Cyclic Contact Research Articles

Guidelines for rail quality selection to increase service life based on wear and fatigue behavior of various wheel and rail materials

The Austrian rail network is very diverse: The lines in the alpine areas have very tight radii while the lines in the flat areas (east and north of Austria) are mainly straight high-speed lines. This leads to different levels of wheel and rail wear. Accordingly, the objective of this study was to experimentally model a laboratory-scale cyclic wheel-rail contact to investigate wear behavior and rolling contact fatigue at different radii (RCF).A novel test system was set up at AC2T research GmbH to reproduce unidirectional loading on a real track. Tribotests were performed with rail (grades R260, R350HT & R400HT) and wheel (grades ER7 & ER9) parts from the field. This newly designed principle is an effective method to characterize the wheel-rail contact. Wear characterization was done after defined testing cycles to detect (i) area loss and (ii) change in surface topography.

Nonproportionally multiaxial cyclic plastic deformation of U75 rail steel: Experiment and modeling

U75V rail steel exhibits cyclic softening and hardening under uniaxial and multiaxial strain-controlled cyclic loadings, respectively. The ratcheting behavior of U75V rail steel depends on the loading paths due to nonproportionally additional hardening. New evolution equations incorporating Tanaka’s nonproportional parameter are introduced into the isotropic and kinematic hardening rules of the cyclic plastic constitutive model to describe the cyclic softening/hardening features and multiaxial ratcheting behavior. The simulated results agree with the corresponding experimental results. A finite element model of wheel-rail cyclic rolling contact is established to investigate the nonproportionally multiaxial effect on the rolling contact fatigue life.

Study on surface fatigue and metamorphic layer of raceway of hybrid ceramic ball bearing in high-speed spindle for machine tool

Performance degradation of the precision bearing originates from the surface fatigue damage, which results in the premature failure of precision bearings in high-end spindle for machine tool. A metamorphic layer forms on the upper surface of the bearing raceway under the cyclic contact stress. The microstructure of the metamorphic layer is consisted of the nanocrystalline layers paralleled to the surface of the raceway. Stress-induced effect plays an important role in the transformation from the common martensite structure to the metamorphic layer, i.e., the grain refinement. Under the cyclic contact stress, the dislocation pile-up groups form in carbide particles, and the carbide particles can even be elongated and crushed. The cracks initiate and propagate near the metamorphic layer, which lead to the peeling band, the surface fatigue damage and premature failure of the precision bearings. This finding indicates that one solution to mitigating the surface fatigue in bearing might be the surface technology application.

Lab-on-a-Chip Electrochemical Immunosensor Array Integrated with Microfluidics: Development and Characterisation

Lab-on-a-chip has recently become an alternative for in situ monitoring for its portability and simple integration with an electrochemical immunoassay. Here, we present an electrochemical cell-on-a-chip configured in a three-electrode system to detect benzo(a)pyrene (BaP) in water. 11-Mercaptoundecanoic acid (MUA), a self-assembled monolayer (SAM), was used to modify a gold chip surface to reduce the randomness of antibody binding. A carboxylic acid group was activated with -ethyl-3-(3-dimethylaminopropyl) (EDC) in combination with N-hyrodsuccinimide (NHS) before antibody immobilisation. The mechanism of the electrochemical reactions on a gold surface and SAM formation were investigated by cyclic voltammetry and contact angle measurements. The data revealed a lower contact angle in the modified chip and a scan rate of 50 mV/s. Through the addition of modification layers and thiol end groups to the SAM, our design allowed the chip surface to became more insulated. All were tested by amperometric detection using the developed Q-sense system. This novel technique detected multiple samples, and completed the analysis reasonably quickly. While the integrated system proved successful in a lab setting, the aim of the research is to use this system for in situ analysis, which can be brought into a water environment to carry out tests with existing processes. In this way, any issues that may arise from an environmental setting can be rectified in an efficient manner.

