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The rated output of USFD machines are as under:-

1. One SRT machine with two operators working 7 days a week should give a rated output of 600Kms per year 2. One DRT machine with two operators working 7 days a week should give a rated output of 1200 Kms per year 3. To convert the number of welds to work load of equivalent track Kms, 30 welds are considered to be one Track Km 4.What are the causes of defects in rail? .Material defects[It is during manufacturing process] .Defects due to residual stresses[ like cooling,rolling etc] .Defect due to incorrect handling[like plastic deformation,scoring,denting] .Defect due to faulty welding[Like gas pore,lack of fusion,inclusions cracks] .Defect due to dynamic stresses .Defect due to excessive thermal stresses. 5.Where is the location of defect? A.Defect starting from the rail end or reaching the rail end. B.Defect in fish plated zones. C.Defect not covered in both of above 6.write the classification of nature of defects in rail? A-Horizontal crack in rail head B-Vertical longitudinal split in rail head C-Horizontal crack at head web junction D-Horizontal crack at web foot junction E-Vertical longitudinal spliting of the web

USFD

FLAWS IN RAILS

SURFACE DEFECTS

  • Wheel Burns
  • Corrugations
  • Plastic flow of metal
  • Surface cracks
  • Battered joints
  • Hogged joints
  • Squats
  • Head checks
  • Shelling of gauge face

INTERNAL DEFECTS

  • Horizontal defects

horizontal_defect_1.jpg

  • Transverse defects

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  • Gauge face corner defects

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  • Longitudinal vertical defects

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  • Star cracks or Bolt hole cracks

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PRINCIPALS OF ULTRASONIC FLAW DETECTION IN RAILS

Any defect in the rail or any material which may ultimately lead to fracture or breakage is called a flaw.The development of flaws in the rails is inevitable because of inherent defects in the rails and fatigue of rails due to passage of traffic. With the increase in the axle loads and the speed of the trains,the rail stresses are increasing day by day which in turn results in high defect generation rate in the rails.

When a particular object is subjected to stresses and strains, it becomes necessary that detection of flaw is carried out in advance so that the timely action is taken to avoid in-service breakage. This is very important in case of rails where the consequences of in-service failures may be disastrous.

PROPAGATION OF WAVE THROUGH ELASTIC MEDIUM

A sound wave can be transmitted through any material which has the elastic properties. The speed of propagation of sound waves depends upon the elastic properties and densities of that particular medium because it involves particle movement in the medium. That is how the velocities of sound waves in solids is more than that in liquids and gases. The denser the material, the more is the velocity of sound waves in that medium.

Depending upon the direction of propagation of wave and the relative direction of the particle vibration, the ultrasonic waves (used for flaw detection in rails, and having frequency greater than 20 kHz) are divided into the following three types:-

1. Longitudinal waves : Oscillation of particles in a medium is in the same direction in which the wave is propagating. This is also known as compression waves and can travel through solids, liquids and gases and their velocities are constant in a given material.

2. Transverse waves : Oscillation of particles in a medium is in the perpendicular direction in which the wave is propagating. This is also known as shear waves because of the energy transfer between two particles is taking place by the shear movement between these two planes. The transverse waves can propagate only through solids as there is no shear resistance in liquids and gases. The velocity of transverse waves is also approximately 50% of that of longitudinal waves in the same material.

3. Surface waves : When the wave is propagated over the surface of material. This consist of both longitudinal and transverse type particle movement.

DETECTABLE FLAW SIZES

The detectable size of the flaw depends upon the wavelength of the ultrasonic wave. Normally a flaw size of λ/2 is reliably detectable. Since the velocity of a particular wave in a medium is constant, a higher frequency wave having lesser time period (f=1/T where f= frequency in cycles/sec and T is the time taken in completing one cycle i.e. time period in seconds) will have a lesser wavelength (λ =v/f). If we use transverse waves, the velocity of which is almost 50% of that of longitudinal wave in the same medium, for the same frequency the wave length of this transverse will be almost 50% thereby reducing the minimum detectable size of flaw by 50%.

PROPERTIES OF SOUND WAVES USED IN FLAW DETECTION

1. Reflection : Principles of reflection are

(a) Angle of incidence is equal to angle of reflection

(b) Incident ray, reflected ray and normal lies in the same plane

2. Refraction : Principles of refraction are

(a) Ratio of sines of angles of incidence and refraction is equal to ratio of velocities in two media.

