Some Considerations In Rolling Fatigue Evaluation

A report is made in the present paper of a series of tests that were conducted by devising a testing apparatus of a rolling cylindrical body. Special attention was paid to both the line contact and the point contact. It was planned to obtain quantitative information ragarding the chracteristics of friction in the rolling contact of the rolling body in the main shaft bearing of aircraft engines, as well as to measure its lifetime. The testing apparatus was designed on the following bases: (1) accurate measurement of the rolling frictional torque and (2) automatic detection of the lifetime of the rolling body by means of an oscillograph. In the present tests the slip ratio was nearly zero.The present tests were performed under static load, and under conditions of dry rolling, water lubrication and #60 spindle oil lubrication. The following conclusions have been obtained from the present tests.(1) From the tests of the line contact of the rolling cylindrical body under static load, the equipment has been found to be satisfactory for performing rolling fatigue tests, and specially for the automatic detection of the lifetime of the rolling body.(2) There are two inflexion points on the characteristic curve of the rolling friction coefficient, which are considered to show respectively the strength of the oil film and the rolling fatigue limit.(3) The three regions thus divided by these two inflexion points are respectively the region of rolling wear, the region of rolling fatigue damage and the region where remarkable evidence points to its complete freedom from rolling fatigue.

In rolling contact fatigue (RCF), cracks are usually initiated from inclusions under the surface and propagated to the contact surface. In the present study, RCF cracks was observed using synchrotron radiation computed laminography (SRCL) in SPring-8. For observation of crack propagation, ex situ rolling fatigue test and SRCL were applied to same sample. Laminography is suitable for the observation of sheeted material and sheeted sample is useful for operation of rolling fatigue tests. The specimen with flaking damage was measured by SRCL. It was found that two types of cracks, vertical crack and horizontal crack under the contact surface, which is typically rolling fatigue crack, can be measured by SRCL. For observation of RCF crack propagation, vertical crack which initiate from inclusion and propagated in the direction perpendicular to the contact surface, could be detected. SRCL and fatigue tests were conducted three times. As results, crack propagation could be observed between first and third measurements, while the different part of crack was detected in second measurement. Residual compression stress is considered to affect the observation of RCF crack. SRCL is concluded to be effective for evaluation of initiation and propagation behavior of crack in the rolling fatigue.

Rolling fatigue tests were carried out on hot isostatically pressed silicon nitride bearing balls. At three load levels of the maximum Hertzian contact pressure pmax=5900, 6740 and 7110MPa, 13 balls each were tested, and the life distribution was studied. It was found that the rolling fatigue life followed approximately a 2-parameter Weibull distribution with the shape parameter of α≅1 at each load level. The relationship pmaxκL50=const (κ≅10) was found between pmax and the median life L50. Microscopic observation of flaking was also performed, and two types of flaking were found. One is flaking with the depth of the order of 100μm, and the other is a shallower one. The depth of the former type was found to coincide approximately with the depth at which the maximum shear stress occurred. At lower load levels, the former type was dominant. Frequency of appearance of the latter type increased with an increase in load level. Comparing the shape parameter of α≅1 and the exponent of κ≅10 mentioned above with those for the cyclic bending fatigue of plain specimens of silicon nitride, it was suggested that the mechanism of rolling fatigue of silicon nitride was different from that of cyclic bending fatigue of the same material.

Scuffing resistance of polyalphaolefin (PAO)-based nanolubricants with oleic acid (OA) and iron oxide nanoparticles

Tribologically transformed structure in fretting

Gas turbine units are widely used in KSA and other countries particularly during the peak demands and in inland regions. They produce about 50% of the total capacity of power generation in the kingdom. Despite their numerous advantages, their thermal efficiency remains very low and their resulting environmental impacts are significant. In this study, the effect of ambient conditions on the performance of a typical gas turbine used in KSA has been studied theoretically using the average hourly temperature and relative humidity for three regions of the country (Eastern, Central, and Western) which have almost the same power demand. Mass and energy balance equations with typical and realistic specifications of power plant units have been used to develop the model. The results present time variations of power generation, fuel consumption and efficiency for several typical cities. The maximum monthly power loss due to weather variation in Riyadh, Ad Dammam, and Jeddah are estimated at 8.9, 9.41 and 9.32 GWh respectively. While the annual power production loss in Riyadh, Ad Dammam, and Jeddah are 7.1, 8.2, and 11.2%, respectively. Power generation increases to about 4220 and 3028 kW when inlet air is cooled to 8.9 and 10.15oC, respectively. In conclusion, the effect of weather conditions of several Saudi areas on the performance of gas turbine units is significant. Therefore, the incorporation of inlet cooling technologies should be considered seriously.