A general mathematical modeling and finite element analysis of a strain wave gear possessing a double-circular-arc-with-a-common-tangent profile

In this study, the characteristics of a strain wave gear (SWG) with a double-circular-arc with a common tangent (DCACT) profile were simulated using two-dimensional (2D) and three-dimensional (3D) finite element analysis (FEA). First, theoretically generated tooth profiles for the flexible spline (FS) and circular spline (CS) of an SWG were developed according to the theory of gearing. An automated mesh-generating computer program was developed in Visual C++ to establish 2D and 3D finite element models of the SWG based on its derived geometry. An FEA was then performed to explore the meshing characteristics, including the stress distribution, torsional angle, torsional stiffness, and number of engaged teeth, of the SWG. Continuous meshing of multiple tooth pairs between the FS and CS from the 2D FEA verified the conjugation of tooth profiles. The coning effect on the FS during assembly of the SWG was also predicted and discussed from the 3D FEA results. The 3D FEA produced more accurate and practical results than the 2D FEA did. Additionally, 3D FEAs of a modified (with longitudinal crowning) and unmodified (without longitudinal crowning) FS were performed and compared. Finally, a 3D FEA of a modified FS was successfully conducted to predict cyclic contact stress, root stress, and transmission error.

Performance of selective laser sintered polyamide spur gear under dry and lubricated conditions

The durability of a selective laser sintered (SLS) polyamide gear was evaluated using an in-house gear test rig under dry and oil-lubricated conditions. The lubricated SLS gear exhibited inferior contact fatigue performance at lower torques than that under dry conditions. Cyclic contact stress coupled with sliding motion diffuses the lubricating oil into the porous subsurface and reduces the contact fatigue strength of the gear tooth. The failure of the SLS gear under dry conditions exhibited the polishing effect, thermal softening with surface cracks, and thermal bending at 1.8, 2.5, and 3.5 Nm, respectively, whereas failure of the SLS gear under the oil-lubricated condition was only pitting for all the considered conditions. The lubricated gear exhibited superior performance at a higher torque than the gears in the dry-running condition, owing to its improved thermal stability.

Numerical study on ratcheting performance of heavy haul rail flash-butt welds in curved tracks

Ratcheting at the rail heads due to cyclic rolling contact stresses is often observed in heavy haul railways, especially when there are large steering forces developed in low-radius curves. Additionally, ratcheting behaviour can be exacerbated when associated with material strength loss in the heat affected zones of rail welds. However, the numerical evaluation of this failure mode requires a sophisticated representation of elastic–plastic contact pressures and traction distributions, which, to the best of the authors’ knowledge, cannot be directly measured by any existing tools. In this paper, in order to accurately evaluate the ratcheting performance at rail welds in curved tracks, attempts were conducted to modify the FaStrip algorithm to analytically estimate the traction distributions based on contact pressures and creepages obtained from static finite element analysis and multi-body dynamic simulations, respectively. Cyclic rolling contact was then simulated by repeatedly applying contact pressure and traction on the rail surface consisting of both the rail weld and the parent rail regions. The weld region was divided into 23 sub-zones in the rolling direction to represent the material inhomogeneity within the heat affected zones. Cases on a 2000 m radius curved track and a tangent track were studied for comparison. It was found that the ratcheting strain rate in curved tracks can be significantly elevated due to the higher magnitude of traction, and the location of the elements with the highest ratcheting strain rate was dependent on the traction distribution patterns. The proposed method provides a general tool for predicting rolling contact fatigue initiation life and location, which will benefit the maintenance efficiency of heavy haul rail welds in curved track.

