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The microstructural and hardness properties of welded joints of AA 6005-T6 aluminum alloy obtained with metal inert gas (MIG), fiber laser–MIG hybrid, and cold metal transfer (CMT) welding were analyzed immediately after welding and after 3 years of natural aging. The fine hardening precipitation within the heat-affected zone, fusion zone, and base material was characterized by differential scanning calorimetry for the three different welding techniques. The elastic–plastic properties of the welded joints were characterized by means of finite element. For all investigations, the analysis on AA6005 aluminum alloy confirms a better behavior of CMT and fiber laser–MIG hybrid joints with respect to the conventional MIG processes.

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.

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.

The dynamical behaviour of the Bronze Age war chariots was studied, considering the different assembly solutions that were found in evidences and pictures. In particular, the chariots with the axle in rear position, typical of Near-East and Egypt, were compared with the European ones, which generally had the axle in central position under the chariot cockpit. Furthermore, the role of the floor, usually realized in woven leather or other organic fibres, was investigated. Dynamical finite element and multibody simulation software was used for studying the behaviour of the chariots in occasional overloading events, such as bumps or curves. An experimental device was set up for studying the difference of the response of a woven floor with respect to a wooden one. Finally, a finite element frequency response analysis was carried out to investigate the behaviour in full run over a rough ground. The results showed that position of the axle did not significantly influence the response of the chariot to occasional overloads. On the contrary, it had a strong influence on the stability of the passengers: rear axle chariots were much more effective in cutting the vibrations transmitted to the passengers when running at high speed. This effect was amplified by an increased floor flexibility, which was achieved with a woven floor. These findings could explain the diffusion of the rear-axle chariots in the Near-East and Egypt, where the chariotry was the most important part of the armies: indeed, the effectiveness in cutting the vibrations at high speed should be a crucial factor for ensuring the required precision to the transported archers. On the contrary, the likely marginal role of the chariots in the European armies could lead to the diffusion of central axle model, which ensured a lower burden on the draught horses.

Wheel microstructure significantly affects railway wheel performances. Although wheel microstructure traditionally consists of pearlite-ferrite, it is possible to detect some intermediate structures traces, for instance bainite, generated during the thermal treatment process. The difference in mechanical properties between pearlite and bainite causes stress and strain incompatibilities at their interface, and this gives rises to early crack initiation and propagation. This paper aims at investigating the presence of bainitic traces in a ferritic-pearlitic microstructure of a railway wheel through semi-analytical and numerical finite element simulations. The effect of bainitic spot shape, dimension and distribution is studied as well as stress and plastic strain in the proximity of the heterogeneities is analysed. The radius-interaxis ratio parameter, defined as the ratio between the bainitic spots radiuses and the distance between two neighbouring spots centres, is introduced to study the effect of circular-sectioned bainitic spots on the steel plasticity. It is observed that the bainitic traces form regions subjected to overstress and overstrain in the ferritic-pearlitic matrix and that the area of the bainitic spots influences the size of these regions. The bainitic spot distribution is found out to increase further the overstrain in case the radius/interaxis ratio is higher than 0.2, corresponding to about 11% of bainite in the ferritic-pearlitic matrix. Beyond this threshold value, bainite traces can compromise both RCF and wear response.

The effect of material plastic properties in solid-contaminated wheel-rail couplings was studied by simplified finite element models, considering typical couplings of steels following European and ...

An improved railway wheel steel containing higher contents of C, Mn, Si, and V than the traditional ER7 steel was developed by alloy design for shoe-braked high-speed train applications. The effects of the thermal load on the microstructure and mechanical properties of a wheel made from this steel were investigated using a combined experimental and numerical approach. The wheel braking was studied using finite element simulations that account for the thermal loading of the wheel in order to find the temperatures reached in the wheel rim. Hardness measurements, tensile tests, toughness tests, fatigue crack growth tests, and microstructural observations were carried out on samples extracted from real wheels, with and without heat treatments simulating the modification of the microstructure due to the shoe braking. The results on the un-treated samples showed that the improved steel has a better combination of strength and toughness than ER7 steel. The results on the heat-treated samples showed that the improved steel maintains acceptable mechanical properties provided the maximum temperature during braking is below the A3 temperature of the steel (around 790 °C).

