rail fish scale injury

rail fish scale injury

The Occurrence, Development and Solutions to Rail Fish Scale Injury

Fish scale injury is a small surface crack caused by long-term stress on the contact part of rail and wheel set. Because it looks like fish scales, it is called fish scale damage. The rail fish scale injury is a common rail disease in railway operations. Fish scale injury has little impact on the line at the beginning, but this kind of damage is secondary disease, on-site maintenance personnel will continue to track and deal with such diseases, and timely treatment before the occurrence of rail block and rail damage to ensure driving safety.

Fish scale refers to the densely distributed surface cracks at the gauge Angle of the rail head tread, called wheel-rail contact fatigue cracks, because of the appearance of fish scale, so it is also known as “fish scale”. Before and after the long ramp change point, the train climbs, braking, starting, wheel and rail intense friction, so that the rail surface quenching, high hardness, low toughness, easy to crack and peel under the action of wheel-rail contact stress, usually the resulting injury is called a fish scale injury.

The fish scale pattern feels prickly when you touch it with your fingers in the direction of driving. After initiating at the track gauge angle tread, it develops obliquely downward (usually at an angle of 10°~15°) along the direction of the deformation streamline, and then gradually turns to expand longitudinally. The depth of expansion is related to the depth of the plastic deformation layer of the rail head tread.

Causes of fish scale injury

After the new rail is used for a period of time, due to the high density of traffic, the fast speed of running, the heavy load of vehicles, the one-way operation of the train in the double line, especially the lack of anti-fatigue of the rail on the curve, and the full-length hardening of the rail, the hardness of the hardened rail surface increases, and the rail surface will gradually form a large number of fish scale injuries under the above conditions. Fish scale injuries are mainly distributed in the section of the curve, the slope, and the straight line, but relatively few, basically due to the fatigue cracks formed by wheel-rail contact. The surface metal deformation of wheel-rail contact surface changes the geometry of the rail, which is manifested as the width of the tread of the rail head, grinding edge, vertical wear and side wear. The plastic deformation degree and wear rate of the rail are directly proportional to the wheel-rail contact stress and friction force, and inversely proportional to the hardness of the rail. On the one hand, the plastic deformation of the rail surface makes the metal work hardened and the hardness increases, on the other hand, the fatigue cracks are easy to start on the surface and develop along the deformation flow line. When the plastic deformation reaches a certain depth, the fatigue cracks formed on the surface will develop downward along the direction of the deformation flow line under the action of the contact shear force. When the expansion rate of the fatigue cracks is greater than the wear rate, there will be different degrees of fish scale cracks and peeling blocks on the working edge of the rail (especially on the curve strand). The direction of this fiscal-shaped peeling crack is consistent with the direction of traffic.

How do rail fish scale injury develop?

In the initial stage of damage, the rail is subjected to the rolling contact stress of wheelset, the maximum shear stress occurs within 6-8mm below the tread of the rail head, and the material on the rail surface will appear subtle plastic deformation and gradually accumulate. When the plastic deformation exceeds the deformation limit of the material, the rail surface will appear small cracks.

In the middle stage of damage, the cracks continued to develop along the direction of wheelset operation and the transverse side of the rail head, and the original small fish scale developed into diagonal peeling cracks, resulting in local depression of the tread, and the surface was dark porphyritic. In addition to the original diagonal cracks, the depression may be accompanied by curved or V-shaped surface cracks.

In the later period, when the cracks on the rail surface continue to widen, the light band at the rail head expands, continuous pieces fall off, and multiple local depressions appear along the entire length of the rail, there is a high probability that nuclear damage has occurred below.

The fish scale injuries generally appear in pieces, and the injuries are closely connected and develop rapidly. When the horizontal force is large, the crack develops along the horizontal direction, develops into a falling block when it is connected with the peripheral crack, and finally leaves a sharp knife type falling block bottom with stress concentration. When the vertical force is large, the cracks develop downward, forming a nuclear injury under fish scratch, and eventually leading to the risk of rail fracture.

