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
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
The tribological characteristics of cotton fibers play an important role in engineering and materials science,and real contact behavior is a significant aspect in the friction behavior of cotton fibers.In this study,t...The tribological characteristics of cotton fibers play an important role in engineering and materials science,and real contact behavior is a significant aspect in the friction behavior of cotton fibers.In this study,the tribological characteristics of cotton fibers and their relationship with the real contact behavior are investigated through reciprocating linear tribotesting and real contact analysis.Results show that the friction coefficient decreases with a general increase in load or velocity,and the load and velocity exhibit a co‐influence on the friction coefficient.The dynamic change in the real contact area is recorded clearly during the experiments and corresponds to the fluctuations observed in the friction coefficient.Moreover,the friction coefficient is positively correlated with the real contact area based on a quantitative analysis of the evolution of friction behavior and the real contact area at different loads and velocities.This correlation is evident at low velocities and medium load.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
The effects of surface roughness, strain rate, friction coefficient and pressure on real contact area were analyzed based on the research of Stupkiewicz. The real contact area model taking account of the effect of fri...The effects of surface roughness, strain rate, friction coefficient and pressure on real contact area were analyzed based on the research of Stupkiewicz. The real contact area model taking account of the effect of friction and deformation of material was obtained. The model of contact conductance at the rolling interface was obtained by integrating the specific feature of heat transfer through the interface of continuous roll-casting. The results indicate that the real contact area increases obviously when the material is under yield, and the real contact area varies inversely with surface roughness, whereas it varies exponentially with friction coefficient, strain rate and pressure, and the power factor depends on strain rate.展开更多
The spatial and temporal evolution of real contact area of contact interface with loads is a challenge.It is generally believed that there is a positive linear correlation between real contact area and normal load.How...The spatial and temporal evolution of real contact area of contact interface with loads is a challenge.It is generally believed that there is a positive linear correlation between real contact area and normal load.However,with the development of measuring instruments and methods,some scholars have found that the growth rate of real contact area will slow down with the increase of normal load under certain conditions,such as large-scale interface contact with small roughness surface,which is called the nonlinear phenomenon of real contact area.At present,there is no unified conclusion on the explanation of this phenomenon.We set up an experimental apparatus based on the total reflection principle to verify this phenomenon and analyze its mechanism.An image processing method is proposed,which can be used to quantitative analysis micro contact behaviors on macro contact phenomenon.The weighted superposition method is used to identify micro contact spots,to calculate the real contact area,and the color superimposed image is used to identify micro contact behaviors.Based on this method,the spatiotemporal evolution mechanism of real contact area nonlinear phenomena is quantitatively analyzed.Furthermore,the influence of nonlinear phenomenon of real contact area on the whole loading and unloading process is analyzed experimentally.It is found that the effects of fluid between contact interface,normal load amplitude and initial contact state on contact behavior cannot be ignored in large-scale interface contact with small roughness surface.展开更多
Urinary incontinence is the most common health problem in aged people. Leaving incontinence events unmanaged will make a negative influence on the aged and the patient both mentally and physically. This paper presents...Urinary incontinence is the most common health problem in aged people. Leaving incontinence events unmanaged will make a negative influence on the aged and the patient both mentally and physically. This paper presents a design and implementation of a real time wireless monitor system for urinary incontinence, which has been applied in two nursing homes in Beijing and Shanghai. We collect real time moisture information by using non-contact humidity sensor designed and manufactured by us. When urinary incontinence, the sensor will send alert to mobile device via Bluetooth. If got the alert, the mobile device will send the alert to relatives of the aged or disabled people and cloud computing platform, through which we can make this information managed and displayed and the paramedic can be informed about the alarm. This paper gives a brief introduction to the framework of this system, the design of the non-contact sensor, the transmission of wireless data and the results of tests.展开更多
The real contact area(RCA)of randomly rough contacts has received a great deal of attention because it correlates strongly with friction,lubrication,sealing,and conductivity.Simulations have revealed that the RCA asso...The real contact area(RCA)of randomly rough contacts has received a great deal of attention because it correlates strongly with friction,lubrication,sealing,and conductivity.Simulations have revealed that the RCA associated with deterministic normal squeezing loads increases when tangential loads are also applied,in a phenomenon called junction growth.However,experimental investigations of the junction growth of randomly rough contacts are rare.Here,we used X-ray computed tomography(CT)to measure junction growth when two aluminum alloy surfaces were in contact.A high-resolution experimental setup was used to apply loads and observe contact behaviors at a resolution of 4μm.The RCA and average contact gaps were computed using a three-dimensional(3D)geometric model constructed from gray CT images using the Otsu thresholding method.The results showed that the RCA increased as the normal load increased.The RCA increased by 22.67%after a tangential load was applied(junction growth),and the average gap decreased by 14.01%after a tangential load was applied.Thus,X-ray CT accurately measured the junction growth as a novel quantitative method.展开更多
针对传统非接触式法向校正技术在弱刚性薄壁上的不足,在接触式压脚结构的基础上对传统法向校正方案进行了研究,提出一种仅适用于接触式压脚结构的两点校正算法,同时设计了一套基于激光跟踪仪的法向测量系统标定方法。针对弱刚性薄壁受...针对传统非接触式法向校正技术在弱刚性薄壁上的不足,在接触式压脚结构的基础上对传统法向校正方案进行了研究,提出一种仅适用于接触式压脚结构的两点校正算法,同时设计了一套基于激光跟踪仪的法向测量系统标定方法。针对弱刚性薄壁受到压脚单向压紧力产生回退对制孔位置精度的影响,提出一种工具中心点(Tool center point, TCP)变位补偿技术,该技术利用激光位移传感器监测壁板回退量,在法向校正前动态调整TCP位置,实现对壁板回退量的实时补偿。搭建试验平台并通过制孔试验验证了接触式法向校正技术与TCP变位补偿技术可有效保证孔的垂直度与孔位精度,实现孔垂直度误差小于0.25°,孔位偏差小于0.4 mm。展开更多
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.