Influence of a decarburised layer on the formation of microcracks in railway rails: On-site investigation and twin-disc study

In the rail manufacturing process, the rail material is exposed to a high-temperature atmosphere, thereby causing decarburisation. It reduces the carbon content on the surface layer of the rails, which degrades the mechanical properties and hardness of the rails’ surface. Squat, a rail defect, results from rolling contact fatigue (RCF) due to the cyclic contact between a wheel and a rail. Generally, RCF initiates microcracks on the running band of a rail; then, when RCF accumulates, the microcracks grow deep below the surface of the running band and finally the squat is visually formed.This study aims to understand the effect of decarburisation on microcrack formation using twin-disc and site tests. Test pieces with and without decarburisation were compared during the twin-disc tests under the same test conditions. The results show that decarburisation affects the formation of microcracks and plastic flow. Furthermore, we investigated the effect of decarburisation on the formation of microcracks in the test pieces taken from the actual tracks. The comparison between the test pieces with and without a decarburised layer showed that the crack density of the ground rail was ∼2.7–5.7 times higher than that of the underground rail at a cumulative tonnage of 23 MGT.

Effect of mixed plastic hardening on the cyclic contact between a sphere and a rigid flat

This paper studies the effect of mixed plasticity mode (combined with isotropic and kinematic hardening law) on the cyclic contact between an elastic-plastic sphere and a rigid flat. Assuming power-law hardening with different levels of mixed plasticity for the sphere, we derived a semi-analytical expression of load versus interference during the first loading and unloading process. During cyclic loading, our results indicate that the isotropic plasticity model shows no variation of residual interference, while kinematic plasticity has the cyclic effect on the residual interference, and this effect is bigger for the material with a higher hardening exponent. In addition, we provided the semi-analytical expression for the evolution of residual interference, which is accurate for the strain hardening exponent from 0.1 to 0.5.

ПОСТРОЕНИЕ КРИВЫХ ГЛУБИННОЙ КОНТАКТНОЙ УСТАЛОСТИ ПОВЕРХНОСТНО УПРОЧНЕННЫХ ЗУБЧАТЫХ КОЛЕС

Laws of fatigue resistance changes of surface-hardened toothed gears are established. It is shown that the main criterion of serviceability of gear wheels of transmissions of power-saturated machines is deep contact pitting of active surfaces of teeth. Dependence is investigated between the number of cycles before the deep contact destruction of material and the maximal tension of cycle with taking into account the mechanical characteristics and structural parameters in the critical zones of the diffusion layers of the teeth, in which fatigue processes of deep contact destruction of material are developed. Lines of deep contact fatigue of teeth material are constructed by the criterion of equality of hardness value and structural parameters in the zones of fatigue processes development under cyclic contact loading. It is shown, that the deep contact fatigue of gear wheels hardened to a considerable degree is determined by hardness value and structural components of local volumes of teeth subsurface layers. Slope index of the deep contact fatigue lines of gear wheel material have been determined. Correlation coefficients and equations of linear regression have been obtained for each deep contact fatigue line depending on hardness value in the subsurface dangerous zone of the diffusion layer, in which formation and development of fatigue processes occur, leading to the limit state of the highly stressed gear wheels.

Experimental and analytical studies on the influence of weathering degree and ground-environment analog conditions on the tribological behavior of granite

Weathering is a geological/geo-environmental process that degrades and transforms rocks, contributing to landslides and instability of geo-systems and infrastructures. The effects of weathering on rock characteristics at the grain scale is crucial for understanding the impact of geological environment-processes on the physical and mechanical behavior of rocks, as well as how these processes are linked to the large-scale behavior of geological systems. In the present study, an investigation on the tribological behavior of highly and completely decomposed granitic rock (termed HDG and CDG, respectively) is presented using micromechanical-based experimental and analytical methods on miniature specimens. Emphasis is placed on examining the influence of the weathering degree and by taking into account analog ground-environment interaction effects on the constitutive (contact) behavior and interface friction of the weathered rocks. The high roughness and coating exposed on the surfaces of the tested samples because of the decomposition of feldspars and micas had important influences on the tribological behavior of the rock. Significant differences were also observed based on the state of the specimens, i.e., nominally dry, immersed in water, or by removing the surface coating from a washing process. An attempt was made to understand the differences between variously weathered materials and provide some basic modeling that can be useful in discrete-based analyses of geomechanics and engineering geology problems. Comparing a broad range of geological materials, including granite of grade I-II revealed that the transportation-sedimentation process and in-situ chemical weathering or rock crushing resulted in significant variabilities in the surface morphological characteristics of the grains. In some aspects of their tribological behavior, HDG grains (grade IV) were rather similar to grade I-II compared with the observed response of grade V. Analog wetting-drying cycles, simulated based on cyclic normal contact tests revealed significant accumulation of plastic displacements for the weathered rocks compared with sedimentary type materials, which implies higher sensitivity of weathered rocks to stress state changes caused by various geo-environmental influences. This is crucial to be considered in the constitutive modeling of ground-environment interaction.

