In railroad transportation, railroad tracks support and lead the locomotives to move forward. If the tracks crack, the trains may derail or turn over which will cause serious damage to people’s lives and property. Railroad track inspection is the process of using non-destructive testing (NDT) methods to detect rails in a planned way to find and deal with flaws in railroad tracks that may lead to disastrous results. As the fast development of high-speed rails and heavy-haul rails, the rails are being squeezed and impacted more and more. Therefore, the probability of rail damage is increasing. To get adequate railroad track inspection has become important.
Non-destructive testing (NDT)
Non-destructive testing is to measure the surface and internal parameters or performance of the tested object without destroying the physical and chemical properties and geometric integrity of the tested object.
Compared with “detection”, the more important thing is “non-destructive”. It is possible to perform repeated and full-coverage inspections on the measured object. Of course, the conclusions of non-destructive testing are not precise enough, and often it can only give the result of “yes”, which is difficult to be more precise.
Ultrasound testing
- Principle
Utilizing the propagation, attenuation characteristics of sound waves in the medium and the reflection, refraction characteristics on the interface.
- Application
Forged, rolled parts and other workpieces or materials with finer grains. It is mainly for the detection of internal flaws. Surface defects can also be detected.
- Advantage
Good directivity, strong penetrating power, high sensitivity, few blind areas (the combination of sound wave emission angles can be used to minimize blind areas), low consumption (basically only use electricity, and a small amount Coupling agent).
- Disadvantage
Defect display is not intuitive enough (non-professionals can’t understand it), surface preparation is required (good coupling is required), quantitative performance is not good.
Magnetic particle testing
- Principle
Using magnetic force or the interaction between the magnetic field and magnet for flaw detection. When there is a defect, a part of the magnetic field lines are exposed to form a leakage magnetic field, and the leakage magnetic field attracts the magnetic powder to form a magnetic trace, indicating the existence of the defect.
- Application
Detect non-ferromagnetic defects in ferromagnetic materials such as iron, nickel, cobalt and certain alloys.
- Advantage
Intuitive display of defects (you can directly see where there are any problems), high detection accuracy (detection of defects in micrometers), and unlimited shapes.
- Disadvantage
Only can detect the surface or shallow subsurface of ferromagnetic materials.
Radiographic testing
- Principle
Utilizing the penetration and attenuation of rays and the photochemical action of film for defect detection.
- Application
Suitable for damage detection of different materials, but mainly used for detection of castings and welds.
- Advantage
The results are intuitively displayed and can be stored for a long time.
- Disadvantage
Strict protection is required, and continuous operation is generally not possible.
Eddy current testing
- Principle
Using the principle of electromagnetic induction, the damage situation can be judged by the change of the surface impedance of the workpiece.
- Application
Eddy current is basically can be used by any metal.
- Advantage
Wide range of applications and can be used at high temperatures.
- Disadvantage
It can only display defects near the surface, and the shape of the workpiece is required. The strip, tube shape is suitable because it needs to be installed in the coil.
Penetrating testing
- Principle
The method of using the capillary phenomenon to detect with the infiltrating detection agent is also called color detection.
- Application
Special needs inspection. For example, the tip rails where the detection angles for ordinary rails are not applicable.
- Advantage
There are no requirements for the detection object, the equipment is simple (just a few bottles), and it can be used without water and electricity especially suitable for harsh environments.
- Disadvantage
Can only detect surface damage, low efficiency, and environmental pollution (colorants and imaging agents are both toxic).
Rail inspection method
Not all the non-destructive testing (NDT) method are suitable for rail inspection. Ultrasound testing and eddy current testing are recognized as the most practical methods for the train tracks. The ultrasound testing method can not only detect the internal defects, but also find out cracks on bolt holes, head and web, and the longitudinal crack at the bottom of the rail. It is the most effective rail inspection method. The eddy current testing method is particularly suitable for steel rail factories to inspect steel rails even under high temperature.
As for the inspection equipment, there are mainly hand-pushed trolley and rail inspection car. The hand-pushed trolley is a flexible inspection device when a section or specific location need detection. The rail inspection car is a large-scale rail vehicle with flaw detection equipment mounted on. Compared with the hand-pushed trolley, the rail inspection car has the advantages of high accuracy, high efficiency. Of course, the cost is higher. At present, the mainstream vehicle-mounted flaw detection probes are wheel probes and sliding shoe probes. Because sliding shoe probes have high requirements for rail profile, consume large amounts of water, and are not suitable for cold weather, we now mainly use wheel probe sensors.
Defects of railroad track
The main defects of railroad track are corrosion, inclusions, seams, shelling, transverse fissures, wheel burn, etc. There are not only defects in the production of train track, but also problems in the process of transportation, laying and use. Defects can be found in different parts of the railroad tracks such as the head, web, foot, switchblades, welds, bolt holes. The rail defects always develop from small to large, from the surface to the inside (partial inclusions, pores, white cores and other internal defects excluded). The process is just like people’s illnesses, they all progress from minor illnesses to serious ones. This gives us time for inspection, anticipation, and treatment.
What should I do if I find a flaw? Should I continue to observe, make a treatment plan or practice an emergency treatment? Well, it depends on the injury or damage. This is a consideration of both economy and safety. For those that have already affected safety, they will be treated urgently, and those that have not been affected temporarily will continue to be observed or scheduled for treatment.