G. Donzella

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.

Stress intensity factor range and propagation mode of surface cracks under rolling–sliding contact

Abstract Finite element analyses were conducted in order to evaluate the mode I and mode II stress intensity factors for inclined edge cracks under cyclic contact load under rolling and rolling–sliding condition. The SIF range depends on crack orientation, crack length to Hertzian contact zone half-width ratio, friction between the crack faces and friction on the contact surface. The results were combined in two compact functions that determine the Δ K I and Δ K II values. The crack propagation mode and direction were investigated using both the maximum stress criterion and the minimum strain energy density criterion. The results are displayed in graph form, which allows a fast evaluation of the crack growth condition.

Analysis and design of a low-noise railway wheel

Abstract The design and construction of a new ‘constrained layer’ damping treatment for railway wheels is presented. A numerical procedure was used for the loss factor calculation. This procedure, first verified on a plate by means of experimental modal analyses, allowed the best treatment thickness and arrangement to be chosen among those commercially available and technologically feasible. The work ended with the construction of a prototype and subsequent tests in the laboratory and in the field. The success of these tests justified the start of production of low-noise wheels treated in this way.

The effect of block braking on the residual stress state of a solid railway wheel

Abstract A finite element, numerical model of a solid railway wheel was perfected permitting the simulation of a block braking operation. The analyses performed, backed by experimental tests, permitted the degree of variation in the residual stress state (induced by differential quenching heat treatment and successive tempering) caused by particularly heavy braking to be evaluated. The results highlight the influence of the main braking parameters (force and time) and the thermal history previously suffered by the wheel. Finally, the thermal fatigue strength of the component, although in an approximate way, was checked on the basis of the calculated stress state and through the introduction of the data of a real and particularly significant route into the model. This demonstrated the broad safety margin with which the wheel operates and also brought to light the dangerous nature of particularly severe braking which can drastically modify the residual stresses induced by the heat treatment, moving them towards tensile stresses.

Some experimental results about the correlation between Barkhausen noise and the fatigue life of steel specimens

Abstract The results of Barkhausen Noise (BN) measurements during pulsating axial fatigue tests on notched steel specimens are reported. The aim of these measurements was to study the cyclic signal in the first stage of the fatigue process, in order to find a correlation between this magnetic parameter and the fatigue life of the specimens. The hysteresis loop of the BN intensity versus the applied load, during the initial stage of the process up to the crack nucleation, for different load levels has been studied. The loop amplitude is found to change significantly during the life period investigated, according to the load level. This behaviour is connected to the degree of cyclic plastic deformation and reflects the micro-structural settling phenomena that happen during the first stage of the fatigue process. The loop amplitude measured at the first cycle of each specimen has been related to the corresponding failure cycles number, showing a possible way for the prediction of the fatigue life.

Levator ani deformation during the second stage of labour

A very important medical problem for females is urinary incontinence, sometimes associated with faecal incontinence and pelvic organ prolapse. One of the most common reasons these issues are increasing is clearly the muscle damage during childbirth. This article focusses on understanding the complex behaviour of the levator ani muscles involved in the second stage of labour. A geometrical model obtained from a 23-year-old nulliparous woman was used to simulate childbirth. Several assumptions were introduced in order to simplify the problem without significantly affecting the global response of the system. An anisotropic hyperelastic model was used to characterize the material behaviour; the muscle fibres were assumed to be mostly orientated circumferentially. In addition, particular attention was also put to the boundary conditions of the model. The introduction of the constraints imposed by the coccyx bone in the central area of the levator ani group represents one the most important improvement compared to previous computational models. The maximum deformation and stress were found in the pubococcygeus muscle of the levator ani group. A stretch value close to 2.2 was determined by considering different material parameters. The results seem convincing with respect to medical observation and previous analysis. However, there are still some limitations concerning the material definition and the geometry and trajectory of the head that can be further improved.

A computational procedure for life assessment of UHP reciprocating seals with reference to fatigue and leakage

A calculation procedure is shown for assessing the working life of reciprocating plastic seals for Ultra High Pressure (UHP) applications. Two main damage phenomena are considered, low cycle fatigue and wear, associated with structural failure or leakage respectively. An integrated approach, based on a finite element analysis of the seal, is proposed to evaluate both fatigue failure and leakage occurrence. The model takes into account non-linear effects due to seal contact interactions, material cyclic behaviour and its dependence on hydrostatic pressure. Fatigue life is predicted by combining equivalent strain range calculated in the critical points with life curve of the material. Leakage prediction is made by calculating a leakage index, based on instroke and outstroke pressure gradients and related to contact pressure profile change due to wear. Seal design can be optimised by choosing seal material and geometry so to delay fatigue failure and leakage, scheduling their occurrence at the same time.

Experimental Investigation on the Crack Closure Phenomenon Using Barkhausen Noise 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.

Failure analysis and life prediction of polymeric rollers for industrial applications

The present study investigates the behaviour of polymeric rollers employed in a cam mechanism of an industrial machine, consisting of two concentrically fitted rings made of polyurethane and short fibre-reinforced PEEK. A cyclic mechanical characterisation of the materials was at first carried out by strain controlled fatigue tests. The safe range of working conditions of the rollers in terms of rotating speed and contact load was then investigated by rolling contact tests carried out on roller prototypes with a dedicated test rig. These tests allowed obtaining important information about the different damage mechanisms occurring in the component: above a critical working temperature, rollers failure resulted from rings unfitting due to excessive deformations, while fatigue crack nucleation in the outer ring at the interface with the inner one was observed below this threshold, at high contact load levels. No contact fatigue damage appeared. FEM models of the roller were also developed, based on cyclic material data, in order to analyse the cyclic stress and strain field and to predict the fatigue life, finding a satisfactory agreement with the experimental rolling contact tests results.

Numerical Analysis of Residual Stress State in Clad Components

A numerical model was realized, by means of ADINA f.e.code, of a low alloy steel specimen with INOX cladding and it was submitted to a thermical cycle similar to that of a relieving heat treatment. The residual stress state was then calculated, due to the coupling of the two different materials, and as a function of the cooling rate.