A peridynamic frictional contact model for contact fatigue crack initiation and propagation

Cyclic contact between components can cause cracks in the contact area. Peridynamics can overcome the difficulty of solving cracks and other discontinuities in classical continuum mechanics. A new peridynamic sliding frictional contact model is proposed and verified by a two-dimensional point contact sliding simulation. Then, a two-dimensional plane strain elastoplastic peridynamic model of a single micro-contact asperity is established, and the fatigue crack initiation and propagation in the micro-contact area during fretting wear is analyzed. The results show that in fretting wear, crack initiation originates from the area of maximum shear stress below the surface. The subsurface crack initially propagates in the direction parallel to the surface, and then gradually propagates to the surface. Finally, the crack grows to the free surface and forms wear debris. The contact pressure and the coefficient of friction in the contact surface can affect the crack initiation depth and crack propagation path, thereby affecting the size of wear debris. The crack propagation path is not sensitive to the change of the coefficient of friction of the crack surface. This study helps to understand the mechanism of fretting wear and provides a reference for predicting fretting wear.

Influence of induction hardening residual stresses on rolling contact fatigue lifetime

Rolling contact fatigue is a unique mode of fatigue that components under cyclic contact loading experience. In this work, the impact of induction hardening residual stresses in rolling contact fatigue lifetime is investigated experimentally and numerically using the Dang Van multiaxial criterion. Various residual stress fields from induction hardening are simulated using the finite element method and are mapped into a classical mono-contact finite element model. The impact of induction hardened residual stresses on the lifetime of a component has been investigated, and the importance of incorporating the residual stress profile into fatigue life assessments is affirmed.

Cyclic contact fatigue behavior of baria-silicate glass-ceramics as a function of crystal aspect ratio

Ceramic materials are potentially useful for dental applications because of their esthetic potential and biocompatibility. However, evidence of contact fatigue damage in ceramics raises considerable concern regarding its effect on the survival probability predicted for dental prostheses. To simulate intraoral conditions, Hertzian indentation loading with steel indenters was applied in this study to characterize the fatigue failure mechanisms of ceramic materials. Baria silicate glasses and glass-ceramics with different aspect ratios of crystals were selected because the glass and crystal phases have similar density, elastic modulus, and thermal expansion coefficients. Therefore, this system is a model ceramic for studying the effect of crystal geometry on contact cyclic fatigue failure. The subsequent flexural strength results show that the failure of materials with a low fracture toughness such as baria-silicate glass (0.7 MPa m1/2) and glass-ceramic with an aspect ratio of 3.6/1 (1.3 MPa m1/2) initiated from cone cracks developed during cyclic loading for 103 to 105 cycles. The mean strengths of baria-silicate glass and glass-ceramics with an aspect ratio of 3.6/1 decreased significantly as a result of the presence of a cone crack. Failures of baria-silicate glass-ceramics with an aspect ratio of 8.1/1 (Kc = 2.1 MPa m1/2) were initiated from surface flaws caused by either grinding or cyclic loading. The gradual decrease of fracture stress was observed in specimens with an aspect ratio of 8.1/1 after loading in air for 103 to 105 cycles. A reduction of approximately 50 % in fracture stress levels was found for specimens with an aspect ratio of 8.1/1 after loading for 105 cycles in deionized water. Thus, even though this glass-ceramic with an 8.1/1 crystal aspect ratio material is tougher than that with a 3.6/1 crystal aspect ratio, the fatigue damage induced by a large number of cycles is comparable. The mechanisms for cyclic fatigue crack propagation in baria-silicate glass-ceramics are similar to those observed under quasi-static loading conditions. An intergranular fracture path was observed in glass-ceramics with an aspect ratio of 3.6/1. For an aspect ratio of 8.1/1, a transgranular fracture mode was dominant.

