186,366 research outputs found
Transient analysis of frictionally excited thermoelastic instability in multi-disk clutches and brakes
No full textA 2D multilayered model has been considered to estimate the transient evolution of temperature and pressure perturbations in multi-disk clutches and brakes during operation. The model proposed by Decuzzi et al. [1] has been modified here to estimate the variation of b-perturbation growth rate-with V-relative sliding speed. It has been verified that the perturbation with the lowest critical speed has also the highest growth rate, and that low frequency perturbations are less critical than high frequency perturbations, at fixed critical speed. Therefore, when comparing perturbations with identical critical speed, those with higher wave numbers are responsible for more intense thermomechanical damages. Also, for perturbations with wave number smaller than the critical m(er), the temperature increases with m; vice versa for perturbations with wave number larger than mer the temperature decreases with m. A reduction in thickness ratio a(1)/a(2) between friction and metal disks has the effect of increasing the temperature and growth rate. An approximate formula for the temperature variation with time has been derived for a linearly decreasing engagement speed. (C) 2002 Elsevier Science B.V. All rights reserved
Transient analysis of frictionally excited thermoelastic instability in multi-disk clutches and brakes
No full textA 2D multilayered model has been considered to estimate the transient evolution of temperature and pressure perturbations in multi-disk clutches and brakes during operation. The model proposed by Decuzzi et al. [1] has been modified here to estimate the variation of b–perturbation growth rate—with V—relative sliding speed. It has been verified that the perturbation with the lowest critical speed has also the highest growth rate, and that low frequency perturbations are less critical than high frequency perturbations, at fixed critical speed. Therefore, when comparing perturbations with identical critical speed, those with higher wave numbers are responsible for more intense thermomechanical damages. Also, for perturbations with wave number smaller than the critical mcr, the temperature increases with m; vice versa for perturbations with wave number larger than mcr the temperature decreases with m. A reduction in thickness ratio a1/a2 between friction and metal disks has the effect of increasing the temperature and growth rate. An approximate formula for the temperature variation with time has been derived for a linearly decreasing engagement speed
Elastic beam over an adhesive wavy foundation
No full textThe adhesive behavior of a thin infinitely long elastic beam resting over a wavy rigid foundation with wavelength lambda is studied. Three governing parameters have been identified describing the physical and geometrical properties of the system: the dimensionless surface energy gamma=gamma/Es, the dimensionless amplitude h=h/lambda of the substrate, and thickness of the beam s=s/lambda. Analyzing the variation of the total energy of the system as a function of the governing parameters three different adhesive regimes have been individuated: full adhesion, partial adhesion, and no adhesion (point contact). An effective surface energy has been considered showing that the effect of surface waviness could be beneficial in increasing the adhesive strength of the system. In particular for gamma=1.0 and s=0.1, it has been evaluated a maximum effective interface energy of about 1.4gamma under full contact conditions. Larger amplifications are expected for higher gamma and smaller s
Stress-driven morphological instability and catastrophic failure of microdevices
No full textIn microdevices, the competition between surface energy and elastic energy could lead at the phenomenon known as stress-driven morphological instability (MI), causing an increase of Surface roughness with time. Several different mass transport mechanisms can trigger such a morphological alteration and operate simultaneously: surface and bulk diffusion, evaporation and condensation, chemical reactions. Unstable solids could eventually evolve towards crack-like surfaces thus altering mechanical, electrical and optical properties of the devices or even leading to catastrophic failures by supercritical crack propagation. In this work, a more general kinetic law is employed to estimate the onset of MI, considering the effect of the stress field on the atomic mobility. A more intuitive and straightforward approach is used to determine the stability conditions, where the rate of atomic mass motion is introduced, as a stability parameter-The critical loads and wavelengths for the onset of MI, determined as a function of material parameters alpha and beta, are compared with the limiting conditions for the supercritical crack propagation (SC) of a crack-like surface in order to asses if and under which situations catastrophic failures by SC can be observed. Two practical cases are investigated: fixed wavelength (Case I) and arbitrary rough surface with a fixed remote load (Case II). In Case I, absolute and relative threshold loads are found below which MI could never occur and a transitional wavelength over which MI would always lead to SC is introduced. In Case II, it is shown that dominant perturbation for NIT would always lead to SC given enough time for the surface to evolve towards a crack-like profile. The influence of the material properties a and on the critical parameters is also addresse
Frictionally-excited thermoelastic contact of rough surfaces
No full textFrictional sliding contact between two elastically similar half-planes, one of which has a sinuisoidally wavy surface, is studied in the full-contact regime. The steady-state regime is evaluated, within the limits imposed by the well-known phenomenon of thermo-elastic instability (TEI). TEI gives a critical speed whose value deqends on the wavelength of the perturbation, and above which the perturbation itself grows arbitrarily with time. It is found that the TEI critical speed, V-cr, is clearly identified by the steady-state solution only in the special and limiting case when the flat half-plane is non-conductor; in that case, V-cr is the speed for which the steady-state predicts infinite amplification. In all other cases, V-cr (appropriate to the wavelength of the profile) does not correspond to infinite amplification, nor to the maximum one, V-M. In the limiting case of thermoelastically similar materials, not only the system is unconditionally stable (V-cr = infinity) for fH(1) < 0.5, where f is the friction coefficient and H-1 a certain thermoelastic constant, but the regime at the maximum amplification is also always stable, and arbitrarily large amplification is obtained for fH(1) tending to infinity. However, it is found that in most practical cases of braking systems, V-cr much less than V-M, and so the limiting conditions are reached at V-cr. At this speed, the amplification is typically not extremely high. (C) 2000 Elsevier Science Ltd. All rights reserved
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