Table of Contents ECNDT '98
Copenhagen 26 - 29 May 1998
Ultrasonic Evaluation of Stresses in the Rims of Railroad WheelsE. Schneider *, R. Herzer Fraunhofer Institut Zerstörungsfreie Prüfverfahren (IZFP) Saarbrücken, Germany
* Corresponding Author: Eckhardt Schneider
Head of Department Process integrated Nondestructive Testing at
Fraunhofer Institut Zerstörungsfreie Prüfverfahren IZFP,
University Bldg 37, D-66123 Saarbrücken
Phone: +49 681 302 3840
Fax: +49 681 39580
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Texture is often going along with a significant direction dependence of the grain dimensions resulting in direction dependent ultrasonic attenuation. The direction dependences of the ultrasonic attenuation and of the shear wave time-of-flight have been measured using more than 60 wheels of the older types. It is found that both quantities change significantly if there is a texture in the wheel and small changes are observed for wheels without texture or with a slightly developed texture. A threshold value is defined to separate wheels with a strong texture from those with no or with a slightly developed texture. The stress analysis for wheels with a strongly developed texture is only possible if the texture is homogeneous along the circumference of each wheel. The characterization of the stress state in wheels with a slightly developed texture is possible as first results show.
Fig.1: Circumferential stress [MPa] and stress profiles in a wheel, heavily braked in a braking test stand (left part) and in a wheel used in traffic (right part).
European Railroad Research Institute: Document ORE B169/RP2, Utrecht (1989).
Ultrasonic techniques enable a fast and nondestructive evaluation of stress states in the rims of railroad wheels. The applied approach is based on the fact that changes of the stresses in the surface near areas yield also changes of the stress state in the bulk of the rim. Hence, the velocities of ultrasonic waves propagating the bulk of the rim are influenced by the stress state and its change. The ultrasonic beam is also illustrated in Fig. 1, showing the uppermost and deepest measuring position. The application of a shear wave, propagating the width of the rim, vibrating along the height (radial direction of the wheel) and then vibrating along the circumferential (tangential) direction results in the evaluation equation:
tan - rad = K (trad - ttan.) / ttan.
tan and rad are the principal stresses along the tangential and radial direction, respectively. t is the time-of-flight of the shear wave vibrating along the direction indicated as index. K is a material dependend constant, evaluated in the laboratory using representative material samples. The technique is described in more detail by Schneider et al (1994).
As to be seen in Fig.1, the sound propagates the whole width of the rim. The result of the investigation is a mean value of the principal stress differences along the ultrasonic path. Depending on the material of the different types of wheels, a maximum value of acceptable stress difference has been defined in order to assure the safety of the wheels during their service life between the periodical inspections. Besides the material dependent quantities and the results of fracture mechanical investigations, the characteristics of the detected cracks (number, length, orientation) is taken into account, as explained by Rode (1994). The German Railroad (DB AG) specified maximum allowable values of tan - rad for different types of wheels. A nondestructive technique to evaluate the crack depths in wheels is developed by Salzburger and coworkers (1996).
As already mentioned earlier, there are different set-ups available to evaluate the stress states in rims using the same ultrasonic approach. The DEBRO 30 and DEBBIE set-ups developed by Deputat (1989 and1995), the set-up developed by the French Railroad SNCF presented by Limal (1995) and the UER set-up developed by Herzer and Schneider (1994) are the ones known to the authors.
Figure 2: Change of stress state with the distance from the running surface of a wheel heavily braked immediately before the stress analysis (upper part) and of a wheel heavily braked long before the stress analysis (lower part).|
The major part of the monoblock wheels inspected so far are forged and heat treated. This procedure has, among others, the advantage that there is no significant texture. Investigations showed that there is a slight texture in some wheels, but the texture caused error in the result of the stress analysis is less than about ( 15 MPa.
But in the recent past, more and more older wheels and wheels made from other materials have been measured and a large number of wheels could not be inspected by the UER system. Investigations show that some of these wheels have coarse grains or large segregations or other microstructural anomalies causing a very high ultrasonic attenuation. Some other wheels are found to have a strong texture which changes in strength at different positions of the same wheel. Other wheels have a texture which is uniformely developed along the circumference of the wheel. Other wheels again have a slight texture, homogeneously developed around the circumference.
In order to recognize textured wheels, the amplitude and the time-of-flight of the shear wave are measured during the automated change of the shear wave polarization direction from the radial (0°) into the circumferential (90°) direction.
|Fig. 3: Change of ultrasonic signals amplitude with the distance from the running surface measured using a wheel of a new type (solid line) and of an older (dashed line) type.||
Fig. 4: Change of the ultrasonic signals amplitude with the distance from the running surface measured using a wheel of an older type.|
In Fig. 5 the variations of the ultrasonic signals amplitude with the change of the shear wave polarization direction are displayed. The significant change as shown in the left part of Fig. 5 is found using wheels with strongly developed textures. The result as displayed in the right part of the figure is a typical result for wheels without or with a slightly developed texture.
Fig. 5: Ultrasonic amplitude versus the change of shear wave polarization from the radial (0°) into the circumferential (90°) direction as measured using a wheel with a strongly developed texture (left part) and using a wheel with no or a slightly developed texture (right part).|
Based on results of 60 wheels a threshold value is defined to separate wheels with a strong texture from those with no or with a slightly developed texture. The UER system indicates a wheel with a strong texture by the information Textured Wheel in the display. The stress analysis on wheels with a strongly developed texture is only possible if the texture is homogeneous along the circumference of each wheel and if the texture influence on the measuring quantities is within only small variations for wheels of the same type or for wheels of the same manufacturing periode.
The characterization of the stress state in wheels with a slightly developed texture is possible as first results show. The texture influence in one type of wheels is found to be always in the same range of magnitude. Hence, the texture influence can be taken into account by a correction term and the stress analysis yield as result Tensile Stress Smaller Than xxx MPa which again is shown in the display of the UER system.
It will certainly not be possible to separate the texture influence in each individual wheel and to evaluate the stress state with the accuracy of about ( 20 MPa as it is possible using wheels without texture and without other microstructural anomalies.
The additional measurements of the ultrasonic amplitude and the time-of-flight as function of the shear wave polarization direction is automatically started by the UER system if the usual evaluation procedure results in stress values which are not within certain predetermined limits. The additional measurements prolong the inspection time for one wheel from about 1.5 to about 2.25 minutes.
Fig.6: The base unit and the manipulator of the portable UER-T set-up for the automated evaluation of stress states of railroad wheels.|