Rolling Contact Fatigue Tests Research Articles

Research on Key Control Technology of Intelligent Rolling Contact Fatigue Test Facility

An intelligent rolling contact fatigue test equipment is developed, and the control methods are presented. For obtaining the slip accurately, the control method based on master-slave synchronization is proposed. For controlling the loads in high precision, the control method took into consideration the influence by two factors, displacement and the load. The nonlinear interference and excess torque in load control are effectively suppressed. Based on the SIMOTION D425 which is the Siemens integrated motion control system, the control system architecture of the intelligent rolling contact fatigue test equipment is constructed. The solutions of slip ratio and the experimental load controlled by these methods are satisfactory with the requirement of design precision. In the validation experiment, the load control accuracy is ±3%, the average error of load control is 1.77%, and the average error of slip control is 0.26%. The experiment results show the proposed control methods are feasible and effective.

Re-investigation of dark etching regions and white etching bands in SAE 52100 bearing steel due to rolling contact fatigue

Microstructural alterations such as dark etching regions (DERs) and white etching bands (WEBs) have been known to manifest in the subsurface of steel bearings due to rolling contact fatigue (RCF) under medium-high stress cycles. Even though such manifestations have been reported substantially in literature for decades, their formation and their evolution mechanisms are not fully understood. As part of the re-investigation of DERs and WEBs, this paper presents the results from characterization of DER and WEBs, including both low angle bands (LABs) and high angle bands (HABs), formed in SAE 52100 bearings at different stages of bearing life. Angular contact ball bearings (ACBBs), subjected to RCF testing under different contact pressure, stress cycles and steel cleanliness, have been examined to reveal new information related to the microstructural alteration processes, e.g. the 3-D structure of the WEBs and their growth pattern. Contrary to some of the literature results, the experimental results have shown that the mean depths and density of WEBs (LABs and HABs), as opposed to DER, are correlated with the position and distribution of the principle shear stress. The results also show that HABs form as a consequence of LABs especially in densed LAB areas in the ACBBs studied here.

Tribochemical degradation of vacuum‐stable lubricants: A comparative study between multialkylated cyclopentane and perfluoropolyether in a vacuum ball‐on‐disc and full‐bearing tests

AbstractIn this study, the decomposition of a multialkylated cyclopentane (MAC) and a perfluoro‐poly‐ether (PFPE) lubricant was investigated in a ball‐on‐disc sliding test under vacuum and in a rolling contact fatigue (RCF) test on full bearings. In situ mass spectrometry was conducted to study the tribochemical formation of hydrogen in a steel‐on‐steel ball‐on‐disc sliding contact. Further investigations on lubricant degradation in rolling‐sliding contact indicated fragmentation and oxidation reaction of the MAC oil by interaction with the oxide layer and the nascent bearing steel.Finite element method (FEM) analysis was carried out to model the contact in the ball‐on‐disc experiments and to extract contact surface temperature fields.Post‐RCF hydrogen analysis of bearing components indicated increased hydrogen content in the samples lubricated with the MAC fluid. The RCF tests with PFPE did not indicate any signs of surface damage.

Subsurface Crack Propagation from Internal Defect in Rolling Contact Fatigue of Railway Wheel Steel

A rolling contact fatigue failure is one of the main fatigue damages in railway wheels due to cyclic rolling contact with rails. This fatigue failure caused by internal defects such as nonmetallic inclusions or voids might occur in heavy haul freight car wheels. Subsurface crack propagation behaviors from the internal defects are evaluated by twin disc type rolling contact fatigue tests using test specimens with artificial defects and by finite element analyses simulating the rolling contact fatigue tests. The subsurface cracks are more likely to propagate in the test specimens with larger artificial defects. In addition, the cracks initiating from the trailing side of defects propagate faster than those from the leading side. Shear mode equivalent stress intensity factors obtained from the finite element analyses correspond well to the results of rolling contact fatigue tests. The test specimen models with larger defect have larger equivalent stress intensity factor ranges than the test specimen model with smaller defect in both leading and trailing side. The results of finite element analyses also suggest that the crack propagations are affected by the deformation of artificial defects and this leads to have higher resistance to crack propagation in the test specimens with smaller internal defects.

