Angelo Mazzù

Competition between wear and rolling contact fatigue at the wheel—rail interface: some experimental evidence on rail steel:

Abstract 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.

Rolling Contact Fatigue and Wear Behavior of High-Performance Railway Wheel Steels Under Various Rolling-Sliding Contact Conditions

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.

Valorisation of crude earth as sustainable building material: a case of international cooperation in the Logone Valley (Chad–Cameroon)

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 set, particularly based on a new press designed for this purpose.

Design of a Press for Densification of Waste Biomass in Developing Countries

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.

Effect of Wear on Surface Crack Propagation in Rail–Wheel Wet Contact

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.

Failure Analysis of a Cyclic Contact Testing Machine Pin

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.

Experimental and Numerical Characterization of a Rail Steel Behavior under Cyclic Contact Stresses

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.