- Rail track scrutiny is put into practice of investigative rail tracks for flaws that might lead to devastating failures. According to the Railroad Administration Office of Safety Analysis, railway track defects are the consecutive principal cause of accidents on railways all over the world the most important reason of railway accidents is recognized to human error. Rolling contact fatigue (RCF) is a family of smash up phenomena that become visible on and in rails due to overstressing of the rail material. This damage may appear first on the surface (e.g. head checks, shelling, and squats) or the subsurface (deep seated shell). In either case, these phenomena are the result of repeated overstressing of the surface or subsurface material by the hundreds or thousands or millions of intense Wheel-rail contact cycles.
There are numerous effects that persuade rail defects and rail failure. These belongings include falsification and shear stresses, wheel/rail contact stresses, thermal stresses, residual stresses and dynamic effects.
Defects due to contact stresses or rolling contact fatigue (RCF):
- tongue lipping
- head inspection (gauge corner cracking)
- squats - which start as small surface breaking cracks
Other forms of surface and internal defects:
- oblique fissures
- wheel smolder
Image: crack in rail track
Reasons for rolling contact fatigue: -
- Welds, profile irregularities: The steel in welds everlastingly has stiffness diverse from that of the parent rail steels, even with the best post heat-treating efforts.
- Track Gauge – altering the aloofness between the two rails frequently modifies the arrangement and geometry of the wheel/rail contact. Tight gauge in tangent track promotes gauge corner contact, trucks hunting and RCF, whereas at seeming gauge more of the contacts will be carried towards the put the finishing touch to of the rail where contact conditions are usually less severe.
- Other factors that affect get in touch with stress take account of cant excess and cant deficiency, hunting of wheel sets in tangent track and mild curves, track geometry errors, string-lining forces on grades, tie-plate suspend and poor obligatory, patchy car loading, skewed trucks and mismatched wheel diameters.
Characteristics of contact rolling fatigue: -
- Axle load: directly influences state stress.
- Asymmetric loadings: the strain will also be posed asymmetric on the rail head and the rail wheel contact will get irregular and discontinuous.
- Track gauge: “tight gauge in digression track provokes gauge corner contact, hunting of the bogies and thus rolling make contact with fatigue In curves, scheming wide gauge is indispensable for mitigating low rail damage associated with hollow wheels.
- This damage map have been established for medium radius curve but virtually it be capable of be extend to other
- Wheel transversal profile;
- rail transversal profile and profile irregularities;
- cant excess/deficiency: wheel sets will tend to shift and to heavily offset to the inner or outer rail respectively;
- welds: at too soft/flexible welds a dip will be produced or, in case too hard high spots will be produced this will both leading to geometry irregularities;
- hunting of wheel set in tangent tracks;
- string lining forces on grades tie plate cut in and poor fastening;
- Skewed trucks.
2. The stresses and defects in rail tracks-
All major rail defects require some form of stress to initiate and develop. It is necessary to have some considerate of the terms that is normally used, the loads that are applied to the rails, and the consequential stresses. Vocabulary used for directions in rails is: -
Longitudinal direction: along the rail.
Transverse direction: across the rail.
Vertical direction: normal to the rail.
Vertical plane: vertical along the rail.
Horizontal plane: horizontal along the rail.
Transverse plane: transverse across the rail.
Short pitch corrugations generally develop under lighter nominal axle load (< 18tonnes) passenger operations. The depth of these corrugations is usually less than 5mm and long pitch corrugations generally develop under higher nominal axle load (> 18 tonnes) mixed stowage or unit train operations. The depth of these corrugations can range from 2mm to above 5mm, and can be changeable.
Causes of Corrugations: diminutive pitch corrugations are contemplation to form from the discrepancy wear caused by a repetitious longitudinal descending accomplishment of the wheel on the rail, whether throughout acceleration, braking or imaginative motion across the rail.
Long pitch corrugations, on the other hand, develop because of the plastic flow of the rail material, which is due to extreme wheel/rail make get in touch with with stresses and the mutual vertical reverberation of the wheel set un sprung mass and the track.
belongings of corrugations: Rail corrugations are of concern because they increase the dynamic wheel loads, vibration and therefore the rate of worsening and malfunction of a variety of track and vehicle constituent.
