Rolling Contact Fatigue or Damage (RCF/RCD) is the surface and near-surface damage that occurs on the rail head and wheel treads of rail cars. The damage in the rail head due to progressive cyclic loading from the contact between the wheel and the rail head can lead to formations of small cracks that can ultimately grow and join up to form a flake that falls loose, leaving behind a cavity in the running surface of the rail or turn downward to a limited depth forming a fatigue crack commonly referred to as head checks and gauge corner cracks. Quantifying RCF/RCD crack depths and density in rails is important for all the railroad authority and industries to manage their grinding programs effectively and efficiently. Detecting RCF/RCD can be challenging due to the size of the cracks, which typically starts out at 2-10 Îơm and progressively can grow up to depths of 3 mm to 5 mm. It becomes impossible to characterize these early stage RCF cracks without physically destroying the sample to get to the area of interest. To gain a better understanding, the cracks that are formed from RCF/RCD can be simplified into four different types: (I) vertical/normal, (II) oblique, (III) branched, and (IV) clustered cracks. Methods that can accurately detect and characterize these cracks non-destructively have been of high interest for the rail community. This work focuses on utilizing Surface Acoustic Waves (SAWs) for detection and characterization of RCF/RCD defects through numerical simulations the using finite element method(FEM). A transient, elastodynamics wave propagation model was used to simulate SAW propagation. Parameters such as the transmission (Tc), reflection (Rc), scattered (Ps), and time off light (TOF) were extracted from the model and quantified to build relationships for understanding the mode conversion and interaction phenomena. The different type of defects that were modeled in FE included vertical, oblique, and branched defects. First, SAW interaction with a set of vertical, oblique and branched RCF defects were studied by quantifying Tc. The Tc values exhibit duality at certain crack angles, which makes it challenging to accurately characterize oblique RCF/RCD type of defects. Experiments have been done to validate vertical and oblique defects: the results also exhibit a duality in Tc for the oblique defects. To understand branched crack morphology, the complex crack geometry can be simplified into a series of varying angled elastic wedges, which is part of a classical problem within elastodynamics. Finally, SAW interaction with clustered cracks for two sets of densely packed RCF/RCD type of defects: a uniform cluster and a non-uniform cluster to further develop characterization techniques using Tc/Rc relationships and through signal processing methods. The impact of this work is to provide a proof of concept that the presented numerical results can be validated through experiments and become field implemented.
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