the increase of the real contact area, which reduces the pressure between the contact spots and finally leads to the slipping. Once slipping occurs,the real contact area drops to the original level resulting in the pressure increase to the original level, which makes the sticking happen again.
文摘Friction is a complex phenomenon that depends on many parameters.Despite this,we still rely on and describe friction in the vast majority of cases with a single value,namely,the coefficient of friction(µ),as first proposed by Amontons in 1699.Later,Coulomb introduced a two-parameter description by separating the adhesive and load-dependent terms.However,experimental evidence that determines under what conditions either of the two historical models is more appropriate has not been investigated in detail.In particular,to take full advantage and achieve better accuracy with the two-parameter equation,the real contact area must be well characterized to determine the constant adhesive component sufficiently and accurately.In this study,we performed sliding experiments and measured friction,but at the same time,we also measured the real contact area with sub-micron lateral resolution,which allowed us to design a two-parameter(Coulomb-type)friction equation.We compared these results with the historical friction models of Amontons and Coulomb to better understand the actual differences between them and how this corresponds to the linearity between the friction force and the normal force,which is a key assumption in the more common and simpler Amontons relation.A strong scaling effect of roughness was observed,as well as the related nonlinearity between the normal load,friction,and real contact area.Under high loads and roughnesses,the one-or two-parameter friction descriptive models differed from experiments by only a few percent,and the variation among them was small in the same range.However,for very low roughnesses and loads(close to the nanoscale region),the two-parameter Coulomb model was required for any relevant friction prediction due to the strong adhesive contribution,while the one-parameter description was not appropriate.
基金supported by the National Natural Science Foundation of China(Grant Nos.52235004,51991371,and 52205495)the Natural Science Foundation of Sichuan Province,China(Grant Nos.2023NSFSC0026 and 2022NSFSC1927)supported by the Fundamental Research Funds for the Central Universities,China(Grant No.2682023GF025).
文摘Simulation model optimization plays a crucial role in the accurate prediction of material removal function in bonnet polishing processes,but model complexity often poses challenges to the practical implementation and efficiency of these processes.This paper presents an innovative method for optimizing simulation model parameters,focusing on achieving consistent contact area and the accurate prediction of the material removal function while preventing increase in model complexity.First,controllable and uncontrollable factors in bonnet simulations are analyzed,and then a simplified contact model is developed and applied under constant force conditions.To characterize the bonnet's contact performance,a contact area response curve is introduced,which can be obtained through a series of single spot contact experiments.Furthermore,a rubber hyperelastic parameter optimization model based on a neural network is proposed to achieve optimal matching of the contact area between simulation and experiment.The average deviation of the contact area under different conditions was reduced from 22.78%before optimization to 3.43%after optimization,preliminarily proving the effectiveness of the proposed simulation optimization model.Additionally,orthogonal experiments are further conducted to validate the proposed approach.The comparison between the experimental and predicted material removal functions reveals a high consistency,validating the accuracy and effectiveness of the proposed optimization method based on consistent contact response.This research provides valuable insights into enhancing the reliability and effectiveness of bonnet polishing simulations with a simple and practical approach while mitigating the complexity of the model.
基金supported by the National Natural Science Foundation of China(Grant Nos.12372100,12302126,and 12302141)the China Postdoctoral Science Foundation(Grant No.2023M732799)+1 种基金the Fundamental Research Funds for the Central Universities(Grant No.xzy012024020)Sihe Wang also thanks the support from the China Scholarship Council(CSC).
文摘In this paper,an incremental contact model is developed for the elastic self-affine fractal rough surfaces under plane strain condition.The contact between a rough surface and a rigid plane is simplified by the accumulation of identical line contacts with half-width given by the truncated area divided by the contact patch number at varying heights.Based on the contact stiffness of two-dimensional flat punch,the total stiffness of rough surface is estimated,and then the normal load is calculated by an incremental method.For various rough surfaces,the approximately linear load-area relationships predicted by the proposed model agree well with the results of finite element simulations.It is found that the real average contact pressure depends significantly on profile properties.
基金supported by the National Natural Science Foundation of China(Grant Nos.10047006,10172048,10232050)the Special Funds for Major State Basic Research Projects(G2000077305).
文摘Molecular dynamics simulations of nanoindentation are performed on monocrystal copper. A new contact atoms method is presented for calculating the contact area. Compared with conventional methods, this method can provide the contact area more accurately not only for sink-in but also for pile-up situation. The effect of tip radius on indentation is investigated too. The results indicate that the measured hardness of the material will become higher as the tip radius increases.
基金Project supported by the National Basic Research Program of China(973 Program)(No.2009CB724406)
文摘A new expression for contact deformation is given, and the normal contact stiff- ness between single asperities is derived according to Hooke＇s law. A contact model between two ellipsoidal asperities is simulated by the FE method, the result compared with the theoretical solution. It is found that the curves of the normal contact stiffness versus the included angle in the principal curvature direction show similar trends and evolve as a cosine feature. The effects of the parameters on normal contact stiffness are found to show that normal contact stiffness increases and reaches the upper limit gradually with an increase in these parameters.
文摘Mechanism of frictional vibration and the difference of kinctic-static friction is an open question. Research shows that the change of real area of contact during transition process of friction results in the phenomena mentioned above. In substance, the F-Vr, descent and F-T ascent characters are caused by A-Vr, and A-T characters respectively. Based on ultrasonic experiment the mechanism of the phenomena is explained and the frictional formulas using the parameter-real area of contact are given in this article.
文摘1.The problem The term'concussion'refers to a common form of traumatic brain injury,which typically occurs after a blow or injury to the head.It has been described as a'complex pathophysiological process affecting the brain,induced by biomechanical factors',and shear forces induced by rotational acceleration are believed to be the primary mechanism of injury in concussion.~1The incidence of concussion in the UK has been shown to be
基金Supported by National Natural Science Foundation of China(Grant No.51275140)
文摘In non-conforming rolling contact, the contact stress is highly concentrated in the contact area. However, there are some limitations of the special contact model and stress model used for the theoretical study of the phenomenon, and this has prevented in-depth analysis of the associated friction, wear, and failure. This paper is particularly aimed at investigating the area of rolling contact between a sphere and a cone, for which purpose the boundary is determined by the Hertz theory and the geometries of the non-conforming surfaces. The phenomenon of stick-slip contact is observed to occur in the contact area under the condition of no-full-slip（Q 〈 μ·P）. Using the two-dimensional rolling contact theory developed by CARTER, the relative positions of the stick and slip regions and the distribution of the tangential force over the contact area are analyzed. Furthermore, each stress component is calculated based on the Mc Ewen theory and the idea of narrow band. The stress equations for the three-dimensional rolling contact between the sphere and the cone are obtained by the principle of superposition, and are used to perform some numerical simulations. The results show that the stress components have a large gradient along the boundary between the stick and slip regions, and that the maximum stress is inversely proportional to the contact coefficient and proportional to the friction coefficient. A new method for investigating the stress during non-classical three-dimensional rolling contact is proposed as a theoretical foundation for the analysis of the associated friction, wear, and failure.
