Michele Carboni

Application of fatigue crack growth algorithms to railway axles and comparison of two steel grades

Abstract The reduction of the total life cycle cost of the whole wheelset requires an optimization of inspection intervals for axles. Such components can, in most European countries, endure more than 30 years of service life. During this time, they are subjected to frequent non-destructive testing (NDT) inspections. It is therefore important to assess the influence of design options, such as hollow versus solid axles or mild versus high strength steel, on the prospective crack propagation life of an axle. In the present paper two dedicated crack propagation algorithms (namely AFGROW and NASGRO) will be applied to the estimation of inspection intervals of railway axles by comparing the properties of two widely used steels.

An Investigation of the Effects of Corrosion on the Fatigue Strength of AlN Axle Steel

Abstract Corrosion fatigue is an important topic in the evaluation of the structural integrity of railway axles. In fact, several recent axle failures have been attributed to the presence of corrosion pits and axle surface corrosion. Despite the importance of this issue, existing EN standards do not precisely quantify the effect that corrosion or corrosion-fatigue might have on the fatigue strength of a railway axle. Consequently, there is a need to have a better understanding of this problem. In this paper, two main lines of research have been adopted with a view to investigating the issues regarding the corrosion fatigue properties of AlN steel. The first line of research assesses the morphology and the statistical distribution of corrosion defects present on the surface of retired axles together with the detrimental effect of the defects detected on the in-air fatigue strength of AlN steel. The second line of research examines the fatigue properties of AlN steel in the presence of artificial rainwater. The results of the analysis show that the corrosion typically found on axles is significant from a point of view of fatigue properties and that the fatigue life of AlN steel is strongly affected by the presence of a mildly corrosive substance like rainwater. Fatigue test results are consistent with the fatigue strength reduction outlined in the BASS technical recommendations.

Analysis of Crack Growth at R =−1 Under Variable Amplitude Loading on a Steel for Railway Axles

Railway axles are designed for infinite life, but occasional failures have been and are observed in service. The failures, which are usually due to fatigue crack propagation, are typically positioned at the press fits which correspond to the wheels, gears, brakes, or they appear at the axle body close to the notches and transitions. The way to addressing this problem is to adopt the concept of “damage tolerance” in order to determine the inspection intervals of the axle depending on the NDT method. The issue open to discussion is the choice of the correct algorithm for simulating crack growth at R=−1 at variable amplitude loading especially when considering mild steels such as A1N. The aim of the present research is to discuss the application of state-of-the-art algorithms to estimate the effect of block loading at R=−1 (derived from real load spectra experienced by the axle) upon constant crack growth rate. More specifically the current research addresses: (i) crack growth experiments at R=−1 under constant amplitude loading on two novel types of small-scale specimens; (ii) crack growth experiments under block loading on small-scale specimens; and (iii) the prediction of the experimental evidences by well-known and wide-spread analytical tools.

Effect of Probability of Detection upon the Definition of Inspection Intervals for Railway Axles

Abstract The need to optimize inspection intervals of railway wheelsets involves, among other aspects, a reliable estimation of fatigue crack growth in the axles. With this in mind, the present paper focuses on the description of the considered axle made of A1N steel, and on some typical load spectra. Application of an opportune algorithm, derived from the dedicated soft-ware NASGRO®, made it possible to obtain life predictions in terms of cycles, and to discuss the influence of service loads. Life estimates were then analysed together with probability of detection curves of different non-destructive testing techniques in order to determine the total probability of observing, during periodical inspections, cracks or defects.

Feasibility study of a multi-parameter probability of detection formulation for a Lamb waves–based structural health monitoring approach to light alloy aeronautical plates

In view of an extensive literature about guided waves–based structural health monitoring of plate-like structures made of metallic and composite materials, a lack of information is pointed out regarding an effective and universally accepted approach for characterizing capability and reliability in detecting, localizing and sizing in-service damages. On the other hand, in the frame of traditional non-destructive testing systems, capability is typically expressed by means of suitable ‘probability of detection’ curves based on Berens’ model, where a linear relationship is established between probability of detection and flaw size. Although the uncertain factors are usually different between a non-destructive inspection technique and a structural health monitoring approach, it seems that a similar mathematical framework could be assumed. From this point of view, this research investigates the feasibility of application of the very recent ‘multi-parameter’ probability of detection approach, developed within the traditional non-destructive testing field, to guided waves–based structural health monitoring. In particular, numerical simulations as well as experimental responses from flawed aluminium alloy plates were combined to bring about a ‘master’ probability of detection curve. Once established, this curve can be used to study the intrinsic capability of the system in terms of probability of detection curves, overcoming the intrinsic limitation of a single predictor (like the crack size) and a statistical model typically based upon a linear behaviour between the predictor and the response.