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 ...

The goal of this paper is to investigate the failure of a mandrel pin mounted on a rolling contact test bench. For this purpose, different methods of analysis were conducted, including metallography and stress analysis. The stress values in the failed pin were determined by Finite Elements. The high notch effect and the incorrect mounting of the specimens were identified as the failure causes. A pin design change is proposed to reduce the maximum stress even in the case of incorrect specimen mounting.

The introduction of two-wheel chariots, pulled by horses, was a key technological innovation in the Bronze Age. Archaeological evidences, found in Northern Africa, Europe and Asia, allowed identifying various chariot typologies and understanding their main features. However, many questions about the function of single pieces and the behavior of the vehicles are still open. In a previous work, a war chariot wheel, found in northern Italy and known as the “Mercurago wheel”, was studied with an engineering approach. In this paper, the whole vehicle, to which the wheel was hypothesized to belong, was studied. In particular, two chariot typologies, differing mainly for the axle position, were analyzed. The stiffness of the various chariot parts were characterized by means of static finite element analyses. These data were subsequently used as input parameters in running multibody dynamics simulations. Finite element dynamical simulations were carried out as well. The analyses results allowed determining the crucial effect of some parts, particularly the cockpit floor, in favoring the passenger stability in dangerous conditions, such as bumping. The effect of the axle position on the passenger and wheel trajectory were evaluated as well.

An investigation was carried out to study the effects caused by shoe braking on the microstructure and mechanical properties of ER7, CLASS B, CLASS C, and SANDLOS® wheels as a result of the thermal load. Particular attention was given to cases of exposure to medium and high temperatures, namely from 700 °C to 970 °C, followed by air cooling. Hardness measurements, tensile tests, toughness tests, fatigue crack growth tests, and microstructural observations were carried out on samples extracted from new wheels, with and without heat treatments simulating the microstructural modifications caused by shoe braking. The experiments showed that the hardness, yield strength and ultimate tensile strength of the steels all decrease with the heat-treatment temperature up to 750 °C due to globular pearlite formation. However, after the heat treatment at 970 °C, these properties show a trend inversion, as the globular pearlite no longer appears, while, in some cases, traces of bainite and martensite are observed. On the other hand, the fracture toughness, crack propagation threshold, and rate are not significantly altered by the heat treatments, showing the steels to have a good stability under the thermal loads caused by shoe braking.

An analytical model for the assessment of a contact in the presence of a solid contaminant is presented, mainly aimed at studying the damage to railway wheels and rails operating in third-body-cont...

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.

Unidirectional plastic strain accumulation (ratcheting) is one of the main causes of surface crack nucleation in rails and wheels in dry condition. It is related to frictional forces which develop at the contact interface due to sliding, especially in curve and in braking. Surface cracks generated by ratcheting can subsequently lead to severe damage when environmental fluid contaminants (such as rain or snow) are added at the contact interface, due to the complex solid-fluid interaction, which can enhance crack growth. In dry condition, wear and ratcheting can reach an equilibrium, such that the strain field and the crack depth are stationary. Understanding such steady-state deformation regime can be a key factor for predicting the expected crack depth and scheduling a correct maintenance programme. In this paper a model for predicting the strain field at steady-state regime is proposed. Such model, based on an integral equation, allows predicting the strain field and crack morphology at high cycle number with no need of iterative numerical simulation. The potentiality of the model was proven both in characterizing the cyclic plasticity behaviour of materials and in predicting the maximum expected crack depth in full scale railway wheels.

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.

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.

Deforestation is one of the most dramatic threats to environmental equilibrium and food safety in many regions of the world, and particularly in sub-Saharan Africa. It is enhanced by many kinds of human activities, among which is the earth brick-firing process that needs large amounts of wood for attaining and maintaining temperatures required during this process. In order to reduce deforestation, the present study investigated from different points of view the potential of crude earth to be used as building material, taking as a case study the Logone Valley located at the border between Chad and Cameroon. First, an on-site investigation was made to verify the state-of-the-art of brick production technologies and to evaluate their impact on the environment. Subsequently, morphological and geotechnical analyses on soil samples collected from different sites of the Logone Valley were carried out, to evaluate their suitability to brick production. Finally, a pilot plant for crude earthen brick production was...

