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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.

The purpose of this work is to gain a better understanding of the complex damage phenomena taking place at the railway wheel/brake block interface due to thermo-mechanical loading. Initially, full-stop braking was studied using Finite Element (FE) simulations to estimate the temperature reached in the wheel rim. Experiments to reproduce wheel damage were conducted with a two-disc machine using test conditions that were based on the results of the FE simulations. Three different wheel steels were tested against the same cast iron shoe material. The evolution of the wheel disc damage was studied at various numbers of cycles under fixed contact pressure and sliding speed. The friction coefficient and the temperature on the wheel disc surface were measured during the tests. At the end of the experiments, the wheel disc was examined and characterized. Cross-sections were observed with an optical microscope and the hardness was measured as a function of the depth to investigate the damage mechanisms that occurred at surface and subsurface. Material transfer from the shoe specimen to the wheel specimen results in the formation of a discontinuous “third body” layer, and that layer plays a key role in the evolution of the wheel disc damage. When the transferred layer of brake material is worn away, detachment of steel from the wheel disc surface occurs, probably promoting the crack nucleation. In addition, wear debris from both disc materials promotes three-body abrasive wear of the wheel disc surface.

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.

Twin-disk tests are an effective method to characterize the material response to rolling sliding contact, to reproduce the damage phenomena of real components at a laboratory scale in controlled working conditions. Usually the monitoring is performed by means of “gross” parameters, such as weight loss, coefficient of friction and Barkhausen noise, and micrographs of the sample sections, at the end of the test. Visual inspection of the sample contact surface at the macro-scale yields further information about the process under analysis. In twin-disk tests, the samples are visually inspected at predetermined steps, typically by acquiring the image of their surface when the samples are stationary. The availability of a system able to capture the images of the samples while they roll during the tests is of interest to better monitor the damage evolution. In this paper, we present the results of the experiments carried out to extract quantitative information from the images captured on railway wheel samples during rolling contact tests: suitable image processing has been designed with the objective of finding meaningful, synthetic indices for the monitoring and the interpretation of the wear process, also in relation to prior knowledge about the process. The experimental work was focused on the definition of the indices, on the analysis of their behavior on different steels, and on their usefulness to predict uneven wear during the tests on the test bench.

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 ...

Fatigue resistance of Direct Metal Laser Sintered (DMLS) AlSi10Mg alloy after sand-blasting was investigated in the present study. A preliminary characterization of the samples was carried out in order to identify material microstructure, surface roughness and superficial residual stresses. It was found that the applied post-processing treatment was responsible for a strong decrease in average surface roughness as compared to the as-built condition and induced compressive residual stresses on the samples surface. Axial fatigue tests were performed in both finite and infinite (i.e. 2x106 cycles) life regimes and the obtained results were compared with literature values for the same alloy after various post-treatments, including heat treatment, machining, polishing and shot peening. A general positive effect of the applied sand-blasting on fatigue resistance was observed. This, coupled with the improvements of surface finishing, encourages the use of sand-blasting as a simple and effective post-treatment. Finally, observations of the fractured surfaces allowed also the identification of porosities located on the surface as the main crack initiation sites. Once the crack has started, it moves along a large flat area, as typical of fatigue propagation, with a small final overload region.

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

This work studies the effect of desert sand at the wheel-rail contact interface on two standard railway wheel steels (ER8 and CLASS C) and two upgraded steels (SANDLOS ® S and SANDLOS ® H) specifically designed for sandy environments. Rolling contact tests were carried out by means of a two-disc testing machine with and without sanding at the contact interface. The tests were performed for various durations in order to study the disc damage evolution. The response of the steels to the same contact pressure, rolling speed, sliding/rolling ratio and sand feed rate was evaluated in terms of wear rate, strain hardening and surface and subsurface damage. The ER8 discs performed worst, incorporating sand into the subsurface layer by extremely high plastic flow and were, therefore, subject to material detachment by rolling contact fatigue. The SANDLOS ® H discs performed best, with a wear rate 2.5 times lower than that of ER8 discs, due to the steel having the highest mechanical properties, and thereby restricting the sand embedding mechanism to a thin surface layer. The sanding increased the wear rate of the wheel discs by a factor which varies from 1.4 for SANDLOS ® H discs to 2.2 for ER8 and CLASS C discs. The damage mechanisms were qualitatively studied by finite element simulations, which recognised two different layers for the plastic strain. These layers are related to the dimensional scales of the disc-disc and sand-disc contact areas, respectively, and their influence on the total damage depends on the material cyclic yield stress.

