•A CDM-FE model investigated the effects of surface roughness on RCF.•Optical profilometer measurements established tribo-surface parameters.•Fatigue life is reduced as specific film thickness ...(λ-ratio) is decreased.•Surface failures reduce fatigue life more than subsurface failures.•The competitive RCF failure mechanism exists with the introduction of roughness.
In this study, a continuum damage mechanics (CDM) finite element (FE) model was developed to investigate the effects of surface roughness on rolling contact fatigue (RCF) life of non-conformal contacts. In order to assess the surface roughness of tribo-components, twelve deep groove rolling element bearings from various companies in different sizes were procured and measured using an optical surface profilometer. The roughness average (Ra) and the root mean square of surface roughness (RMS, σ) ranged from a low of 0.03, 0.05 µm to a high of 0.14, 0.20 µm, respectively. The number of peaks and valleys per 400 µm were measured and calculated. The number of peaks ranged from 11 to 31 (greater than99.5% Confidence Interval). The measured surfaces also revealed that a sinusoidal pattern can be used to accurately represent the surface patterns. The sinusoidal surface pattern was used to determine the elastohydrodynamic lubrication (EHL) pressure distribution between an equivalent rough surface in contact with a smooth surface. Four roughness amplitude were used to generate specific film thicknesses (λ-ratios) resulting in full to mixed EHL lubrication regimes. The EHL pressure distributions were replaced with representative symmetric Hertzian pressure distributions in order to remove the effect of asymmetry of an EHL pressure distribution. The resulting symmetric pressure distributions were used in a finite element continuum damage mechanics model to determine RCF life of machine elements operating in specific film thickness range of 1 < λ < 10. The RCF results from the FE model indicate that as roughness amplitude increases or lambda ratio decreases, the fatigue lives decrease for the various frequencies. Additionally, subsurface failure fatigue lives are reduced as roughness frequency increases regardless of amplitude or Hertzian pressure. The RCF results also indicate that for the low frequency pressure distribution the contact is most susceptible to surface failure, whereas for high frequency pressure distribution the contact resists surface failure. The results from this investigation were used to develop surface roughness effects for various RCF life equations commonly used in rolling element bearing application.
The quest for high‐performance energy efficient aircraft turbine engines has led to the development of a number of high‐performance rolling element bearing materials and engine lubricants with the ...aim of providing superior mechanical component durability. The heat treatments/surface treatments used to achieve the desired physical and mechanical properties for these newer alloys can result in altered surface chemistry from currently used materials. Surface chemistry plays an important role during lubricant‐bearing material interactions and the formation of beneficial tribological films during component operation. The objective of this study was to analyse the tribo‐films formed on bearing surfaces and investigate the interaction of lubricant additives, specifically the phosphorus‐based anti‐wear additive tricresyl phosphate (TCP), with different bearing materials under relevant bearing operating conditions. Bearing tests were conducted on 208‐size (40 mm bore) angular contact bearings at 127°C and 154°C using gas turbine engine lubricants conforming to MIL‐PRF‐23699G at maximum Hertzian contact stresses of 3.1 GPa and 3.55 GPa. Bearing materials evaluated included AISI M50, M50NiL, nitrided M50NiL (N) and three variants of Pyrowear 675 with silicon nitride rolling elements. Tribo‐films were analysed using Energy dispersive X‐ray spectroscopy and Auger Electron spectroscopy. Results indicate that phosphorus‐rich anti‐wear tribo‐films form on all of the bearing materials studied. The applied thrust load and heat treatment had a significant effect on tribo‐film thickness. The study also suggests that current gas turbine engine lubricants formulated with TCP should form beneficial tribo‐films that enhance bearing fatigue life and performance.
