To make the ultrasonic examination more effective, detailed metallurgical investigations into the failures have been undertaken. Simultaneously rejection pattern of axles during ultrasonic examination have also been analysed statistically. These have provided valuable information regarding failure mechanism (fretting corrosion, crevice corrosion, crack initiation from biting marks, tool marks, surface imperfections) and most vulnerable locations of crack initiation. Fracture mechanics studies have also been carried out to determine the critical crack size in axles and its relationship with the service fractures have also been established. This has indicated that a crack size of 80-100 mm is critical for axles and accordingly the inspection limit for in-service axles has been fixed to 3-5 mm, depending on the flaw location.

Ultrasonic testing procedures which detect such size of defects have been developed along with suitable equipments and transducers. Far-end scanning, low-angle near end scanning, high angle scanning, scale expansion, trace delay, journal probing are some of the techniques employed for ultrasonic examination for axle in-service. Such examinations are conducted to coincide with regular periodic overhaul (POH) schedule. In the case of wagon axles, the crack propagation has been found to be faster and hence a fresh schedule of examination has been introduced. These have resulted in withdrawal of a large number of fracture- prone axles from service. Majority of cracks have been detected on the wheel-seat inner fillet, axle journals and in rare cases on axle body. Examination through diametrical probing also reveal that longitudinal defects in the centre of the axles do not lead to fractures.

Fracture toughness and metallurgical investigations have provided valuable clues to proper design of axle examination system through ultrasonic.