(b) Incident ray, reflected ray and normal lies in the same plane

3. Transformation : This is the property by virtue of which redistribution of energy takes place. The longitudinal waves are transformed into transverse waves and vice-versa while getting reflected or refracted at the boundary.

4. Diffraction : A wave passing near an edge of an object has a tendency to bend towards and around it. This bending of wave is called diffraction.

5. Absorption : The sound energy is getting absorbed at the boundary and in the medium and hence does not last indefinitely. Thus the sound is not audible after a particular distance which depends upon the intensity, frequency and type of medium.

6. Scattering : It is a process where a wave is forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. Whenever there is abrupt changes in the densities of the two media, at the boundary of these two materials scattering of ultrasonic waves takes place resulting into loss of energy.

The total loss of sonic energy mainly through diffraction, absorption and scattering is termed as Attenuation.

Attenuation = D³ f4 / v4 where, D = average grain size of material, f = frequency and v = velocity

At higher frequencies losses are more, thereby implying that penetration of the sound in that medium becomes poor. This explains as to why the frequencies chosen cannot be still higher for finer crack detections. Also rail steel have fine grain structure but the thermit joints are relatively coarse grained which requires different frequency probes since there will be more attenuation in weld material

FLAW DETECTION

• An ultrasonic wave is first introduced into the rail steel.

• The ultrasonic wave will travel through the rail until it comes across a boundary with a dissimilar medium.

• At the boundary the wave will either get reflected or refracted depending upon the acoustic impedance of the two media.

• The boundary could be the other surface of the rail or an internal flaw.

• A flaw in rail is air void /crack or any other material (slag) having acoustic impedance much different from that of steel.

• The reflected US wave can be detected and the location & size of the source of reflection can be interpreted.

• This is called “pulse echo” or reflection technique.

• Due to the shape & fixity of rail, the transmission & reception of signals has to be done from the same side ( rail head)

• The other but less commonly used method is called “transmission technique”

• A plane (two-dimensional) discontinuity (e.g. material separation, crack) OR a volumetric discontinuity (hollow space, foreign material) reflects the ultrasonic waves mostly in a certain direction.

• If the reflected portion of the sound wave is not received by the probe then it is unlikely that the discontinuity will be detected. The possibilities of detection only increase when the plane discontinuity is hit normally by the sound beam.

PROBE

It is an assembly having crystal(s) and damping material in a metallic housing. The crystal can transmit ultrasonic waves as well as can receive them back simply by reversing the procedure of generation of ultrasonic waves. Probes can be single crystal or double crystal depending upon whether the same crystal is used both as transmitter and receiver or separate crystals are used for this purpose.

Probes are designated by an angle at which the wave enters the rails i.e. angle of refraction from normal.First critical angle is the one for which only transverse wave travels in the second medium. Its value is 27.7° and the corresponding angle of refraction (Probe angle) is 33.3°. Second critical angle is the one for which no waves travels in second medium. Its value is 57.7° and the corresponding angle of refraction (Probe angle) is 90°.

Depending upon the angle at which the waves are transmitted into the test specimen, the probes are classified as follows:

1. Normal Probe : This probe generates longitudinal waves and transmit them into the specimen at zero angle of incidence. The frequency of probe is 4 MHz for rail testing and this probe can detect horizontal flaws, longitudinal vertical flaws (LVF) and bolt hole defects. It is a double crystal probe.

2. Anglular Probe : These probes transmit waves at an angle into the specimen. The angular probes used in IR are 45°, 70° and 70° shifted, all generating transverse waves at 2 MHz. 70° forward/backward probe is used to detect transverse flaws and 70° shifted forward/backward are used to detect Gauge face corner defects.

FLAWS DETECTABLE BY VARIOUS PROBES

HORIZONTAL FLAWS - 0° PROBE

TRANSVERSE FLAWS - 70° PROBE

LONG. VERT. FLAWS (LVF) - 0° PROBE

BOLT HOLE FLAWS - 0° PROBE

GAUGE FACE CORNER(GFC) FLAWS - 70° SHIFTED PROBE

ACOUSTIC IMPEDENCE

It is the property of medium defined as the product of density of the medium and the velocity of the wave in the medium. Relative values of acoustic impedence (Z) in two media decides the percentage of energy to be reflected or refracted.

