Contact Fatigue Phenomenon Research Articles

Numerical and Experimental Investigation of the Effects of Thin Hard Coatings on the Strength of Spur Gears

Abstract: The use of thin films to coat mechanical components is a well-established practice. The main uses concern areas in which scratch resistance, corrosion resistance, surface hardness and particular aesthetic characteristics are required. In addition to these properties, it has been demonstrated that Physical Vapor Deposition (PVD) processes bring further benefits from a structural point of view, especially as regards fatigue and contact fatigue phenomena. In this research work, predictive models have been proposed which allow to evaluate the mechanical behavior of coated components to model the initiation and propagation of cracks. Spur gears used in the automotive and motorcycle industries were examined. Therefore, advanced numerical models were developed to accurately evaluate the stress-strain state of the components under examination, both as regards the fatigue at the tooth base and for the contact fatigue on the tooth flank. The results of the numerical models were finally compared with the results of the experimental tests to verify their accuracy.

Modelling of delamination in rolling and sliding contacts

The contact fatigue phenomenon in conditions of cyclic sliding or rolling is studied based on the model of contact fatigue damage accumulation in the subsurface layers of the material. It is assumed that the rate of the damage accumulation depends on the principal shear stress amplitudes. The results of numerical modelling show that the contact fatigue fracture process in the conditions of a constant load, acting on the sliding or rolling body, consists of the following stages: the incubation period, the running-in period with the alternating detachment of material’s fragments of certain thickness (the mechanism of delamination) and the surface wear, and then the steady-state stage, characterizing by the surface wear with a constant rate. The influence of the sliding friction coefficient, the relative slippage and the strength properties of contacting bodies on the evolution of the accumulated damage in the surface layers and on the fatigue wear kinetics in sliding and rolling contacts has been analyzed.

Researches and Finite Element Simulations of Radial Loading Deformations Applied to Steel Ball Bearings

The appearance of quick wear of rolling paths and rolling elements (balls) through contact fatigue phenomena and through surface defects lead to a decrease in the life of ball bearings in service. One of the causes is the interruption of the lubricating film between surfaces with relative motion under load conditions and high speeds, which leads to plastic micro-deformations and micro-welding between contact surfaces. Diagnosing defects by visual observation involves disassemble of the bearing and is an ineffective method of investigation in serial production. In this context, the necessity of: (i) comparative study of the stresses to which two types (100Cr6 steel and 100CrMnSi6-4 steel type) of radial ball bearings are subjected, (ii) the influence of contact between the ball and the inner ring during rotation, and (iii) the analysis with Finite Element Method (FEM) of the deformations occurring at the application of radial bearing forces. FEM is a current approach to real phenomena in mechanical assemblies, which allows a validation of study assumptions; the results of FEM can lead to the detection of tribological causes of increased bearing wear at inner ring to bearing balls contact. In this paper, FE simulation of radial loading deformations was performed in SolidWorks software using the Simulation module and the stresses and deformations in the bearing rings for the two materials were obtained. It has been found that the stresses that appear during simulations are similar for both materials and do not exceed the limits allowed for heavy duty steel bearings.

Critical plane analysis of multiaxial fatigue experiments leading to White Etching Crack formation

Various researchers have shown that rolling contact fatigue can be reproduced with cyclic compression-torsion experiments, with the load components either in-phase or out of phase. As reported previously, the authors used such experiments to reproduce the rolling contact fatigue phenomenon “White Etching Cracks” which can cause premature failures of rolling element bearings. It is characterized by subsurface crack initiation and propagation coupled with microstructural changes alongside the crack flanks. Surprisingly, only in-phase load superposition caused these microstructural changes to occur. This suggests that White Etching Crack formation is somehow linked to the multiaxial stress state in the specimens, as this was the only variable that changed between in-phase and out-of-phase testing. In this study, the multiaxial stress state in the two experiments is analysed and compared using different critical plane criteria. In contrast to common ways of characterizing the stress state, e.g. equivalent stress approaches, this class of criteria is explicitly designed for multiaxial stress states. Special attention is given to the Dang Van criterion, which has been used in a number of rolling contact fatigue studies.

