Michael D. Bryant

On the thermodynamics of degradation

The science base that underlies modelling and analysis of machine reliability has remained substantially unchanged for decades. Therefore, it is not surprising that a significant gap exists between available machinery technology and science to capture degradation dynamics for prediction of failure. Further, there is a lack of a systematic technique for the development of accelerated failure testing of machinery components. This article develops a thermodynamic characterization of degradation dynamics, which employs entropy, a measure of thermodynamic disorder, as the fundamental measure of degradation; this relates entropy generation to irreversible degradation and shows that components of material degradation can be related to the production of corresponding thermodynamic entropy by the irreversible dissipative processes that characterize the degradation. A theorem that relates entropy generation to irreversible degradation, via generalized thermodynamic forces and degradation forces, is constructed. This theorem provides the basis of a structured method for formulating degradation models consistent with the laws of thermodynamics. Applications of the theorem to problems involving sliding wear and fretting wear, caused by effects of friction and associated with tribological components, are presented.

An experimental study of the correlation between wear and entropy flow in machinery components

Based on first principles, a hypothesis was made on the potential correlation between entropy and degradation of machinery components in an earlier investigation of stochastic characterization of degradation dynamics. This paper reports on an experimental study in which degradation in the form of wear of model machinery component pairs was made on an accelerated testing basis. Concomitant measurement of entropy flow was made by means of a simple calorimeter. Results show a strong correlation between the referenced wear and the production of entropy flow.

Dynamic Modeling of Rolling Element Bearings With Surface Contact Defects Using Bond Graphs

Multibody dynamics of healthy and faulty rolling element bearings were modeled using vector bond graphs. A 33 degree of freedom (DOF) model was constructed for a bearing with nine balls and two rings (11 elements). The developed model can be extended to a rolling element bearing with n elements and (3 × n) DOF in planar and (6 X n) DOF in three dimensional motions. The model incorporates the gyroscopic and centrifugal effects, contact elastic deflections and forces, contact slip, contact separations, and localized faults. Dents and pits on inner race and outer race and balls were modeled through surface profile changes. Bearing load zones under various radial loads and clearances were simulated. The effects of type, size, and shape of faults on the vibration response in rolling element bearings and dynamics of contacts in the presence of localized faults were studied. Experiments with healthy and faulty bearings were conducted to validate the model. The proposed model clearly mimics healthy and faulty rolling element bearings.

Attenuation and transformation of vibration through active control of magnetostrictive terfenol

Abstract A special magnetostrictive material (terfenol), when subjected to a properly controlled magnetic field, induces giant magnetostrictive motions. In an actuator (termed a mount) described in this paper, these motions are used to reduce and/or alter unwanted vibrational motions. Experimental results indicate that properly controlled magnetostrictive extensions can yield vibration isolation that exhibits significant low-frequency performance with small static deflection. In other experiments energy is added to and subtracted from an isolated system (through mount control) to alter the vibrational noise signature of the system. This work also reports on such experiments and demonstrates the ability to modify the vibrational response to disturbances.

Active Vibration Control in Structures Using Magnetostrictive Terfenol with Feedback and/or Neural Network Controllers

Experimental results are described in which a rod of magnetostrictive terfenol was used in the dual capacity of a passive structural support element and an active vibration control actu ator and artificial neural networks were used for the adaptive real-time control algorithm. Tests were performed on a three-legged table, where the terfenol actuators mentioned above are the table legs. For the table experiment, shaker vibrations generated in the ground and transmitted to the tabletop (via the legs) were attenuated by counter vibrations synthesized in the table leg actuators. The goal of this experiment was to maintain a quiescent tabletop in the presence of floor vibrations. Utilizing a proportional-integral derivative and a neural network controller, actuated forces were used to cancel applied disturbance forces. The neural network architecture identifies (learns) and adapts to the tabletop forced disturbance through a fast adaptation law known as Adaptive Back Propagation, generating the required counter vibration. The architecture and hence the control was designed to be modular so cross talk (coupling in the control signal) is minimized. This puts an extra burden on the controller to decouple the spillovers but maintains modularity, an important feature for large scale implementations. This article describes work in this area and demonstrates the ability to cancel disturbances from static to the hundred hertz frequency range.

