Rolling Contact Fatigue Tests Research Articles

Multiaxial fatigue criteria versus experiments for small crack under rolling contact fatigue

Rolling contact fatigue (RCF) is traditionally a very critical loading condition for fatigue and, moreover, material defects (as inclusions and inhomogeneities) play a significant role in determination of the service life of materials exposed to out-of-phase stresses, which typically occur at the interface and below the surface of contacting bodies.In this paper we summarize the results previously obtained for two different hard steels (that is, a bearing and a gear steel), together with a new set of experimental data for a mild railway wheel steel, that have been subjected to out-of-phase multiaxial fatigue loading, simulating RCF conditions in presence of small shallow pre-cracks.Then, the experimental results obtained have been discussed by employing the Dang Van and Liu–Mahadevan criteria, which are criteria extensively applied in the case of RCF problems.The results show that the Liu–Mahadevan criterion is close enough to experimental RCF tests, while the Dang Van criterion needs a substantial modification for the load cases when a negative hydrostatic stress component is present.

Effect of PTFE Retainer on Friction Coefficient in Polymer Thrust Bearings under Dry Contact

Polyetheretherketone (PEEK) is a tough semi-crystalline thermoplastic polymer with excellent mechanical properties. While abilities of polyphenylenesulfide (PPS) are similar to PEEK, former material cost was lower than later. Polytetrafluoroethylene (PTFE) is well known because of its low friction coefficient and self lubrication ability. The objective of this study is to observe the friction coefficient of hybrid bearings, PTFE retainer sandwiched with PPS-races or PEEK-races. Rolling contact fatigue tests were performed and in situ friction forces wear measured. It is concluded that the PTFE retainer reduced friction coefficient.

Relationship between Life, Load and Rotation Speed of UHMWPE Bearing under Dry Rolling Contact Fatigue

The polymer bearings have been widely used in recent years. In this study, ultra-high-molecular-weight-polyethylene (UHMWPE) is investigated. In order to investigate the relation between the lives, loads and rotation speeds, rolling contact fatigue tests were conducted. It was found that rotation speed related to the bearing life and wear loss.

Surface Profile Observation of PTFE Radial Bearings under Rolling-Contact-Fatigue in Water

Groove shape effect on wear behavior in PTFE radial bearings under rolling contact fatigue (RCF) was investigated. RCF tests in water-lubricated conditions were carried out at different loads and rotation speeds. The groove surfaces after testing were observed by using a laser confocal microscope and a two-dimensional shape measurement sensor. It was found that PTFE bearings under RCF generate wear debris, however no cracking or flaking failure could be observed. It is concluded that groove deformation depend on load rather than rotation speed. The biggest changes in groove profiles occurred when tested at loads close to 400N.

Fourier Transform Infrared Spectroscopy for Wear Debris Adhesion on PEEK Bearing Surface

Polyetheretherketone (PEEK), a tough semi-crystalline thermoplastic polymer with excellent mechanical properties and polytetrafluoroethylene (PTFE), valued for its low friction coefficient are popular materials used for the production of bearings. In this work, rolling contact fatigue (RCF) tests were performed in order to investigate wear on bearing surfaces by using the Fourier Transform Infrared Spectroscopy (FT-IR). It is reported that PEEK’s specific peak at 1243cm-1was shifted and PEEK’s polymer bearing crystalline content level on wear surface remained unchanged by contact stress or wear debris adhesion.

Single-Ball Rolling Contact Fatigue of 13Cr-2Ni-2Mo Stainless Steels Quenched by Induction Heating Method

Rolling contact fatigue tests of 20mm-diameter 13Cr-2Ni-2Mo stainless steel bars was carried out using a newly developed single-ball system. The bars were quenched by induction heating method and after that tempered. Sectional observations of the subsurface cracks were made after the surface layer separations. Based on the observations, stress analysis was performed in order to investigate the relation between flaking failures and subsurface crack growth. From these calculations it was found that the shear stress propagated subsurface cracks and finally caused the flaking failures. Furthermore, eight specimens were fatigue tested and Weibull statics distribution of RCF life were calculated using Johnson's statistical method. It was found that the basic life (L10) of the bars under 5.3GPa Hertzian stress was 8.33×106 cycles.

