Damage and defects in railway materials
Casey Jessop, PhD student at Engineering Materials IMS, defends her doctoral thesis.
Opponent: Professor Sabine Denis, Univ Lorraine, Frankrike
Doctoral thesis defence
June 13 2019, 10.00
VDL, Chalmers Tvärgata 4c
Railways are one of the cleanest forms of transport in terms of environmental footprint and CO2 emissions, so how can we convince more people to travel by train? We need to assure safety, efficiency, reliability, and comfort to the passengers. There are many parts that go into making sure all these conditions are met, including proper maintenance systems and a good knowledge of the damage in the metal parts. To predict and prevent delays, derailments, and rail fractures, we need to know why and how the materials fail. The focus of this work is on gaining a better understanding of the damage which occurs in railway component materials.
In order to accommodate increasingly large demands on the railway infrastructure, the materials are sometimes subjected to extreme loads, speeds, and weather conditions. The combination of loadings affects the materials in a multitude of ways, including thermal and mechanical damage. Considering that the equivalent of ten tonnes is pushing down on a contact patch roughly the size of a dime on the rail head with each wheel passing, it is not hard to imagine why the materials eventually fail. This failure is usually manifested as cracks in the materials, which could potentially lead to rail breaks, for example. What’s more, the components are also subjected to extreme heating for short periods of time during braking and acceleration. These thermal events affect both cracking and bulk material properties.
The combination of both types of damage has been studied in the current work, including extensive characterization of materials taken from the field, recreation of certain types of defects in the lab, as well as investigation of their effect on crack initiation and growth in experiments. It was found that a combination of several characterization techniques can give an accurate description of crack networks below the surface. Furthermore, thermal damage in the form of white etching layers behave as crack initiation sites, and reduce the life of rail and wheel steels.