Reductions in wear rate of carbon samples sliding against wavy copper surfaces

Wear rates (μgm/s) versus rotor speed for carbon samples sliding against smooth and wavy copper rotors (250 μm thick copper sheets were attached to smooth and wavy steel and polycarbonate backings) were identical at some speeds, but at other speeds wear rates for the wavy rotors were almost half those of the smooth rotors. Slider vibrations (periodic, with period set by rotation) perpendicular to the sliding surface were measured and Fourier analyzed. Comparison of vibration spectral amplitudes to spectral amplitudes derived from surface profiles identified vibration modes dynamically enhanced by surface waviness on the wavy rotor. At speeds where wear rates on the wavy rotor were most reduced, amplitudes of certain modes in the vibration spectrum were most enhanced. For all these cases, the product of mode number times speed was nearly constant, suggesting resonance. Contact forces and contact voltage drop (due to a mA current flowing from slider to rotor) were measured and plotted versus time during all experiments. Friction coefficients rapidly varied between 0.1 and 0.4, but averaged 0.2. Traces of friction coefficient versus time for both wavy and smooth rotors were similar, even when wear rates plunged on the wavy rotor. There were no large jumps in the contact voltage drop data, suggesting that the slider never disconnected from any of the rotors. Photoelastic visualizations (Bryant and Lin, 1993) of slider-rotor interfaces revealed concentrated contact on the smooth rotors, but none on the wavy rotors. The absence (induced by vibration) of concentrated contact may have caused differences in wear rates. Appreciable reductions (up to 50 percent) in wear rate are possible by adding small surface waves to a rotor that induce micro-vibrations of the slider-spring-rotor contact system. The effect appears most pronounced at resonance.

Hydrodynamic performance of gas microbearings

Hydrodynamic performance of gas microbearings, fabricated with deep X-ray lithography and electroplating, will be presented in this article. Static performance in terms of load parameters and attitude angles was calculated using Molecular Gas Lubrication (MGL) theory. Threshold speed and allowable rotor mass were calculated to investigate safe operating conditions of the fabricated gas microbearings using orbit simulation. Finally, improved bearing designs were proposed to increase stability.

Photoelastic visualization of contact phenomena between real tribological surfaces, with and without sliding

Abstract The design, construction and operation of a machine for visualizing real contact interfaces and testing sliding wear is described. The bodies that form the interface are both opaque—the slider is a carbon graphite sample (National Electrical Carbon Company brush grade 634) and the rotor surface is copper. The visualization technique uses contact indentation stresses and photoelasticity to generate optical fringes (shadows) directly beneath the contact interface. The resultant fringes indicate the size and location of the islands of real contact that exist within the apparent contact region. Resolution is sufficient to distinguish two neighbouring millimetre-sized contacts. For calibration and comparison purposes, photoelastic pictures of sharp graphite pencil points and hard plastic spheres of various diameter indenting the copper slip ring surface are presented. Photographs of photoelastic images of contact interfaces between an opaque carbon sample sliding against an opaque copper slip ring are also presented. Photographs are presented that show the formation of concentrated contact (and possible evolution to thermal mounding) on the carbon slider surface. Collective data from other photographs measures the relative occurrence of concentrated contact and suggests operating and surface conditions that can discourage concentrated contact and formation of thermal mounds.

On irreversible thermodynamics for wear prediction

In June of 2000, the authors of this paper reported an experimental study in which degradation in the form of wear of model machinery component-pairs was made on an accelerated testing basis. Concomitant measurement of entropy flow was made by means of a simple calorimeter. Results showed a strong correlation between the referenced wear and the production of entropy flow. We designate this previous work as Category I, in which the normal load is sufficiently high that the heat generated by wear, in a boundary lubrication regime, is much larger than the heat generated by shearing of the lubricant. The present paper treats what we shall call Category II, in which the heat generated by shearing of the lubricant is larger than the heat generated by wear. This includes the limiting case wherein there is neither wear nor production of entropy flow. Again we invoke the laws of thermodynamics, which led us to the conclusion that normalized wear is strongly correlated to normalized entropy flow.

Transient nonlinear thermal runaway effects in carbon graphite electrical brushes

A numerical analysis is formulated and used to determine the transient temperature fields associated with Joule heating of a cooled stationary electrical contact. The temperature field equations are developed using integral transforms, complex variables, and conformal transforms; approximate solutions are developed using finite-difference methods. Transient nonlinear temperature fields are obtained and compared with temperature fields obtained from linear and nonlinear studies. Thermal runaway effects are noticed and studied. >