Contact Stiffness Of Rough Surfaces Research Articles

Analysis, Modeling and Experimental Study of the Normal Contact Stiffness of Rough Surfaces in Grinding

Grinding is the most important method in machining, which belongs to the category of precision machining processes. Many mechanical bonding surfaces are grinding surfaces. Therefore, the contact mechanism of grinding a joint surface is of great significance for predicting the loading process and dynamic characteristics of precision mechanical products. In this paper, based on the collected grinding surface roughness data, the profile parameters and topography characteristics of the asperity were analyzed, the rough surface data were fitted, the asperity profile was reconstructed, and the parabola y = nx2 + mx + l of the cylindrical asperity model was established. After analyzing the rough surface data of the grinding process, the asperity distribution height was fitted with a Gaussian distribution function, which proved that asperity follows the Gaussian distribution law. The validity of this model was confirmed by the non-dimensional processing of the assumed model and the fitting of six plasticity indices. When the pressure is the same, the normal stiffness increases with the decrease in the roughness value of the joint surface. The experimental stiffness values are basically consistent with the fitting stiffness values of the newly established model, which verifies the reliability and effectiveness of the new model established for the grinding surface. In this paper, a new model for grinding joint surface is established, and an experimental platform is set up to verify the validity of the model.

Simulation and experiment of secondary contact stiffness of rough surface

Simulation and experiment of secondary contact stiffness of rough surface

A Novel Micro-Contact Stiffness Model for the Grinding Surfaces of Steel Materials Based on Cosine Curve-Shaped Asperities.

Contact stiffness is an important parameter for describing the contact behavior of rough surfaces. In this study, to more accurately describe the contact stiffness between grinding surfaces of steel materials, a novel microcontact stiffness model is proposed. In this model, the novel cosine curve-shaped asperity and the conventional Gauss distribution are used to develop a simulated rough surface. Based on this simulated rough surface, the analytical expression of the microcontact stiffness model is obtained using contact mechanics theory and statistical theory. Finally, an experimental study of the contact stiffness of rough surfaces was conducted on different steel materials of various levels of roughness. The comparison results reveal that the prediction results of the present model show the same trend as that of the experimental results; the contact stiffness increases with increasing contact pressure. Under the same contact pressure, the present model is closer to the experimental results than the already existing elastic–plastic contact (CEB) and finite-element microcontact stiffness (KE) models, whose hypothesis of a single asperity is hemispherical. In addition, under the same contact pressure, the contact stiffness of the same steel material decreases with increasing roughness, whereas the contact stiffness values of different steel materials under the same roughness show only small differences. The correctness and accuracy of the present model can be demonstrated by analyzing the measured asperity geometry of steel materials and experimental results.

Analysis of Normal Elastic Contact Stiffness of Rough Surfaces Based on Ubiquitiform Theory

Abstract In this study, the normal stiffness of elastic contact between rough surfaces with asperities following Gaussian distribution is investigated using ubiquitiform theory, developed from fractal theory. In the generalized ubiquitiformal Sierpinski carpet model, the rough surface including contact asperity is controlled for, given the lower bound to scale invariance of rough surfaces. Considering the stiffness of a single asperity deduced from the Hertz contact model, we deduce the theoretical relation between the normal stiffness and the elastic contact of rough surfaces based on ubiquitiform theory. The results show that the normal contact stiffness of a rough surface increases as the normal load rises. If the ubiquitiformal complexity of a rough surface increases or the lower bound to scale invariance of a rough surface decreases, the normal contact stiffness of the rough surface should increase. The larger the ubiquitiformal complexity of a rough surface is, the more obvious the impact of the lower bound to scale invariance on the normal contact stiffness of the rough surface becomes. The results based on the ubiquitiformal model and the experimental results are in closer agreement. Therefore, the introduction of scale invariance is crucial to the surface contact problem.