1.5Cr – 0.2Mo 焼結合金鋼ローラの面圧疲労強度

The rolling contact fatigue tests were carried out using case-carburized rollers made of 1.5Cr – 0.2Mo alloyed sintered steel and SCM415 wrought steel. Two kinds of P/M rollers having the initial density of 7.25 × 103 kg/m3 and 7.60 × 103 kg/m3 were prepared. The effects of the case-hardening depth on the rolling contact fatigue strength were mainly investigated. The failure mode of all P/M rollers was spalling caused by the cracking under the contacting surface. When the effective case-hardening depth was set to 1.2 mm or more, the contact fatigue strength after 2 × 107 cycles of surface rolled P/M rollers reached 2 GPa and the maximum surface damage depth remarkably decreased of about 0.2～0.3 mm, which is almost equivalent to the depth that maximum shear stress becomes the peak. The finite element analysis of contact stress shows that the maximum shear stress distribution descends suddenly at the position that is slightly deeper than the effective case-hardening layer, which is corresponding to the location of the maximum failure depth of spalling when the effective case-hardening depth of P/M rollers was less than 1 mm.

OS1504 放射光μCT及び小型転動疲労試験機を用いた転動疲労き裂進展過程の観察

In rolling contact fatigue (RCF), cracks are usually initiated from inclusions under the surface and propagated to the surface. In the present study, the micro computed tomography (μCT) imaging and laminography using synchrotron radiation in SPring-8 is applied for observation of RCF cracks. The compact RCF testing machine is developed to observe RCF crack behavior m-situ. In this testing machine, the steel ball rolls on the specimen, linearly and reciprocally, and a specimen can be attached and removed easily. Since the size of the sample for μCT imaging has to be limited by penetrative power, the sample for μCT imaging has 500μm×500μm cross-section. On the other hand, laminography can be applied to a sheeted material. The sample, where an artificial defect is introduced, is used for observation of crack initiation and propagation. It is found from this experiment that cracks initiate and propagate in the depth direction along the artificial hole with increasing the number of cycles. Flaking and RCF crack in sheeted sample are observed by laminography. RCF cracks which initiate and propagate under contact surface can be detected. The shape and location observed by laminography coincide with that by SEM.

풍력발전시스템의 주 베어링용 0.18C-3.5Ni-1.5Cr-0.2Mo강의 침탄 표면특성

Characteristics of carburized surface layers in 0.18C-3.5Ni-1.5Cr-0.2Mo steels for main shaft bearings of wind turbines have been analyzed and evaluated before and after rolling contact fatigue tests. Mixed microstructure consisting of retained austenite and tempered martensite has been formed with compressive residual stresses in the surface hardened layers of the specimens showing uniform hardness distribution with value about Hv700 after vacuum carburizing and tempering. It has been found on the raceway of the layers of the specimens after rolling contact fatigue tests that the amount of retained austenite decreased and compressive residual stresses increased, resulting from cyclic contact stresses applied during the tests. It has been also revealed that higher durability of the bearings can be obtained through controlling the amount of the retained austenite in the surface of the bearing steels to be lower in this study.

Micro-pitting fatigue lives of lubricated point contacts: Experiments and model validation

This study employs the two-disk rolling contact fatigue test methodology to investigate the impacts of various parameters on micro-pitting performance of lubricated point contacts of rough surfaces. A test matrix that spans the operating ranges of the sun-planet gear pair of a wind turbine gearbox is constructed using the Fractional Factorial technique to rank the order of the influences of contact pressure, rolling velocity, slide-to-roll ratio, roughness amplitude and run-in process. The test results indicate that a run-in stage with higher contact pressure and lower rolling velocity reduce the amount of micro-pits. For the normal operating stage that follows, lower roughness amplitude, lower slide-to-roll ratio, higher rolling velocity and lower contact pressures are observed to lead to reduced micro-pitting activity. To quantify micro-pitting failure, the micro-pitting severity index (MSI) which is defined as the cumulative probability of fatigue failure is proposed. The micro-pitting test outcomes are compared to the predictions of a recently developed physics-based micro-pitting model [1] to describe the failure mechanism and assess the model accuracy.

Application of the local strain approach on a rolling point contact model

A newly developed cold forming process, the linear flow splitting process, allows the bifurcation of thin metal sheets by severe plastic deformation (SPD). This process produces flanges with an ultra-fine grained (UFG) microstructure, which is characterized by an increased hardness and lower surface roughness in comparison to the material in as-received state. The technology is researched within the Collaborative Research Centre 666 (CRC 666) “Integral Sheet Metal Design with Higher Order Bifurcations”. The increased hardness and reduced surface roughness of the flanges make this technology suitable for manufacturing linear guide rails. In order to achieve the required reliability and fatigue strength, the assessment of the performance of the linear flow-split profiles with gradient microstructure is of great importance. When considering rolling contact fatigue, the examination of the mechanical behavior of the side of the flange with higher hardness due to the UFG microstructure is required. In order to reduce time and costs, a numerical method to determine the rolling contact fatigue behavior would be useful minimizing the normally applied experimental effort. The present work aims to check the applicability of the local strain approach for the evaluation of the rolling contact fatigue behavior of linear flow split components by a Finite Element (FE) analysis. The Hertzian theory is only valid for homogeneous materials and cannot be directly applied to the flanges with gradient microstructure. Therefore an accurate modeling of the material in order to take into account the gradient in the mechanical properties is required. In this paper the characterization of the material behavior under axial cyclic loading will be presented. These results will be used for the Bergmann damage parameter to analyze the rolling contact fatigue behavior. Some parameters required for the application of the Bergmann approach are determined by a numerical simulation of the rolling contact. Results are compared to rolling contact fatigue tests performed on a recently developed test rig under the assumption of rolling without sliding and neglecting the effect of surface roughness and residual stresses.