Simulations of stress distributions in crankshaft sections under fillet rolling and bending fatigue tests

There are many defects on gear flank such as indentation marks and machined scar. In this study the influence of indentation mark on the rolling fatigue behavior of steel roller was investigated. The rolling fatigue test was conducted using steel rollers with artificial indentation marks on the contact surface. The rollers were made of JIS SCM440 (around 450 HV) which was thermally refined, and JIS SUJ2 (around 780 HV) which was quenched and tempered. The change of indentation mark and the location of failure during tests were evaluated. In the tests using SCM440 roller, the wear of indentation mark was comparatively great. As a result in high load condition where the indentation marks wore out at early stage the fatigue life was comparatively long. In the SUJ2 rollers with high hardness wear and plastic deformation of indentation marks were rather small. The failure portion, however, was not around indentation marks but near the contact edge. This behavior would suggest that the edge contact played the important role of the surface failure of high hardness roller. Stress state around the indentation mark and near contact edge was evaluated with elastic-plastic FEM analysis. Though the stress amplitude around indentation mark was equivalent to that near contact edge, the stressed volume near contact edge was larger.

A typical damage of bearings used in contaminated lubricants is caused by increasing of shearing stress at dents due to harder wearing particles in the lubricants.In this study, SUJ2 steel was used as the test material in which the amount of retained austenite and hardness were changed under various heat treatments. Rolling fatigue tests were repeated by the use of specimens having sharp dents intruded by a Vickers indenter. From the results of the rolling fatigue tests, the effect of the retained austenite on the fatigue life was investigated, considering the work hardening of the dent ridge.The main results were obtained as follows.(1) Retained austenite changed into martensite under rolling load and induced work hardening of the ridge, and then further shape changes of the ridges were virtually prevented.(2) As the dent with ridge caused the flaking of the rolling surface that occurred under shearing stress, the optimizing amount of retained austenite must be found for rolling fatigue life.(3) The upper amount of retained austenite was 20∼30 vol%, judging from the surface hardness, work hardening, life range and toughness.Based on the present results, the taper roller bearings which were controlled 20∼25 vol% retained austenite in the surface layer (10 μm depth from surface) and the surface hardness of 800∼860 HV were made. These bearings showed 11.3 times longer life than the conventional ones in contaminated oil. It was proved that the retained austenite had a significant effect on the formation and elimination of these ridges.

This study explores the development law of concrete internal stress and strain under rolling wheel loads. To do this, strain gauges are embedded in different positions inside concrete, and the stress and strain values at different positions inside the concrete are measured using the rolling fatigue test. A numerical model is established using COMSOL Multiphysics software, and the concrete internal stress and strain values are simulated. A comparison between the calculated and measured values shows that the numerical model can simulate the actual stress state inside the concrete. The numerical model can predict the internal stress and strain variation of concrete specimens under more complicated load conditions. The prediction results show that under the same fatigue load conditions, the concrete stress peak is hardly affected by the fatigue loading times, but there is a marked increase in the strain value inside the specimen. When the stress peak of concrete does not reach its design strength, an excessive increase of the internal strain will cause hidden damage to the concrete. By investigating the internal stress and strain of the concrete, a theoretical basis can be determined for the deterioration mechanism of concrete microstructure and the durability design of concrete.