Investigation into the contact characteristics of rough surfaces with surface tension

Surface tension is a kind of force that exists between the molecules of the material surface, which has a significant influence on the contact characteristics of rough surfaces. Therefore, the Nayak’s random process model is introduced to describe the height and curvature distribution of asperity on isotropic rough surfaces, and a single conical asperity system considering surface tension is established. Then, the single conical asperity system is extended to the entire rough surface, and a new contact model of the rough surface is presented based on statistical methods. The contact stiffness from the proposed model is added to the finite element model for comparison with experimental results. The effects of surface tension and semi-cone angle of conical asperity on the contact load, the contact area, and the contact stiffness of rough surfaces are revealed. The results show that the new model has larger normal load and contact stiffness, and a smaller real contact area than the classical elastic model where the dimensionless separation is constant. In addition, the contact load and contact stiffness between rough surfaces increase with increasing surface tension, but the real contact area decreases. When the surface tension is constant, the larger the semi-cone angles of the conical asperity, the greater the load, the real contact area, and the stiffness will be.

Normal contact stiffness of rough surfaces considering oblique asperity contact

The contact stiffness of the machined surface has an important effect on the performance of the complex mechanical product. A modified fractal model based on oblique asperity contact is proposed in this research. First, the contact radius and the critical contact area are analyzed based on oblique contact condition. The normal contact stiffness and elastic–plastic force are calculated. The ratio of the actual contact area and a new parameter related to the current contact angle are introduced. Second, numerical simulations indicate the difference. The results show that the stiffness of the oblique contact is smaller, and with the increment of the fractal dimension, the extent of the stiffness reduction is larger. In contrast, the uniform distribution has the lower proportion of the elastic force in the total normal contact force. Finally, experiments including specimen surface observation and load-deformation measurement are utilized to obtain contact stiffness of the machined surface. To some extent, the modified fractal stiffness model is more reasonable and accurate from the result.

Investigation into the Normal Contact Stiffness of Rough Surface in Line Contact Mixed Elastohydrodynamic Lubrication

ABSTRACT In this work, the statistical asperity microcontact models in combination with the acoustic spring model and the load sharing concept are utilized to study the interfacial normal contact stiffness for a rough surface in line contact elastohydrodynamic lubrication (EHL). Two different statistical microcontact models of Greenwood and Williamson (GW) and Kogut and Etsion (KE) are employed to derive the normal contact stiffness expressions for a dry rough line contact considering the purely elastic contact and the multiple regimes elastic–elastoplastic–fully plastic contact, respectively. The liquid film stiffness is calculated based on the relationship between film thickness and bulk modulus of the lubricant. The lubricant film thickness equations are employed in conjunction with the load sharing concept and the empirical formulas for the maximum contact pressure in a dry rough contact are fitted for the GW model and the KE model, to evaluate the relationship between film thickness and motion velocity for the purely elastic GW microcontact model and the multiregime KE microcontact model, respectively. The comparison with experimental results shows that the KE model predicts closer total contact stiffness results than the GW model. The stiffness contributions from the solid asperity contact and lubricant film are obtained and effects of surface roughness, applied load, motion velocity, and type of lubricant on the normal contact stiffness are analyzed.

Numerical Analysis of Normal Contact Stiffness of Rough Surfaces

A numerical model was proposed to investigate the contact behaviour of a solid with a rough surface squeezed against a rigid flat plane. We considered simulated hierarchical surface structures as well as scanned surface data obtained by the profilometry of isotropically roughened specimens. The simulated and treated surfaces were characterised using statistical and fractal parameters. The evolution of contact stiffness under increasing normal compression was analysed through the total truncated area at varying heights, in order to relate contact mechanics to different surface parameters employed for surface characterisation. For a relatively small surface interference, the predicted stress-dependent normal contact stiffness of both scanned and simulated surfaces is in good agreement with experimental observation from nanoindentation tests, revealing a power-law function of the normal load, with the exponent of this relationship closely depending on the fractal dimension of rough surfaces. The numerical results show that the amplitude of a fractal rough surface mainly contributes to the magnitude of the contact stiffness at a given normal load.

Fractal contact spot and its application in the contact model of isotropic surfaces

All engineering surfaces are rough and composed of asperities with different scales. In conventional fractal contact models, asperities are used to build contact models between two rough surfaces. However, it is found that the deformation of asperities in the contact process is rather complex because only a small part of the asperities deform elastically or plastically. Moreover, the asperities used in conventional fractal contact models are not constructed exactly from contact mechanism and, of course, they are not fractal or self-affine. So, these fractal contact models seem like statistical models using fractal parameters rather than pure-fractal contact models. In the present paper, instead of asperities, fractal contact spots are utilized to describe the contact process of isotropic rough surfaces. The differences between fractal contact spots and asperities are analyzed to prove that fractal contact spots are more suitable to reveal the nature of contact process. Numerical method is used to simulate the contact process and study the characteristics of fractal contact spots, and the load-separation relationship of fractal contact spots is obtained using a fractal method. Then, a pure-fractal contact model is proposed and extended to calculate the contact stiffness of rough surfaces. To verify the contact model proposed, an experiment of stiffness identification of isotropic metal surfaces is carried out. The identified stiffness is in good agreement with that calculated by the contact model proposed, which indicates that the pure-fractal contact model applying the fractal contact spots is more suitable to reveal the contact mechanism of isotropic surfaces.

