12.1.1 Continuous testing of rails by manual flaw detectors is strenuous and demands continuous concentration on CRT screen, also the speed achievable is only 2/6 track km in a day. An equipment which can shortlist likely defective spots quickly can improve the effectiveness of the testing.
12.1.2 Self Propelled Ultrasonic Rail Testing (SPURT) Car has been procured for this purpose and is in operation since May’88 over Indian Railways. This car tests both the rails while traveling up to 30 km/h. Most of the testing operations have been automated in this car.
12.2 Description of SPURT Car
12.2.1 SPURT Car has water supply system with a capacity of 4000 litres guiding system for probe beam positioning and air system for lifting and lowering of probe. Probes used are of the following types:
Probes
Defect type
0o (Normal)
Horizontal flaws
70o Central
Vertical transverse flaw
(forward and backward)
70o side wise
Vertical transverse flaw
35o (forward and backward)
Star cracks
550
Vertical longitudinal flaws.
12.2.2 As SPURT car tests at higher speed, observation for flaws can not be done manually. Identification of defect is done electronically according to laid down logics. Also it has computer for tabulating, printing and storage of data.
12.2.3 Comparative characteristics of manual USFD tester and SPURT car are tabulated below:
SPURT car
USFD manual machine
Real time recording of defects
No automatic recording of defect.
Analysed by computer hence less strenuous Concentration.Testing is objective
Testing requires sustained and intense Testing depends on the interpretation by operator
Testing of 80-120 track km per day.
Testing of 2 to 6 track km per day.
Testing of 2 to 6 track km per day.and severity classification is dependent on visual information. So classification is not possible
Ultrasonic information is supplemented by visual information also. So severity classification is done
Repetition of test at a spot is difficult due speed and path constraint
Repetition of test at a spot can be conveniently to done
To take advantage of both machines, the method used by Railways having SPURT car is to use both as complementary to each other. SPURT car short lists the likely defective spots and manual equipment is used to check only these locations. During this check the severity classification is done.
12.3 Locations not tested by SPURT Car
12.3.1 Fishplated joint causes damage to SPURT car probes and also efficacy of testing reduces depending upon condition of joints, therefore only LWR/CWR lengths are normally tested by SPURT car. Also probes are lifted on points and crossings, SEJ’s and buffer rails and these remain untested. At locations fitted with ground rail lubricators, probes are also lifted and hence testing is not done at such locations.
12.3.2 Testing of rail with surface defects like wheel burn and scab is also to be done manually.
12.3.3 The real time out put as well as final defect report contains the information about stretches not tested by SPURT car.
ULTRASONIC TESTING OF RAILS AT MANUFACTURERS’ WORKS
The defects in rails may be due to manufacturing deficiencies, defects arising during service or defects generated due to a combination of manufacturing defects and service conditions. The most important source of defects is the manufacturing deficiencies during steel making, rolling and subsequent processing.
2.1 Testing methods in Steel Plants : Testing in the Steel Plant is carried out with the help of an on-line ultrasonic testing equipment having multiple probes covering entire cross-section of the rail. A typical arrangement of probes is shown inFig.1.
All these probes are 0deg (Normal probes) of frequency varying between 2.5 – 4.0 MHz. They detect defects oriented perpendicular to the direction of the beam.
2.2 Criteria for acceptance : The criteria of acceptance is as per clause 10 of IRS :T-12-96 (specification of flat bottom railway rails). These are reproduced below:
10 Freedom from defects
10.1 The rails shall be of uniform section throughout and shall be free from all detrimental defects such as cracks of all kinds, flaws, piping or lack of metal, etc. having an unfavourable effect on the behaviour of the rail in service.
10.2 The absence of harmful internal defects shall be ensured by the continuous on-line ultrasonic examination. This examination shall be carried out for all rails by the manufacturer to the satisfaction of the inspecting agency.
10..3 The manufacturer in his offer shall furnish the detailed method of on-line ultrasonic testing of rails to be followed by him. The limits of permissible defects for ultrasonic testing of rails shall be as follows and the standard test piece shall be as shown in Annexure-I
Head : 1.5 mm dia through hole
Web : 2.0 mm dia through hole
Web & Foot junction : 2.0 mm dia through hole
Foot : 0.5 mm deep, 12.5 mm long and 1.0 mm wide notch (Inclined at 20deg with vertical axis)´
2.3 In case initial testing of rails has not been done in the rail manufacturing plant, the rails shall be tested either at Flash Butt Welding Plant or at site. In no case rail should be laid in track without USFD testing.
Information regarding the type and nature of rail failures and their service conditions is primarily obtained through personnel responsible for maintenance of permanent way and it is of utmost importance that they are familiar with the various types and nature of rail defects and their possible causes to enable them to report the rail failures accurately. With this objective in view, it is necessary to devise a suitable coding system for reporting rail failures.