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 Bronze Age crossbar wheel found in the XIX century in Mercurago (Italy) is an amazing example of the technical innovations stimulated by the diffusion of horse draught war chariots in Europe across the third and second millennium B.C. It is a tripartite wheel, with two felloes attached to a diametric crossbar by means of internal dowels, and four thinner rods departing from the crossbar near the hub and ending into the felloes. A separate nave is inserted into the crossbar hub. Many question are still open about this wheel, concerning in particular its constructive concept, the materials and the role of each member. In this paper the tools of modern engineering, in particular the laser-scanner acquisition technology and the finite element method, were used to study the structural issues concerning the crossbar Mercurago wheel under the hypothesized operating conditions. Verisimilar explanations of some technical choices, such as the shape and the materials of the members, were obtained; furthermore, the presence of a leather tire was hypothesized on the basis of the rim-soil contact analysis. Finally, the role of the inserted nave was investigated. It was hypothesized that it could have a role similar to modern bushings, thus being a very innovative device for that time.

Subsurface crack mode II propagation parallel to the contact surface is a damage mechanism leading to dramatic failure in many components subjected to cyclic loading. A weight function (WF) was elaborated for calculating the applied mode II stress intensity factor (SIF) of a crack in a two-dimensional half-space in plane strain condition, for crack completely closed and frictionless contact between the crack faces. With respect to other methods, the WF allows faster SIF calculation, thus being suitable for simulation of many repeated load cycles and fatigue crack propagation. The WF was applied for simulating a case of rolling contact experiments found in the literature, and good agreement between experimental and numerical results was obtained, showing the effectiveness of the WF method in damage tolerant design.

The paper presents an extension to finite life of a failure assessment diagram recently proposed for components subjected to rolling contact loading. The approach considers inherent defects, like inclusions, as potential sources of crack initiation and propagation. A predictive model based on the critical distance theory was proposed for this aim, allowing to define iso-life curves, represented by a quarter of ellipse in the diagram. For working conditions exceeding shakedown, an approach to calculate finite life due to ratcheting was also presented, by comparing cumulated shear strain with the critical one. The proposed approach was used to predict the number of cycles to failure of disk specimens made of SAE 5135 gear steel and compared with the results of previously carried out rolling contact fatigue tests. The agreement between predicted and experimental specimens life resulted good, showing the effectiveness of the approach.

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.

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.

In many developing countries wood is the main energy source, and its exploitation is accelerating deforestation and desertification. In order to reduce wood consumption, an innovative biomass press was designed, aimed at providing alternative fuel by compacting dry waste biomass. The machine is based only on mechanisms, and can work by animal power, in order that it can be used in places where there is not electric power. The main machine element is a cam, designed according to the criterion of making manual running possible, on the basis of an experimental biomass behaviour law. The whole machine design was conceived with the aim of allowing construction in the developing countries. A prototype for research was realised, equipped with an electric motor and electronic control system. Biomass compaction tests were carried out, from which the machine characteristics were derived, and the validity of the biomass behaviour model was confirmed. Pilot plants, both manual and electric, were realised in Burundi and Chad, obtaining encouraging results as far as the appropriateness of this technology to developing countries is concerned.

In many isolated settings electricity is not available or is supplied by fossil fuels, while biomass is used for cooking purposes, not exploiting all of its potential. We are building a prototype of a biomass powered 1kWel Ringbom- Stirling engine that doesn't require high technology, but nevertheless brings relevant innovations. This solution aims to generate electricity using the same source used for cooking. It is meant to be used and to be producible in developing countries, where distributed electricity generation, using locally available primary sources, can help reducing the dependence from foreign aids or suppliers. The system we are building is composed of a down-draft gasifier which supplies the syngas that is then burned to heat the Ringbom-Stirling engine, which produces electricity, while the remaining heat is available for cooking. We discuss here the general scheme of this solution with particular regard for some new applications that greatly simplify the operation of traditional Stirling engines, such as metal bellows, used instead of the typical cylinder-piston arrangement, to totally eliminate the friction due to sealing needs between moving parts.

