This article is the first part of a three-part series that investigates the rolling contact fatigue (RCF) initiation and spall propagation characteristics of three bearing materials, namely, AISI ...52100, VIM-VAR AISI M50, and VIM-VAR M50NiL steels. Although there is substantial prior work published on the rolling contact fatigue initiation of these materials, little has been published on their spall propagation characteristics after spall initiation. It is recognized that rapid spall growth can lead to catastrophic bearing failure. Hence, understanding the spall growth phase and factors that may cause accelerated growth rates is key to achieving a reliable and robust bearing design. The end goal is to identify control parameters for optimizing bearing materials for improved spall growth resistance. This first part study features the experimental results from 208-size (40 mm bore) angular-contact ball bearings endurance life tested at maximum Hertzian contact stress levels of 3.10 GPa and bearing outer race temperatures up to 131°C. Spall propagation experiments were conducted on new and life tested bearings at 2.10 and 2.41 GPa maximum contact stress. Spall propagation experiments show that all materials exhibit a rapid or critical spall growth rate after undergoing an initial low-rate spall growth period. The time-to-critical growth rate is dependent on contact stress and was swiftest in AISI 52100 steel. To better understand the underlying physics, the driving factors, and failure mechanisms, the state of stress is modeled using finite element analysis in Part II and an in-depth microstructural analysis of selected bearings is presented in Part III.
Display omitted
•Aircraft engine oils 4 h dermal exposures induced slight skin irritation in rabbits.•Used oils enhanced irritation under semi-occlusive conditions compared to new oils.•Test site ...occlusive dressing less irritating compared to semi-occlusive dressing.•Data suggest irritation enhancement under semi-occlusive dressing as the oils age.
There is little data available on toxicity levels of used aircraft engine oils relative to their unused (new) versions. This study was conducted to determine if new engine oils and their used versions have the potential to induce dermal irritation. Twelve male New Zealand White rabbits (Oryctolagus cuniculus, 19 weeks old) were used to determine the acute dermal toxicity potential of four aircraft turbine oils including MIL-PRF-7808 Grades 3 and 4 and MIL-PRF-23699 Grade 5 High Thermal Stability (HTS) and a Grade 5 experimental aircraft engine oil in their unused and used or laboratory stressed states. Five fur-free test sites (6 cm2 each) located lateral to the midline of the back were treated with two undiluted (0.5 ml) new engine oils and their used versions. The fifth site received reverse osmosis deionized (RODI) water as a control. Each treatment was repeated 3 times (3 rabbits/oil type). Each oil was tested under both semi-occluded and occluded conditions. The 4 h exposure was followed by gauze plus wrappings removal, and gentle cleaning of sites prior to scoring for erythema and edema at 0.5–1, 24, 48 and 72 h post exposure based on Draize (1959). E-collars were placed on each animal for at least 72 h to prevent ingestion of the test substance and/or gauze and wrappings and/or disturbance of site recovery. Additional observations were made on days 7, 10 and 14 to determine recovery. Exposure to both used and new oils produced dermal irritation consisting of no more than very slight to well-defined erythema and very slight edema. The calculated Primary Dermal Irritation Index (PDII) indicated that all the oils were slightly irritating (means ranged from 0.42 to 1.08). Although the PDII values for new oils and their used versions were not significantly different from each other, they were all statistically higher (p < 0.05) than those obtained for the control regardless of the type of occlusion binding applied. The used oils under semi-occlusion conditions yielded larger size effects (Cohen’s d) relative to their unused versions suggesting an enhancement in irritation when the oil is aging. Grade 4 in the used state yielded the largest size effect which was d = 5.9 versus 2.6 for its unused version. The slight dermal irritation resulting from four hours of exposure to oils raises concerns about the magnitude of impact related to prolonged and/or repeated exposure.
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.
•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.
•Non-linear dynamic model with a realistic representation of the spall edge.•Rolling element-spall edge interaction Finite Element model: simple and efficient.•Physics-based damage driven mechanism ...of the spall propagation process.•Damage evolution simulations within the spall edge.
This work presents a new method for a reliable modeling of a rolling element (RE)-spall edge interactions. The model aims to describe the material response within the spall edge as a result of repeated RE impacts. Two complementary RE-spall edge interaction models were developed and integrated, non-linear dynamic and finite element (FE). In order to demonstrate the advantages of the developed model, qualitative damage initiation simulations were conducted. The simulation results were validated and showed a good agreement with the published experimental results. To our best knowledge, this is the first physics-based attempt to simulate damage evolution within the spall edge.
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.
The rolling contact fatigue and wear characteristics of uncoated and diamond-like carbon coated VIM-VAR M50 bearing steel were investigated at room temperature and 177°C (350°F) and a Hertzian stress ...level of 4.8 GPa (700 ksi). The coatings were deposited via ion beam enhanced deposition and were approximately 33 nm thick. Rolling fatigue and wear tests were conducted using a ball-on-rod type tester. Results did not indicate any significant difference in the fatigue life of coated and uncoated specimens at room temperature, at 90% confidence level. However, the coating significantly improved the fatigue life (90% confidence level) at 177°C and wear resistance at both temperatures. Some correlation was noted between wear and fatigue life for the uncoated specimens at both temperatures, but none for the diamond-like carbon coated (the fatigue life was independent of wear) specimens. Scanning electron microscopy did not show any sign of coating delamination.
The effect of the oil anti-wear additive tricresyl phosphate (TCP) on rolling contact fatigue (RCF) life of advanced bearing steels--AISI VIMVAR M50, CSS 42L, Pyrowear 675, and Cronidur 30 was ...investigated with silicon nitride balls at 177 °C and at a maximum Hertzian stress of 5.5 GPa. TCP at 1% additive concentration was blended into a synthetic polyol ester turbine engine lubricant basestock having a nominal viscosity of 3 cSt at 100 °C. Additionally, the basestock was fortified with the anti-oxidants dioctyl-diphenyl amine (DODPA) and phenyl-α-napthyl amine (PANA) at 1% concentration each. The presence of TCP has a measurable positive effect on RCF life and wear. Also, all the advanced bearing materials exhibited superior fatigue life compared to conventional bearing steel M50, both with and without TCP. The study indicates that current gas turbine lubricant formulations with TCP have positive effects on fatigue life and wear performance of M50, Pyrowear 675, CSS 42L, and Cronidur 30.
The fatigue and wear characteristics of AISI VIM-VAR M50 steel were evaluated using two high-temperature synthetic turbine engine lubricants. Rolling contact fatigue (RCF) tests were conducted on a ...ball-on-rod type rig at a cyclic Hertzian contact stress of 4.8 GPa (700 ksi) and temperatures of 177 °C (350 °F) and 274 °C (525 °F). The two lubricants tested included a five-ring polyphenylether (MIL-L-87100, 12.64 cSt at 100 °C) and perfluoropolyalkylether (25.51 cSt at 100 °C). RCF results were compared with those obtained with a polyolester (MIL-L-7808J) at 177 °C. Weibull analysis and analysis of variance conducted on the RCF data show significant increases in fatigue life and reductions in wear rate when compared with MIL-L-7808J lubricant at 177 °C. However, at 274 °C, fatigue life was substantially reduced and wear of M50 steel increased with both high-temperature lubricants.
PDF