Experimental investigation on evolution of surface damage and topography parameters during rolling contact fatigue tests

AbstractNowadays, micropitting is perceived as one of the prevalent wear mechanisms, which is responsible for the breakdown of high power density (power throughput/weight) machines. The current work demonstrates the evolution of surface damage and surface topography parameters during micropitting tests. The test surfaces are examined at different load stages utilizing 3D optical profiler, optical microscope, and scanning electron microscope. The effect of slide‐to‐roll ratio (SRR) on surface damage and topography parameters is also presented. It is observed that the severity of micropitting increases with an increase in the number of cycles. It is concluded that surface bearing index (Sbi), kurtosis (Sku), maximum valley height (Sv), and autocorrelation length (Sal) are positively correlated with surface damage, whereas mean summit curvature (Ssc) and skewness (Ssk) are negatively correlated with surface damage. Surface texture ratio (Str) and power spectral density (PSD) of test surfaces are used for analysing surface texture, and it is found that deterioration in surface texture occurs after 6 million cycles. It is also concluded that geometrical isotropy increases with an increase in SRR.

Optimization of the rolling contact fatigue (RCF) performance of carbidic austempered ductile iron (CADI)

Abstract Austempered ductile iron (ADI) has a good combination of mechanical properties and castability allowing it to replace forged steels while offering lower costs. CADI is thought to improve ADI's abrasion resistance by the reinforcement with carbides. However, there are other potential applications where different wear mechanisms are present. This study is a continuation of a previous work on CADI in which the influence of the microstructure on the RCF performance was evaluated. Hence provides tools for microstructural design to obtain an optimal performing CADI for each application. Four CADI microstructures were evaluated in a flat washer rolling contact fatigue testing rig with parameters leading to interaction of asperities. It was found that the RCF performance of CADI can be improved by microstructure control.

Observation of Subsurface Crack of Carburized Steel (SCM415) under Single-Ball Rolling Contact Fatigue over 10<sup>7</sup> Cycles

Bearing fails due to the flaking failure which is caused by the subsurface cracks. The observation of the subsurface cracks is not easy beacause the cracks propagate inside the material. In order to observe the whole subsurface cracks, we performed rolling contact fatigue (RCF) tests of carburized SCM415 until over 107 cycles with the single-ball RCF machine. After the RCF tests, we directly observed the subsurface cracks.

Failure Observation of 3D-Printed Thrust Bearing Specimens with Inner Defects in Water Conditions

Additive manufacturing (AM) methods are developing and have become popular, but questions remain about the strength of parts made by this method. We have already investigated the effect of the lamination direction on the fractures in bearing specimens. In this study, we made inner defects in the 3D-printed bearing specimens and performed rolling contact fatigue (RCF) tests on this specimens immersed in a water bath in order to investigate the fracture of 3D printed bearings.

Observation of Tribological Fatigue Fracture on PEEK Shaft with Artificial Defect under One-Point Rolling Contact by Using 2.5D Layer Method

In order to explore the mechanism of tribological fatigue fracture in PEEK (Poly-ether-ether-ketone) polymer mechanical element application, one-point contact type RCF (rolling contact fatigue) tests were carried out by using a PEEK shaft with an artificial defect. An alumina ball contacted a PEEK shaft specimen under maximum Hertzian stress 380 MPa. Flaking and internal fatigue crack propagation under the rolling track of the tested PEEK shaft were investigated through 2.5D layer observation method. The main fatigue crack occurred near the artificial defect on the rolling track of the PEEK shaft, and propagated into depth direction. In addition, the main fatigue crack branched due to internal shear stress. The branching crack as internal fatigue crack propagated into the ball’s rolling direction. After the linkage of the branching crack and another semicircular surface crack, the horseshoe-shaped flaking as tribological fatigue fracture occurred on the rolling track of the PEEK shaft.

Twin disc assessment of wear regime transitions and rolling contact fatigue in R400HT – E8 pairs

Twin disc tests were carried out to evaluate the wear resistance and Rolling Contact Fatigue (RCF) of premium R400HT rail samples in contact with E8 wheel samples. The wear rate and friction coefficient were correlated with the frictional work expended at the contact interface (the Tgamma approach). Accelerated RCF tests were also carried out on the premium R400HT rail and the results were compared to those obtained for standard R260 rail. The wear rates of rail samples were consistently lower than those reported in the literature for other contacting pairs in which the rail material studied is softer than R400HT. Also, the energy needed for the transition from the moderate to severe wear regime significantly increased for the hardened rail. Fatigue cracks were shallower for R400HT when compared with standard rail material. Hardened rails also showed lower mean spacing between fatigue cracks. This new information can be used to improve wear simulations of wheels and rails by using more realistic wear equations.