The development of fish scale injuries can lead to fatigue sources and other causes of rail breakage

After the 60kg rail of Baotou Steel was put into use, after 10 years of observation and summary, it was found that the crack tip developed into a larger fatigue source due to fish scale crack, which was generally above φ30 mm in diameter, causing more than 20 rail fractures, and the main source was fish scale. In addition to the vertical forces generated by locomotives and vehicles, the primary consideration for railways is the lateral force induced by eccentricity when a train negotiates a curve. Another significant aspect is the design of the gauge baffle used by the outer rail in curved sections, which is based on the straight section. This results in a gap between the rail bottom and the gauge baffle. When a train traverses the curve, a transverse force exceeding the design parameters is generated, leading to an instantaneous triangular state of the rail. Furthermore, the positive vector of small-radius curves is not circular, causing the working surface of the rail head on the inside of the curve to differ. In some sections, the inner wear amount ranges from 3 to 8 mm over a distance of 3 m, indicating severe localized side wear and resulting in insufficient local fatigue strength of the rail.

Problems caused by fish scale injuries

Interference flaw detection

First, although the depth of some fish scale injuries is not deep, generally 4 to 6 mm, because the damage tends to be good, the waveform displayed on the instrument is also very strong, which is similar to the wave position and displacement of early small nuclear injuries. . At the same time, due to the continuous distribution of fish scale injuries, the same waveform appears continuously, which seriously interferes with our injury diagnosis and results in early missed detection of small nuclear injuries. Secondly, due to the existence of the horizontal cap, ultrasonic waves are blocked from detecting nuclear damage under the horizontal cap. In recent years, most of the nuclear damage and rail breakage caused by missed inspections are due to fish scale damage on the rail surface. Therefore, it is key to detect the waveform of instruments to distinguish fish scale injuries and nuclear injuries.

The further development of fish scale injuries will form nuclear injuries, which will directly cause the rail to break and seriously affect driving safety. Therefore, it is recommended that the relevant departments at higher levels formulate a set of injury criteria for fish scale injuries so that there is evidence to rely on during the flaw detection work and to minimize the occurrence of rail breaking accidents caused by fish scale injuries.

The development trend of fish scales

With the expansion of the flow line direction of the plastic deformation, when it reaches a certain depth, the horizontal cap is formed under the action of the orthogonal shear stress of the symmetrical pulse ring in the horizontal direction, and the horizontal cap continues to expand, some of them form rail surface falling, and some encounter the chain inclusions existing in the rail itself, and form longitudinal and horizontal nuclear damage.

The crack in the fish scale slopes directly downward to form a nuclear injury.

Fish scale injury shows the following characteristics in the development process

(1) Due to the hardness of the surface of the hardened rail, the fine cracks formed in the process of wheel rolling are not easy to be worn off by the wheel, often with the extension of the use cycle, the fish scale is inclined to the direction of the vehicle, and the depth is constantly deepening.

(2) In order to delay the side wear of the strand on the curve, it is necessary to apply oil on the head of the rail every day, in order to reduce the wear of the wheel on the rail. With the infiltration of new cracks, the development rate of fish damage is accelerated. In addition, there is a possibility that during the train operation, the wheel and rail through friction, the temperature will gradually rise, and the oil is often at the tail of the train, at this time the oil on the rail may produce a quenching process, will lead to changes in the internal grain structure of the rail.

(3) When the fish scale injury develops to a certain extent, the law is to first develop downward and then develop upward, and eventually cause the inside of the rail head to peel off the block, such injuries are generally open injuries, and can be found and judged by flaw detection personnel and road maintenance personnel.

(4) Fish scale injury in the development process will cause another phenomenon, that is, continue to develop downward, it is different from the general nuclear injury and rail head metal rupture and stripping, but often to the fish scale crack tip as the source of fatigue, from the outside to the inside gradually developed into fatigue damage. It is characterized by rapid development and multi-faceted, that is, when it has just formed, there are at least 3 different quadrants of small cracks on its top surface, and the detection surface is complex and small, which is not easy to be detected by the flaw detector.

How to treat fish scale injuries

Rails with fish scale injuries patterns can be repaired and polished according to the situation. The grinding plan should be determined according to the degree of surface damage and wheel-rail contact. Before grinding, the condition of the rails in the area to be polished should be investigated and measured using a rail profiler or profile template. According to the surface condition of the rail, rail damage and wheel-rail contact, a polishing plan is formulated based on the design profile.

During the grinding process of large-scale road maintenance machinery, the results after grinding should be measured and analyzed, and the grinding plan should be adjusted in a timely manner to ensure that the grinding is in place according to the designed profile. The quality of rail grinding operations should comply with the acceptance standards for rail grinding operations.