Contact Fatigue Mechanism Research Articles

Mechanical and metallurgical characterization of contact fatigue mechanisms in ADI spur gears

Surface pitting of gear teeth flanks is one of the most common causes of gear failure. The increase of the contact fatigue strength of gears is usually achieved through the development of new combinations of materials and heat-treatment processes. Austempered Ductile Iron (ADI) is one of the results of this research field that leaded to a material which is replacing conventional steel in many mechanical applications. This paper shows the experimental results obtained from tests performed on ADI spur gears in order to characterize the contact fatigue crack nucleation and growth and understand the possible interaction with the graphite spheroids and casting defects. Using a four-square test rig, it was carried out a systematic monitoring of the evolution of the damage, collecting several images concerning crack development at gear surfaces. At the end of the tests several metallurgical analyses of surface and sub-surface areas were performed to understand the fracture mechanism at the base of the pitting failure in ADI. All experimental analyses were complemented by theoretical calculations, useful for a better understanding of the phenomenon.

Comprehensive surface integrity evaluation during early stages of gear contact fatigue

The study aims to evaluate the microstructural sensitive aspects of contact fatigue crack initiation and its evolution during the gear lifetime. Tests were performed to evaluate different stages in the evolution curve of gear contact fatigue. The magnetic Barkhausen noise (MBN) technique was used to characterize variations in the magnetic response in gears during the incipient gear contact fatigue mechanisms initiation. For a complete comprehension of the surface degradation, residual stresses, microstructure, and microhardness were explored. A substantial increase in the MBN signal is identified before the fatigue failure occurs, indicating the occurrence of microstructural alterations that change the magnetic properties. The early stages of contact fatigue are accompanied by a surface softening in the near-surface region, up to approximately 40 µm depth. These phenomena were also followed by a less compressive residual stress region at 20 µm depth. A lower influence of microstrains on the diffractogram can be observed by the full width at half maximum parameter (FWHM), indicating a higher amount of dislocation annihilation during the appearance of the initial stages of the contact fatigue mechanism. The study concludes by proposing a comprehensive approach to understand the mechanisms behind the early stages of gear contact fatigue and how the MBN signal can be used to detect fatigue damage.

Elastic–plastic cylindrical rolling contact fatigue mechanism evolution and life prediction considering random surface topography

This paper proposes a semi-analytical fatigue life prediction method for the evolution of elastic–plastic cylindrical rolling contact fatigue mechanism under random surface topography, and reveals the evolution process of three contact fatigue mechanisms under high load conditions: micro-pitting, surface-originated pitting (SOP), and subsurface-originated spalling (SOS). Firstly, an elastic–plastic line contact model of random rough surface is proposed to analyze the contact condition between rolling elements considering random surface topography. Subsequently, a contact pressure spatial distribution model of rough surface is established, which can fully reflect the variation law of pressure distribution in the elastic–plastic stage. Finally, a semi-analytical life prediction (SALP) method is developed by combining the newly proposed analytical model with finite element method to illustrate the evolution process of the contact fatigue mechanism. The prediction accuracy of the SALP method is verified by comparing the results with those of the finite element method. Furthermore, the influence of random surface topography parameters (root mean square of roughness δ, correlation length λ) on contact fatigue life is further discussed, which helps to provide theoretical support for good surface process control for high-load rolling elements.

A study on the rolling contact fatigue mechanism of ZC-L cast wheel steel

In this paper, the rolling contact fatigue (RCF) failure mechanism of ZC-L cast steel wheels was analyzed by optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The results show that obvious fatigue flaking is formed on the macroscopic surface of the wheel tread after RCF failure. The RCF cracks are formed at the maximum shear stress region of the sun-surface of the wheel (1–5 mm below the tread). There are a lot of subsurface white etching microstructure (WEM) and obvious casting defects in the subsurface of the wheel tread. The RCF cracks are mainly formed at the interface between the subsurface WEM and the matrix and at the edge of the casting defects. The subsurface WEM is composed of nano-ferrite grains and residual cementite, and its hardness is about 920HV, which is much higher than that of the matrix structure. During the wear process, the plastic deformation of the subsurface WEM and the matrix is not coordinated, resulting in the formation of RCF cracks. As a hard phase, the hardness of casting defects is much higher than that of matrix. In the process of wear, under the action of subsurface stress, the casting defects can cause obvious plastic deformation in the soft orientation region at the edge of the casting defects. When the plastic deformation reaches the plastic limit, the RCF cracks are formed at the edge of the inclusions due to ratchet failure. The stress at the crack tip is caused by the RCF crack propagation at the edge of inclusions will further lead to the refinement of the microstructure at the edge of the RCF crack. Under the action of the subsurface contact stress, the subsurface WEM and casting defects are the main reasons leading to the RCF failure of the ZC-L casting wheel. Improving the microstructure uniformity and reducing casting defects are the main methods to improve the fatigue property of casting wheels.