Fatigue Crack Growth Monitoring in Composite Bonded Lap Joints by a Distributed Fibre Optic Sensing System and Comparison with Ultrasonic Testing

In this paper, the back-face technique is exploited to monitor fatigue crack growth (FCG) in a composite, single lap adhesive bonded joint, using distributed sensing by Optical Backscatter Reflectometry (OBR). Some preliminary results are presented, indicating that, by measuring accurately the strain profile in the overlap region, the correlation between the minimum peak of the strain profile and the position of the crack tip can be exploited for monitoring the structural health of joints. The proposed structural health monitoring technique was validated on the basis of the results obtained by a non-destructive technique using phased array ultrasonic testing (PAUT). The comparison between the two methods yielded encouraging results, suggesting that, thanks to its distributed sensing capabilities, the OBR technology could allow for improving the back face (BF) technique, as well as any other strain field-based measurement technique, for the health monitoring of adhesive joints.

A Lamb waves based statistical approach to structural health monitoring of carbon fibre reinforced polymer composites.

This research investigates a Lamb-wave based structural health monitoring approach matching an out-of-phase actuation of a pair of piezoceramic transducers at low frequency. The target is a typical quasi-isotropic carbon fibre reinforced polymer aeronautical laminate subjected to artificial, via Teflon patches, and natural, via suitable low velocity drop weight impact tests, delaminations. The performance and main influencing factors of such an approach are studied through a Design of Experiment statistical method, considering both Pulse Echo and Pitch Catch configurations of PZT sensors. Results show that some factors and their interactions can effectively influence the detection of a delamination-like damage.

Load Interaction Effects in a Medium Strength Steel for Railway Axles

As is well known, an interaction effect arises on crack propagation when a specimen or a component is subjected to variable amplitude fatigue loading. Depending on the applied load sequence, a certain amount of retardation or acceleration can then be observed, on the fatigue crack growth rate, with respect to the constant amplitude case. In the case of structural ductile materials, the interaction phenomenon is mainly addressed by the local plasticity at the crack tip and can be explained, from a global point of view, by adopting the crack closure concept. From this point of view, in the present research, load interaction effects in a medium strength steel for railway axles are analyzed. An experimental campaign was carried on this material, using SE(T) specimens, in order to understand and quantify the interaction effects arising from relevant load sequences derived from service. The experimental outcomes were then modeled adopting both a simple no-interaction approach and a more sophisticated strip-yield model in order to quantify the possible interaction effects. The modeling was carried out considering different experimental techniques for deriving the crack growth and threshold behaviors of the material, i.e., the traditional △K-decreasing technique and the compression pre-cracking one.

A Preliminary Analysis about the Application of Acoustic Emission and low Frequency Vibration Methods to the Structural Health Monitoring of Railway Axles

Railway axles are safety-critical components whose failure could result in large additional costs for the infrastructure manager and the railway operator and, in most serious cases, even lead to unacceptable human losses. For this reason, they are periodically inspected by means of non-destructive techniques usually requiring expensive service interruptions. Considering the special case of solid axles for freight cars, this paper investigates the feasibility to apply two structural health monitoring (SHM) techniques to increase vehicle safety and reliability and, at the same time, to decrease the costs of damage inspection. In particular, the considered SHM techniques are Acoustic Emission and the measurement of low frequency vibrations. In the present paper, some preliminary results about the application of both these SHM techniques to freight railway axles are introduced and discussed.

A preliminary study of multi-parameter POD curves for a guided waves based SHM approach to lightweight materials

In view of an extensive literature about guided waves propagation, interaction and numerical simulation in plate-like structures made of metallic and composite materials, a lack of information is pointed out regarding their reliability in structural health monitoring approaches. Typically, because of uncertainties in the inspection process, the capability of non-destructive testing systems is expressed by means of suitable probability of detection curves. Based on Berens’ model, a linear relationship is established between probability of detection and flaw size. Although the uncertain factors differ from a non-destructive inspection technique and a structural health monitoring approach, the same mathematical framework can be assumed. Hence, the authors investigated the application of a recently developed non-destructive testing Multi-Parameter POD approach to a guided waves based SHM one: numerical simulations as well as experimental data from flawed plates were combined to bring about a “master” POD curv...