The braking process used on railroad cars is known to create tensile stresses in the circumferential direction due to the thermal expansion and subsequent cooling of the wheel rim. The presence of tensile residual stresses can contribute to, and are often the main cause of fatigue failure and cracking.
Compressive residual stresses are generally induced by different means in the wheel material during manufacturing process as these are beneficial since they prevent origination and propagation of fatigue cracks and increase wear and corrosion resistance. Over the wheel’s service life, especially due to the braking process, tensile residual stresses appear becoming a potential danger for safety.
Evaluating periodically the level of residual stress measurement in train wheels making use of non-destructive testing methods is of major importance, as railroad companies can prevent wheel failures and derailments that can be extremely dangerous and costly.
Innerspec Technologies has developed the first, portable, battery-operated handheld system that can be used to provide rail-side inspections and facilitate operation in any environment.
EMATs (Electro Magnetic Acoustic Transducers) are best suited for this application due to a variety of technical advantages, especially the lack of couplant for the inspections, as the ultrasound is generated within the test piece, not in the transducer.
EMATs can generate shear waves with different polarizations and do not mechanically load the surface under test allowing a greater precision of measurement of ultrasonic arrival time variation. Furthermore, EMATs do not suffer from “beam steering” errors caused by misalignment of the transducer with the surface.
The technique uses two orthogonal linearly polarized Shear Horizontal waves at 0 and 90 degrees. A stress change in a material results in a change in the speed of ultrasonic waves passing through that part. The difference in the wave speeds in the hoop (circumferential) and radial directions allows for calculation birefringence value, which is directly correlated with internal stress.
Being K the acousto-elastic constant which can be determined empirically and σ0 the value of texture, both depending on the material.
Innerspec PowerBox H is the only hand-held EMAT system available in the market for residual stress measurement. The portability of the equipment allows the user to determine residual stress in wheels already mounted in axles or vehicles.
Stress measurements can be performed on the different sections of the wheel rim. In contrast with alternative systems, the dual EMAT sensor allows for instant acquisition of stress values avoiding transducer rotation and facilitating user intervention.
Although it is possible to position the sensor manually, the use of a high performance semi-automatic scanner is desirable to guarantee accurate inspection results. Innerspec´ s rail wheel sensor positioner guarantees extended accuracy (0.1 mm) in comparison with manual handling which helps to comply with the requirements set by the European Norm EN13262:2015.
Innerspec has developed proprietary software for stress measurement to meet industry’s needs, with capabilities of generating automatic reports showing inspection results through a user-friendly interface.
Innerspec PowerBox H system allows for storage of wheel specific configuration parameters which facilitate user intervention. In addition, inspection results are reported automatically in a document which includes inspection parameters, table of results and graphs. These results are presented in standard form to secure wheels compliance with railway standards.
Learn more about Innerspec’s applications for internal stress measurement.