When it is good to be rusty!


Traveling by train compared to an airplane generates lesser carbon-footprint, and these days responsible citizens are availing this mode of transport to a greater extent. 


But for a safe train journey, the wheels – the foundation of this journey – need to be in a sound condition and well connected by a healthy axle.


Railway axles are one of the most highly stressed components of the railway-system.


Mechanical stresses cause fatigue-cracks in them, which propagates with time making them vulnerable. The safety and consistency of these axles are, therefore, of utmost importance. Train axle failures have triggered a significant loss of life, and researchers are devoting their time to avoid such incidents.


The research group of Dr. Pavel Hutař at CEITEC-IPM has recently presented an exclusive observation (1), which could solve the problem mentioned above.


They teamed up with the Bonatrans group – one of the biggest manufacturers of railway axles in Europe. 


A fatigue crack that caused a fracture of a railway axle (Image courtesy: BONATRANS GROUP)


Dr. Hutař describes the backdrop of their research aim, “One of the key problems is to outline the strategies to maintain and endure the railway axles. Therefore, it’s necessary to estimate – how long it takes for the fatigue crack to propagate through the axle. Numerous phenomena are taking place during this propagation. One of them is oxide-induced crack closure (OICC), and the effect of humidity on the crack deceleration during the process.”


Dr. Tomáš Vojtek, the lead author of the study, explains, “The railway tracks are of thousands to 100s of thousand kilometers long. The railway operator has a certain interval of kilometers, at which the vehicle should go for inspections.“


“During the inspection, they search for fatigue cracks in the axle component. If they do not find any crack, it means either there is no crack, or the crack is smaller than the detection level.” 


Dr. Pavel Pokorný and Dr. Pavel Hutař (Photography by Emil Gallík)


“Our task is to observe these small cracks and estimate the fatigue-crack propagation stretch with the use of simulations. The operator wants to know the number of kilometers the component with a crack can still be operated safely without failure.”


Dr. Vojtek added, “We are trying to find out ways to resist crack propagation. Initially, the scientific community thought that oxide is one of the small influences, but Dr. Pavel Pokorný (another researcher associated with the study) comprehends this as a great influencing factor, and therefore the effect of OICC was found to be very important for the propagation resistance.”


OICC is a well-established phenomenon where corrosion (AKA rust) occurs during crack propagation.

When the base material at the crack-surface is exposed to gaseous and aqueous atmospheres, it gets oxidized, filling the cracks as a result. 



The key outcome of the study: humidity matters!


Dr. Vojtek delves more in-depth, “Our study covered two scenarios: one involves basic-science, and the other applies this knowledge for a better railway axle. The cooperation with the Bonatrans group helped us establish how long an axle can run. The scientific part was to study the crack-propagation in terms of the influencing factors.”


“The most important aspect of the story is the influence of air humidity on the materials’ resistance to crack propagation, which is more essential than was believed. Many laboratories measure the air humidity, but they don’t test its influence, e.g., performing experiments under a range of humidity.”


Dr. Pavel Pokorný carefully measured the temperature and humidity and then created a controlled environment for the experiments. He designed a special chamber to control humidity and observed, in dry air (lower humidity), the cracks propagate faster. 



The chamber designed to control humidity (Photography by Emil Gallík)



Dr. Vojtek mentions, “This is quite unexpected! Usually, when you have humidity, it leads to corrosion of the material, deteriorating the axle properties. In this case, the humidity works as a paradox which improves the properties.”


“In a laboratory of material-testing, the humidity is varied, but the temperature remains more or less constant,” states Dr. Hutař.


“When the axles are outside in nature, with changing temperature, the absolute humidity of the air also oscillates substantially. This important factor remained hidden until we observed the consequence.”


By explaining the effect, Dr. Hutař and Dr. Vojtek utter, “Actually, the mechanism is quite easy to understand.“


“Imagine a crack: during operation, when the components are always rotating and accelerating, the crack is cyclically opened and closed, and subsequently propagates. When the material is corroded (with higher air humidity), it produces oxide debris - which fills in the crack, and the structure cannot deform.”



Gauging the future


Dr. Vojtek says, “Our research shows how the crack can be filled naturally, but we need to think about ways to artificially fill the cracks. We have a wide-open field of potential chemicals that we could put in there, which can increase in volume and do that task.”


“The know-how will not only stabilize railway axles but has the potential even to protect steel bridges - a huge problem nowadays.”


“You can imagine, it is much harder to replace a bridge than an axle,” says optimistic Dr. Vojtek.  



References

  1. Vojtek, T; Pokorny, P; Kubena, I; Nahlik, L; Fajkos, R .; Hutar, P, 2019: Quantitative dependence of oxide-induced crack closure on air humidity for railway axle steel. INTERNATIONAL JOURNAL OF FATIGUE 123, pp. 213 - 224, DOI: 10.1016 / j.ijfatigue.2019.02.019




Written by Somsuvro Basu


Publication date: 23.01.2020