Advanced ultrasonic techniques to determine the structural integrity of rail steel

Laser generation coupled with air detection of ultrasound was used for the inspection of the structural integrity of rail steel. The work described in this thesis is divided into two parts. The first part is concerned with detection of internal and surface-breaking cracks in rail tracks and wheels. Rail tracks were tested for surface breaking cracks in the rail head along the vertical, horizontal and transverse planes; internal cracks in the rail head along the transverse plane; and transverse cracks in the rail base and flowed rail. The experiments demonstrated the flexibility and capability of a Laser-Air Hybrid Ultrasonic Technique (LAHUT) for testing hard-to-reach areas in the rail and find critical defects that remain outside the scope of current technology available to the railroad industry today. The non-contact and remote nature of LAHUT also allows for a substantial increase in the speed at which these inspections may be conducted. The current inspection speed operated by the industry is 13 mph.; LAHUT was also used for the inspection of rail wheels. The signals were strong and provided full coverage of the 360° circumference of the wheel. With one transducer, it was possible to provide full coverage in the inspection of each of the tread and flange of the wheel. LAHUT was capable of providing 90° coverage for the inspection of the wheel rim and 15° coverage in the detection of a small hole on the side of the wheel. All of these results exceed the capacity of currently used methods, where a minimum of 56 transducers are used to provide full overage in the detection of cracks in the wheel tread or flange. Furthermore, with the remote and noncontact nature of this technique, tests were possible while the wheel was in rotation, such that, full coverage was possible even when the detection capability was limited to less than 360°.; The second part of this thesis is concerned with fatigue monitoring of railway steel using advanced ultrasonic techniques. A narrowband toneburst was used to drive contact piezoelectric transducers, the reflected echoes were gated and phase and amplitude measurements were taken while specimens were fatigued. The coefficient of attenuation and velocity shift were calculated and plotted as a function of fatigue cycles. In the reverse bending fatigue experiment, the coefficient of attenuation was directly related to the formation and growth of cracks. It followed a certain pattern, rising slightly between 30% and 40%, then rapidly between 70% and 80% of fatigue life. This pattern was independent of the magnitude of applied load, the number of cycles to failure N_f, the origin of the specimen when it was taken from the rail track, or the number of cracks causing failure. In the cyclic tensile loading fatigue experiment, the coefficient of attenuation and velocity shift were calculated and used to derive a value for the dislocation loop length (L) and density (Λ). The results were interpreted in terms of dislocation mobility, pile up, pinning and depinning. Dislocations were found to tangle up and become immobile relatively early in the fatigue life due to the fine pearlitic structure of the rail steel. This resulted in a decline in L and a rise in Λ between 5% to 15% of fatigue life.