METALLURGICAL ASPECTS RELATED TO CONTACT FATIGUE PHENOMENA IN STEELS FOR BACK-UP ROLLS

The need of even longer rolling sessions is driving the improvement of back up rolls in terms of wear resistance. This is also aimed to reduce costs. In this paper the effect of steel chemical composition on contact fatigue phenomena, bringing to the macroscopic damage named spalling is reported. Results show that the removal by grinding operations of damaged portion of rolls surface should be not sufficient to restore the initial performances of material. Experimental tests showed that a portion of material below the damaged one keeps memory of the last fatigue cycle, and has to be removed.

Influence of multiaxial fatigue loading conditions on white etching crack formation

The rolling contact fatigue phenomenon “White Etching Cracks” (WEC) was investigated in this study. The load conditions in a rolling element bearing causing WEC formation were reproduced with a simplified testing setup, using cylindrical specimens and compression-torsion loads. In-phase and out-of-phase loads were tested. It was found that under the same equivalent stresses, the experimental results varied greatly between the two load cycles. Multiple characterisation techniques were used for comparing crack behaviour and microstructural evolution. It was found that only in-phase loading resulted in WEC-like structures. A critical plane approach was employed to find possible reasons for the differing results.

Laser cladding of rail steel with Co–Cr

Degradation and subsequent failure of rail tracks are commonly caused by rolling contact fatigue among other mechanisms of wear. Rail crossings are known to exhibit more of these failures due to increased localised traffic and environmental conditions. A high proportion of the costs associated with the repair of rail tracks was due to the rolling contact fatigue phenomenon. In order to mitigate these costs, laser cladding of worn regions has been proposed for the repair of used tracks in situ to limit the need for them to be replaced and for the preservice protection of newly rolled rails and cast crossings. A Co–Cr, Stellite 6, alloy is chosen to demonstrate repair and also surface coating/protection of R260 rail steel. Results showed that cladded Stellite 6 possessed improved hardness, good tribological performance and excellent workhardening ability when compared with rail steel. These demonstrate laser cladding as a viable solution for repair worn rail track.

Rolling Contact Fatigue of Wheel and Rail

Abstract This investigation describes rolling contact fatigue phenomena in the wheel/rail-contact area of railway vehicles. In the scope of this paper new calculation, experimental results of stresses and strains within the wheel as well as a new method the determination of fatigue strength are presented. The stress fields in railway wheels introduced by different loads are calculated using the finite element method. The results are validated on a full-scale test rig. The results of the measurements are used to draw conclusions for the fatigue strength of wheels. Furthermore, the fast and cost saving “PHYBALLIT” method for the determination of fatigue properties is described.

Material Inclusion Factors for Lundberg-Palmgren–Based RCF Life Equations

Material inclusions in the form of hydrogen embrittlement, carbides, etc., are by-products of manufacturing processes and commonly present in bearing steel. The objective of this study was to develop a life equation for rolling contact fatigue phenomenon that accounts for the effects of inclusions. The life equation was developed using the fatigue results previously obtained using the damage model (Jalalahmadi and Sadeghi, Journal of Tribology vol. 132, 2010). Four modifying factors counting for effects of the stiffness, size, depth, and number of the inclusions are used to modify the life equation. These modifying coefficients are extracted from the simulations obtained from the Voronoi Finite element (FE) model and the Lundberg-Palmgren-based fatigue criterion (Jalalahmadi and Sadeghi, Journal of Tribology vol. 131, 2009). These simulations predict Weibull slopes and L 10 lives that are in good agreement with the previous theoretical and experimental results. It is seen that as inclusions become larger or shallower, they cause a larger decrease in the fatigue lives and their scatter. Also, introduction of more inclusions to the material significantly reduces the fatigue lives and their Weibull slopes.

Rolling contact fatigue testing of peek based composites

Rolling contact fatigue phenomenon was investigated on unfilled PEEK and on three different PEEK composites: 10% carbon micro-fiber, graphite and PTFE filled matrix, 30% carbon micro-fiber filled matrix, 30% glass micro-fiber filled matrix. For this aim, roller-shaped specimens were machined from extruded bars of these materials and subjected to rolling contact tests at different contact pressure levels by means of a four roller machine. Contact pressure-life diagrams and wear rates were so obtained and compared, highlighting a relationship with monotonic and hardness materials properties. Microscopic observations of contact surfaces and transversal section of the specimens also allowed observing the damage mechanisms occurred in the materials tested and the effects of the filler. In particular way, deep radial cracks appeared on unfilled PEEK, while spalling and delamination phenomena where found on composites. Diffuse microcracks were found at the filler-matrix interface of the composites specimens, confirming that the fatigue life of these materials is essentially determined by the crack propagation phase, also under rolling contact loading.

