In order to facilitate the statistics and analysis of rail damage, it is necessary to classify rail damage. According to the location, appearance and causes of the damage on the rail section, the damage is divided into nine categories and 32 kinds of damage, which are classified by two digits, with ten digits indicating the location and state of the damage and single digits indicating the cause of the damage. The specific content of rail damage classification can be found in Railway Engineering Technical Manual (Track).
Rail fracture refers to one of the following situations: the whole section of the rail is broken into at least two parts; The crack has penetrated the whole rail head part or rail bottom part; On the top surface of the track, there is a falling block with a length of more than 50 mm and a depth of more than 10 mm. The broken track directly threatens the driving safety and should be replaced in time. Rail crack refers to the separation of some rail materials and the formation of cracks except rail fracture.
There are many kinds of rail damage, such as wear, peeling, rail head core loss, rail waist bolt hole crack and so on. Several common rail injuries are described below. Rail wear mainly refers to the side wear and fluctuating wear of rails on small radius curves. As for vertical wear, it is generally normal and increases with the increase of axle load and total passing weight. Improper setting of track geometry will accelerate the vertical wear rate, which is to be prevented and can be solved by adjusting track geometry.
(1) Side wear
Side wear occurs on the outer rail of small radius curve, which is one of the main damage forms on the curve at present When the train runs on the curve, the friction and sliding of wheel and rail is the fundamental reason for the side wear of the outer rail. When the train passes through a curve with a smaller radius, there are generally two points of wheel-rail contact, and the side wear is the largest at this time. The size of side wear can be expressed by the product of guiding force and impact angle, that is, the wear coefficient. Improving the conditions of train passing curve, such as using worn wheel treads and radial bogies, will reduce the side wear rate.
From the point of view of public works, rail materials should be improved and wear-resistant rails should be adopted. For example, the wear resistance of high-hardness rare earth rail is about 2 times that of ordinary rail, and the quenching rail is more than 1 time.
Strengthening maintenance, setting appropriate gauge, superelevation and inclination of outer rail, increasing the flexibility of the line and oiling the rail side can reduce the influence of side wear.
(2) Corrugated wear
Wavy wear refers to uneven wavy wear on the top surface of rail, which is essentially wavy rolling. Wave grinding will cause high wheel-rail dynamic action, accelerate the damage of locomotive and track components, and increase maintenance costs; In addition, the violent vibration of the train will make passengers feel uncomfortable, and even threaten driving safety in serious cases; Wave mill is also a source of noise. In some freight trunk lines in China, serious wave wear has appeared. Its development speed is faster than that of side grinding, which is the main reason for rail changing.
Wave grinding can be divided into short wave (or ripple) and long wave (or wave). The ripple is periodic and irregular, with a wavelength of about 50~ 100mm and an amplitude of 0.1~ 0.4 mm; Long wave is a periodic irregular wave with a wavelength greater than 100 mm but less than 3000 mm and an amplitude less than 2 mm. ..
Wave wear mainly occurs on heavy-haul transportation lines, especially on coal and ore transportation lines, and also occurs to varying degrees on high-speed and high-passenger transportation lines, and is also very common on urban subways. On the railway with high train speed, corrugated wear mainly occurs, and mainly occurs in straight lines and braking sections. Wave wear mainly occurs in low-speed and heavy-load transportation lines, and generally occurs in curved sections. There are many factors affecting the occurrence and development of rail corrugation, involving rail materials, lines and rolling stock conditions. All countries in the world are committed to the theoretical study of rail corrugation. There are dozens of theories about wave grinding, which can be roughly divided into two categories: dynamic genesis and non-dynamic genesis. Generally speaking, dynamic action is the external cause of rail corrugation, and rail material properties are the internal cause of rail corrugation. In fact, it is quite difficult to summarize all the causes of rail corrugation by analyzing only one aspect, but it is necessary to take the vehicle and track as a system and study the formation of various vibrations as a whole, so as to grasp the whole picture of the causes of rail corrugation.