A finite element model was developed to investigate the influence of near surface orthogonal shear stress (OSS) on the competitive failure mechanism between surface originated pitting (SOP) and ...subsurface originated spalling (SOS), which is intrinsic to rolling contact fatigue (RCF). Surface roughness in heavily loaded non-conformal contacts causes competition between SOS and SOP. In this investigation, tribo-surface roughness has been represented as sinusoidal waveform based on surface measurements of rolling element bearings. These measurements outlined the range of roughness frequency and amplitude. The effects of these surfaces on the contact were investigated and the resulting pressure distributions were used in a finite element model in order to quantify the effects of pressure distribution on near surface orthogonal shear stress concentration. The resulting pressure distributions obtained from rough surfaces were also used in a continuum damage mechanics finite element model (CDM-FEM). The results indicate that a contact with a low frequency surface roughness (pressure distribution) is more susceptible to surface failure, whereas the contact with high frequency surface roughness frequency will resist surface failure. To quantify surface originated failure for a given surface roughness, the probability of surface failure parameter (πsf), which is defined as the ratio of contacts exhibiting SOP characteristics to the total tested is proposed. The near surface stress analysis and failure mechanism results were used to establish a relation between the near surface OSS concentration and πsf. This relation is described by a 2-parameter Weibull cumulative distribution function (CDF). The results indicate that roughness frequency and half contact width are the main parameters controlling the probability of surface failure.
•2 FE models explored the influence of near surface shear on RCF failure mechanism.•The competitive RCF failure mechanism exists with the introduction of roughness.•Long wavelength surfaces form distinct, pronounced surface stress concentrations.•Long wavelength surfaces are the most susceptible to surface failure.•A 2-parm. Weibull CDF is capable of relating OSSC to probability of surface failure.
Historically, high carbon steels have been used in mechanical applications because their high surface hardness contributes to excellent wear performance. However, in aggressive environments, current ...bearing steels exhibit insufficient corrosion resistance. Martensitic stainless steels are attractive for bearing applications due to their high corrosion resistance and ability to be surface hardened via carburizing heat treatments. Here three different carburizing heat treatments were applied to UNS S42670: a high-temperature temper (HTT), a low-temperature temper (LTT), and carbo-nitriding (CN). Magnetic force microscopy showed differences in magnetic domains between the matrix and carbides, while scanning Kelvin probe force microscopy (SKPFM) revealed a 90⁻200 mV Volta potential difference between the two phases. Corrosion progression was monitored on the nanoscale via SKPFM and in situ atomic force microscopy (AFM), revealing different corrosion modes among heat treatments that predicted bulk corrosion behavior in electrochemical testing. HTT outperforms LTT and CN in wear testing and thus is recommended for non-corrosive aerospace applications, whereas CN is recommended for corrosion-prone applications as it exhibits exceptional corrosion resistance. The results reported here support the use of scanning probe microscopy for predicting bulk corrosion behavior by measuring nanoscale surface differences in properties between carbides and the surrounding matrix.
Polyol‐ester lubricants have been used and developed for aviation gas turbine engines for many decades. The newest MIL‐PRF‐23699 lubricant class, called enhanced ester (EE), provides the best ...combination of thermal stability, load carrying capability, boundary lubrication and compatibility with fluoroelastomer O‐rings. Two candidate EE Class formulations and one high thermal stability class formulation conforming to MIL‐PRF‐23699G were evaluated for oil degradation with up to 3000 h of bearing operation. Lubricant degradation was studied using VIM VAR M50 bearings with M50 and silicon nitride balls under two operating conditions using two bearing test rigs. Oil degradation in terms of oxidation time, total acid number and viscosity was studied as a function of time with varying results for the three lubricants.
Carburizable martensitic stainless steels (MSSs) are attractive candidates for bearings due to their high corrosion resistance, high hardness, and high temperature performance. Wear performance in ...tribocorrosion applications is strongly influenced by the surrounding environment. Electrochemical testing was used to evaluate three different surface treatments on AMS 5930 steel developed for advanced gas turbine engine bearing applications: low temperature (LTT), high temperature (HTT), and carbonitrided (CN). HTT had a higher corrosion rate that increased with time, whereas LTT and CN had lower corrosion rates that were stable over time. Accelerated testing revealed that surface treatment significantly influenced how corrosion propagated: HTT was more uniform; conversely, LTT and CN showed localized attack. Degradation mechanisms developed from electrochemical methods provide rapid insight into long-term wear behavior.