The Role of Surface Structure in Normal Contact Stiffness

The effects of roughness and fractality on the normal contact stiffness of rough surfaces were investigated by considering samples of isotropically roughened aluminium. Surface features of samples were altered by polishing and by five surface mechanical treatments using different sized particles. Surface topology was characterised by interferometry-based profilometry and electron microscopy. Subsequently, the normal contact stiffness was evaluated through flat-tipped diamond nanoindentation tests employing the partial unloading method to isolate elastic deformation. Three indenter tips of various sizes were utilised in order to gain results across a wide range of stress levels. We focus on establishing relationships between interfacial stiffness and roughness descriptors, combined with the effects of the fractal dimension of surfaces over various length scales. The experimental results show that the observed contact stiffness is a power-law function of the normal force with the exponent of this relationship closely correlated to surfaces’ values of fractal dimension, yielding corresponding correlation coefficients above 90 %. A relatively weak correlation coefficient of 60 % was found between the exponent and surfaces’ RMS roughness values. The RMS roughness mainly contributes to the magnitude of the contact stiffness, when surfaces have similar fractal structures at a given loading, with a correlation coefficient of −95 %. These findings from this work can be served as the experimental basis for modelling contact stiffness on various rough surfaces.

Research on normal contact stiffness of rough surface considering friction based on fractal theory

The friction which plays an essential role on normal contact stiffness is usually omitted in existed researches, thus a modified fractal model for normal contact stiffness considering friction is discussed in this paper for calculating normal contact stiffness precisely and reasonably. The model is obtained by introducing a specific coefficient factor into the equation of the normal contact stiffness which is deduced by employing the theory of fractal contact and the definition of stiffness. Through the simulation and comparison with the original model and experiment result, it is clearly concluded that the modified model can reveal the influence of friction on contact area as well as calculate the normal contact stiffness more accurately. Furthermore, the numerical results show that the normal contact stiffness increases with the normal load; it is advantageous to improve the normal contact stiffness of rough surface by adding fractal dimension-D and reducing fractal scaling parameter-G. Also, the friction coefficient decreases with adding D and G and the fractal dimension has a marked impact on the friction coefficient. The presented achievements will help to analyze the performance for surface contact dynamics in the future.

Ultrasonic Measurements of Contact Stiffness Between Rough Surfaces

We have used an ultrasonic method to determine the normal and shear stiffness for three different surfaces. The degree of hysteresis for the loading/unloading and stiffness ratio is a function of roughness. Nonlinear contact stiffness characteristics are obtained. The ratio of tangential to normal stiffness KT/KN slowly increases in proportion to normal loading. The novelty of our setup is that at the same time we can measure the reflection coefficient, obtain results for three transducers simultaneously, and measure the approach as a function of load. The presented experimental results of normal contact stiffness measurements have been used for the verification of our theoretical model based on a fractal description of rough surfaces (Buczkowski et al., “Fractal Normal Contact Stiffness of Rough Surfaces,” Arch. Mech. (submitted for publication).

Investigations of the influence of geometrical irregularities and roughness on the normal contact stiffness of joints

AbstractConducting experiments is necessary to determine the normal contact stiffness of rough surfaces. In compression tests the interface of specimen pairs is loaded in normal direction and the deflection is measured by extensometers with strain gages. The results can be used for example to describe the contact behaviour of joints in finite element analyses. It is shown that geometrical irregularities which are superposed by the surface roughness have a large influence on the elastic deformation of a joint and on the determination of the contact stiffness. They can be characterized by a skewed and a waved form of the surface with wavelengths which are larger than the surface roughness. In the experimental setup several different surfaces are compressed. In addition, the influence of the size of the contact area on the deformation of a joint is analysed. The results of the normal contact tests are compared with numerical simulations. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)