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The effect of asperity flattening of a rough spherical contact during cyclic loading is investigated experimentally. Two types of surfaces are examined; the first is an “as-manufactured” isotropic surface and the second a smooth “laser-polished” surface. Both the surfaces exhibit a large amount of hysteresis of the load–displacement curve during the first load–unload cycles. This hysteresis is found to decrease as a function of the number of load cycles. A comparison of the experimental results with results obtained from a numerical model for a rough spherical contact shows good correlation. The model shows that for rough surfaces the total displacement is a function of the contacting asperities while for smooth surfaces the main contribution comes from the bulk displacement.

The Cattaneo--Mindlin concept of interfacial slip in tangentially loaded compliant bodies is revisited and its basic simplifying assumptions are critically examined. It is shown that these assumptions, which, in the absence of modern numerical techniques, were essential in 1949 to enable an elegant quantitative solution of the basic problem of presliding between contacting bodies, may be nonphysical. An alternative approach to the same problem that is based on treating sliding inception as a failure mode involving material plastic yield is discussed. This alternative approach was suggested even before 1949 but for the same lack of modern numerical techniques could only be promoted qualitatively. Some recent theoretical models, which are based on this earlier alternative approach, and in which the simplifying assumptions of the Cattaneo--Mindlin concept were completely relaxed, are described along with their experimental verification. It is shown that the presliding problem between contacting bodies can be accurately solved by these models using realistic physical assumptions and failure criterion. [DOI: 10.1115/1.4001238] Keywords: spherical contact, presliding, static friction, junction growth, contact condition

The failure due to accidental drop of magnetic recording disks made of brittle or ductile materials is of great interest in the design of small form factor hard disk drives. In this study, fracture of glass disks (brittle material) and plastic deformation of aluminum disks (ductile material) at very high shock levels caused by accidental drop are investigated using finite element analysis. It is found that failure inception for both disk types occurs at the inside perimeter of the disk. For glass disks, cracks are found to propagate toward the outer perimeter of the disk along distinct radial lines associated with the largest bending moment of the disk. The critical shock level at which failure originates increases with an increase in the clamp diameter, a reduction in the disk diameter, and an increase in the thickness of the disk. Some experimental results are presented to validate the numerical model. [DOI: 10.1115/1.4000238] Keywords: shock, ductile, brittle, hard disk drive, magnetic recording

Surface texturing has emerged in the last decade as a viable option of surface engineering resulting in significant improvement in load capacity, wear resistance, friction coefficient etc. of tribological mechanical components. Various techniques can be employed for surface texturing but Laser Surface Texturing (LST) is probably the most advanced so far. LST produces a vet), large number of micro-dimples on the surface and each of these microdimples can serve either as a micro-hydrodynamic bearing in cases of full or mixed lubrication, a micro-reservoir for lubricant in cases of starved lubrication conditions, or a micro-trap for wear debris in either lubricated or dry sliding. The present paper reviews the current effort being made world wide on surface texturing in general and on laser surface texturing in particular. It presents the state of the art in LST and the potential of this technology in various lubricated applications like mechanical seals, piston rings and thrust bearings. The paper also describes some fundamental on going research around the world with LST.

A model that predicts the static friction for elastic-plastic contact of rough surfaces is presented. The model incorporates the results of accurate finite element analyses for the elastic-plastic contact, adhesion and sliding inception of a single asperity in a statistical representation of surface roughness. The model shows strong effect of the external force and nominal contact area on the static friction coefficient in contrast to the classical laws of friction. It also shows that the main dimensionless parameters affecting the static friction coefficient are the plasticity index and adhesion parameter. The effect of adhesion on the static friction is discussed and found to be negligible at plasticity index values larger than 2. It is shown that the classical laws of friction are a limiting case of the present more general solution and are adequate only for high plasticity index and negligible adhesion. Some potential limitations of the present model are also discussed pointing to possible improvements. A comparison of the present results with those obtained from an approximate CEB friction model shows substantial differences, with the latter severely underestimating the static friction coefficient. [DOI: 10.1115/1.1609488] Keywords: Friction Modeling, Contacting Rough Surfaces, Static Friction, Adhesion

A finite element analysis, for an elastic perfectly plastic sphere normally loaded by a rigid fiat, is combined with an approximate analytical solution to evaluate the maximum tangential load (static friction) that can be supported by the spherical contact at the inception of sliding. Sliding inception is treated as a failure mechanism based on plastic yield rather than a Coulomb friction law with a certain friction coefficient. Two different failure modes are identified, either on the contact area or below it, depending on the elastic-plastic status of the normal preloading. A limiting normal preload is found above which the contact cannot support any additional tangential load. Simple analytical expressions for an 'internal static friction coefficient' are presented for both the elastic and the elastic-plastic regimes. [DOI: 10.1115/1.1538190]

A critical examination of the analytical solution presented in the classic paper of Greenwood and Williamson (1966), (GW) on the statistical modeling of nominally fiat contacting rough surfaces is undertaken in this study. It is found that using the GW simple exponential distribution to approximate the usually Gaussian height distribution of the asperities is inadequate for most practical cases. Some other exponential type approximations are suggested, which approximate the Gaussian distribution more accurately, and still enable closed form solutions for the real area of contact, the contact load, and the number of contacting asperities. The best-modified exponential approximation is then used in the case of elastic-plastic contacts of Chang et al. (1987) (CEB model), to obtain closed-form solutions, which favorably compare with the numerical results using the Gaussian distribution.

A model for calculating the static friction coefficient of contacting real (rough) surfaces in the presence of very thin liquid films (sub-boundary lubrication) is developed. The liquid has a very high affinity for the surfaces and its thickness is of the order of the surface roughness average. An extension of the Greenwood and Williamson (GW) asperity model and an improved Derjaguin, Muller and Toporov (DMT) adhesion model are utilized for calculating the contact and adhesion forces, respectively. The effects of the liquid film thickness and the surface topography on the static friction coefficient are investigated. A critical film thickness is found above which the friction coefficient increases sharply. The critical thickness depends on the surface roughness and the external normal load. This phenomenon is more profound for very smooth surfaces and small normal loads, in agreement with published experimental work on magnetic hard disk interfaces.

A finite element model was developed to study adhesion of elastic-plastic microcontacts in a previous investigation. An interesting result was the identification of two distinct separation modes, i.e. brittle and ductile separation. In the current study, that model is used to conduct a series of simulations to determine the influence of four nondimensional parameters (including the maximum load parameter) on the contact and on the separation modes. The results show that the parameter S (the ratio of the theoretical stress to the hardness) and δf/δc (representing the loading level) are the most important. Smaller S can only lead to brittle separation, while larger S can cause either separation mode depending on δf/δc. Ductile separation is more likely to occur at smaller δf/δc and brittle separation at greater δf/δc. The transition between the two separation modes occurs at about S=1.2 (for δf/δc=30) which corresponds to the theoretical stress for adhesion being 20% greater than the hardness. This result is qualitatively similar to the existing simplified analytical models, in that the adhesion energy, the hardness, and the loading level play important roles in the occurrence of ductile separation. However, there are important quantitative differences. Comparisons are also made with molecular dynamics simulations of a contact and with a fracture mechanics model of crack propagation. [ABSTRACT FROM AUTHOR]

A finite element model was developed to study the impact of four parameters, S, E/[[sigma].sub.[gamma]], [mu], and the load level [[delta].sub.f]/[[delta].sub.c] on the separation modes of adhesive contacts. The model could be used to solve variety of single contact problems such as the contact in a micromechanical switch.