Physical Examination of Rails–Rail Flaw Detection

Rail Flaw Detection

A journey of a thousand miles begins with a single step, but whether the road is good or bad is also an important condition that determines whether we can travel thousands of miles a day. As the main part of the foundation of the railway line, the steel rail directly bears the huge pressure from the vehicle. These forces keep impacting the rail. Over time, some defects such as fatigue cracks that are invisible to the naked eye may appear on the rail over time, endangering the safety of railway traffic. At this time, rail flaw detection operations are required to detect and eliminate defects in time.

The purpose of rail flaw detection

Although the damage to the rail is varied, the development is regular, basically from small to large development, from the table to the development (part of the inclusion, pores, white nucleus and other internal defects are not), and people are sick, from the development of minor disease to serious disease, which gives us the inspection, prediction, processing time.

Therefore, rail flaw detection is the process of using non-destructive testing methods to detect rails in a planned way, so as to find and deal with the damage before it causes serious consequences.

What to do if the damage is found? Continue to observe, and plan treatment or emergency treatment according to the damage situation. This is a consideration of both economy and safety. For those that have affected safety, emergency treatment should be performed, and for those that are not affected for the time being, continuous observation or planned treatment should be arranged.

Non-destructive testing method

Nondestructive testing is a method of testing without changing the physical properties of the object under test. It is equivalent to “testing” and more importantly, “nondestructive”, so it can be repeated and full coverage testing of the object under test. Of course, the conclusion of nondestructive testing is not precise enough, often is intelligent to give a “no” result, difficult to be more accurate.

The corresponding “lossy detection method”, although the conclusion is more accurate, with the process of detection, the measured object also disappears and cannot be used again. It can neither be repeated nor fully covered, so it can only be used as an auxiliary sampling inspection method.

Generally, there are five kinds of nondestructive testing methods, which are ultrasonic testing (UT), magnetic particle testing (MT), X-ray testing (RT), eddy current testing (ET), and penetration testing (PT).

Ultrasonic flaw detection

Basic principle: the directional radiated ultrasonic beam reflects on the defect interface or reduces the transmitted sound energy, and a method to indicate damage by measuring the echo information and the change in the intensity of the transmitted sound wave.

Advantages: good directivity (can directional launch), strong penetrating power (large beam energy), high sensitivity, fewer blind spots (through the combination of sound wave emission angles, the largest reduction of blind spots), low consumption (basically only consumes electricity, and also with a small amount of coupling)

Insufficient: defect display is not intuitive enough (non-professionals can’t understand it basically), surface preparation is required (good coupling is required), and quantitative performance is not good.

Magnetic particle detection

Basic principle: use magnetic force or magnetic field and ferromagnet interaction for flaw detection. When there is a defect, a part of the magnetic field line is exposed to form a leaky magnetic field, and the leaky magnetic field absorbs magnetic powder to form magnetic marks, giving the existence of defects.

Advantages: visual defect display (can directly see where the problem is), high detection accuracy (can detect micron defects), the shape is not controlled

Shortcomings: only the workpiece that can be magnetized (such as austenitic stainless steel can not be detected), only the damage near the surface (the skin of the magnetic field), the need for magnetization and demagnetization, and the directional blind area (when the defect is parallel to the direction of the magnetic field, it cannot be detected without magnetic leakage).

The leakage magnetic field generated by the longitudinal magnetization crack

the characteristic of magnetic particle inspection is intuitive

Need to excite the object under test

Radiographic testing

Basic principle: use the penetration, attenuation, and photochemical action of film for flaw detection

Advantages: the test results can be saved and displayed intuitively

Disadvantages: need to use radioactive sources (like X-rays in hospitals, need strict protection), high cost

This type of method is the same as the principle of taking X-rays and CT in hospitals, basically X-ray detection of welds. It is generally used in weld flaw detection of important equipment, such as flight deck weld flaw detection of aircraft carriers.

Eddy current testing

Basic principle: using the principle of electromagnetic induction, the damage is judged by the change of the co-constructed surface impedance.

Advantages: a wide range of applications (basically as long as it is metal), it can be used at high temperatures

Insufficient: it can only display defects near the surface (skin tendency) and has requirements for the shape of the workpiece (strip and tube are suitable because they must be installed in the coil)

Therefore, the method of eddy current flaw detection is especially suitable for rail factories to inspect rails. For high temperature, strip, and metal, it is only necessary to set up the coil and let the rail pass through.

Eddy current testing uses the principle of electromagnetic induction to detect changes in the electromagnetic field

Penetration test

Basic principle: the use of capillary phenomenon, the use of infiltrating detection agent detection method, also called color detection.

Advantages: no requirements on test objects, simple equipment, can be used in the absence of water and electricity (especially suitable for harsh environment)

Shortcomings: only surface damage can be detected, low efficiency and environmental pollution (both the main color agent and the developing agent are toxic)

Method of rail inspection

Not every nondestructive testing method described above is suitable for rail inspection. After a long period of exploration, now rail inspection basically only uses ultrasonic inspection hair (online rail) and eddy current inspection (rail manufacturers), and sometimes other methods will be used to assist judgment.

Rail on-line inspection

Online inspection is also the main direction of rail inspection. Online flaw detection is divided into manual flaw detection and vehicle flaw detection.

Manual inspection

Artificial flaw detection is the method of using rail flaw detection car or universal flaw detector, which is flexible, low cost but low efficient.

Inside, the flaw detection car can be pushed on the rail, used for full-sweep flaw detection, the speed is about the same as walking speed. Universal and phase-controlled flaw detectors are used to review or reinforce individual locations, with better accuracy but less efficiency.

The phased array flaw detector is an upgraded version of the universal instrument. Using the principle of a phased array, it can detect flaws more flexibly, with higher efficiency, and looks more like a B-ultrasound. After comparison, it takes 10 minutes for the general-purpose instrument to inspect the same weld and about 2 minutes for the phased array flaw detector.

Off-line flaw detection of rails (factory inspection)

Before the rail leaves the factory, ultrasonic flaw detection is required one by one to rule out internal damage. However, many rail manufacturing enterprises also set up rail online eddy current flaw detection equipment to detect rail surface defects (cracks, dropped blocks) as an important supplementary means. And the actual situation is that the eddy current flaw detection is in the front, and the ultrasonic flaw detection is in the back. After these two procedures, you can leave the factory.

Vehicle flaw detection

Vehicle flaw detection is a method of detecting flaw detection equipment installed on large rail vehicles, which has high accuracy, high efficiency, and high cost. At present, the mainstream vehicle-mounted flaw detection probes are wheel probes and sliding shoe probes. Because the sliding shoe probes have high requirements on the profile of the rail, are not suitable for cold weather, and consume a lot of water, so now we mainly use wheel probe sensors.

The advantages of the wheel probe sensor are:

①adapt to different rail head shapes, and will not cause signal loss due to the change of the rail head shape;

②low water consumption can work at a lower ambient temperature;

③reasonable use of ultrasonic far-field flaw detection, will not form multiple interface reflection interference at the outer membrane of the probe wheel; ④ non-contact ultrasonic flaw detection, the probe is installed inside the wheel, which is not easily damaged by mechanical collisions, and can realize flaw detection at steel track forks, level crossings, and bad joints.