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.

Wear and rolling contact fatigue interaction in rolling/sliding contact is an important topic in the research on structural integrity of rails and railway wheels. Wear is in competition with rolling contact fatigue, as it removes surface material layers, reducing cracks length and hindering their propagation. Cracks nucleate by accumulation of cyclic unidirectional plastic strain (ratcheting). In this paper a model for ratcheting assessment is discussed and applied to the UIC 900A steel, after a calibration based on experimental results. The experimental tests allow also a characterization of the crack formation condition for this material. By this model, a computer program is developed in order to simulate in a very short time the effect of a large number of load cycles, providing a tool for predicting crack formation and propagation rates.

An animal traction piston press was studied for applications in developing countries, aimed at producing biomass briquettes as alternative household fuel. Experiments were carried out with different biomasses for determining the minimum compaction pressure and the relationship between applied pressure and volume reduction. Two cams were designed for moving the machine mechanisms: their shape was determined with the aim of minimising the applied draft force, considering the experimentally determined biomass behaviour. The machine was prototyped and tested in Senegal, giving satisfactory results.

A model for assessing wear and rolling contact fatigue (RCF) in rails is proposed, with particular emphasis for their competitive role. RCF was studied taking into account different mechanisms of crack nucleation and propagation in different working conditions, and failure indexes are proposed to predict which damage mechanism prevails. Wear was investigated in terms of wear rate under different working conditions; a method to compare it to crack propagation rate is proposed in order to predict if wear can remove surface cracks due to RCF. The effectiveness of the method was verified by experimental tests.

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.

The general properties of a mode II Weight Function for a subsurface crack in a two-dimensional half-space are discussed. A general form for the WF is proposed, and its analytical expression is deduced from the asymptotic properties of the displacements field near the crack tips and from some reference cases obtained by finite elements models. Although the WF has general validity, the main interest is on its application to the study of rolling contact fatigue: its properties are explored for a crack depth range within which the most common failure phenomena in rolling contact are experimentally observed, and for a crack length range within the field of short cracks. The accuracy is estimated by comparison with several results obtained by FEM models, and its validity in the crack depth range explored is shown.

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.

Access to modern energy services is still low in developing countries and this lack of access affects in particular the Least Developed Countries (LDCs) and sub-Saharan Africa. The large majority of population in the Logone Valley at the border between Chad and Cameroon still relies on wood fuel burnt in smoky and inefficient fireplaces for cooking. The promotion of wood saving stoves locally produced and appropriate for the traditional cooking practices has been implemented by an international cooperation project. Two stove models were compared to the traditional 3-stone fire and a gas stove by Water Boiling Tests and Controlled Cooking Tests. The results showed significant fuel savings thanks to the use of the improved stoves. Data collected during the tests, crossed with information about the local cooking habits, allowed to estimate the impact, in term of money savings, on each household adopting an improved stove. The Centrafricain improved stove resulted being the most performing model occurring in a 25% reduction of the expenditure per family for cooking purposes in a short-medium term. This study witnesses that the use of improved wood stove is likely to be a sustainable way to achieve an appropriate minimum level of energy access for cooking purposes for the poor people in the LDCs, in particular in rural areas.

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 previously published model for plasticity assessment in rolling contact, based on a simplification of the non- linear kinematic and isotropic hardening model of Chaboche and Lemaitre, is discussed, and an update is introduced in order to improve its accuracy in the plastic strain prediction within the region just underneath the contact surface. The update is based on a correction of the yield limit and of the strain rate as a function of the load ratio of the tensile stress in the direction parallel to the contact surface. The effectiveness and the accuracy of the updated model in not too severe conditions are demonstrated through comparisons with results obtained by finite element model (FEM) analyses. An application of the model to some experimental results obtained on rail and railway wheel steels is also carried out, and quite good agreement is found in plastic strain prediction, although some discrepancies are found. The method appears to be a valid tool for practical application, especially for its ability of combining the effects of different phenomena and of simulating a number of cycles of the order of millions in a reasonable time.