Relationship between Load, Rotation Speed and, Strength in All - PEEK and PEEK Race – PTFE Retainer Hybrid Polymer Bearings under Dry Rolling Contact Fatigue

The production of bearings is focusing on polymers, such as polyether ether ketone (PEEK), a tough semi-crystalline thermoplastic polymer with excellent mechanical properties and polytetrafluoroethylene (PTFE), a material with low friction coefficient. The purpose of this study was to establish the properties of hybrid PEEK races – PTFE retainer bearings. Rolling contact fatigue tests were performed in order to investigate the wear properties of such components and the relation between load and rotation speed. It is concluded that by using a PTFE retainer, no wear occurs under thrust load ranging from 200N to 800N at 300rpm.

Microstructure and Rolling Contact Fatigue Strength of Induction Heated AISI 52100 Bearings

In the present work the microstructure and the rolling contact fatigue properties of induction heated AISI 52100 bearings are investigated. The bearings were heat treated by using a flat coil at 30 kW power and 60 kHz frequency, cooled with water and subsequently tempered for 1 hour at 180 °C. The hardness at the surface of the material was close to 900 HV0.3kgf/15s. The hardening depth of the induction heated sample was higher than 5 mm. The retained austenite content was around 18% at the surface and decreases along with the depth. The samples were rolling contact fatigue (RCF) tested up to 107 and 4.5x107 cycles, at Hertzian stress 4 GPa. No flaking failure was observed on the bearing races. For tests up to 107 cycles the track size was around 690 um and this remained unchanged up to 4.5x107 cycles. The residual stresses at the material surface before testing were close to zero and became highly compressive after the RCF testing. Stress induced transformation occurred at the surface and the retained austenite content after testing reached around 10%. Induction heating was successfully applied to induce martensitic transformation in AISI 52100 steel and the bearings showed very high fatigue strength under rolling contact.

Observation of Crack Propagation in PEEK Polymer Bearings under Water-Lubricated Conditions

Radial ball bearings made of metal, ceramics and plastics are commonly used as important components in various types of industrial machinery. Due to the latest markets demands for elements capable of withstanding e.g. corrosive environment, metallic bearings are being gradually replaced by components produced from high performance engineering plastic polymers. In order to investigate the failure mechanism of polymer bearings and further improve their performance in practical applications in an underwater environment, in this research crack propagation in Poly-ether-ether-ketone (PEEK) was studied by rolling contact fatigue (RCF) testing under water. Crack propagation in the inner ring raceway surface and subsurface areas of PEEK bearings after testing was observed by a laser confocal microscope. Cracks and flaking failure were found on the bearing raceway surface. From the RCF tests results, it was found that the detected cracks could be divided into three groups: Main Surface Cracks, Semi-circular Cracks and Main Subsurface Cracks. It is concluded that flaking occurs on the inner ring raceway due to the fusion of semi-circular cracks and a main subsurface crack.

Early Stages of the Wear Behavior of AISI 440C Stainless Steel under Rolling Contact in Water

In the present work, rolling contact fatigue (RCF) tests in water were performed on AISI 440C stainless steels under different loading. Each test was interrupted at 3.6×104, 7.2×104, 1.44×105, 2.16×105, 2.88×105 and 2.88×105 rotating cycles and the wear track at different stages was observed by using a 3D laser confocal microscope. The wear loss at 2100 N was a significantly higher compared to 500 N or 1000 N. The contact surface roughness in samples tested at 2100 N increased during the rolling contact and severe adhesion wear was present at the entire surface. In case of 500 and 1000 N tests, the surface roughness remained low with mild adhesion wear occurring. It is concluded that adhesion force levels are higher under high load rolling contact. They greatly influence the surface conditions and cause high wear loss.

