Fatigue crack behaviour in pearlitic railway rails subjected to large shear deformation
Doctoral thesis, 2024
The impacts from global warming and climate change continue to rise, and securing the needs of future generations requires a transition to a climate-neutral society. Rail transportation, as one of the safest and most energy-efficient modes of transportation, offers a sustainable alternative to fossil-fuel-based transportation. There are, however, many challenges that must be addressed for rail transportation to be a more competitive option. Safety, functionality, reliability, and economic feasibility must be ensured. The major challenge related to materials is rolling contact fatigue, which impairs safety and economic reliability. The imposed loadings from the wheel/rail contact induce severe deformations in the near-surface region of the rail, leading to the formation of an aligned and anisotropic microstructure. Rolling contact fatigue cracks are often initiated in this region, and crack propagation is affected by the direction of the microstructure alignment.
The aim of this thesis work is to better understand how the anisotropy developing in service changes the fatigue and fracture characteristics of rail steels. Fatigue crack propa-gation experiments under uniaxial, pulsating torsional, and non-proportional multiaxial loading, on both virgin and predeformed pearlitic rail steel R260 have been conducted. The material state of the predeformed material is similar to the material state in the near surface of used rails and was obtained by bi-axial large shear deformation under compression.
The uniaxial and torsional fatigue crack propagation experiments showed that the fatigue life is dependent on the material state, with predeformed material exhibiting a longer fatigue life. The effect of predeformation on the crack growth direction was limited in uniaxial loading but dependent on the material state in torsional loading. For the multiaxial fatigue crack propagation experiments, the crack growth direction was significantly influenced by predeformation. The early crack growth rate was found to be similar for both the undeformed and predeformed material states. In addition to these experiments, in-situ fatigue crack propagation experiments were also conducted on single edge notched specimens machined from predeformed test bars along two different directions. This was a first attempt to characterize the effect of microstructure alignment locally on the crack growth and crack paths. Lastly, a method for in-field railhead crack detection using digital image correlation was proposed. The method was evaluated under laboratory conditions, and the detected cracks correlated well with the crack network in the analyzed rail section.
The aim of this thesis work is to better understand how the anisotropy developing in service changes the fatigue and fracture characteristics of rail steels. Fatigue crack propa-gation experiments under uniaxial, pulsating torsional, and non-proportional multiaxial loading, on both virgin and predeformed pearlitic rail steel R260 have been conducted. The material state of the predeformed material is similar to the material state in the near surface of used rails and was obtained by bi-axial large shear deformation under compression.
The uniaxial and torsional fatigue crack propagation experiments showed that the fatigue life is dependent on the material state, with predeformed material exhibiting a longer fatigue life. The effect of predeformation on the crack growth direction was limited in uniaxial loading but dependent on the material state in torsional loading. For the multiaxial fatigue crack propagation experiments, the crack growth direction was significantly influenced by predeformation. The early crack growth rate was found to be similar for both the undeformed and predeformed material states. In addition to these experiments, in-situ fatigue crack propagation experiments were also conducted on single edge notched specimens machined from predeformed test bars along two different directions. This was a first attempt to characterize the effect of microstructure alignment locally on the crack growth and crack paths. Lastly, a method for in-field railhead crack detection using digital image correlation was proposed. The method was evaluated under laboratory conditions, and the detected cracks correlated well with the crack network in the analyzed rail section.