Sandwich structures are manufactured using multiple combinations of materials for core and laminates. The real performances are influenced by variability in the composing layers and even by the uncertainties introduced while bonding them together. Therefore, experimental tests are usually the preferred way to assess the most important parameters required to develop and to characterize the product, the main downsides lying in their cost and time consumption. This work explores a practical application of the wave propagation approach by means of a case study, in which some significant properties of an aluminum honeycomb panel are obtained starting from simple vibro-acoustic tests carried out on beam specimens. After determining the frequency-dependent bending stiffness function, the sound transmission loss is predicted and compared with the experimental results obtained in sound transmission suites. The same vibro-acoustic tests are used to estimate the core shear modulus. Finally, a parametric study is proposed to show how this technique can be effectively used in the early design stage, when producing physical samples is often impossible due to time and money constraints. The method proved to be a reliable and powerful tool in all the tested applications, providing good results with limited computational effort.

A procedure for damage assessment in wheel–rail contact is still an open task: much work has been done for understanding failure mechanisms such as wear, cyclic plasticity and Rolling Contact Fatigue, but only recently the competition between them has been taken into account. In this study, a numerical procedure for damage assessment of railway wheel steels, considering the complex interaction between various failure phenomena that can occur at the loaded region, is proposed. The procedure is based on various damage mechanism models, integrated into an overall simulation method able to take into account their reciprocal influence. The model was applied for characterising a railway wheel steel (ER8 EN13262), through the elaboration of the results of rolling–sliding experiments, determining the plasticity and the wear model constants that can be used for damage assessment of real wheels. The experiments showed that the main damage phenomenon was surface crack formation due to unidirectional plastic flow (ratcheting); in the presence of water these cracks propagated causing very severe damage. A role of manganese sulphides non-metallic inclusions as preferential site for subsurface crack nucleation was observed. A quantitative evaluation of these phenomena was provided.

30NiCrMoV12 quenched and tempered samples were coated with 4 μm of CrN produced by cathodic arc deposition technique. Micro-blasting treatment was performed on the coated samples, after the deposition, to remove defects like droplets from the surface. The fatigue limit was assessed and compared to that of the bare steel. A basic characterization of the coating/substrate was performed (hardness, roughness and adhesion). The residual stress distribution next to the surface of the samples was measured and its effect on the fatigue limit was evaluated. The micro-blasting treatment does not adversely affect the hardness and the adhesion of the coating nor the fatigue resistance of the coated samples.

The effect of shoe braking on three railway wheel steels was studied by means of bi-disc tests, in order to evaluate the applicability of shoe braking as an emergency stop braking system in high-speed passenger trains. Disc-shaped specimens, extracted from real wheels, were initially paired with discs extracted from cast iron brake blocks, in condition of rolling and sliding such to reach the typical temperature of the surface of a wheel rim during stop braking. Subsequently, the wheel discs were coupled with rail steel discs in rolling and sliding dry contact. Some of them were cut and analysed; other ones were subsequently subjected to rolling and sliding contact with rail specimens in wet condition, with various test duration. Measurements of the coefficient of friction and of the weight loss were carried out during the tests; at the end of the tests, hardness measurements and microstructure observation were made on the disc cross-sections. The main damage phenomena observed in the specimens subjected to wheel-brake contact and dry wheel-rail contact were ratcheting, wear and nucleation of surface cracks. Traces of cast iron, previously transferred from the brake specimens to the wheel ones, were detected. The main damage phenomenon observed in the specimens subjected to wheel-brake, dry wheel-rail and wet wheel-rail contact was rolling contact fatigue, developing by the propagation of surface cracks due to the pressurization of the water entrapped inside the cracks. All these phenomena were found to be similar to those observed in wheel specimens subjected to wheel-rail contact only. The effect of the cast iron traces on surface crack propagation was evaluated by means of finite element simulations and it was found to be limited. Therefore, the application of shoe braking as emergency stop braking system to high speed trains did not show any relevant contraindications.

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.