Polishing rail is the most effective measure to eliminate wave wear at present. In addition, there are some measures to slow down the development of wave grinding: eliminating rail joints through continuous welding to improve track smoothness; Improve the strength and wear resistance of rail materials, improve the quality of heat treatment process and eliminate the residual stress of rail; Improve the track quality, improve the track elasticity, and make the longitudinal and transverse elasticity continuous and uniform; Keep the curve direction smooth, the superelevation setting is reasonable, and the outer rail is oiled when working; The wheel-rail system should have enough resistance, etc.
(3) Allowable limit of rail wear
The allowable wear limit of rail head is mainly determined by strength and structural conditions. That is, when the rail wear reaches the allowable limit, first, it can ensure that the rail has sufficient strength and bending stiffness; Second, it is necessary to ensure that the rim does not collide with the joint splint in the most unfavorable situation. According to the different degree of rail head wear, railway line maintenance rules can be divided into two categories: minor injury and serious injury. When the valley depth of the wave grinding rail exceeds 0.5mm, it is a minor injury rail. The formation of contact fatigue damage can be roughly divided into three stages: the first stage is the change of rail tread shape, such as the irregularity of rail tread and saddle wear at weld, which will increase the impact of wheels on rail; The second stage is the destruction of metal on the rail head surface. Due to the cold working hardening of the metal on the tread of the rail head, the hardness of the working face of the rail head is increasing. When the total mass is 150~200Mt, the hardness can reach HB360. Since then, the hardened layer has not changed. For carbon rail, when the total mass is 200~250Mt, micro-cracks are formed on the rail head surface. For lines with unequal elasticity, when the wheels and rails are obviously irregular, the tensile force on the top surface of the rail is almost equal. If there are micro-lines, the bending stress and residual stress are the same, and the strength of the rail will be greatly reduced. The third stage is the formation of rail head contact fatigue. Due to the insufficient fatigue strength of metal contact and the repeated action of heavy-duty wheels, when the maximum shear stress exceeds the shear yield limit, this point will become a plastic zone, and the wheels will inevitably slip from the metal microstructure every time. After running for a period of time, this slip will accumulate and aggregate, which will eventually lead to the formation of fatigue cracks. With the increase of axle load, the inadaptability of mass transportation conditions, rail materials and rail types, the initiation and development of contact fatigue cracks will accelerate.
Stripping near the fillet of the rail head working edge is mainly caused by the following three reasons: longitudinal fatigue cracks caused by inclusions or contact shear stress lead to peeling; The alternating shear stress produced by the guide wheel on the curved outer rail promotes the fatigue of the outer rail head and leads to peeling; Poor maintenance of wheels and tracks accelerates the development of stripping. Usually peeling will cause stress concentration in the notch area, affect ride comfort, increase dynamic impact, and promote the generation and development of cracks in the notch area. The existence of notch area will also hinder the development of metal plastic deformation and reduce the rail plastic index.
Rail head nuclear damage is the most dangerous form of damage, which will suddenly break under the action of trains, seriously affecting traffic safety. The main reason of rail head core loss is that there are tiny cracks or defects (such as nonmetallic inclusions and white spots) inside the rail head. Under the action of repeated dynamic loads, extremely complex stress combinations appear in the rail head below the running surface of the rail, so that tiny cracks nucleate first and then develop around the rail head until the steel around the core is not enough to provide enough resistance, and the rail suddenly breaks under the warning of milliyuan. Therefore, the defect of rail internal material is the internal cause of nuclear damage, while the external load is the external cause, which promotes the development of nuclear damage. The development of nuclear loss is related to traffic volume, axle load, running speed and line plane state. In order to ensure traffic safety, rails should be inspected regularly.
The measures to slow down the contact fatigue damage of rail are as follows: purifying rail steel and controlling the shape of debris; Adopt quenched rail, develop high-quality heavy rail and improve the mechanical properties of rail steel; Reform the old rail recycling system and rationally use rails; Rail grinding; Track laying is classified according to rail steel materials.