Current Issue : October - December Volume : 2011 Issue Number : 1 Articles : 6 Articles
In the design process, one of the main targets is to reduce the peak values of the contact stresses. This can be attained by layering the contacting bodies by layers of different material characteristics. Viscoelastic materials are characterized by either a stress relaxation or a creep deformation; therefore, the contacting bodies can be layered with such materials to attain this target. This paper discusses effects of the material characteristics of viscoelastic layers upon the unbounded contact configuration. Three material parameters are considered: the layer/contact solids stiffness ratio, the delayed/instantaneous elasticity ratio, and the material relaxation time. The results are obtained by using a two-dimensional time-dependent nonlinear computational model, developed by the authors, capable of analyzing quasistatic viscoelastic frictional contact problems....
An experimental study, including three experimental devices, is presented in order to investigate dry friction phenomena in a wide range of sliding speeds for the steel on steel contact. A ballistic setup, with an air gun launch, allows to estimate the friction coefficient between 20?m/s and 80?m/s. Tests are completed by an adaptation of the sensor on a hydraulic tensile machine (0.01?m/s to 3?m/s) and a pin-on-disk tribometer mounted on a CNC lathe (1 to 30?m/s). The interactions at the asperity scale are characterized by a white light interferometer surface analysis....
Advanced materials, like nickel-based alloys, gain importance in turbomachinery manufacturing, where creating complex surfaces constitute a major challenge. However, milling strategies that provide high material removal rates at acceptable tooling costs demand optimized tool geometry and process parameter selection. In this paper, a description of circular milling is given, focusing on resulting engagement conditions. Regarding this, a test bench was designed to investigate the chip formation process in an analogy milling process. Furthermore, the methodology for the approach in the analogy process was developed. Results of a first test run in Inconel 718 verify the presented approach....
An aerodynamic journal bearing that is capable of self-generating squeeze-film pressure is presented and its dynamic characteristics investigated numerically and experimentally. A numerical method based on a time-marching static model was applied to assess the orbit trajectory path of the rotor upon a perturbation. Experimental results were obtained to validate the effect of the self-generated squeeze-film pressure on the stability of the rotor. Analyzing the Fast Fourier Transform (FFT) responses of the rotor orbits enabled identification of self-excited whirling instabilities. Both numerical and experimental results showed that increasing the squeeze-film effect of the bearing could raise the threshold speed of instability....
This paper provides an overview of the problems pertaining to the sensing and identification of nonlinear dynamics of slider with clearance in sub-5 nanometer regime. This problem is complex in nature because the nonlinear dynamics of slider in sub-5 nanometer clearance regime involves different sources of nonlinear, nonstationary, and uncertainty characteristics. For example, the involved forces such as air-bearing force, intermolecular force, and contact forces are all nonlinear. The complex interface interaction with mobile lubricant makes the slider response be nonstationary. Furthermore, the interfacial parameters are available only by assumptions in the sense of statistics. Most of the reported studies either focused on physics-based simulations by using assumed interfacial parameters or focused on experimental characterization. The issues of the sensing and identification of the nonlinear dynamic properties of slider in nanometer clearance regime will be discussed with an aim at illustrating the promising approaches for improving the correlation between test data and physics-based simulations....
Lubrication is the science of friction at moving interfaces. Nanomaterials acting as interfacial modifiers can minimize friction and thereby improve energy efficiency. To test this hypothesis, single- (SWNT) and double-walled (DWNT) carbon nanotubes and an ionic fluid are tested individually and compared to SWNTs and graphite as additives within the ionic fluid. The minimum coefficient of friction is correlated with the longest lifetime using a ball-on-disc tribometer, in air, at atmospheric pressure. Results are interpreted in terms of the nanotubes' mechanical properties and the formation of transfer layers upon the tribosurfaces....
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