Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Mechanical measurement system
Reexamination Certificate
2001-10-10
2003-07-29
Barlow, John (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Mechanical measurement system
C702S185000
Reexamination Certificate
active
06600999
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates broadly to non-destructive testing of steel rails and, more particularly, to a rail inspection system having a magnetic induction sensor system that can be used by a test vehicle that can be configured for either highway or railroad use.
BACKGROUND OF THE INVENTION
Basic Rail Testing Approaches
In the wake of several train derailments in the 1920's, it was determined that nondestructive testing methods for locating structural flaws in railroad rail was needed. Initial work focused on an approach wherein a current was applied to the rail and the drop in voltage used to determine the presence of a discontinuity within the rail. This voltage drop technique, although successful statically, proved to be unreliable when testing was carried out using a test car moving over the rails being tested. Subsequent research focused on magnetic induction techniques.
Induction testing is based on simple physical principles. A large direct current is injected into the rail using two sets of contacts or brushes as shown in FIG.
1
. Discontinuities in the railhead section cause a disturbance of the current flowing through the railhead between the contacts. The discontinuity is detected using a sensing head that responds to the accompanying magnetic field disturbance. Perturbations in the magnetic field around the railhead are detected as induced voltages in search coils in the sensing head.
Magnetic induction was the dominant rail inspection technique until the introduction of ultrasonic techniques. Initially seen as complementing magnetic induction, ultrasonics later became the dominant technique. In the typical ultrasonic inspection unit, ultrasonic transducers are installed in pliable wheels that ride over the upper surface of the rail. The pliable wheels are filled with a coupling fluid and are in contact with the rails under pressure. The transducers are arranged to send ultrasonic signals at different angles into the rail and especially the railhead. The return signals are processed and used to map the locations of flaws in the rail.
Types of Rail Defects
Rail defects can occur in the rail head, web or base. Defects are usually a result of impurities in the original ingot that were elongated during the forging process. Depending on the nature of the impurity, the resulting flaw can grow along the axis of the rail or transverse to this axis. Transverse defects may also result from service-induced anomalies, such as work hardening of the railhead. Some of the more common defect classifications are as follows:
Transverse Fissure. This type of defect is usually centrally located in the railhead and results from an oxide inclusion or other small impurity that causes a “stress riser” in the rail. See FIG.
2
. Growth of the inclusion flaw is promoted by the constant flexing of the rail. This growth generally continues until the rail eventually fractures. A fracture of this type exhibits “growth rings” as shown in FIG.
2
.
Detail Fracture. This type of transverse defect usually occurs as a result of the work hardening of the railhead. This causes a split in the railhead and a transverse separation that typically begins on the gage side of the rail as shown in FIG.
3
. (The “gage side” is defined as the side of the rail along which rail car wheel flanges run.) Another mechanism for this type of rail failure is an anomaly known as a “shell.” A shell is usually caused by a horizontally oriented, axial, linear impurity (a “stringer”) that becomes elongated and flattened during use. A shell is not usually classified as a defect in itself; however, it is common for such a condition to subsequently result in a transverse defect.
Vertical Split Head. A railhead stringer that is vertically oriented can grow in the vertical plane along the axis of the rail. This is referred to as a vertical split head and is potentially an extremely serious type of defect as it can result in the loss of the running surface of the rail. See
FIG. 4. A
horizontal split head usually originates from a longitudinal seam or inclusion. Growth usually occurs rapidly along the length of the inclusion and spreads horizontally as shown in FIG.
5
.
Head and Web Separation. This type of defect is usually found at the end of the rail (i.e., at a joint). Such separation is believed to occur due to eccentric loading at the end of the rail. The separation occurs at the weakest point, which is where the railhead joins the web at the fillet.
FIG. 6
shows a head and web defect that has progressed into the fillet area.
Bolt Hole Cracks. These defects are usually as the result of stresses applied to the edge of a bolt hole by the bolt. Such stresses are produced due to the cycling up and down of the joint as a train passes over it. The effect may be worsened by worn joint bars or improper drilling. A severe case is shown in FIG.
7
.
Engine Burn Fractures. These defects result from wheel slippage during acceleration of a locomotive from a standstill. Rapid heating and cooling causes thermal cracks that are exacerbated by the train wheels pounding the area. Transverse separation can occur as a result. An example is shown in FIG.
8
.
Defective Welds. Weld defects vary according to the weld type. In general, there are welds that are made during rail manufacture and there are welds that are made on site while the rail is being installed or repaired. Manufacturing welds are usually “flash butt” welds. Welds made in the field are mostly “thermite” welds. Defects that are germane to the flash butt type of weld are for the most part fusion type flaws. Thermite welding is actually a type of casting operation where a mold is situated around the profile of the rail and molten metal is allowed to flow between the mating surfaces. The flaw possibilities from a thermite weld can be more diverse, ranging from lack of fusion to porosity or other non-metallic inclusions.
Statistically, defects and associated failures can be broken down as follows:
Type of Defect
Percentage of Defected Defects
Defective Welds
22%
Bolt Hold Defects
19%
Transverse Defects
18%
Vertical Split Heads
9%
Head and Web Separation
7%
Detail Fractures
6%
Engine Burn Fractures
6%
Percentage of Notified Failures
Transverse Defects
33%
Defective Wells
30%
Bolt Hole Defects
9%
Vertical Split Heads
8%
Detail Fractures
4%
Factors in Flaw Detection
Defect detection in railroad rails is complicated by the fact that rails come in a variety of shapes and sizes. The accessible scanning surface, which is usually the railhead, is extremely non-uniform. In addition to variability of the rail as manufactured, head shape changes over time as a result of use by high speed, high axle-load trains. The resulting non-uniformity of the rail geometry renders it difficult to maintain the contact of sensor equipment with the rail head. The difficulty is exacerbated by curves, crossings and switches. In addition to affecting data, these track components can be hazardous to the sensor equipment that contacts the rail.
The surface condition of the railhead can be an important limitation on sensor sensitivity. A railhead having rust, grease or other foreign matter such as leaves on its surface can severely inhibit the transfer of energy from an ultrasonic transducer mounted within a rail search unit tire. Search unit tires may also be punctured by steel slivers that develop on the railhead surface.
Weather can be a significant factor in flaw propagation. Contraction of the rail due to cold temperatures combined with heavy train axle loads are very conducive to flaw separation, particularly when a train has a flat spot on a wheel that happens to contact the rail at a critical location relative to the flaw. Weather can also have a significant impact on flaw detection. Formation of ice in particular can make testing extremely difficult.
Regardless of the system quality or its ability to detect defects, personnel and their training are an integral part of the equation. Experience has shown that proper personnel sel
Boyle Jeffery L.
Clark Robin
LaMacchia Brewster W.
Main, Jr. Douglas T.
Barlow John
Hunton & Williams
Sperry Rail, Inc.
Washburn Douglas N
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