Image: crack width and depth
For the case of contact rolling: -
- Gauss corner checking: -This is a surface circumstance that occurs mainly on the outer rails in sharper curves, and can be described as being like “fish scales”. The cracks are initiated at or very close to the rail surface, normally occur at about 2?5 mm intervals by the side of the rail, and can grow to 2?5 mm in depth.
- Squats defects: - Squats are surface or near?surface initiated defects, which can be of two types. The more common type of squats, which are initiated on the crown or ball of the rail head. These are easily identified visually, as they appear as dark spots or “bruises” on the running surface of the rails. The malfunctioning area seems pitch-black because of the sub?facade cracking which occurs naturally on a horizontal plane, just about 3?5 mm below the rail surface, and which causes a depression on the rail surface.
Image: scanning direction of defects of eddy current
Eddy-current crack depth measurement: -
Current methods for practical crack-sizing using eddy-current NDE are based either on the use of calibration cracks or on the estimation of crack depth from measurements of the surface crack length assuming that the crack has a known aspect ratio. In contrast to the techniques described above, both approaches have serious limitations. A major drawback in using the surface crack length to infer crack depth is that the assumed length-to-depth ratio may be incorrect for the particular crack under investigation. For example, the crack depth will be overestimated if the crack has full-fledged through coalescence of superficial cracks having multiple origins rather than through the growth of a single crack, or may be underestimate if the crack has initiated from an unpredicted sub-surface defect. A supplementary question is what length-to-depth ratio should be assumed in the absence of any fractographic or detailed crack mechanics information. Such problems are less severe in the case of quadrant or bend cracks, where the crack intersects two surfaces and uncharacteristic crack geometry can be more straightforwardly ruled out.
In principle it is possible to estimate crack depth by comparing the eddy-current signal from an unknown crack against data from a calibration crack of the same surface length in the same constituent, conceited the depth of the calibration crack is already recognized and that equipment factors such as crack closure, crack branching and crack-face contact are equivalent. In practice, such a library of calibration cracks is rarely available, leading to considerable uncertainty in depth estimation because the influence of factors additional than depth cannot be isolated from the signal. It is appealing therefore to substitute a set of electro-discharge machined (EDM) slots for actual fatigue cracks and to attempt to determine crack depth by compare the signal from an mysterious crack against that of an EDM slot of known depth. Even though convenient, this come near is wrong.
A new method of crack detecting using eddy current-
Swept frequency methods:
DSTO is budding a first-principles move toward to eddy-current crack sizing in which the crack size is contingent from measurements of the probe response using arithmetical models that are appropriate in the bound of small skin depth.
The electromagnetic method which helps in estimating the crack depth-
The ACFM Method: The Alternating Current Field Measurement (ACFM) technique is an electromagnetic technique capable of both detecting and sizing (length and depth) facade contravention cracks in metals. The basis of the modus operandi is that an alternating current can be induced to flow in a thin skin near the facade of any conductor. By introducing an inaccessible uniform current into an area of the constituent under test, when there are no defects present the electrical current will be without disruption. If a crack is present the uniform current is disturbed and the current flows in the region of the ends and down the faces of the crack. Because the current is a flashing current (AC) it flows in a thin skin close to the facade and is impassive by the overall geometry of the constituent.
Image: ACFM crack detection using electromagnetic technique
Associated with the current flowing in the surface is a magnetic field above the surface which, like the current in the surface, will be concerned in the presence of a defect. An imperative factor of the ACFM technique is its potential to relate dimensions of the magnetic field disturbance to the size of defect that caused that disturbance. The breakthrough came from a combination of studies which provided arithmetical modeling of the magnetic field rather than electrical fields, and advances in electronics and sensing technology.
Although the magnetic field above the surface is a composite 3D field, it is possible, by choosing suitable orthogonal axes, to measure components of the field that are indicative of the nature of the disturbance and which can be related to the physical properties of any cracks present. Figure 1 presents a plan view of a surface breaking crack where a homogeneous ac current is flowing. The field component denoted Bz in figure 1 responds to the poles generated as the current flows around the ends of the crack introducing current rotations in the plane of the component. These responses are principally at the crack ends and are indicative of crack length. The field ingredient denoted Bx responds to the lessening in current surface density as the current flows down the crack and is investigative of the depth of the defects. Usually the current is introduced at right angles to the predictable way of cracking so for a shaft or axle subjected to exhaustion, the current would be introduced in an axial direction to be uneasy by cracks in a circumferential way
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