Rail degradation due to thermite weld discontinuities: Field experience

This paper details the field-testing approach and results of thermite welds used in railway applications. Rail steels made from R260 grades are welded by two different processes: the standard process according to the European standards, and a recent technology. Welds are introduced in the same testing location to ensure comparable loading conditions in the aim of studying their degradation behaviours. The total testing-period is 6 months. During the in-service period, the surface hardness of the running band, in the welded area, is recurrently measured. For an accurate assessment of field results, a vehicle/track interaction (VTI) model evaluated the expected dynamic loads induced by the initial vertical irregularities.The simulations show that the highest dynamic load at the wheel/rail contact happens at the location of the maximum absolute gradient, in accordance with previous research. Particularly, dipped welds show relatively high dynamic forces inducing a high loss of the transversal profile. In respect of the field results, the comparison of initial and final surface hardness indicates a significant increase for ‘ALFONS’ welds over the welded areas. Additionally, all welds depicted a cyclic increase and decrease of the running band hardness. This result is discussed according to the ratcheting susceptibility of welds and eventual wear progression. For a testing-period of 10 weeks, the gauge corner of one ‘ALFONS’ weld developed a crack. The assessment of the longitudinal profiles revealed changes of the vertical irregularities that may modify the dynamic loads, and further the rolling contact fatigue mechanisms and degradation rates.

Rolling contact fatigue crack propagation in nitrided alloyed steels

Experimental investigations were carried out to better understand the rolling contact fatigue mechanisms in nitrided layers of the 33CrMoV12-9 steel grade. Surface-initiated pitting failure mode was reproduced on a twin-disc machine to analyse crack growth and compressive residual stress behaviour within the nitrided layers. Metallographic examinations, 3D observations by means of high-resolution X-ray computed tomography and residual stress analysis were realised on nitrided 33CrMoV12-9 specimens before and after rolling contact fatigue tests. The study revealed that if the initial compressive residual stresses associated with the surface treatment are released during the process of rolling contact fatigue, pre-existing superficial cracks propagate in the nitrided layers along the intergranular carbides. These precipitates induced by the nitriding process therefore act as preferential crack propagation sites.

Multi-Physics Calculation and Contact Degradation Mechanism Evolution of GIB Connector Under Daily Cyclic Loading

A 3-D electromagnetic-thermal-mechanical coupling finite element model of a gas-insulated bus plug-in connector is proposed in this paper with a subsequent analysis for the contact fatigue mechanism. Special attention has been paid to the contact degradation mechanism under normal cyclic operation conditions, which is important for the optimal designing and condition monitoring of the equipment. The current constriction effect and the contact resistance are considered through modeling the equivalent contact bridges between contact interfaces, and the current densities derived from the electromagnetic field are applied as load inputs in a electrical-thermal-mechanical analysis where varying of friction coefficient is considered. The validity of the calculation model has been demonstrated by temperature rise and relative motion experiments, and the influence of daily changing current and environmental temperature on the thermal and mechanical characteristics of plug-in connector has been analyzed. Analysis results show that the temperature rise among different contact fingers of plug-in connector is not the same with different contact forces, and the plastic deformation can be induced on contact spots. The insert depth of connector can be changed under the action of alternating thermal loading induced by daily change of current and environmental temperature, leading to contact degradation and overheating fault of plug-in connector.

Investigation of stress of Fe–Cr alloy coating under rolling contact

The aim of the paper is to investigate the rolling contact fatigue mechanism of the Fe–Cr alloy coating under different stress conditions. The Fe–Cr alloy surface coatings and Ni–Al alloy undercoatings were respectively deposited on the steel substrates using a high efficiency plasma spraying system. The distribution of maximum stress and shear at the interface and the inside of the coating were obtained using finite element simulation and experiment. The maximum stress is the critical failure stress, and shear stress does the main driving force for coating failure. The different failure modes are due to the synergy of the maximum stress and the shear stress.