Observations of Cracks from Microscopic Holes of PEEK Bearings under Rolling-Contact Fatigue in Water

The behavior of surface cracks in PEEK thrust bearings under rolling contact fatigue (RCF) was investigated. Eight small holes were introduced along the bottom race surface, and RCF tests underwater using different loads were carried out. The cracks growing from the holes were observed by using a microscope. It was found that the surface cracks could not be explained by Hertzian crack theory or the ‘wedge effect’ models. This indicates that the standard theories cannot be generally applied to investigation of PEEK bearings working under RCF in water.

Observation of Crack Initiation from Inclusions in Rolling Contact Fatigue Tested Specimens, Using a Newly Developed Single-Ball Testing Device

In order to obtain experimental data to investigate the mechanism of crack initiation and propagation, an innovative rolling contact fatigue (RCF) machine was developed. Compared to the conventional thrust type RCF machine the new device enables more efficient RCF testing and observation of subsurface cracks. Experimental data and information on inclusions and micro-cracks were obtained through observation by a laser confocal microscope and comparison with stress analysis. The depth of detected crack initiation is strongly correlated with the stress distribution.

The effect of nanoclay reinforcement on the rolling contact fatigue behaviour of polyamide

Polymer nanocomposite has received great research interest in design of engineering components owing to its remarkable improvement in tribological characteristics. Rolling contact fatigue is a predominant failure mode of the many functional components like gears, bearings, cams, ball screw rods and rail wheels. Nanosize clay fillers were dispersed into the polymer using melt intercalation method to produce polymer nanocomposite material. Injection-molded disc-type specimens were made to run against each other on twin-disc test rig designed in-house. To understand the behaviour of the materials, rolling contact fatigue behaviour against different contact loads under constant speed were studied. Rolling contact fatigue testing of polymer nanocomposite has revealed significant difference from the pristine PA6 behaviour in failure modality and temperature rise. Addition of nanoclay to polymer although improved the modulus and strength of the nanocomposite, deteriorated the rolling contact behaviour. Rolling contact fatigue performance of PA6 was better than that of the clay-reinforced polymer nanocomposite under all tested conditions. Polymer nanocomposite materials have shown poor wear resistance as the specific wear rate was calculated to be higher for PNC materials under rolling contact fatigue conditions. Difference in surface temperature rise in PA6 and polymer nanocomposite has led to the different failure mechanism and reduction in rolling contact fatigue life of polymer nanocomposite materials.

Crack Propagation Behavior of Silicon Nitride Using a Ball-on-Flat Rolling Fatigue Bench Test Methodology

Crack propagation behavior prior to flaking in rolling contact fatigue (RCF) tests has been analyzed by microscopic observation to gain insight into the mechanism for final spall of specimen. Balls-on-flat RCF tests under constant and stepwise load were carried out for three bearing grade silicon nitrides and one general purpose one at several stress levels. Detailed observations of crack propagation on a contact track during tests under constant load reveal that a linear crack, which shows up just outside of the track, grows at both ends of the crack with an increase in stress cycles. This growth, opposite to the rolling direction, eventually leads to the formation of hook-like cracks. Immediately before spalling, a major hook-like crack and associated arc-like cracks are formed. From the surface and section views of spalled regions of the specimens tested under stepwise load, we found supporting evidence for the hypothesis that major crack growth inclined toward the rolling direction forming at the bottom surface of the spall, with subsequent upward crack growths from the major crack and associated arc-like ones on the specimen surface, eventually developing into a crack network leading to the dislodging of fragments.

Experiments under pure shear and rolling contact fatigue conditions: Competition between tensile and shear mode crack growth

In the present paper, the mechanism of shear crack growth under both pure torsion and mixed mode loadings, simulating rolling contact fatigue testing conditions, has been investigated for a bearing steel and the role of the superimposed compressive stress in subsurface RCF has been clarified both numerically and experimentally. In particular a previous data set of fatigue tests on micro-notched specimens subjected to torsion and out-of-phase loads with |σmin|/τmax≅3.5 (LP1) has been complemented with the new tests onto micro-notched specimens loads with |σmin|/τmax≅0.7 (LP2) and a test under pure compression. The same tests have been also simulated numerically with a non-linear FE analysis of crack advance. The numerical analyses have been conducted with the aim of demonstrating that the compressive stress fully suppresses the tendency to tensile mode growth as the crack extends.Eventually, the competition between tensile and shear mode growth during a fatigue cycle has been investigated theoretically in terms of local branch SIFs. In particular, the conditions for the branch crack growth have been examined on the basis of the effective SIFs: the crack tip shielding effects due to the crack surface interference (both the mode I contribution caused by the asperity mismatch and the shear attenuation produced by the frictional stresses) have been quantified by employing a model for crack sliding interaction under pure mode III and mixed mode I+III loadings.