An acoustic emission based structural health monitoring approach to damage development in solid railway axles


Railway transport offers a safe and economical means of moving people or goods to various destinations. Electric railway systems are seen as part of a future where systems run on clean and renewable energy resources. As such, upgrading and construction of new and effective well-interconnected rail-track systems is on the rise globally. From a mechanical perspective, the wheelset subsystem of a railway vehicle is fundamentally controlled by axles/loading transferred to the railway track. These sub systems are designed against fatigue limits according to relevant standards. As with most engineering systems, the design is usually based on an expected service life. The system is subject to various in-service damage, that at some point can lead to premature failure. This can have serious implications on cost, operation or in the worst case, can be fatal. With recent technological advances, railway systems designers complement contemporary techniques with a touch of modernism; either empirically or numerically by using modelling. Modeling, for instance, has been employed during design to assess various aspects of the systems. In addition, field inspection techniques, such as: non-destructive testing, have been well advanced and reported noteworthy results; nonetheless, some fatigue-induced failures of axles are still observed.

In essence, the in-service safety of railway axles is a very important engineering challenge, as it has a large impact not only from the economic point of view of the railway operator, but it has cascading effects on an economy. It is therefore vital that the structural integrity of such components is known, during their lifecycle, with the highest possible accuracy via precise modelling, reliable inspections and, more recently but still at research level, effective condition monitoring. In light of this, researchers Professor Michele Carboni from Politecnico di Milano and Dr. Davide Crivelli, proposed to apply structural health monitoring by Acoustic Emission to the special case of a deep-rolled solid axle subjected to a variable amplitude full-scale crack propagation test. Their work is currently published in the research journal, International Journal of Fatigue.

In their approach, a fatigue crack propagation test was carried out in the lab subjecting the axle to many repetitions of a block load sequence defined from real service measurements. Acoustic emission data was continuously recorded during the test, whilst crack size was periodically measured by conventional non-destructive techniques. Eventually, a first-approximation correlation was highlighted between Acoustic Emission data, post-processed by a machine-learning algorithm, and crack propagation ones.

Carboni and Crivelli reported that the physical nature of the collected data was clearer after the application of unsupervised classification, excluding effectively signals due to background noise. Consequently, the possibility to clearly identify and separate the initiation and propagation stages of damage development was observed, providing an important advantage when compared with the conventional application of non-destructive testing. Simply put, acoustic emission detected damage before traditional non-destructive testing.

In summary, the objective of the study was to highlight the feasibility of using acoustic emission for the real-time structural health monitoring of in-service railway axles with initiating and developing damage. Remarkably, by focusing on the propagation stage, an empirical and reasonable relationship, linking suitable acoustic emission features to the measured crack size, was found. In a statement to Advances in Engineering, Professor Michele Carboni mentioned that their work showed that there certainly exists a scope for using acoustic emission to reduce the frequency and the cost of periodic nondestructive testing inspections, and of maintenance as a whole, of railway axles.

An acoustic emission based structural health monitoring approach to damage development in solid railway axles - Advances in Engineering

About the author

Dr Davide Crivelli has worked in structural health monitoring and signal processing for over a decade. He obtained his PhD in “Structural Health Monitoring with Acoustic Emission and Neural Networks” at Politecnico di Milano, Italy, where he developed signal processing and classification methods for identifying damage type and location in composite and metal structures.

Since then, he worked at Cardiff University as a research associate first, and as a Lecturer afterwards. He published 15 international peer-reviewed journal publications mainly focused on sensors, signal processing, and structural health monitoring, and presented at over 20 national and international conferences.

His main research topics deal with signal processing, high and low frequency vibration, and early stage medical applications of structural health monitoring techniques. Whilst working as a lecturer, he led research projects totaling over £400k, and taught undergraduate and master’s degree courses including Engineering Analysis, Design, Computer aided design, and Automotive Design.

He is a Fellow of the UK Higher Education Association (now Advance HE), a Chartered Engineer and member of the Institution of Mechanical Engineers in the UK.

He currently works at a UK national research facility as senior engineer.

About the author

Dr. Michele Carboni got a PhD in Mechanical Behaviour of Materials, at the University of Pisa (Pisa, Italy), with a thesis regarding fatigue crack growth and the influence of defects on the mechanical strength of components subjected to random fatigue conditions. Being a mechanical designer specialized in fatigue, fracture mechanics and the influence of defects on mechanical strength, in 2006 he started to be interested to NonDestructive Testing (NDT) and Structural Health Monitoring (SHM). Today, he is Associate Professor of Machine and Vehicle Design at the Department of Mechanical Engineering of Politecnico di Milano (Milano, Italy).

He is Appointed Member of Academia NDT International, Elected Member of the Board of Directors of the European Federation for Non-Destructive Testing (EFNDT) and Vice President of the Italian Society for NonDestructive Testing Monitoring Diagnostics (AIPnD).

His main research topics, related to Structural Integrity, include the stress analysis of mechanical components, the characterisation of mechanical, fatigue and crack growth behaviours of materials and components, the environmental and technological effects on fatigue and fatigue crack propagation, the defect-tolerant and damage-tolerant designs of components and the development of methods for structural reliability and integrity under in-service conditions and loads. On the other hand, his main research topics, related to NDT and SHM, include the reliability and the capability of NDT and SHM methods (POD, MAPOD, MPPOD), the role of NDT in the damage-tolerant design approach, traditional and advanced (phased array, TOFD, EMAT, creeping waves) ultrasonic testing of materials and components, SHM by ultrasonic guided waves and by acoustic emission, eddy currents testing of corrosion-fatigue phenomena, the characterization of materials and defects by computed micro-tomography and non-destructive experimental stress/strain analysis by digital image correlation and optical fibres (optical backscattered reflectometry and Bragg gratings). The main applicative fields of such research topics are railway transportation (rolling stock and infrastructure) and lightweight design (light materials and adhesive bonding).

His bibliography contains 175 references including: 80 reviewed contributions, indexed in Web of Science/SCOPUS, 82 contributions in the proceedings of international conferences and 4 chapters in scientific books. He is the winner of ten scientific prizes and awards (including IGF Young Researcher Prize, AIAS “Capocaccia” Prize, Best Paper Award ECF12, Best Paper Award WCNDT18, Best Paper Award IWC19) and person in charge of numerous research projects/contracts.

His recent lecturing activities include “Experimental Methods for Structural Diagnostics” (experimental stress analysis, NDT and SHM), “Mechanical Design and Structural Analysis” and “Mechanical Design Laboratory B” (Mechanical Design of Machine Elements).


Michele Carboni, Davide Crivelli. An acoustic emission based structural health monitoring approach to damage development in solid railway axles. International Journal of Fatigue: volume 139 (2020) 105753.

Go To International Journal of Fatigue

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