Formation mechanism of the distinctive granular fracture surface in subsurface fracture of Ti6Al4V alloy

The formation mechanism of the granular regions often observed in subsurface fractures of Ti6Al4V alloy was experimentally investigated. To simulate and simplify the phenomenon of crack opening and closure, which is strongly related to fatigue crack propagation and strength properties, uniaxial cyclic compression tests were conducted in air and vacuum using test pieces with rounded surfaces under contact conditions. The results showed ring-like patterns on the parts of the rounded surfaces subjected to cyclic contact regardless of the ambient environmental conditions. However, in the case of vacuum only, fine surface asperities were observed, which were quite similar to those in the subsurface fractures. Microstructural refinement was confirmed by cross-sectional observation of the concave and convex parts. In the refined microstructure, voids and cracks were observed, which were formed during the refinement and adhesion processes. Microstructural refinement and metallic adhesion under compression loading in vacuum are key phenomena for the formation of fine surface asperities. Based on the experimental results, a new model for the formation mechanism of the granular region was proposed. This model can explain the characteristics of the granular region and the properties of subsurface fractures, which occur at lower stress and have longer lifetimes than surface fractures.

Assessment of tribotechnical properties and resistance to wear of plasma coatings

In order to determine the operating conditions of parts for which restoration of worn surfaces is acceptable by the method of plasma spraying of various powder alloys, contact fatigue tests were carried out under cyclic contact impulse loading. In addition, tribotechnical tests were carried out with various wear-resistant coatings under conditions of liquid sliding friction.
 Bench and operational tests showed the use of coatings obtained using modern plasma technologies, the feasibility of protecting parts operating in conditions of corrosion, waterjet and cavitation wear, as well as in sliding friction.
 The coating sprayed with Ni-Al intermetallic alloy powder provides the most reliable protection against shock cyclic impact and abrasion during liquid friction than other materials studied.
 Coating with wear-resistant self-fluxing powder Ni-Cr-B-Si-C alloy, hardened by solid carboboride phases, without its additional heat treatment for restoration of surfaces working in sliding friction pairs, is not recommended.

Effect of long-period stacking ordered structure on very high cycle fatigue properties of Mg-Gd-Y-Zn-Zr alloys

Magnesium alloys with a long-period stacking ordered (LPSO) structure usually possess excellent static strength, but their fatigue behaviors are poorly understood. This work presents the effect of the LPSO structure on the crack behaviors of Mg alloys in a very high cycle fatigue (VHCF) regime. The LPSO lamellas lead to a facet-like cracking process along the basal planes at the crack initiation site and strongly prohibit the early crack propagation by deflecting the growth direction. The stress intensity factor at the periphery of the faceted area is much higher than the conventional LPSO-free Mg alloys, contributing higher fatigue crack propagation threshold of LPSO-containing Mg alloys. Microstructure observation at the facets reveals a layer of ultrafine grains at the fracture surface due to the cyclic contact of the crack surface, which supports the numerous cyclic pressing model describing the VHCF crack initiation behavior.