1.3.1UIC has adopted an Alphanumeric system of codification of rail failures. In view of its international status and the facility afforded for computerized statistical data analysis, this system has been adopted by Indian Railways for reporting rail failures. The code for reporting rail failures consists of two parts viz first – Alphabetic, consisting of three code letters and second numeric, consisting of three or four digits.
1.3.1.1 First part of the code ± Alphabetic
(a) First code letter indicates the type of rail:
O
indicates
plain rail
X
indicates
point & crossing rail
(b) Second code letter which follows the first code letter, indicates the reasons for withdraw of rail. Thus,
F
indicates
Fractured rail
C
indicates
Cracked rail
D
indicates
Defective rail other than F&C
(c) Third code letter which follows the second code letter, indicates probable cause of failure or rail. Thus:
R
indicates
Cause inherent in rail (attributable to faults in the steel making stage and / or its rolling into rails).
S
indicates
Fault of rolling stock
C
indicates
Excessive corrosion
J
indicates
Badly maintained joint
M
indicates
Other maintenance conditions (Defects which develop due To ineffective maintenance or delayed renewal of rails).
D
indicates
Derailment
W
indicates
Associated with welding defects (through or adjacent Within 100 mm of a welded joint)
O
indicates
Other causes
1.3.1.2 Second part of the code ± Numeric :
This part of the code consisting of three or four digits indicates the location of the failure in the rail as well as its characteristics :
(a) The first digit indicates the location of rail failure (head, web or foot)
(b) The second digit indicates:
(i) The position in the rail section from which failure has started except where failure is associated with welding.
(ii) In case of welding, the second digit indicates the type of welding.
(c) The third digit is interpreted in relation to the first two digits of the code viz :
(i) If failure is due to internal defect (first digit 4 or second digit 1,3 or 5), it shows the direction of the crack or fracture.
(ii) If failure is due to surface defect (second digit 2 or 5), it indicates the nature of defect.
(iii) If failure is by damage (first digit 3), it indicates the cause of the damage (details have been given in succeeding pages)
(d) The fourth digit gives further details.
1.3.1.3 The summary of codification of rail and weld failures is given as under:
(a) First Part – Alphabetic codes
1st letter
2nd letter
3rd letter
Type of railO-Plain rail X-Points & Crossings rail
Reason for withdrawalF-FracturedC-CrackedD-Defect other than F&C
Probable cause of failure R-Inherent in rail S-Fault of rolling stockC-Excessive corrosionJ-Badly maintained joint M-Other maintenance conditions D-Derailment (failure developed later) W-Associated with welding O-Other causes
(c) Second Part – Numeric codes
1st digit
2nd digit
3rd digit
4st digit
Location & characteristics
Origin/Type Of welding
i) For internal defects in welding shows direction: if 2nd digit 1,3,5
1. At rail seat2. Not at rail seat1. Top fillet2. Bottom fillet3. Not at fillet radius
2.Other location
2. Surface of rail head3. In web5.In foot
5. Diagonal at hole 8. Diagonal not at holeii) For surface defects it shows nature of defects if second digit 2,3,50. Corrugation1. Shallow surface defect 2. Breaking out3. C rushing continuous4. Battering local 5. Wheel burns9. Lap, seam, roll mark
1. Short pitch 2. Long corner 1.Running surface 2.Guage corner1. Isolated 2.Repeated
3.Damage done
0
1 Brushing or arcing2 Incorrect machining drilling or flame cutting
4.Associated with welding
Type of welding1. Flash butt2. Thermit 3. Electric arc joint4. Oxy-acyty lene joint7. Building up8. Traction bond
Direction of fracture1. Transverse 2. H orizontal 5. Diagonal at hole8. Diagonal not at hole
5.Corrosion
0
0
1.3.1.4 On the basis of the system of classification described in para
1.3.1.2, a list of the failure code group to be followed is given as under, along with the meaning of the codes.
Within fish-plate limits
Elsewhere on rail
Description
100
200
Transverse breakage without apparent origin (i.e. sudden fracture)
111
211
Internal flaw in head, transverse breakage
112
212
Internal flaw in head, horizontal crack.
113
213
Internal flaw in head, vertical longitudinal split.
1211
2211
Head, surface, shallow surface defect (flaking)
1212
2212
Head, surface, shallow surface defect (line).
1221
2221
Head, surface, breaking out running surface (scabbing)
(a) Transverse crack in head and foot: It is caused by inclusions entrapped during welding, which leads to crack initiation on the foot and its growth in the web region causing fracture. Such cracks can be detected by USFD.
(b) Horizontal cracks in web: These cracks occur in AT welds in which the ends having bolt holes have not been removed. The presence of holes result in unfavorable stress distribution caused due to non-uniform cooling. USFD can easily detect such flaws.