Axisymmetric fretting analysis in coated cylinder

Fretting is essentially a contact fatigue phenomenon, although bulk stresses and material properties contribute to final failure. The near surface state of stress developed under oscillatory contact between machine elements plays a major role in deciding the severity of fretting. It is possible to enhance tribological properties by coating the surface. There is rather scanty literature available on fretting analysis of coated components. Presence of such coatings has a large influence on the near surface state of stress. The effect of coatings on the severity of fretting is the focus of this paper. Results obtained for both hard and soft coatings are compared with the results obtained for the homogeneous case. The component geometry and loading are chosen to be cylindrical to enable 3D elastic axisymmetric fretting analysis. The results are compared with 2D models (strip and half-plane) to examine their utility and validity for understanding axisymmetric fretting. Contact pressure and frictional shear loading cases are solved separately and superposed appropriately depending on the coefficient of friction considered. Results for different values of coefficient of friction and elastic mismatch are illustrated through contour plots of stresses and strains. These results are expected to be helpful for identifying fretting failure zones and fracture mechanisms in coated components. Analytical results presented here could serve as useful benchmarks for calibrating numerical codes and experimental techniques.

Fatigue of railway wheels and rails under rolling contact and thermal loading—an overview

An overview of rolling contact fatigue phenomena occurring at wheels and rails is given. The paper outlines mechanisms behind the various phenomena, means of prediction, influencing parameters and possible means of prevention.

特殊環境用軸受鋼の開発

Recent improvements in the quality of steels have meant that rolling bearings are largely free from materials defects. Therefore, damage generated during the operating life of the component becomes the main source of failure. This damage is frequently caused by overheating at a higher speed operation or by the overrolling of debris paricles in the lubricant. A new bearing steel, named STJ2, for high temperature service was developed in the view of supressing rolling contact fatigue phenomenon, which is accelerated by the heat occur at raceway. STJ2 showed a longer life than cnventional JIS-SUJ2 steel in a bearing test at 473K, and have been applied to spherical roller bearings produced by NTN Corp. Another bearing grade for contaminated lubricant service was designed with the control of retained austenite in a steel. Adequate amount of retained austenite that prolonged the rolling contact fatigue life under a contaminated lubricant was obtained by the addition of manganese more than 2 mass%. This new steel has just been begun to introduce to domestic and foreign bearing manufacturers.

Modern State of Experimentation and Modeling in Contact Fatigue Phenomenon: Part I—Contact Fatigue. Normal and Tangential Contact and Residual Stresses. Nonmetallic Inclusions and Lubricant Contamination. Crack Initiation and Crack Propagation. Surface and Subsurface Cracks

The purpose of this paper is to describe the modern understanding of the contact fatigue phenomenon which occurs in different materials under various loading conditions. Part I presents experimental and theoretical knowledge of the influence of normal and factional contact and residual stresses on contact fatigue. It analyzes relationships between contact fatigue and material defects, inclusions, and lubricant contamination. Furthermore, it discusses crack initiation and crack propagation, the effect of material microstructure on contact fatigue life as well as some theoretical results for surface and subsurface cracks. Part II is devoted to reviewing the existing mathematical models of fatigue life for bearings and gears and to describing a new statistical model of contact fatigue based on contact and fracture mechanics. This new model is based on the assumptions derived from the analysis of the data presented in Part I. Presented as a Society of Tribologists and Lubrication Engineers Paper at the STLE/ASME Tribology Conference In Orlando, Florida, October 11–13, 1999

Shear Stresses Below Asperities in Hertzian Contact as Measured by Photoelasticity

The effect of surface roughness on shear stresses below the surface of an unlubricated Hertzian contact is analyzed using a three dimensional stress freezing photoelastic technique. The shear stresses in the micro-Hertzian contact in each asperity are shown to combine and form, at a greater depth below the surface, shear stresses generally associated with contact stress theory. These macro-Hertzian stresses are compared with the micro-Hertzian stresses in the asperities. The results are also correlated with an existing asperity contact theory and are discussed in relation to the contact fatigue phenomenon.