Electricity: measuring and testing – Magnetic – Railroad rail flaw testing
Reexamination Certificate
1999-06-01
2003-04-15
Snow, Walter E. (Department: 2862)
Electricity: measuring and testing
Magnetic
Railroad rail flaw testing
C324S235000, C324S242000, C324S225000
Reexamination Certificate
active
06549005
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for magnetically detecting discontinuities in magnetic materials and relates particularly, though not exclusively, to such a method and apparatus for detecting surface fatigue cracks in steel rails of railway track.
BACKGROUND TO THE INVENTION
Surface fatigue cracks form in the running surface material of rail heads due to the accumulative effect of wheel loading stresses in the region of the rail-wheel interface. If surface cracks propagate over a sufficient distance along the rails, spalling of the rail surface material occurs which greatly reduces the service life of the railway rails. It is common practice in the management of railway tracks to periodically grind the running surface material down to restore the rail profile as close as possible to the desired shape and to prolong the service life of the rail. The frequency of grinding may be determined by such factors as profile degradation, maximum crack depth and crack geometry, the exhaustion of ductility of the surface material and grinder scheduling limitations. It is therefore desirable to be able to monitor the extent and severity of surface fatigue cracking across and along the running surfaces of the rail heads in order to determine appropriate grinding strategies.
Surface fatigue cracks rarely result in catastrophic failure of the rail. Rail failure is more closely related to internal defects such as transverse defects (TDs), sometimes called “rolling contact fatigue” defects. The most commonly employed non-destructive method for detecting such internal defects is ultrasonic flaw detection. Railway rails are regularly monitored using ultrasonic detectors applied to the rail heads in an attempt to detect the presence of internal defects before catastrophic failure occurs, which may lead to derailment. However, it is thought that surface fatigue cracks hide or mask the presence of internal defects from ultrasonic detection so that the internal defects may not be detected in time. In the USA, the failure to detect internal rail flaws, which led to a major derailment of a train, was blamed on the presence of rail surface fatigue defects. (National Transportation Safety Board (1994), Derailment of Burlington Northern freight train No. 01-142-30 and release of hazardous materials in the town of Superior, Wis., Jun. 30, 1992. Hazardous Materials Accident Report NTSB/HZM-94/01. Washington DC., Report Number PB94-917003). This illustrates the importance of being able to reliably detect the presence of surface fatigue cracks in railway rails.
Magnetic stray flux techniques in non-destructive testing for defects in ferromagnetic materials are well known. The test object is typically magnetised by an imposed magnetic field and the magnetic flux generally stays largely within the object unless it encounters cracks, flaws or other discontinuities in the metal object. At the location of such discontinuities a portion of the magnetic flux is expelled into the air as stray flux. The discontinuity in the metal object results in a region of increased resistance to the flux density which causes some of the flux to deviate around the discontinuity, a portion of which “strays” into the surrounding air. Various techniques have been developed for measuring this stray flux in order to infer the presence of defects in the object.
U.S. Pat. No. 4,792,755 discloses one such prior art apparatus and method for non-destructive testing of metal tubes having surfaces adjoining each other along edges or corners. An imposed magnetic field is generated in the tube between two magnetic pole shoes using an electromagnet. Stray flux is detected using a plurality of magnetic field sensors, typically Hall probes, connected in pairs to a multiplexer by way of differential amplifiers. Stray flux values are measured and subtracted from stored reference values and the results examined in a threshold value discriminator to ascertain whether a preset threshold value, which is indicative of the existence of a structural fault, is exceeded.
One disadvantage of the known prior art magnetic techniques for detecting defects lies in the methods employed for magnetising the test object. Generally the test object is magnetised by an imposed magnetic field generated by the two poles of a magnet (permanent magnet or electromagnet) positioned either side of a detector. Alternatively the imposed magnetic field is generated or induced by passing an electric current through the test object. The difficulty with using an imposed magnetic field is the increased complexity it necessitates in the engineering of the structure and arrangement of the test apparatus. In the case where an electromagnet or the induced magnetic field technique is employed, an electric current source with the appropriate control circuitry must also be provided. These prior art magnetisation techniques do not easily lend themselves to high speed testing of rails installed in railway tracks.
The present invention was developed with a view to providing a simple and effective magnetic technique for assessing rail surface fatigue in railway tracks which is capable of deployment along the tracks at high velocities. However it will be understood that the technique may have wider application and is not necessarily limited to measuring rail surface fatigue.
The term “magnetic material” is used throughout the specification to describe materials, such as iron and steel, characterised by the property known as spontaneous magnetisation and capable of acquiring remanent magnetisation after exposure to and subsequent removal of an applied magnetic field.
SUMMARY OF THE INVENTION
According to one aspect of the present invention there is provided a method for magnetically detecting surface defects in a magnetic material, the method comprising the steps of:
magnetising the magnetic material in an area of interest by passing an applied magnetic field over the area of interest, leaving a remanent magnetic field; and,
sensing stray flux generated in the vicinity of surface defects in the magnetic material by the remanent magnetic field after the applied magnetic field has been removed whereby, in use, the presence of surface defects in the magnetic material can be detected.
Typically said step of magnetising the magnetic material involves touch magnetisation using a permanent magnet. Touch magnetisation requires one magnetic pole of the permanent magnet to be moved parallel to and in close proximity to the surface of the object which is to be left in a state of remanent magnetisation.
Preferably said step of sensing stray flux involves passing a magnetic field sensing means over a surface of the magnetic material in the area of interest and measuring fluctuations in the remanent magnetic field strength in the vicinity of said surface defects.
According to another aspect of the present invention there is provided an apparatus for magnetically detecting surface defects in a magnetic material, the apparatus comprising:
magnetic field generating means for magnetising the magnetic material in an area of interest by passing an applied magnetic field over the area of interest, leaving a remanent magnetic field; and,
magnetic field sensing means for sensing stray flux generated in the vicinity of surface defects in the magnetic material by the remanent magnetic field after the applied magnetic field is removed whereby, in use, the presence of surface defects in the magnetic material can be detected.
In one embodiment said magnetic field generating means is a permanent magnet, for example, a plurality of high energy Neodymium Iron Boron (Nd—Fe—B) permanent magnets stacked one on top of the other. An electromagnet with a suitable configuration could also be used. The magnetic sensing means may, for example, be a Hall effect sensor which is sufficiently sensitive to detect fluctuations in the magnetic field strength caused by surface defects in the magnetic material. Alternatively, the magnetic sensing means may be a detector of the inductive type.
Typica
Hay Sid
Street Robert
Vanselow Robert Gordon
Michael & Best & Friedrich LLP
Snow Walter E.
Technological Resources Pty. Ltd.
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