Predeformation
Fatigue crack propagation
Multiaxial loading
Anisotropy
Torsional loading
Uniaxial loading
Pearlitic steel
Virtual Developement Laboratory (VDL), Chalmers Tvärgata 4C
Opponent: Dr Anton Hohenwarter
Author
Daniel Gren
Chalmers, Industrial and Materials Science, Engineering Materials
Fatigue Crack Propagation on Uniaxial Loading of Biaxially Predeformed Pearlitic Rail Steel
Metals,;Vol. 13(2023)
Journal article
Effects of predeformation on torsional fatigue in R260 rail steel
International Journal of Fatigue,;Vol. 179(2024)
Journal article
D.Gren, J.Ahlström, M.Ekh. Fatigue crack characteristics in gradient predeformed pearlitic steel under multiaxial loading
D.Gren, J.Ahlström. In-situ fatigue crack propagation of pearlitic rail steel subjected to large shear deformation
A method for in-field railhead crack detection using digital image correlation
International Journal of Rail Transportation,;Vol. 10(2022)p. 675-694
Journal article
As the planet warms and climate change accelerates, reducing greenhouse gas emissions has become more critical than ever. These changes bring significant challenges that we cannot afford to ignore. To address our current and future needs, we must transition to a climate-neutral society. One promising solution is rail transportation, known for being energy-efficient and safe. If we can ensure its safety, functionality, reliability, and economic viability, rail transport can be a key player in achieving net-zero emissions. A major obstacle in rail transportation is rolling contact fatigue (RCF) which damage the rails by causing cracks. Over time, these cracks can grow and cause significant damage. This issue affects the safety, reliability, and cost-effectiveness of rail systems. To tackle RCF, we need to understand how the mechanical behavior and properties of the materials in wheels and rails change over time, and how these changes influence the initiation and propagation of cracks.
The contact patch between a railway wheel and rail is about the size of a small coin, yet it bears enormous loads, typically between 6-12 tonnes. This results in extremely high contact stress. Additionally, the rail endures frictional forces from traction, turning, and flange contact, which can severely deform the surface material. This deformed material has an aligned microstructure and exhibits anisotropic mechanical behavior, meaning its properties vary depending on the direction of the force applied. Many RCF-related defects start at this severely deformed surface, but we still do not fully understand the material properties and mechanical behavior of this layer. By gaining a better understanding of these factors, we can develop strategies to mitigate the effects of rolling contact fatigue, enhancing the safety, reliability, and economic feasibility of rail transportation.
This thesis explores how the unique properties of pearlitic rail steel, specifically the R260 grade, affect the formation and growth of cracks due to repeated stress. By mimicking this surface condition of severely deformed rails, this thesis work aims to understand how this anisotropy influences crack growth behavior under various loads. The goal is to enhance our knowledge of crack behavior in rail steel, ultimately contributing to safer and more durable railway tracks.
The contact patch between a railway wheel and rail is about the size of a small coin, yet it bears enormous loads, typically between 6-12 tonnes. This results in extremely high contact stress. Additionally, the rail endures frictional forces from traction, turning, and flange contact, which can severely deform the surface material. This deformed material has an aligned microstructure and exhibits anisotropic mechanical behavior, meaning its properties vary depending on the direction of the force applied. Many RCF-related defects start at this severely deformed surface, but we still do not fully understand the material properties and mechanical behavior of this layer. By gaining a better understanding of these factors, we can develop strategies to mitigate the effects of rolling contact fatigue, enhancing the safety, reliability, and economic feasibility of rail transportation.
This thesis explores how the unique properties of pearlitic rail steel, specifically the R260 grade, affect the formation and growth of cracks due to repeated stress. By mimicking this surface condition of severely deformed rails, this thesis work aims to understand how this anisotropy influences crack growth behavior under various loads. The goal is to enhance our knowledge of crack behavior in rail steel, ultimately contributing to safer and more durable railway tracks.
Characterization of crack initiation and propagation in anisotropic material (CHARMEC MU35)
European Commission (EC) (EC/H2020/730848), 2019-06-10 -- 2024-06-09.
Chalmers Railway Mechanics (CHARMEC) (MU35), 2019-06-10 -- 2024-06-09.
Driving Forces
Sustainable development
Areas of Advance
Transport
Materials Science
Subject Categories
Applied Mechanics
Metallurgy and Metallic Materials
Roots
Basic sciences
ISBN
978-91-8103-069-3
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5527
Publisher
Chalmers