Investigate the occurrence and development trend of fish scale damage at the scene, and classify them according to different situations through anatomy, fatigue test and other means. Put forward the standards and proofreading methods for fish scale injuries.
1. Causes of fish cuts
Fish scale injuries are mainly distributed on curves, ramps, and straight lines, but they are rare.
Basically due to the interaction between the wheel and the rail, the top surface of the rail repeatedly appears contact stress, and the surface metal of the wheel-rail contact surface undergoes plastic deformation. When the contact stress approaches the shear yield limit of the rail, the geometry of the rail changes, which is expressed as Rail head tread width, edge grinding, vertical wear and side wear.
The degree of plastic deformation and wear rate of the rail are proportional to the wheel-rail contact stress and friction and inversely proportional to the hardness of the rail.
On the one hand, the plastic deformation of the surface of the rail makes the metal work hardening on the top surface of the rail, and the hardness increases and fatigue cracks appear on the surface, resulting in flaky peeling.
On the other hand, fatigue cracks are easy to germinate on the surface and develop along the deformation streamline.
When the plastic deformation reaches a certain depth, the fatigue cracks formed on the surface will develop obliquely downward in the direction of the deformation streamline under the action of contact shear stress.
When the crack growth rate is greater than the wear rate, scale-like peeling cracks appear at the small arc on the inner side of the rail top where the contact stress is greater. The direction of this scale-like peeling crack is consistent with the driving direction, and the depth of the peeling crack corresponds to the plastic deformation. , Generally, it can reach more than 2mm at the outer rail of the small radius curve. exist
The adhesion and slippage of the curved outer rail wheel and rail promoted the development of cracks.
On the long ramps and the lines before and after the signal, the train brakes, starts, and the wheels and rails are violently rubbed. The steel rail surface produces a quenched martensite metallographic structure, which has high hardness and low toughness. Cracks and peeling are prone to occur under the action of wheel-rail contact stress.
At present, we collectively refer to the flaking and falling pieces caused by fish scale injury or fish scale injury.
2. The development trend of fish cuts
According to the results of our investigation, there are three trends in the development of scale injuries.
1. As the train runs, it wears off, and the fish cuts do not develop.
2. With the development of the streamline direction of plastic deformation, a horizontal cap will be formed under the orthogonal shear stress of the symmetrical vein ring in the horizontal direction when it reaches a certain depth.
The horizontal caps continue to expand, and some form pieces. Due to the unevenness of the track, the wheel-rail impact is increased and the crack development is accelerated. If there are chain-like inclusions in the rail itself, vertical and horizontal nuclear damage will be formed downwards.
3. The scale cracks directly tilt downward to form a nuclear injury.
3. The current situation of fish cuts and scales on the rails in our section
1. The Nei-Kun Line and the Shanghai-Kun Line are single-track sections. Due to the reciprocating operation of the train, the two tendencies of fish-scale crack propagation are compared, and the stripping of the rail surface is mainly caused.
Due to the small radius of the curve of the Shanghai-Kunming Line, the wheel-rail contact pressure on the strands on the curve is very large, so the small rail surface is often peeled off to form a broken rail, which is very similar to the principle of using a rail bending device to dissect nuclear injuries.
2. There are large areas of fish scale injuries in the curved section of the Zhuliu complex line. Densely distributed, it is a typical fish cut.
Due to the large transportation volume and fast speed of the Zhuliu complex, the abrasion rate is similar to or greater than the growth rate of the fish scale cracks. The depth is generally not deep, and the depth is generally only 1 to 2 mm.
4、 fish cuts and scales cause problems
1. Although the depth of some fish scale injuries is not deep, but because the damage tends to be good, the waveform on the instrument is also very strong and the position and displacement of the early small nucleus injuries are similar. At the same time, the continuous distribution of fish scale injuries seriously interferes. Our sentence hurts. Missed detection of early small nucleus injuries.
2. Due to the increase of axle load, the contact fatigue strength of the rail is insufficient, and the surface of the rail head is stripped (or dropped), and the rail surface is black and continues to extend forward. It is easy to cause nuclear damage in the extension direction, and the upper part of the nuclear damage With a peeling layer "hat brim". Because the existence of the "horizontal cap" prevents the ultrasound from being able to detect nuclear injuries under the "horizontal cap".
3. Heavy-duty track section with high traffic density and fast driving speed. As the train runs in one direction in a double track, scale-like damage often occurs on the inner surface of the strand rail head on a small radius curve.
It is different from ordinary metal fragmentation and peeling. It often takes the crack tip as the source of fatigue and gradually forms a nuclear injury. It is characterized by rapid development and a multi-faceted core, which causes the rail to break and seriously affects driving safety.
Combining the above descriptions, it is a key to distinguish the waveforms of fish scale injuries and nuclear injuries.
5. Discussion on the judgment and proofreading of five fish cuts and bruises
1. Strengthen the second wave flaw detection. Due to the scale damage of the strands on the curve of the high-volume section, it is characterized by the vertical and horizontal inclined planes. When the instrument is pushed in the direction of the train in the double-track section, the rear probe 70° should be offset inward, which is conducive to the damage of the scales. Detection of nuclear injuries.
2. Due to insufficient contact fatigue strength of the rail, scale-like peeling is formed on the curve or part of the straight section. The channel emitted to the inner side of the rail head, the fluorescent screen scale of the A-type display will appear regular, continuous, cyclic, and echo display (due to rail head wear, different probe positions and instrument detection range correction errors, echo position Will be different).
3. Because the width of some fish scales is close to or more than half of the rail surface, while the secondary wave is displayed, there will be a first damage wave display, which is the function of the longitudinal inclined surface near the surface of the rail head.
Where the first wave is displayed in the forward direction and the second inspection wave is displayed in the long waveform displacement or in the reverse direction, it indicates that the fish scale injury has split into the rail head to form a nuclear injury.
4. According to the condition of the line, establish the standard of the damage degree of the fish scale injury itself, and quantify it to the wave output of the instrument.
When the depth exceeds the serious injury standard, it will be judged as serious injury, so as to avoid the development of nuclear injury, which will cause rail breakage and affect transportation production.
5. As the axle load increases and the speed increases, the scale damage of the rail head has the characteristics of wide distribution, multiple forms, and rapid expansion.
In view of the above characteristics, various inspection methods should be performed in the inspection of damaged fish scales; you should walk slowly and carefully, and pay attention to echoes with strong amplitude and large displacement, because the peeling end of fish scales is prone to nuclear injury.
6. The echoes with strong amplitude and large displacement shall be strictly verified and verified, and the false ones shall be preserved in order to prevent nuclear damage caused by fish scale injuries.
The above is the author's knowledge of fish scale flaw detection, and there are some shortcomings.