Investigation on wear and damage performance of laser cladding Co-based alloy on single wheel or rail material

The aim of this study is to investigate the microstructure and wear behavior of laser cladding Co-based alloy coating on single wheel or rail material (the laser cladding coating is applied only to the wheel or to the rail) using a rolling–sliding wear test apparatus. The coating of laser cladding Co-based alloy consists of dendrite and eutectic. Single-treated wheel or rail specimen undergone the laser cladding treatment effectively decreases rolling friction coefficient and improve wear resistance of both wheel and rail materials. Single-treated wheel or rail roller presents mild wear damage. Furthermore, it is helpful to alleviate the surface damage and plastic flow of wheel and rail rollers. It is concluded that the laser cladding Co-based alloy coating is suitable for single treatment of wheel or rail material. However, further work should be carried out to clarify the rolling contact fatigue mechanism and to characterize the laser cladding Co-based alloy coating.

Surface and subsurface rolling contact fatigue characteristic depths and proposal of stress indexes

The rolling contact fatigue is distinguished into subsurface initiated (spalling and case crushing) and surface initiated (pitting and micropitting). A characteristic depth was identified for each of these mechanism. The characteristic depth of the case crushing is the hardening depth, while for the spalling it is the maximum cyclic shear stress depth. The pitting depth is the size of the crack for which the mode I stress intensity factor range, due to the fluid pressurization, is higher than the threshold. This depth can be similar to the micropitting depth, in the order of 10μm, for heavily loaded small radius contacts. Rolling contact fatigue cyclic shear stress indexes are then defined on the basis of the characteristic depths, and they identify the load intensity of each rolling contact fatigue mechanism. The characteristic depths and the stress index approach can be used to relate specific tests to component design, without any size effect misinterpretation.

Technological provision of contact fatigue resistance for cemented gear wheels made of heat-resistant steels

On the basis of analyzing the effect structural factors on the mechanism of contact fatigue of complexly-alloyed heat-resistant steels requirements are determined for cemented layer parameters corresponding to high contact fatigue resistance. The possibility is demonstrated of implementing these requirements during cyclic vacuum cementation regimes.

Failure analysis of belt/roll tribological pair used for the production of eucalypt fiber panels

The premature failure of a large agglomeration machine used for the annual production of 360,000 m 3 of eucalypt fiber panels was investigated to identify the nucleation and growth mechanisms of cracking in PH stainless steel belts (126 m × 2.9 m × 3.0 mm). These belts are used to compress a cushion composed of eucalyptus fibers and glue, being the pressure transmitted from the pistons by the action of numerous case-hardening steel rolls. Examination of the belt working interfaces (belt/rolls and belt/eucalypt fibers) indicated that the main cracking was nucleated on the belt/roll interface and that there is a clear relationship between the crack nucleation and the presence of superficial irregularities, which were observed on the belt/roll working surface. Used rolls showed the presence of perimetric wear marks and 2 μm silicon-rich encrusted particles (identified as silicon carbide). Lubricant residues contained the presence of helicoidal wires, which were originated by the release of the stainless steel cleaning brush bristles, and 15 μm diameter metallic particles, which were generated by material detachment of the belt. The presence of foreign particles on the tribological interface contributed to an increase of the shear stresses at the surfaces and, consequently, the number of the contact fatigue crack nucleation sites in the belt/roll tribo-interface. The cracking was originated on the belt/roll interface of the stainless steel belt by a mixed rolling/slip contact fatigue mechanism, which promoted spalling and further nucleation and growth of conventional fatigue cracks. Finally, the system lubrication efficiency and the cleaning procedure should be optimised in order to increase the life expectancy of the belt.

Influence of carburizing and nitriding on failure of gears – A case study

Designing of mechanical parts covers many basic aspects of mechanical engineering. Coupling a proper choice of material and surface treatment, the static and the dynamic designing follow well consolidated criteria, often suggested by experience or prescribed by standards. In gear design many efforts are planned to extend life against wear and contact fatigue, hence the main role is played by a proper selection of surface hardening methods. Complex phenomena of failure are involved for such components during their working life, therefore a deeper comprehension of the damaging mechanisms is necessary to prevent them. To this aim a particular case of failure analysis of a pinion gear is considered. It was built in 18NiCrMo5 UNI 7846 case-carburized, quench hardened and tempered steel. By very few working cycles, the transmission gear prematurely failed. Several ruptures on teeth were noticeable to the unaided eye which covered either a small area of the tooth flank (near the root fillet) or a large zone of the flank itself. In any case, the failure involved only upper part of the pinion teeth. A detailed investigation was needed to clarify the reasons of such a premature rupture. The root of failure was determined to be external overloading and the initial stage of the damage was close related to complex surface contact fatigue mechanism. Following on this investigation the effect of carburising was studied in relation to the mechanism of failure and an alternative nitriding treatment was considered to solve this problem.