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The diffusion of brake-shoe system has made it necessary to study brake–wheel–rail interactions. This study aims to elucidate this topic with an innovative small-scale test rig, the ‘4-contact machine’. A test procedure is designed in which an ER7 steel wheel is coupled with a R260 steel rail and two different brake-shoe materials (cast iron and sintered material). The results show that the 4-contact machine reproduces the effects of wheel–rail–brake shoe contact and damage similar to those observed on a railway wheel at the end of its life, highlighting marked differences in behaviour between the two brake materials.

To date, discarded tires are reused in many applications, however, because of the enormous quantity decommissioned annually, it is essential to continue researching new recycling methods as well as applications to reduce waste and preserve new resources. In the present study, a simple recycling technology of end-of-life tire (ELT) powder is proposed, and the influence of steel slag as filler is assessed. Europe produces about 7 Mt of steel slag annually, and although most of it is reused as an artificial aggregate, about 15% is still landfilled. Also in the case of steel slag, the study of new applications is mandatory so that the combination of these two waste materials, in a 100% recycled composite fits across different industrial sectors facing the same environmental issue. It was found that the leaching of the slag incorporated in the rubber matrix is reduced and that the ELT powder recycled by this technology gives rise to a well-cohesive material. A good rubber-filler interaction was found by swelling test and differential scanning calorimetry (DSC) analysis. The slag reduces the friction coefficient and increases the thermal conductivity. The experimental results showed how some properties of recycled ELT can be improved by adding the steel slag.

Worn railway wheels after service in the desert were investigated by means of failure analysis, hardness measurements, tensile tests, fracture toughness tests and low-cycle fatigue (LCF) tests. The results complete both previous small-scale laboratory experiments and FEM simulations performed by the authors on widely used steels and new steel grades allowing to better understand the main damage mechanisms due to the sand. These new steel grades were developed for forged-rolled solid wheels in alternative to AAR Class B/C/D steel grades and were designed to ensure high resistance to wear and rolling contact fatigue in the presence of sand, debris or gravel on the rails. This paper covers the in-depth mechanical characterization of wheels made of the new steels. Hardness measurements, tensile tests, fracture toughness tests and LCF tests were carried out. The results are compared with AAR Class B/C/D steels showing an excellent combination of fracture toughness, cyclic yield strength and ductility.

The interaction between wear and rolling contact fatigue in wet contact was investigated by means of experiments on a two-disk test bench. Specimens of various railway wheel steels were coupled with specimens of the same rail steel, subjected to rolling and sliding wet contact with varying sliding/rolling ratio. Some tests with a dry rolling–sliding contact phase preceding wet contact were carried out as well (dry–wet tests). The pressurization of the fluid entrapped in the surface cracks was the cause of a rapid and severe damage in the dry–wet tests, due to the nucleation of surface cracks by ratcheting in the dry phase, which subsequently propagate in the wet phase. In the wet tests, the fluid pressurization effect was much mitigated due to the absence of initial surface cracks; the specimens subjected to higher sliding/rolling ratio showed the best performance against rolling contact fatigue, due to the effect of wear in reducing the length of surface cracks. A model for assessing the interaction between wear and rolling contact fatigue was proposed, based on the correction of the classical Paris law for crack propagation by taking into account the effect of wear on crack length reduction. The application of the model to the experimental tests allowed finding a correlation between the model predictions and the occurrence of fatigue failure.

A recent interest on emergency shoe braking in high speed trains stimulated the research on the damage due to sliding contact between brake blocks and wheel tread. Bi-disc tests, with rolling/slidi...

A vision-based experimental methodology was developed for monitoring the surface state evolution of specimens during twin-disc rolling contact tests, aimed at providing information for identifying ...

In the present paper, fatigue tests were performed on sand-blasted AlSi10Mg samples produced by Direct Metal Laser Sintering (DMLS). The effect of sand-blasting on surface properties was evaluated by roughness and residual stresses measurements, together with morphological analysis, in comparison with as-built condition. An evident improvement of surface finishing was observed after sand-blasting, which also leaded to the presence of compressive residual stress on the external surface of samples, as revealed by XRD2 measurements. Furthermore, defects analysis allowed the identification of a uniform distribution of porosities in the cross section in terms of number of defects, while larger porosities seem more abundant close to the surface. It was found that the tested material exhibits good fatigue resistance, supporting the positive role of sand-blasting as a simple post-processing treatment. Superficial defects are the preferential crack initiation sites, as demonstrated by SEM analysis of fracture surfaces.