Computer modeling of processes of wear and accumulation of rolling contact fatigue damage in railway wheels using combined criterion

An algorithm for computer modeling of the processes of wear and accumulation of the rolling contact fatigue (RCF) damage in railway vehicle wheels is introduced. The combined criterion of contact fatigue is obtained on the basis of the specific strain energy and of the criterion theories of strength. The proposed criterion is tested on the Hertzian problem with full sliding. RCF curve for wheel steel with a hardness 295 HB using proposed criterion is constructed by processing the results of the RCF tests of the wheel steel specimens. The example of modeling of the processes of wear and accumulation of RCF damage in the car wheel using combined criterion is given.

Fatigue property and microstructure deformation behavior of multiphase microstructure in a medium-carbon bainite steel under rolling contact condition

Bainitic steel is considered to be the best potential candidate steel for rails due to its high strength and excellent toughness. The objective of this work was to investigate the fatigue property and microstructure deformation behavior of a medium-carbon bainitic steel with bainite/martensite/retained austenite multiphase microstructure obtained by a modified Q&P process (B-QP) under rolling contact condition. The analysis results of TEM indicated that the multiphase microstructure consists of lath bainitic ferrite, lath martensitic ferrite and film-like retained austenite which distributes mainly between bainite and martensite laths. The results showed that the multiphase microstructure has a better combination of toughness (103 J), hardness (47.8 HRC) and elongation (18.6%), along with better rolling contact fatigue (RCF) properties and wear resistance than that of full bainite microstructure obtained by the austempering process (BAT). After the steel subjected to 105 cycles of RCF testing, nanocrystalline grains (approximately 93 nm and 67 nm for the BAT and B-QP sample, respectively) were obtained at the deformed layer depth of 10 μm. Meanwhile, the multiphase microstructure demonstrated a higher microhardness increasing ratio (24.5%) than that of the full bainite (21.2%) and a larger plastic deformation zone (103 μm). The EBSD and XRD analysis indicated that the multiphase microstructure has a lower stress concentration possibility and higher percentage of high-angle boundaries as well as some film-like austenite retained in the plastic deformation layer. In addition, many pits on the contact surface of the BAT sample appeared, due to spalling damage caused by cyclic loading, and the crack propagation depth was larger. Conversely, the contact surface of the B-QP sample with a multiphase microstructure looked smooth, and the crack propagation depth was shallow under the contact surface. The enhanced rolling contact fatigue properties and wear resistance of multiphase microstructure can be mainly attributed to better a combination of strength and toughness, larger plastic strain accumulation and lower fatigue damage level.

Rolling contact fatigue crack propagation relative to anisotropies in additive manufactured Inconel 625

Additive manufacturing is steadily gaining acceptance in certain industry segments as a process for the manufacturing of dense metallic components. The Ni-based superalloys belonging to the Inconel family have for many years been in focus for AM research and AM produced components are now becoming commercially available. However, it is still unclear how the microstructural anisotropy, inherent to most AM materials, affects the material performance in a given application. The anisotropy may e.g. influence the mechanical properties and the performance in certain tribological situations, such as when subjected to rolling contact fatigue. Like most AM methods, the powder bed fusion – laser beam process gives the produced components a relatively rough surface. To perform well in demanding tribological situations, the components are commonly machined to a smooth finish.In this work, Inconel 625 produced using PBF-LB is evaluated in a rolling contact fatigue test. Test cylinders (Ø10 mm) have been produced using different build directions and scan strategies, resulting in varying microstructures and textures.In the rolling contact fatigue test, a cylindrical sample is mounted between two Ø 140 mm metal rollers, pulled together via a spring. After testing, the contact tracks are studied using SEM and EBSD to reveal cracks. Cracks were analysed with respect to the microstructure and anisotropies.It was found that the anisotropy influences both the nucleation and growth of cracks. The AM produced specimens were also found to be more prone to transgranular cracking than conventional Inconel 625, which predominantly displayed intergranular cracks.