The origin of these cracks is the imperfection in the weldment such as lack of fusion, inclusions, etc. Fracture usually occurs from these imperfections, which may be in railhead, web or foot. During the course of its propagation USFD testing is extremely effective.
(b) Horizontal cracks:
These cracks develop in the web and propagate both in head and foot. The principal cause is large tensile residual stresses acting in the vertical direction.
Joining rails by improper welding may introduce a variety of defects on the joints as well as in the heat affected zone (HAZ) e.g. lack of fusion, cracks, porosity, slag inclusion, structural variation, etc. The quality of weld depends to a large extent on the careful execution of the welding operation. USFD testing done by manual rail tester suffers from following deficiencies:
(i) Full cross section of weld is not covered by normal USFD examination using manual tester thereby leaving areas in head and foot, which may have flaws.
(ii) Micro structural variations in the weldment cause attenuation of ultrasonic energy.
Therefore, a separate testing procedure for welds has been developed
(a) Horizontal crack in head: These cracks run usually parallel to the rail table at a depth of 10-20 mm and may finally split the material layer. Crushing of the railhead may also be observed in the vicinity of the crack. Clusters of non-metallic inclusions and abnormal vertical service stresses are the factors responsible for this defect. USFD can easily detect such flaws.
(b) Vertical-longitudinal split in head: These cracks run parallel to the longitudinal axis of the rail and are caused by presence of non-metallic inclusions, poor maintenance of joints and high dynamic stresses. It cannot be easily detected in early stages by USFD due to their unfavourable orientation.
(c) Horizontal crack at head web junction: Such flaws may lead to rail head separation. Contributory causes are wheel flats, bad fish-plated joint, inclusions and high residual stresses. USFD is sensitive to such defects and can easily detect them.
(d) Horizontal crack at web-foot junction: Such cracks develop both towards head and foot. They are caused by high vertical and lateral dynamic loads, scoring and high residual stresses. USFD can easily detect these flaws.
(e) Vertical longitudinal splitting of the web: It is primarily due to heavy accumulation of non-metallic inclusions and wheel flats. USFD conducted from rail top can detect it only if the defect is severe and in an advanced stage. Vertical longitudinal defects of minor nature are not amenable to USFD examination conducted from rail top. Probing from railhead sides can detect such defects for which hand probing may be essential.
(f) Bolt hole crack: Such cracks often run diagonally and may run towards head or the foot. They result from inadequately maintained joints and unchamfered fish boltholes and stress concentration. USFD (37o probe) can easily detect these cracks. Normal probes provide indication as diminished back wall echo.
(g) Transverse fracture without apparent origin: These fractures occur suddenly, especially during winter and may emanate from microscopic flaws (embedded or on surface) and are generally very difficult to detect by USFD. These minute flaws manifest suddenly under severe service conditions or when the fracture toughness values are comparatively low.
(h) Transverse fatigue crack in head: They resemble a kidney in shape in the railhead and USFD is ideally suited for detecting them. They are generally inclined at the angle of 18deg-23deg and originate at a depth of 15-20 mm below the running surface. Mainly hydrogen accumulation and non-metallic inclusions cause this defect. These cracks are easily detected by 70deg probe.
(i) Horizontal crack at top and bottom fillet radius: These cracks are caused by accumulation of non-metallic inclusions and high residual stresses introduced at the time of rail straightening. These are difficult to be detected by USFD.
(j) Vertical ± longitudinal crack in foot: Such cracks develop from sharp chamfers on the bottom surface of the rail foot. Cracks occurring in this way are the points of origin of transverse cracks in the foot.
(k) Transverse cracks in rail foot: Due to localized overheating during FB welding, structural changes in the bottom surface of the rail material takes place which result in a minor crack. These cracks under the tensile loading give rise to brittle fracture. Such defects are not detectable by USFD. Transverse cracks originated from AT welds in the rail foot grow as half moon and are detectable by 45oprobe.
In order to study the fractures in rail systematically, they may be divided into the following categories based on their location of occurrence in the rail length:
(a) Defects emanating from the rail end or reaching the end of the rail.
The origin and development of such cracks is due to:
Material defects originating during the manufacturing process such as clusters of nonmetallic inclusions, hydrogen flakes, rolling marks, guide marks etc. which may be present in spite of successful non-destructive tests carried out on the rails during quality assurance examination.
Residual stresses induced during manufacture (cooling, rolling, gas pressing and straightening).
Defects due to incorrect handling e.g. plastic deformation, scoring, denting, etc.
Defects associated with faulty welding i.e. gas pores, lack of fusion, inclusions, cracks etc.
Dynamic stresses caused by vertical and lateral loads particularly by vehicles with wheel flats or when the vehicle runs over poorly maintained rail joints etc.
Excessive thermal stresses due to variation in rail temperature beyond specified limits.