This study investigates the impact of different surfactants and dispersion techniques on the friction and wear behavior of WS2 and MoS2 nanoparticles additives in a Polyalphaolefin (PAO) base oil under boundary lubrication conditions. The nanoparticles were dispersed using Oleic acid (OA) and Polyvinylpyrrolidone (PVP) to investigate their impact on particle agglomeration. The size distribution of the dispersed nanoparticles in PAO was measured by dynamic light scattering. The nanoparticles treated using PVP resulted in the most stable particle size. Friction studies showed that nanoparticle agglomeration reduction and the homogeneity of the suspension did not significantly impact the friction reduction behavior of the lubricant. Reciprocating wear experiments showed that, for our test conditions, both WS2 and MoS2 nano additives exhibited maximum wear depth reduction (45%) when using the PVP surface treatment compared to base oil. The wear results confirmed the significance of minimizing agglomeration and promoting high dispersion in promoting favorable wear resistance under boundary lubricant conditions. Analysis of the wear surfaces showed that a tribofilm formation was the primary wear reduction mechanism for WS2 particles treated by PVP while, in the case of MoS2 treated by PVP, the mechanism was load sharing via particles rolling and/or sliding at the interface.

Effects of sliding velocity and ambient temperature on the friction and wear of a boundary-lubricated, multi-layered DLC coating

Surface roughness plays a significant important role in deciding the tribological behavior of different materials. The aim of the study is to evaluate the effect of surface roughness and nanoparticles concentration on the lubricating performance of synthetic oil. In the present study, diamond nanoparticles have been used as additives in the PAO oil. The nanoparticles were used in varying concentrations from 0–0.8 wt%. The effect on seven surface roughness values has been studied. The testing conditions were selected in such a way so that boundary lubrication regime can be ensured. The results were obtained for steel/steel contacts using pin on disc tribometer. A 3D profilometer was used to quantify the different surface roughness parameters. Optical microscopy was used to find out wear scar diameter, in order to calculate the wear volume. Scanning Electron Microscopy (SEM) was used to analyze the wear behavior of worn out samples. It can be observed that for the oil containing diamond nanoparticles, the COF is a function of nanoparticles concentration and size. For rough surfaces, the higher concentration of nanoparticles reduces the COF by a larger amount and for the smooth surfaces a small concentration of nanoparticles is enough to lower the COF. The minimum COF was obtained for 0.4 wt% concentration of nanoparticles for 0.120 μm surface. This is because of the nanoparticles size, which is in the range of the Ra value of the disk samples as a small amount of nanoparticles is sufficient to fill the valleys and polish the sharp asperities. However, the increased concentration of nanoparticles acts as abrasives, thereby resulting in a slight increase in COF.

To reduce the amount of petroleum-derived fuel used in vehicles and vessels powered by internal combustion engines (ICEs), the addition of bioderived fuel extenders is a common practice. Ethanol is perhaps the most common bioderived fuel used for blending, and butanol is being evaluated as a promising alternative. The present study determined the fuel dilution rate of three lubricating oils (pure gasoline (E0), gasoline–10% ethanol blend (E10), and gasoline–16% isobutanol blend (i-B16)) in a marine engine operating in on-water conditions with a start-and-stop cycle protocol. The level of fuel dilution increased with the number of cycles for all three fuels. The most dilution was observed with i-B16 fuel, and the least with E10 fuel. In all cases, fuel dilution substantially reduced the oil viscosity. The impacts of fuel dilution and the consequent viscosity reduction on the lubricating capability of the engine oil in terms of friction, wear, and scuffing prevention were evaluated by four different tests protocols. Although the fuel dilution of the engine oil had minimal effect on friction, because the test conditions were under the boundary lubrication regime, significant effects were observed on wear in many cases. Fuel dilution was also observed to reduce the load-carrying capacity of the engine oils in terms of scuffing load reduction.

The wear behavior of AISI304 stainless steel was investigated under dry, water-, and oil-lubricated conditions. A block-on-disk wear test was conducted in this work, since the test conditions could be controlled easily. For oil-lubricated contact, a significant amount of thin and elongated cutting chip-like debris was observed. This is attributed to the high lubricating effect of oil. Strain-induced martensitic (SIM) transformation was observed for all AISI304 blocks after the wear test, while AISI304 consisted of a single γ-austenite phase prior to the wear test. The Stribeck curve and the corresponding lubrication regimes were also considered to explain the wear behaviors and lubrication effect of AISI304. In comparison to the dry or water-lubricated conditions, which fall in the boundary lubrication regime at a low rotation speed, it is considered that the high viscosity of the oil-based lubricant causes the lubrication condition to enter the mixed lubrication regime early at a lower speed, thus reducing the specific wear rate over the 100–300 rpm range.