Rolling fatigue life of PM steel with different porosity and surface finish

Gears made of PM steel are of interest for the automotive industry because they can be produced to near net shape with only a few processing steps.Automotive gears experience a complex contact situation with rolling as well as combined rolling and sliding. They also have to be able to withstand high loads and fairly high temperature variations.Earlier work show that the main limiting parameter of the contact fatigue life of PM steel is the porosity. A higher porosity/lower density will decrease the fatigue life of the PM component.In the present study, the pure rolling contact fatigue life of PM steel with different density and surface finish has been investigated. A rolling contact fatigue test rig, where rods of the tested material are mounted between two rolling wheels, was used. Two densities of PM steel, 6.8 and 7.15 g/cm3 and a full density reference steel with two different surface finishes, centerless grinded and superfinished, were tested. The tests were run for a given number of load cycles or until failure (fatigue life). SEM was used to study the surfaces and cross-sections to reveal the mechanisms of crack initiation and propagation.The higher density PM steel (7.15 g/cm3) outperformed the lower density steel (6.18 g/cm3) by a factor of around 4 in fatigue life at the same surface pressure, regardless of surface finish. Cracks are initiated at a depth of around 100 µm. These cracks propagate and eventually they reach the surface, causing surface damage and failure. For the low density PM steel, both sub surface crack initiation and failure occurred earlier (at a lower number of load cycles) than for the higher density PM steel.Severe surface damage or wear were not found until failure occurred. Still, some initial alteration of the surfaces was seen already after 0.5 million load cycles, in the form of removal of the highest asperity peaks on the centerless grinded surfaces, and opening of the surface pores on the superfinished surface. No effect of surface finish was found on the fatigue life. The difference in surface roughness could induce a difference in local stress concentrations at the surface, but in this test the cracks causing fatigue failure are initiated at a depth where the stress distribution is not affected by local surface stress concentrations. This means that for fairly smooth surfaces roughness, the surface of PM steel is not important when it comes to pure rolling fatigue life.

Effect of ultrasonic surface rolling on microstructure and rolling contact fatigue behavior of 17Cr2Ni2MoVNb steel

Ultrasonic surface rolling (USR) treatment with different rolling loads (600, 800, 1000 and 1200 N) was introduced to the 17Cr2Ni2MoVNb steel. The microstructure and rolling contact fatigue (RCF) behavior after USR treatment were investigated. Results pointed out that grain refinement existed in the surface-modified layer of the material after USR treatment. The treatment with rolling load at 1200 N could produce a best surface enhancement effect, compared to the untreated sample, the microhardness of USR treated sample (25 kHz, 1200 N) was increased from 795 HV0.2 to 905 HV0.2, with an increase by 13.8%. The surface residual compressive stress was increased from −334 MPa to −527 MPa, with an increase by 57.8%. However, the samples treated at 1200 N increased the surface roughness and caused some surface cracks on the rolled surface, which was an important factor to reduce the fatigue life. The results from RCF tests showed that USR treatment with rolling load at 1000 N exhibited the maximum mean life of 3.71 × 106 cycles.

Influence of laser strengthening techniques on anti-wear and anti-fatigue properties of rail welding joint

The fatigue damage of an on-site rail welding joint was analysed. The fatigue cracks were longer and deeper on the welding joint than on the normal rail material. The influence of two laser strengthening techniques, laser quenching (LQ) and laser shock peening (LSP), on the anti-wear and anti-fatigue properties of the welding joint were explored through rolling contact fatigue (RCF) tests. The LQ process significantly improved the anti-wear property of the rail welding joint, but the anti-fatigue property got worse. The wear rate of the LQ treated welding joint was decreased by 91.6–92.4%, however, the average crack angle (23–25°) and the average crack length (62–74 μm) on the LQ treated welding joint were larger than those on the untreated rail welding joint (13–15° and 37–59 μm, respectively). The LSP process improved both the anti-wear and anti-fatigue properties of the welding joint. The wear rate of the LSP treated welding joint was decreased by 5.1–10.1%. Meanwhile, the density of cracks (0.9–4.2 mm−1), the average crack angle (10–13°), and the average crack length (16–26 μm) on the LSP treated welding joint were smaller than those on the untreated and LQ treated welding joints.

Investigating the micropitting and wear performance of copper oxide and tungsten carbide nanofluids under boundary lubrication

The potential of copper oxide (CuO) and tungsten carbide (WC) nanofluids in enhancing micropitting and wear behavior of AISI 8620 steel under boundary lubrication conditions was investigated. The nanofluids consisted of 1% nanoparticles by weight and 1% by weight of oleic acid surfactant in Polyalphaolefin (PAO). Rolling contact fatigue tests were conducted using a micropitting test rig (MPR). Both the nanofluids exhibited increased micropitting life compared to the base oil. Tungsten carbide nanofluids showed significantly higher micropitting and wear resistance behavior than the CuO nanofluids under the boundary lubrication regime. Analysis of the surfaces showed different mechanisms to inhibit micropitting and wear for the two nanofluids. The WC nanofluid formed a tribofilm whereas the CuO nanofluids tended to fill surface cracks with the nanoparticles.