The automotive wheel is a critical safety component in the vehicle and, for such a reason, it has also to meet strict requirements about technological properties. Aluminum wheels are produced by low pressure die casting technique and the casting defects related to the process have to be properly considered having a high effect in decreasing both static and cyclic resistance of the component. Effectively, casting defects as porosities influence the fatigue crack initiation and strongly affect the fatigue life too. One of the most common problem in the real component is the mismatch between the experimental and literature data about fatigue life. In fact, many scientific researches are usually carried out on small samples produced in a controlled condition and therefore it is difficult to direct transfer the laboratory results to a real cast component with a well-defined shape and different thicknesses. In the present study, an aluminum alloy A356-T6 wheel was analyzed in order to assess its fatigue performance, taking into account the casting defects. The fatigue limit of the component was calculated by rotating bending fatigue tests executed on the whole wheels. Microfractographic analyses on the broken wheels were carried out on the fracture surfaces using a Scanning Electron Microscope in order to identify the crack initiation zone: it was recognized that the crack always started from shrinkage porosities. The statistical population of these defects was therefore investigated on samples taken from the wheel in crack nucleation positions of the spoke and the maximum expected defect size on the component was estimated by the statistics of extreme values. The experimental fatigue limit was finally compared with the theoretical value predicted with the Murakami’s method.

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.

The paper presents a ratchetting model, based on the non-linear kinematic-isotropic hardening law of Leimatre and Chaboche, able to predict the shear strain accumulation during rolling contact loading and including wear as a competitive phenomenon. A procedure was proposed to calibrate the model with the material constants suitable for rolling contact problems, obtaining them from bi-disk contact tests. For this aim and to study the damage evolution at the wheel–rail interface, some twin disk rolling contact tests were carried out on a common wheel–rail material couple under a dry rolling–sliding condition typical of normal service. The tests were stopped at progressive cycles numbers for different couples of specimens, which were then cut and observed with optical and electron microscope in order to analyse the damage evolution in the zone near to the contact interface. A wear-rolling contact fatigue competition was observed with a progressive crack tip advance following a shear band cracking mechanism and crack tail removal due to wear. The numerical model allowed predicting the experimental strain profile along the depth as a function of contact cycles number, also demonstrating through the critical strain approach that in the analysed conditions wear was able to prevent deep crack formation.

The effect of shot peening treatment on sintered steels plates were analyzed in terms of micro-structural and mechanical properties and residual stress profiles. Two high performances powder metallurgy steels were considered: the former was obtained starting from diffusion bonded powders, whereas the latter starting from pre-alloyed powders. Two different nominal densities were considered, 6.9g/cm3 and 7.1g/cm3. After a preliminary optimization, two shoot peening cycles were selected and carried out on the investigated materials. Residual stresses after the treatment were measured by means of the hole drilling technique and related to the mechanical properties and the surface densification of both steels, varying the nominal density.

A development of a failure assessment diagram for the evaluation of the safe working area of components subjected to rolling contact loading is here presented regarding the application to components whose hardness varies along the depth. The approach takes into account the influence of inherent defects in determining subsurface rolling contact fatigue, depending on working conditions, material properties and hardness profile along the depth. For this aim, crack propagation from inherent defects is assessed in terms of applied stress intensity factor range normalized with respect to short crack growth threshold, defect – free fatigue is assessed in terms of Dang Van stress normalized with respect to shear fatigue limit, where the material quantities depend on hardness variable with depth. These two normalized quantities are the coordinates of the points of a “reference curve” in the failure assessment diagram, which location indicates whether and where failure is expected to occur. By analysing different combinations of loading condition, inclusion dimension and hardness profile, it was possible obtaining a design diagram of general validity, which allows a fast prediction of safety against subsurface rolling contact fatigue.

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.

The assessment of damage in rail–wheel cyclic contact requires considering the combined action of different damage mechanisms, such as wear, ratcheting, surface or subsurface crack nucleation and propagation. Models, usually requiring experimental calibration, are available for assessing these phenomena. The best way to calibrate them is based on cyclic contact tests, as these represent the real working conditions more closely. However, some experimental information, such as microstructural changes or crack paths, can be obtained only by destructive methods at the end of the tests, and their evolution cannot be monitored; other parameters, such as the wear rate, can only be determined during time consuming breaks to the tests. In this work, non destructive measurements of vibrations, torque and Barkhausen noise were introduced as indicators of damage evolution in cyclic contact tests on a high performance steel for railway wheels, coupled with a rail steel. In particular, their correlation with surface state, wear rate, subsurface microstructure and presence of cracks was shown.

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.