基金The authors are grateful to the National Natural Science Foundation of China(Nos.51875152 and 51975174)for financial support of this study.
文摘The tribological characteristics of cotton fibers play an important role in engineering and materials science,and real contact behavior is a significant aspect in the friction behavior of cotton fibers.In this study,the tribological characteristics of cotton fibers and their relationship with the real contact behavior are investigated through reciprocating linear tribotesting and real contact analysis.Results show that the friction coefficient decreases with a general increase in load or velocity,and the load and velocity exhibit a co‐influence on the friction coefficient.The dynamic change in the real contact area is recorded clearly during the experiments and corresponds to the fluctuations observed in the friction coefficient.Moreover,the friction coefficient is positively correlated with the real contact area based on a quantitative analysis of the evolution of friction behavior and the real contact area at different loads and velocities.This correlation is evident at low velocities and medium load.
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.
基金Project(1999064906) supported by the National Key Fundamental Research and Development of China
文摘The effects of surface roughness, strain rate, friction coefficient and pressure on real contact area were analyzed based on the research of Stupkiewicz. The real contact area model taking account of the effect of friction and deformation of material was obtained. The model of contact conductance at the rolling interface was obtained by integrating the specific feature of heat transfer through the interface of continuous roll-casting. The results indicate that the real contact area increases obviously when the material is under yield, and the real contact area varies inversely with surface roughness, whereas it varies exponentially with friction coefficient, strain rate and pressure, and the power factor depends on strain rate.
基金the National Natural Science Foundation of China(Grant No.11872033)the Beijing Natural Science Foundation,China(Grant No.3172017).
文摘The spatial and temporal evolution of real contact area of contact interface with loads is a challenge.It is generally believed that there is a positive linear correlation between real contact area and normal load.However,with the development of measuring instruments and methods,some scholars have found that the growth rate of real contact area will slow down with the increase of normal load under certain conditions,such as large-scale interface contact with small roughness surface,which is called the nonlinear phenomenon of real contact area.At present,there is no unified conclusion on the explanation of this phenomenon.We set up an experimental apparatus based on the total reflection principle to verify this phenomenon and analyze its mechanism.An image processing method is proposed,which can be used to quantitative analysis micro contact behaviors on macro contact phenomenon.The weighted superposition method is used to identify micro contact spots,to calculate the real contact area,and the color superimposed image is used to identify micro contact behaviors.Based on this method,the spatiotemporal evolution mechanism of real contact area nonlinear phenomena is quantitatively analyzed.Furthermore,the influence of nonlinear phenomenon of real contact area on the whole loading and unloading process is analyzed experimentally.It is found that the effects of fluid between contact interface,normal load amplitude and initial contact state on contact behavior cannot be ignored in large-scale interface contact with small roughness surface.
文摘Urinary incontinence is the most common health problem in aged people. Leaving incontinence events unmanaged will make a negative influence on the aged and the patient both mentally and physically. This paper presents a design and implementation of a real time wireless monitor system for urinary incontinence, which has been applied in two nursing homes in Beijing and Shanghai. We collect real time moisture information by using non-contact humidity sensor designed and manufactured by us. When urinary incontinence, the sensor will send alert to mobile device via Bluetooth. If got the alert, the mobile device will send the alert to relatives of the aged or disabled people and cloud computing platform, through which we can make this information managed and displayed and the paramedic can be informed about the alarm. This paper gives a brief introduction to the framework of this system, the design of the non-contact sensor, the transmission of wireless data and the results of tests.
基金supported by the National Natural Science Foundation of China(Nos.U2141217 and 51935003)。
文摘The real contact area(RCA)of randomly rough contacts has received a great deal of attention because it correlates strongly with friction,lubrication,sealing,and conductivity.Simulations have revealed that the RCA associated with deterministic normal squeezing loads increases when tangential loads are also applied,in a phenomenon called junction growth.However,experimental investigations of the junction growth of randomly rough contacts are rare.Here,we used X-ray computed tomography(CT)to measure junction growth when two aluminum alloy surfaces were in contact.A high-resolution experimental setup was used to apply loads and observe contact behaviors at a resolution of 4μm.The RCA and average contact gaps were computed using a three-dimensional(3D)geometric model constructed from gray CT images using the Otsu thresholding method.The results showed that the RCA increased as the normal load increased.The RCA increased by 22.67%after a tangential load was applied(junction growth),and the average gap decreased by 14.01%after a tangential load was applied.Thus,X-ray CT accurately measured the junction growth as a novel quantitative method.
文摘针对传统非接触式法向校正技术在弱刚性薄壁上的不足,在接触式压脚结构的基础上对传统法向校正方案进行了研究,提出一种仅适用于接触式压脚结构的两点校正算法,同时设计了一套基于激光跟踪仪的法向测量系统标定方法。针对弱刚性薄壁受到压脚单向压紧力产生回退对制孔位置精度的影响,提出一种工具中心点(Tool center point, TCP)变位补偿技术,该技术利用激光位移传感器监测壁板回退量,在法向校正前动态调整TCP位置,实现对壁板回退量的实时补偿。搭建试验平台并通过制孔试验验证了接触式法向校正技术与TCP变位补偿技术可有效保证孔的垂直度与孔位精度,实现孔垂直度误差小于0.25°,孔位偏差小于0.4 mm。