Coefficient of Reflection (R) = {(Z1-Z2)/(Z1+Z2)}²

If Z1 = Z2 , R = 0

If Z1 » Z2, R ≅ 1

Air film is always between the probe and rail which donot allow wave to propagate into rail steel as acoustic impedence of rail is very low. To eliminate this problem couplant (water) is used.

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TESTING

USFD is carried out by

(a) Single rail tester (SRT)

(b) Double Rail Tester (DRT)

© Hand Testers

(d) SPURT Car

CALIBRATION OF EQUIPMENT

Probes :- 0°, 70°(F), 70°(B), 70° GF(F), 70° GF(B).

1) USFD tester is to be calibrated for 300 mm depth range (long wave) with 60x50x50 mm steel block.

2) Adjust surface echo at ‘Zero’ using ‘Shift/Delay’ control.

3) Adjust Range by ‘H-shift/Delay’ and range’ control simultaneously to get signals at 2/4/6/8/10 on 60 mm block.

4) It is carried out daily before starting the work.

SENSITIVITY SETTING

1) Adjust gap of 0.2 mm approx. in between the contact face of normal probe and the sole of probe shoe (for normal and angular probes)

2) For 0° probe – Adjust Back Wall echo by ‘gain’ control to 100% of FSH.

3) For 70° Probe – Adjust signal from 12Ф hole in head (at 25 mm depth from rail table) to 60% of FSH. While testing on single line section and ‘D’ marked rails on Dbl/multiple line section, additional gain of 10 dB is to be employed. Rail defects classification – with this additional gain of 10 dB. Weld defect classification – after reduction of this additional gain of 10dB.

4) It is carried out weekly.

5) For 70°GFC probe:- Adjust max. signal from 5Ф FBH in head (at 15 mm from rail top) to 60% of FSH.

6) Sensitivity is to be adjusted to cater for variation in temp also (monthly checking)

SINGLE RAIL TESTER

• Capable of Testing only one rail at a time.

• Provided with 5 probes i.e. normal/ 0° (4 MHz), 70°(F) (2 MHz), 70°(B) (2MHz), 70°(S/F) (2 MHz) and 70 ° (S/B) (2MHz)

• The signal received from the defects by any of the probes is indicated on the cathode ray tube (CRT)/LFT screen.

• In order to find out the origin of detection, provision for displaying the individual probe operation has been made in the equipment.

• To be used for testing sections other than LWR / CWR, new AT welds and flange Testing of AT welds.

DOUBLE RAIL TESTER

• Capable of testing both the rails at a time.

• Probes are same as for SRT.

• Provided with multi-channel facility i.e. signal from each probe can be instantaneously distinguished without taking recourse to process of elimination.

• Also provided with a threshold arrangement, LED display and audio alarm in addition to the CRT screen.

• Due to pre-calibrated arrangement, frequent setting of equipment is not considered necessary.

• Due to frequent misalignment of probes on the fish plated joints and limitations of detection of bolt hole cracks, it is desirable to deploy on LWR/CWR sections.

TESTING PROCEDURE

The signals from the rail are received back and are seen on the CRT screen of the machine. The horizontal axis of the screen is calibrated for the time taken by the ultrasonic wave from entering the rail to come back after hitting a reflector in the rail which could be either the bottom of the rail (back wall) or the defect in the rail. Since the velocity of the wave in the rail is known, we can get the distance from which the signal is received back. Thus we can get the location of the defect in the rail.

The vertical axis of the screen is calibrated for the amount of energy reflected by the discontinuity in the rail. The larger the discontinuity or the defect, the more will be the amount of energy reflected. Thus by seeing the height of the signal received, we can estimate the severity of the defect. The classification of the flaw is based on this principle.

The procedure laid down for ultrasonic testing of rails is broadly divided into following parts:

1. Calibration of the USFD machine using a 50 X 50 X 60 mm block.

2. Checking the machine for various parameters - linearity of time base, linearity of amplification, resolution, dead zone, penetration power with the help of IIW block.