Short carbon fiber reinforced (SCFR) PEEK is a highly attractive material for lightweight structures; improving knowledge about the influence of local imperfections on its fatigue behavior is essential for the design of real components. To this aim, fatigue strength and crack propagation of two grades of SCFR PEEK and neat matrix were investigated by testing at different stress levels specimens with a micro-notch consisting of a small blind hole (range diameter 0.1–1 mm). Overall, the presence of a micro-notch resulted in a decrease of fatigue strength compared to un-notched condition, but with different sensitivity and crack propagation patterns; while a higher fiber volume fraction enhanced fatigue strength and resistance to crack propagation, the combination of a lower fiber content and inclusion of additive particles had a negative effect. Crack propagation in the notched region was also evaluated. The average values of Paris' law exponential coefficients were similar and within the range of literature values, without apparent correlation with reinforcement type. Preliminary investigations in the presence of the smallest micro-notches seem to indicate the presence of a threshold size below which the influence of a small notch is comparable with that of material inherent defects, but further testing is necessary. Keywords: Short carbon fiber, PEEK, Fatigue, Micro-notch

A failure assessment diagram for components subjected to rolling contact with hardness varying along the depth is presented. The approach takes into account the influence of inherent defects on subsurface fatigue: considering a 2D plane strain model, crack propagation from inherent defects was assessed in terms of applied stress intensity factor; defect-free fatigue was assessed in terms of the Dang Van stress. By analysing different combinations of loading condition, defect dimension and hardness profile, it was possible to obtain a general relationship for subsurface rolling contact fatigue prediction. A good agreement was found between predicted contact pressure limit and some published experimental results.

An experimental investigation was carried out to study and compare the response to cyclic loading of the high-performance railway wheel steels ER8 EN13262 and SUPERLOS®. Rolling contact tests were performed with the same contact pressure, rolling speed and sliding/rolling ratio, varying the lubrication regime to simulate different climatic conditions. The samples, machined out of wheel rims at two depths within the reprofiling layer, were coupled with UIC 900A rail steel samples. The wear rates, friction coefficients and hardness were correlated with the deformation beneath the contact surface. The crack morphology was studied, and the damage mechanisms were identified. The distribution of crack length and depth at the end of the dry tests was analyzed to quantify the damage. The main difference between the steels lies in the response of the external samples to dry contact: SUPERLOS® is subjected to a higher wear and lower friction coefficient than ER8, and this reduces the density of surface cracks that can propagate under wet contact conditions. The analysis of feedback data from in-service wheels confirmed the experimental results.

The wear behaviour of two railway wheel steels, ER8 and SUPERLOS®, was studied through pin-on-disc tests, and the results were correlated with those previously obtained with twin-disc tests. The work-hardening of the steels was investigated with Vickers hardness measurements, and the wear mechanisms were studied using scanning electron microscopy. ER8 discs showed higher wear resistance, lower work-hardening ability and less wear damage than SUPERLOS® ones, confirming the results of the twin-disc tests. Therefore, sliding pin-on-disc experiments are recommended as a simple laboratory technique that can be used as a screening method for wheel steel performance prior to more complex and more expensive tests. The damage in both steels was due to the concomitance of oxidative wear, abrasive wear and fatigue wear. Iron oxide formation protects the steels from severe wear, whereas its detachment causes abrasive wear; furthermore, surface fatigue cracks initiate and propagate leading to the detachment of material flakes.

This paper presents an experimental investigation on a middle tension (MT) specimen made of a low carbon structural steel subjected to cyclic loading. The work was aimed to assess the effectiveness of Barkhausen Noise method in detecting the crack closure phenomenon. The Influence of probe orientation and position ahead the crack tip was examined, in order to optimize the magnetic measurements. A filtering procedure was also used to treat the Barkhausen Noise signal. A procedure was proposed to determine the crack-opening load from it. On the same specimen, some strain gauge measurements were carried out to determine the crack-opening load by the local compliance method. The results showed a good agreement of the results obtained with the two experimental techniques.

Fatigue strength and failure mechanisms of short fiber reinforced (SFR) PEEK have been investigated in the past by several research groups. However some relevant aspects of the fatigue behavior of these materials, like cyclic creep and fatigue damage accumulation and modeling, have not been studied yet, in particular in presence of both fillers and short fibers as reinforcement. In the present research these aspects were considered by carrying out uni-axial fatigue tests in load control (cycle ratio R = 0) on neat PEEK and PEEK based composites reinforced either with short carbon fibers only or with addition of fillers (graphite and PTFE). For each material stress-life curves were obtained and compared. Fatigue fracture surfaces were analyzed to identify failure mechanisms in presence of different reinforcement types. The evolution of cyclic creep strain was also monitored as a function of the number of cycles, thus allowing investigation on the correlation between cyclic creep parameters and fatigue life. The evolution of cyclic damage with loading cycles was then compared by defining a damage parameter related to the specimen stiffness reduction observed during the tests. Progressive cyclic damage evolution of short fiber reinforced PEEK composites presented significantly different patterns depending on applied stress level and on the presence of different reinforcement typologies. In order to reproduce the different fatigue damage kinetics and stages of progressive damage accumulation observed experimentally, a cyclic damage model was finally developed and implemented into a finite element code by which a satisfactory agreement between numerical prediction and experimental data at different stress levels for each examined material.