Advanced bearing materials for future military and commercial gas turbines are required to operate at high speeds, high temperature, and higher thrust loads. At elevated operating conditions, the ...bearing and gear materials must be able to operate with ultrathin oil films without suffering detrimental effects of adhesive wear. The development of materials with rolling-element fatigue and corrosion resistance properties without deterioration in adhesive wear attributes is a significant challenge. To meet those performance requirements, the forerunner, martensitic stainless steel Pyrowear 675 (AMS 5930), has been in development for aerospace bearing and gear applications. This article addresses the adhesive wear performance of three variants of Pyrowear 675 with silicon nitride ball material simulating a hybrid bearing evaluated using a WAM8 machine. Baseline testing was conducted using conventional bearing steel AISI M-50. Adhesive wear testing was conducted at a temperature of 200°C and at different contact slips (15, 30, 50, and 70%) and entraining velocities (1.3 to 10.2 m/s). Posttest specimens were analyzed by scanning electron microscopy (SEM) and auger electron spectroscopy (AES). All the hybrid material pairs demonstrated very good adhesive wear performance compared to the baseline AISI M-50-AISI M-50 pair.
During times of restricted supply, bearing refurbishment offers an attractive avenue to maintain operational readiness. However, bearing operation after fatigue spall initiation on refurbished ...bearings has not been extensively studied. In this study, spall propagation characteristics were compared between new and refurbished vacuum induction-melted, vacuum arc remelted AISI M50 208-size angular contact bearings. A control group of new AISI M50 bearings was evaluated for spall propagation characteristics as a baseline. Another group of AISI M50 bearings were subjected to an accumulated 11.5 billion stress cycles at a maximum Hertzian contact stress of 1.93 GPa and a temperature of 127 °C followed by Level II refurbishment. The refurbished bearings were evaluated for spall propagation characteristics and compared to the baseline bearings. Spalls were initiated via seeded Rockwell C hardness indents and propagated at a maximum Hertzian contact stress of 2.65 and 2.41 GPa, respectively, on both groups of bearings. The propagation rates of the bearings were measured in real time using an oil debris monitor. Pre- and post-tested bearings were examined for changes in microstructure, residual stress and retained austenite as a function of depth in the circumferential direction. No statistically significant difference in spall propagation characteristics was observed between new and refurbished bearings at the operating conditions and accumulated stress cycles studied here.
This is the third part of a three-part series that investigates the rolling contact fatigue initiation and spall propagation characteristics of three bearing materials, namely, AISI 52100, VIM-VAR ...M50, and VIM-VAR M50 NiL steels. Though there is substantial prior work published on the rolling contact fatigue initiation of these materials, little is known about their spall propagation characteristics after spall initiation. In Part III, 208-size, 40-mm-bore bearings are examined for changes in appearance of the microstructure as well as residual stress as a function of depth in the circumferential direction. The correlations between the experimental results from Part I and computer modeling in Part II are made.
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•Common aerospace-quality bearing steels were studied under RCF and torsion fatigue.•An analytical model was developed and considered Fatemi-Socie failure criteria.•Good corroboration ...existed between simulation life and experimental results.•Calibrated analytical model offers 2/3rds time save compared to experiments.
In this investigation common aerospace-quality bearing steels was evaluated in rolling contact fatigue both experimentally and analytically. Three aerospace-quality bearing steels was procured and evaluated. First, the bearing steels were evaluated using a 3 ball-on-rod rolling contact fatigue test rig. Next, the same bearing steels were evaluated using a torsion fatigue test rig in order to quantify these materials’ performance against the damage causing stress in RCF – shear reversal. The torsion S-N data provided the foundation for the determination of material constants that were used in a continuum damage mechanics finite element model (CDM-FE model), which considered the Fatemi-Socie critical plane approach as the failure criteria. These material constants captured the material cleanliness effect between the various materials investigated. Additionally, the CDM-FE model utilized Voronoi tessellations to capture the material topological effect. RCF simulations were performed at the same operating conditions as in the 3 ball-on-rod test apparatus. Torsional fatigue results from this investigation indicated which material possessed the largest ultimate shear strength, and which material performed best in low cycle and high cycle fatigue. The three ball-on-rod results established experimentally which material performed superior in RCF. It was observed that good corroboration existed between the analytical simulation life predictions and the 3 ball-on-rod experimental results.
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