3. Sensitivity setting of probes with respect to a reference rail piece having simulated flaws.

4. Testing, interpretation and flaw classification.

FREQUENCY OF TESTING

Safety against failures of rails in track depends upon the inspection frequency and the permissible defect size. The inspection frequency and the condemning defect sizes are related parameters. If the inspection frequency is high, the condemning defect size can be suitably increased. Increase in condemning defect size also enhances the reliability of inspection as chances of non detection for smaller sizes defects are high.

After the initial testing of rails in Rail Manufacturing plant, the first retesting is to be done after 40 GMT in test free period. Whenever rails are not tested in the plant, the test free period shall not be applicable and the rail testing shall be done as per laid down periodicity right from the day of its laying in field.

Test Free Period for Rails

Year of Rolling Test Free Period
Rails rolled prior to April 1999 15% of service life of rail
Rails rolled later to April 1999 25% of service life of rail

Service Life for Rails

Rail Section Service Life (GMT)
72 UTS 90 UTS
60 kg 550 800
52 kg 350 525
90R 250 375

Frequency of Testing for Rails on BG routes

Route GMT Frequency
≤ 5 2 yrs
5 - 8 12 months
8 - 12 9 months
12 - 16 6 months
16 - 24 4 months
24 - 40 3 months
40 - 60 2 months
> 60 1.5 months

Frequency for SKV Welds

• Initial acceptance just after execution (as per AT weld Manual)

• First periodic test after one year

• Further tests based on route GMT

GMT Frequency
>45 2 yrs
30 - 45 3 yrs
15 - 30 4 yrs
<15 5 yrs

Frequency for Conventional AT Welds:

• Initial acceptance just after execution

• First periodic test after one year

• Subsequent tests after passage of each 40 GMT

CLASSIFICATION OF RAIL/WELD DEFECTS

Classification Painting on both faces of web
IMR & IMRW Three cross with red paint
OBS & OBSW One cross with red paint
DFW Two cross with red paint

Action to be taken

1. IMR & IMRW

The flawed portion should be replaced by sound tested rail piece of not less than 6 m length within 3 days of detection.

Interim Action

PWI/USFD shall impose speed restriction of 30 kmph or stricter immediately and to be continued till flawed rail is replaced. He should communicate to sectional PWI about the flaw location who shall ensure that clamped joggled fish plate is provided within 24 hours.

2. OBS & OBSW

The rail/weld to be provided with clamped joggled fish plate within 3 days. PWI/USFD to specifically record the observations of the location in his register in subsequent round of testing.

Interim Action

PWI/USFD to advise sectional PWI within 24 hours about the flaw location. Keyman to watch during his daily patrolling till it is joggled fish plated.

3. DFW

PWI/USFD shall impose speed restriction of 30 kmph or stricter immediately. Sectional PWI to ensure

(a) Protection of defective weld by joggled fish plates using minimum two tight clamps or two far end tight bolts, one on each side after which speed restriction can be relaxed upto 75 kmph for goods train and 100 kmph for passenger trains on BG and 30 kmph for goods train & 50 kmph for passenger trains on MG.

(b) In case the protection has been done using joggled fish plates and clamps, the defective weld shall be replaced within 15 days. However in case the protection has been done using joggled fish plates with two far end tight bolts, the speed restriction imposed in (a) above shall continue till the defective weld is replaced which should not be later than 3 months. The defective weld with speed restriction as (a) above may be continued in track if the track is to be renewed within 12 months.

LIMITATIONS OF ULTRASONIC TESTING

1. The existence of flaw in a region which is not penetrated by the incident ultrasonic beam remains undetected. Consequently defects in the flange of the rail are not detected.

2. The orientation of flaw being unfavorable to the incident ultrasonic beam, e.g. vertical longitudinal flaws, remnant of piping, bolt hole cracks unfavorably placed near joints, etc remain undetected.

3. Flaw size less than the detectable limit of the chosen frequency cannot be detected.

4. Defects within 4 mm from the top of the rail surface cannot be detected as this is the dead zone of the probe.

5. If the top table of the rails have scabs, wheel burns, battering, hogging of rail ends, etc, detection becomes difficult and unreliable because of improper contact.

 
C:/wamp/www/doku/data/pages/usfd.txt · Last modified: 2010/10/01 06:01 by 10.152.4.173