Recent studies of the rail—wheel interface have shown that wear and rolling contact fatigue (RCF) are competitive phenomena. Wear, by removing the material from the contact surface, tends to limit the propagation of cracks formed by cyclic contact stresses. Several parameters influence these phenomena. This article studies the effects of various working conditions on the surface damage of railway steels. The competition between wear and RCF is particularly strong in dry rolling—sliding contacts, in which the damage severity of our test cases can be adequately predicted using the shakedown map. In wet contacts, RCF is prevalent: cracks rapidly propagate into the subsurface layer in response to hydraulic pressure penetration and then branch towards the surface causing severe damage (macroscopic pitting). The severity of this phenomenon depends strongly on the applied load and cannot be inhibited by wear due to the low friction. On the basis of these results, a general procedure is proposed to the structural integrity of rails.

Corrosion and wear resistance of Cathodic Arc Evaporation (CAE) CrN coatings deposited on a tool steel were investigated considering the effect of the number of layers (mono or double) and the influence of defects. The CrN coatings were characterized for mechanical (scratch, nanoindentation test), corrosion behavior (polarizarion tests) and tribological properties (pin-on-disk tests). Scanning Electron Microscope analyses on samples before and after the tests were also made in order to identify the type of the defects and the role they play in the coating damage. Monolayer coating shows a good wear behavior with a stable and low friction coefficient, but its corrosion performance is poor. On the contrary, the presence of an interface into the double layer improves the corrosion resistance, but it is a weak point for the coating delamination during wear tests.

Wear and rolling contact fatigue were investigated on unfilled PEEK and on three PEEK short fiber reinforced composites. For this aim, roller-shaped specimens were subjected to rolling contact tests at different contact pressure levels. Wear rate- pv product and contact pressure-life diagrams were obtained, depending on material hardness. Microscopic observations of rolling contact surfaces and sections of the rollers highlighted damages at two different scales, surface and bulk, leading independently to wear and rolling contact fatigue. These damages were also of different typology for unfilled PEEK and composites: micro-pitting and deep transversal cracks occurred on unfilled PEEK rollers, while delamination and spalling phenomena where found on composite rollers.

Rolling contact fatigue tests were carried out on ring specimens made of quenched and tempered SAE 5135 gear steel with three different steel-production processes, through a bi-disc machine under pure rolling condition and water lubrication. Early formation of micro-pits then coalescing into macro-pits was observed on the rolling surface, while the final failure was caused by subsurface originated spalling phenomena. Microscope analysis of specimens section highlighted the complex surface and subsurface crack layout, and permitted to recognise sulphides as preferential sites for cracks initiation. The inclusion content was analysed throughout the extreme value statistics and the maximum expected inclusion in the Hertzian contact zone was introduced in a failure assessment diagram recently proposed, which resulted effective in predicting the specimen failures.

A failure assessment diagram for the evaluation of the safe working area of components subjected to rolling contact loading is proposed. Rolling contact fatigue limitation is treated in terms of non-propagation condition of inherent defects, following the El-Haddad model for the short-cracks growth threshold. Static fracture and ratchetting limitations are also added to the diagram. In this way, the approach gives an overview of the possible damage mechanisms, automatically indicating which is expected for a specific case. In particular way, the diagram presents different areas: a safe zone (infinite life), a rolling contact fatigue zone almost independent on defects content, a rolling contact fatigue zone dependent on defects, a ratchetting zone and a static fracture zone. Depending on material properties, operating conditions and inclusion content, a reference point can be drawn on this diagram, indicating in which area the component is working and, consequently, if it is safe or which damage mechanism is expected. Some experimental evidences referring to rolling contact tests carried out in the past where re-interpreted and verified by this approach, highlighting the role of working conditions, material properties and inclusion content in determining the damage mechanism.

Rolling contact fatigue (RCF) plays a critical role in railway components, and the characterization of materials used, in terms of RCF life, is still an open task, made complex by the interactions of different phenomena. The contact surface has a direct impact on the pressure exerted and can change during the test, due to wear. The procedure proposed consists in using vibrations of a test bench during RCF?life tests to identify when wear increases and causes a quick flattening of the specimen’s surface, and when this process is complete. The procedure is applied to two case studies regarding wheel and rail steels. In the tests, a wheel steel specimen rotates against a rail steel specimen, while pressed against each other by a constant force. At regular intervals weight loss and surface analysis are performed, while vibrations and torque are monitored continuously. Destructive tests are carried out at the end of each test. Results from non?destructive measurements were used to provide input data to a numerical simulation, used to determine the cyclic plasticity properties of the material. The methodology proposed shows the potential application of vibration measurements for detecting wear rates thus allowing supporting or partially supplanting destructive testing.