Contact fatigue behaviour of CVD coated cemented carbides in dry and wet conditions

Cycling contact fatigue indentation tests in dry and wet conditions were conducted in cemented carbides CVD-coated with Ti(C,N)/α-Al2O3 and Zr(C,N)/α-Al2O3 systems. In dry contact experiments, all imprints presented a deformed area where the coating was still present after the test. Formation of cracks was visible at the edge of the contact point of the imprint. On the other hand, under wet conditions, almost all imprints presented a ring where the α-Al2O3 had been removed, driven by a corrosion-fatigue process. Exceptionally, for Cr-containing cemented carbides combined with Zr(C,N)/α-Al2O3 coating, no delamination of α-Al2O3 was observed after the test under wet conditions. This result suggests that the combination of fine-grained cemented carbides and coating systems with low coefficient of thermal expansion may be of advantage to resist degradation by cyclic contact fatigue. Furthermore, the response to fatigue by contact loads can be enhanced by tailoring/combining cemented carbides and CVD coating systems which present higher chemical stability, and both higher intrinsic plasticity and resistance to plastic deformation.

Contact fatigue of WC-6%wtCo cemented carbides: Influence of corrosion-induced changes on emergence and evolution of damage

The effect of corrosion-induced changes on the contact fatigue response and associated damage in a medium-grained WC-6%wtCo hardmetal grade is investigated. Cyclic contact loading tests are conducted by means of Hertzian indentation techniques. Corrosion damage is introduced in a controlled way by previous immersion of specimens in a stirred acidic medium. Results reveal that corrosion significantly affects the contact fatigue behavior of the material under consideration, in terms of decreasing the effective load bearing capability of the material as well as earlier emergence and evolution of specific damage features, as compared to those discerned under monotonic loading and in a pristine hardmetal respectively. As cycle number increases, damage evolution in the corroded condition is more gradual and diffuse than for the uncorroded one. In general, it is dominated by existence of ill-defined radial cracks together with pronounced carbide grain pull-out which finally evolve into cohesive-like spallation in regions close to the periphery of indentation imprints. These findings are rationalized on the basis of phase assemblage changes linked to corrosion, particularly in terms of lessening of toughening capability and easiness of WC grain pull-out intrinsic to the loose and porous binderless WC skeleton remnant at the surface level, after exposure to the acid medium. Although some removal of carbide grains and discrete cohesive chipping is also observed in the pristine sample for very high number of cycles, such different response, as compared to monotonic loading, is linked to oxidation phenomena of the metallic binder for this reference condition.

A numerical-experimental study of AISI 316L borided steels under cyclic contact loading

AISI 316L borided steels under cyclic contact loading were evaluated. Boriding was carried out by two powder-pack processes: the continuous and interrupted processes. Boriding processes were developed at 900 °C for 1 h (continuous process) and 4 h (interrupted process). A monolayer mainly formed by Fe 2 B was obtained by the interrupted boriding with a thickness of 6.09 ± 0.31 μm and a surface hardness of 22.3 ± 1.4 GPa. In contrast, a FeB-Fe 2 B bilayer was formed by the continuous boriding with a total thickness of 13.08 ± 0.39 μm and a surface hardness of 26.3 ± 2.1 GPa. Adhesion tests were developed, in which the monolayer showed better adhesion than that the bilayer. Cyclic contact tests were carried out by loading a ball on the borided steel surface. Circular cracks because of the applied critical load (monotonic load) were observed on the borided steel surface. Subcritical loads were applied to the borided steels, where cohesive damage was caused in the bilayer, whereas no damage was observed in the monolayer at low subcritical loads. In contrast, circular cracks were observed in both treatment conditions at high subcritical loads. In addition, the high percentage of the FeB phase caused a more severe damage upon the bilayer system. Additionally, the thinner thickness of the boride layer, as well as the presence of compressive residual stresses (−0.75 GPa) in the monolayer system, caused a decrease on the maximum principal stress (2.4 GPa at the highest applied load) caused by contact loading compared to bilayer system (5.2 GPa at the highest applied load). Therefore, the interrupted boriding (monolayer system) showed a better mechanical resistance under cyclic contact loads than the continuous boriding (bilayer system). • FeB-Fe 2 B and Fe 2 B layers were obtained by continuous and interrupted boriding. • The bilayer system showed more severe damage under cyclic contact loads. • The plastic deformation of the substrate decreased the damage in the monolayer system.