Assessment of Topography Parameters During Running-In and Subsequent Rolling Contact Fatigue Tests

Rolling contact fatigue (RCF) is one of the major problems observed in gear mechanisms, which leads to high friction, ultimately resulting in high energy consumption. This paper demonstrates the evolution of surface topography during running-in and subsequent RCF tests under boundary or mixed-elastohydrodynamic lubrication regimes. The case-hardened disks of equal surface finish and hardness are used in the experiments, and the evolution of surface topography is investigated using a white light interferometer. Surface topography at different load stages is measured at three distinct points, on the disks and average roughness and topography parameters are reported. Semi-quantitative techniques are used to determine the asperity-level parameters at different load stages. From the running-in experiment, it is found that running-in is a fast process where substantial change in surface topography occurs due to plastic deformation of most prominent asperity. From the RCF test, it is concluded that within range of the fatigue cycles, the root-mean-square (RMS) roughness (Sq) is negatively correlated with the summit radius (R) and the autocorrelation length (Sal) and positively correlated with the summit density (Sds) and the RMS slope (Sdq). Scanning electron microscope (SEM) analysis reveals the disappearance of grinding ridges, the formation of micropits at a very small scale, and pit growth in the sliding direction.

Mechanical behavior of wheel steels with solid solution and precipitation hardening

Purpose: The aim of the proposed research is to investigate operational properties of a wheel steel treated with simultaneous solid solution and precipitation hardening at various carbon content, in comparison with the standard wheel grade T steel. Design/methodology/approach: The mechanical behaviour of wheel steels with increased content of silicon, manganese, vanadium, and nitrogen at various carbon content has been investigated and compared to that of the standard high-strength wheel grade T steel. The steels were undergo thermal treatment due to austenitic heating up to a temperature of 950ºС with cooling down in water to 550ºС followed by intense blowing of blanks in the air. After that, a tempering was performed at a temperature in the range of 450-650ºС. Static strength (UTS), relative elongation (TEL), impact toughness tests (KCV) were determined on standard specimens. The characteristics of Mode I fatigue crack growth resistance of steel were determined on the basis of fatigue macrocrack growth rate diagrams da/dN–KІ, obtained by the standard method on compact specimens with the thickness of 10 mm at a frequency of 10-15 Hz and the stress ratio R = 0.1 and R = 0.5 of the loading cycle. The characteristics of Mode II fatigue crack growth resistance were determined on the basis of da/dN–KІІ diagrams, obtained earlier method on edge notched specimens with the thickness 3.2 mm at a frequency of 10-15 Hz and R = –1 taking account of the crack face friction. Rolling contact fatigue testing was carried out on the model specimens. Findings: The regularities of the change of mechanical characteristics of the high-strength wheel steel with simultaneous solid solution and precipitation hardening at lowered carbon content under static, impact and cyclic loading are studied. Research limitations/implications: The results obtained using laboratory samples should be checked during a real railway wheels investigation. Practical implications: The investigated steel with simultaneous solid solution and precipitation hardening provides high wear resistance of the tread surface and damage resistance determined on the model wheels. Originality/value: A steel with solid solution hardening due to increased content of silicon (up to 0.7%) and manganese (up to 0.8%) and also with precipitation hardening (at optimal content of vanadium and nitrogen [V‧N]‧104 = 28.9%) at lowered carbon content (0.52) possesses high strength and fatigue fracture toughness in cases of Mode I and Mode II loading, causing better combination of wear and damage resistances of the tread surface of the model wheels, as compared to corresponding parameters for grade T steel.

The effect of over-based calcium sulfonate detergent additives on white etching crack (WEC) formation in rolling contact fatigue tested 100Cr6 steel

Abstract Over-based calcium sulfonate (OBCaSul) additives in oils are considered potentially critical in driving WEC formations, this ‘driving’ effect being unclear. Rolling contact fatigue testing of 100Cr6 steel using FE8 and PCS Micro-Pitting-Rig lubricated with oils containing varying OBCaSul concentrations have been conducted, tested samples being analysed using serial sectioning, Thermal desorption analysis (TDA) to measure hydrogen diffusion, and SEM/EDX of the tribofilms formed. Results show that OBCaSul concentration appears to affect WEC formation propensity, a reversal relationship of formations being shown between test rigs. Evidence shows oils containing OBCaSul form thick Ca dominated tribofilms, potentially promoting hydrogen diffusion and WEC formations in the FE8 rollers, where thinner Zn S tribofilms on the FE8 raceways may demote hydrogen diffusion and WEC formations.